Swift - How to manage turn order management? - swift

I'm working on a local game which relies on turn order.
Rules;
There are a number of phases in the game (ie: Buy, Sell)
During each phase, a player takes a single turn
Each phase is not considered complete until every player (in turn order) has completed their turn.
I'm not sure how to manage this data. There are a number of things to track.
The phase we are in
The current player on turn
When all players have completed their turns
When the end of the turn order has been reached so we can move to next phase.
Resetting all turn completions when all phases are complete
I'm thinking that a subscription model is the best approach to this, but I'm not used to such a pattern.
Currently I'm using a similar system to a to-do where the phase itself can be marked complete or incomplete.
This is the way I'm currently handling turn orders and phases in Swift playground.
// Turn order management
class TurnOrderManager: NSObject
{
static var instance = TurnOrderManager()
var turnOrder: [Player] = [Player]()
private var turnOrderIndex = 0
var current: Player {
return turnOrder[turnOrderIndex]
}
func next() {
if (self.turnOrderIndex < (self.turnOrder.count-1)) {
turnOrderIndex += 1
}
else {
print("Reached end of line")
}
}
}
class Player: NSObject {
var name: String = ""
override var description: String {
return self.name
}
init(name: String) {
super.init()
self.name = name
}
}
let p1:Player = Player.init(name: "Bob")
let p2:Player = Player.init(name: "Alex")
TurnOrderManager.instance.turnOrder = [p1,p2]
print (TurnOrderManager.instance.current)
TurnOrderManager.instance.next()
print (TurnOrderManager.instance.current)
TurnOrderManager.instance.next()
print (TurnOrderManager.instance.current)
// ---------------------------------
// Phase management
enum PhaseType: Int {
case buying = 1
case selling
}
struct Phase {
var id: PhaseType
var title: String
var completed: Bool = false {
didSet {
// Notify subscribers of completion
guard completed else { return }
handlers.forEach { $0(self) }
}
}
var description:String {
return "Phase: \(self.title), completed: \(completed)"
}
// Task queue
var handlers = [(Phase) -> Void]()
init(id: PhaseType, title: String, initialSubscription: #escaping (Phase) -> Void =
{_ in})
{
self.id = id
self.title = title
subscribe(completion: initialSubscription)
}
mutating func subscribe(completion: #escaping (Phase) -> Void) {
handlers.append(completion)
}
}
class MyParentController {
lazy var phase1: Phase = {
return Phase(id: .buying, title: "Phase #1") {
print("Do something with phase: \($0.title)")
}
}()
}
let controller = MyParentController()
controller.phase1.completed = true
Question:
I'm wanting to notify:
Turn is complete
All turns are complete (so that it can move to next phase)
How do I make my TurnOrderManager alert the PhaseManager that the current turn is complete.
How do I make my PhaseManager know that when all turns are complete to move to the next phase.
I apologize for the verboseness of my query.
Many thanks

You're going to want to define a delegate relationship PhaseManager and your TurnOrderManager.
Here is the Apple docs on protocols.
Here is a great article on delegation.
First you'll need to define a protocol. Something like this:
protocol TurnManagerDelegate {
func complete(turn: objectType)
func allTurnsComplete()
}
Next you'll have to conform your PhaseManager to the protocol:
class PhaseManager: TurnManagerDelegate {
...
func complete(turn: objectType) {
// implement
}
func allTurnsComplete() {
// implement
}
...
}
Last you'll have to add a property on your TurnOrderManager with the delegate:
class TurnOrderManager {
...
var delegate: TurnManagerDelegate
...
}
and call the functions whenever needed in your TurnOrderManager:
...
delegate?.allTurnsComplete() //example
...
You'll also have to set your PhaseManager as the delegate before your TurnOrderManager would try to call any of the delegate methods.

Related

The most efficient way to build tree from dictionary data

I have a dictionary with the following structure: [Point:[Line]], where:
Point - custom data structure that contains two coordinates (X, Y)
Line - tuple (Point, Point) that contains the first and last points of the line.
Key - first point of the line.
So, it is a dictionary of lines grouped by their first point, like following:
[a: [(a,b),(a,c)], b: [(b,c), (b,d)], c: [(c,d)] ]
The goal is to convert this dictionary into a list of data structures like following:
(a,b) -> (b,c) -> (c,d)
(a,b) -> (b,d)
(a,c) -> (c,d)
So, basically a tree with the root at the first point.
I tried to use builder pattern and stack to perform this operation, so I created a builder using first point and put in into stack, then started a while loop until stack is empty and then in the loop was removing current builder and creating new ones based on last & first points, the code looked like following:
import Foundation
typealias ProcessedLine = (UUID, [LineModel])
typealias LinePointDict = [Point: [Line]]
class LineAggregator {
var builderStack: Stack<LineModel.LineModelBuilder>
let startPointMap: LinePointDict
var result: ProcessedLine
var currentBuilder: (Point, LineModel.LineModelBuilder)?
let startPoint: Point
init(LineUid: UUID, startPointMap: LinePointDict, startPoint: Point) {
self.builderStack = Stack<LineModel.LineModelBuilder>()
self.startPointMap = startPointMap
self.result = (LineUid, [])
self.startPoint = startPoint
self.currentBuilder = nil
}
func getLineAggregation() -> ProcessedLine {
for Line in startPointMap[startPoint]! {
var bldr = LineModel.LineModelBuilder(initialLineUuid: result.0)
bldr = bldr.addLine(Line: Line)
builderStack.push(bldr)
}
return aggregateModels()
}
func aggregateModels() -> ProcessedLine {
while !builderStack.isEmpty() {
takeBuilderFromStack()
aggregateLine()
}
return result
}
/**
* This functions pops Builder object from stack if the stack is not empty and sets it as a Current object to be processed.
* #param object
* #return
*/
private func takeBuilderFromStack() {
if(!builderStack.isEmpty()) {
let curBuilder = builderStack.pop()!
currentBuilder = (curBuilder.getLastElement(), curBuilder)
}
}
private func aggregateLine() {
if currentBuilder?.1.isLastAdded() ?? true {
//If there is only one Line in the Line model
if(currentBuilder!.1.isLastAddedLineLast()) {
result.1.append(currentBuilder!.1.build())
return
}
if(!builderStack.isEmpty()) {
print("ERROR: Empty builder stack! Such situation should not happen. Pay attention at it.");
builderStack.removeAll()
}
return
}
if currentBuilder != nil {
for Line in startPointMap[currentBuilder!.0]! {
var newBuilder = LineModel.LineModelBuilder(builder: currentBuilder!.1)
newBuilder = newBuilder.addLine(Line: Line)
if Line.isLast {
result.1.append(newBuilder.build())
} else {
builderStack.push(newBuilder)
}
}
}
}
}
This solution is a very straightforward one. It works, but I have a very large amount of data in the dictionary, and the number of combinations is even larger, so this algorithm is extremely slow and not memory efficient.
The main slowness is caused by adding and retrieving data to/from stack which has following implementation:
import Foundation
protocol Stackable {
associatedtype Element
func peek() -> Element?
mutating func push(_ element: Element)
#discardableResult mutating func pop() -> Element?
}
extension Stackable {
var isEmpty: Bool { peek() == nil }
}
struct Stack<Element>: Stackable where Element: Equatable {
private var storage = [Element]()
func peek() -> Element? { storage.first }
mutating func push(_ element: Element) { storage.append(element) }
mutating func pop() -> Element? { storage.popLast() }
func size() -> Int { storage.count }
func isEmpty() -> Bool { storage.isEmpty }
mutating func removeAll() { storage.removeAll() }
}
extension Stack: Equatable {
static func == (lhs: Stack<Element>, rhs: Stack<Element>) -> Bool { lhs.storage == rhs.storage }
}
extension Stack: CustomStringConvertible {
var description: String { "\(storage)" }
}
extension Stack: ExpressibleByArrayLiteral {
init(arrayLiteral elements: Self.Element...) { storage = elements }
}
And another bottleneck is related to copying data and deinit method.
I was trying to find a better solution, but couldn't find anything yet. Would be grateful for any suggestions. Thanks.
While the builder pattern is useful, I think in this case it just complicates the straight-forward solution, although as you'll see, I'll present a couple that are more complicated, but those are based on increased performance optimizations on the first simple solution.
As you you noted, initializing and deinitializing classes is kind of slow. Actually the worst part is the dynamic memory allocation. Classes are powerful and definitely have their uses, but they're not the fastest tool in the Swift toolbox. Unless you make methods final, calling them can require a virtual dispatch. That can happen with protocols too depending on the particulars of their declaration, though in that case it's called "witness table thunking". But the worst part about classes is that their instances can be littered pretty much anywhere in memory. That's hell on the processor's on-chip cache. So for performance try to avoid dynamic dispatch, and reference types (ie, classes), and when you do need to allocate memory (such as Array or Dictionary), try to allocate all you need at once, and reuse it as much as possible. In Swift that requres some thought because of copy-on-write. You can easily end up allocating memory when you didn't intend to.
I present three solutions. Each one is more complicated, but also (hopefully) faster than the previous one. I'll explain why, and what to look out for. I should mention that I am not including the simplest solution. It is much like my first solution but with local variable arrays. The performance would not be especially good, and you're question makes it clear that performance is an issue.
First there's some boiler plate. To code it up and test it, I needed to define your Point and Line types, plus a few others I use for convenience, as well as some extensions purely for generating output. This code is common to all three solutions. Substitue your own definitions for Point and Line.
struct Point: Hashable
{
// This is just to give the points uniqueness, and letter names
static private let pointNames = [Character]("abcdefghijklmnopqrstuvwxyz")
static private var curNameIndex = 0
static private var nextID: Character
{
defer { curNameIndex += 1 }
return pointNames[curNameIndex]
}
let id = nextID
}
typealias Line = (Point, Point)
typealias Graph = [Point: [Line]]
typealias Path = [Line]
// Now we add some extensions for convenient output
extension Point: CustomStringConvertible {
var description: String { "\(id)" }
}
extension String.StringInterpolation
{
mutating func appendInterpolation(_ line: Line) {
appendLiteral("(\(line.0),\(line.1))")
}
mutating func appendInterpolation(_ lines: [Line]) {
appendLiteral(lines.map { "\($0)" }.joined(separator: "->"))
}
}
You mention that Point has an X and Y, but for this problem it doesn't matter. You just need unique "things" to serve as end-points for your Line instances.
Then I declared the inputs from your example:
let (a, b, c, d) = (Point(), Point(), Point(), Point())
let input = [ a: [(a,b),(a,c)], b: [(b,c), (b,d)], c: [(c,d)] ]
With the common code out of the way, here are the actual solutions:
Solution 1
Although the recursion introduces overhead all by itself, the main problem with the most straight-forward solution is that it requres local arrays that are allocated and deallocated up and down the call stack. The dynamic memory allocations and deallocations for them are actually the main performance problem with the simple recursive solution.
The solution is to attempt to pre-allocate working storage, and re-use it all through-out the recursion, or at least make reallocations rare.
One way would be to allocate the working arrays at the top level and pass them in as inout parameters. Another is to make them mutable properties of a struct (actually, a class wouldn't be too bad in this case, because you only allocate one instance). I chose the latter approach.
As I mentioned in my comment, I think of this problem as a graph theory problem.. Point = node. Line = edge. Though it's not explicitly stated, I assume there are no cycles. Putting in code to detect cycles isn't that hard, but would complicate the solution slightly. I also assume that the output should not contain entries with just one Line, because your example doesn't include any examples of that.
struct PathFinderVersion1
{
private let graph: Graph
private var pathList = [Path]()
private var currentPath = Path()
private init(for graph: Graph)
{
self.graph = graph
self.pathList.reserveCapacity(graph.count)
self.currentPath.reserveCapacity(graph.count)
}
static func pathList(for graph: Graph) -> [Path]
{
var pathFinder = Self(for: graph)
return pathFinder.makePathLists()
}
private mutating func makePathLists() -> [Path]
{
for src in graph.keys
{
for edge in graph[src]!
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
return pathList
}
private mutating func appendAllPaths()
{
assert(currentPath.count > 0, "currentPath must not be empty on entry")
guard let src = currentPath.last?.1 else { return }
guard let edges = graph[src] else
{
if currentPath.count > 1 {
pathList.append(currentPath)
}
return
}
for edge in edges
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
}
Apart from the init, and static wrapper function, pathList(for:), the algorithm is really just two functions. The initializer is where I pre-allocate the working storage. Assuming there is an entry in the graph Dictionary for each Point, no path can ever be longer than there entries keys in the graph ... at least not without cycles, so currentPath is initialized with that much capacity. Similar thinking applies to the other working arrays. The pathList is likely to be be larger than graph.count, but unless there are a lot of unconnected Lines, it will need to be at least as big as graph is.
makePathLists() is the part that gets thing started, extracting the Point and array of Line for each of its entries. It initializes the first entry in a currentPath, then calls appendAllPaths() to recursively append new Line instances to currentPath on the way down. When it reaches the end, it has found a complete path, so it adds the currentPath to the pathList. On the way back up, it removes the entry it added from currentPath so that it's in a state that it can be reused again to go down another path in the graph.
Once makePathLists() has iterated over all its keys and appendAllPaths() has recursed for each one, pathList contains the result. I'm not 100% sure it's in the format you want. It's basically a flat list of all the paths it found. So it's kind of one data structure. But all the paths will be grouped together according to the starting point in the line, so splitting it into smaller lists is easy enough, if that's what you actually want.
In any case, the re-use of existing storage is where this version gets most of its performance.
You can call it like this:
print("Version 1")
for path in PathFinderVersion1.pathList(for: input) {
print("\(path)")
}
And here's the output for the data I set up as input:
Version 1
(b,c)->(c,d)
(a,b)->(b,c)->(c,d)
(a,b)->(b,d)
(a,c)->(c,d)
The exact order changes slightly from run-to-run because Dictionary doesn't necessarily hand out its keys in an order that is consistent from run to run, even if they are inserted exactly the same way every time. I tested each version to verify they all emit the same output (and so should you), but I won't include the output again for the other versions.
Solution 2
This version is based on solution 1, so everything about it applies to this version too. What's different is the addition of the dynamic programming technique of caching intermediate values. Basically I cache paths along the way, so that when I encounter them again, I don't have do all that recursion. I can just used the cached path instead.
There is one snag with this caching, it requres allocating some local caches, which introduces dynamic memory allocation again. However hope is not lost. For starters, assuming lots of nodes in the graph have multiple input edges (ie, lots of different lines connect to the same line), the result should be a win overall from being able to avoid a vast amount of recursion. Additionally i, re-use the local caches, so I only ever have to actually allocate a new one when I recurse deeper than the previous maximum depth reached. So while some allocation does happen, it's minimized.
All that cache handling makes the code longer, but faster.
To make the code more readable I put the local caching in nested struct. Here's the code for version 2:
struct PathFinderVersion2
{
private typealias CachedPath = Path.SubSequence
private typealias CachedPaths = Array<CachedPath>
private let graph: Graph
private var pathList = [Path]()
private var currentPath = Path()
private var pathCache = [Point: CachedPaths]()
private struct LocalPathCache
{
var cache = [CachedPath]()
var pathListIndex: Int
var curPathLength: Int
mutating func updateLocalPathCache(from pathList: [Path])
{
while pathListIndex < pathList.endIndex
{
let pathToCache =
pathList[pathListIndex][curPathLength...]
cache.append(pathToCache)
pathListIndex += 1
}
}
mutating func update(
mainCache: inout [Point: CachedPaths],
for src: Point)
{
if cache.count > 0 {
mainCache[src] = cache
}
}
}
private var localCaches = [LocalPathCache]()
private mutating func getLocalCache(
pathListIndex: Int,
curPathLength: Int) -> LocalPathCache
{
if var cache = localCaches.last
{
localCaches.removeLast()
cache.cache.removeAll(keepingCapacity: true)
cache.pathListIndex = pathListIndex
cache.curPathLength = curPathLength
return cache
}
return LocalPathCache(
pathListIndex: pathListIndex,
curPathLength: curPathLength
)
}
private mutating func freeLocalCache(_ cache: LocalPathCache) {
localCaches.append(cache)
}
private init(for graph: Graph)
{
self.graph = graph
self.pathList.reserveCapacity(graph.count)
self.currentPath.reserveCapacity(graph.count)
self.pathCache.reserveCapacity(graph.count)
}
static func pathList(for graph: Graph) -> [Path]
{
var pathFinder = Self(for: graph)
return pathFinder.makePathLists()
}
private mutating func makePathLists() -> [Path]
{
for src in graph.keys
{
for edge in graph[src]!
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
return pathList
}
private mutating func appendAllPaths()
{
assert(currentPath.count > 0, "currentPath must not be empty on entry")
guard let src = currentPath.last?.1 else { return }
if updatePathListFromCache(for: src) { return }
guard let edges = graph[src] else
{
if currentPath.count > 1 {
pathList.append(currentPath)
}
return
}
var localCache = getLocalCache(
pathListIndex: pathList.endIndex,
curPathLength: currentPath.endIndex
)
defer { freeLocalCache(localCache) }
for edge in edges
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
localCache.updateLocalPathCache(from: pathList)
}
localCache.update(mainCache: &pathCache, for: src)
}
mutating func updatePathListFromCache(for src: Point) -> Bool
{
if let cachedPaths = pathCache[src]
{
let curPathIndex = currentPath.endIndex
for path in cachedPaths
{
currentPath.append(contentsOf: path)
pathList.append(currentPath)
currentPath.removeSubrange(curPathIndex...)
}
return true
}
return false
}
}
Solution 3
Solutions 1 and 2 still use recursion. Recursion is elegant, and nice to think about, because you can express a problem as a slightly simpler problem plus a bit. But unless it's tail-recursive, compilers can't optimize it particularly well. So the solution to that problem is to use iteration instead of recursion.
Turning a recursive algorithm into an iterative one is not always so easy, and can result in ugly code. That is the definitely the case here. There might be a simpler iterative algorithm, but I don't know it. So I basically replaced recursion with a state machine + stack. I've used this technique before for recursive code the desperately needed to be faster, and it does work. The code is a total pain for a human to read and maintain, but compilers can optimize the hell out of it.
This version still uses the cached intermediate solutions from version 2.
struct PathFinderVersion3
{
private typealias CachedPath = Path.SubSequence
private typealias CachedPaths = Array<CachedPath>
private let graph: Graph
private var pathList = [Path]()
private var currentPath = Path()
private var pathCache = [Point: CachedPaths]()
private struct LocalPathCache
{
var cache = [CachedPath]()
var pathListIndex: Int
var curPathLength: Int
mutating func updateLocalPathCache(from pathList: [Path])
{
while pathListIndex < pathList.endIndex
{
let pathToCache =
pathList[pathListIndex][curPathLength...]
cache.append(pathToCache)
pathListIndex += 1
}
}
mutating func update(
mainCache: inout [Point: CachedPaths],
for src: Point)
{
if cache.count > 0 {
mainCache[src] = cache
}
}
}
private var localCaches = [LocalPathCache]()
private mutating func getLocalCache(
pathListIndex: Int,
curPathLength: Int) -> LocalPathCache
{
if var cache = localCaches.last
{
localCaches.removeLast()
cache.cache.removeAll(keepingCapacity: true)
cache.pathListIndex = pathListIndex
cache.curPathLength = curPathLength
return cache
}
return LocalPathCache(
pathListIndex: pathListIndex,
curPathLength: curPathLength
)
}
private mutating func freeLocalCache(_ cache: LocalPathCache) {
localCaches.append(cache)
}
private init(for graph: Graph)
{
self.graph = graph
self.pathList.reserveCapacity(graph.count)
self.currentPath.reserveCapacity(graph.count)
self.pathCache.reserveCapacity(graph.count)
}
static func pathList(for graph: Graph) -> [Path]
{
var pathFinder = Self(for: graph)
return pathFinder.makePathLists()
}
private mutating func makePathLists() -> [Path]
{
for src in graph.keys
{
for edge in graph[src]!
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
return pathList
}
struct Stack<T>
{
var storage: [T] = []
var isEmpty: Bool { storage.isEmpty }
var count: Int { storage.count }
init(capacity: Int) { storage.reserveCapacity(capacity) }
mutating func push(_ element: T) { storage.append(element) }
mutating func pop() -> T? { storage.popLast() }
}
private mutating func appendAllPaths()
{
assert(currentPath.count > 0, "currentPath must not be empty on entry")
enum State
{
case entry
case inLoopPart1
case inLoopPart2
case exit
}
var state: State = .entry
typealias StackElement =
(Point, Int, Line, [Line], LocalPathCache, State)
var stack = Stack<StackElement>(capacity: graph.count)
var src: Point! = nil
var edges: [Line]! = nil
var edgeIndex: Int = 0
var edge: Line! = nil
var localCache: LocalPathCache! = nil
outer: while true
{
switch state
{
case .entry:
if let s = currentPath.last?.1 {
src = s
}
else
{
state = .exit
continue outer
}
if updatePathListFromCache(for: src)
{
state = .exit
continue outer
}
if let e = graph[src] { edges = e }
else
{
if currentPath.count > 1 {
pathList.append(currentPath)
}
state = .exit
continue outer
}
localCache = getLocalCache(
pathListIndex: pathList.endIndex,
curPathLength: currentPath.endIndex
)
edgeIndex = edges.startIndex
state = .inLoopPart1
continue outer
case .inLoopPart1:
if edgeIndex < edges.endIndex
{
edge = edges[edgeIndex]
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
// Simulate function call
stack.push((src, edgeIndex, edge, edges, localCache, .inLoopPart2))
state = .entry
continue outer
}
localCache.update(mainCache: &pathCache, for: src)
state = .exit
case .inLoopPart2:
localCache.updateLocalPathCache(from: pathList)
edgeIndex += 1
state = .inLoopPart1 // Simulate goto top of inner loop
case .exit: // Simulate return
if let c = localCache { freeLocalCache(c) }
if let savedState = stack.pop()
{
(src, edgeIndex, edge, edges, localCache, state) = savedState
currentPath.removeLast()
}
else { break outer }
}
}
assert(stack.isEmpty)
}
mutating func updatePathListFromCache(for src: Point) -> Bool
{
if let cachedPaths = pathCache[src]
{
let curPathIndex = currentPath.endIndex
for path in cachedPaths
{
currentPath.append(contentsOf: path)
pathList.append(currentPath)
currentPath.removeSubrange(curPathIndex...)
}
return true
}
return false
}
}
Ummm... yeah, there it is in all its ugly glory. It works. It's fast. Good luck maintaining it.
The question is whether the speed is worth it when weighed against the readability issues and maintenance headaches, and that all depends on the application requirements. In my opinion that speed would have to be pretty darn important to put up with maintaining this version, and I'm normally fine with putting up with some ugliness to get speed in critical code. Somehow, this version, written in a language that doesn't even have a goto statement is nonetheless a poster-child for why goto is considered bad in the first place.
Solution 4 (Bonus)
Originally I just mentioned that you could parallelize it, but I didn't implement it. I decided that for completeness, a parallelization example really should be included.
I chose to parallelize solution 1, but all three of the previous solutions can be parallelized in exactly the same way. The changes that have to be made are to add the split method, and modify the static pathList(for:) method as well as the private makePathLists instance method. You also need a concurrent DispatchQueue. I create a global one for this example, but you can use an existing one. Don't use DispatchQueue.main for this, if you want your app to be responsive while processing.
Here's the code:
import Foundation
let dispatchQueue =
DispatchQueue(label: "PathFinder-\(UUID())",attributes: .concurrent)
struct PathFinderVersion4
{
private let graph: Graph
private var pathList = [Path]()
private var currentPath = Path()
private init(for graph: Graph)
{
self.graph = graph
self.pathList.reserveCapacity(graph.count)
self.currentPath.reserveCapacity(graph.count)
}
public static func pathList(for graph: Graph) -> [Path]
{
let concurrency = min(4, graph.count)
let pointGroups = split(.init(graph.keys), numberOfGroups: concurrency)
var pathLists = [[Path]](repeating: [], count: concurrency)
let waitSems = Array(
repeating: DispatchSemaphore(value: 0),
count: concurrency
)
for groupIndex in pointGroups.indices
{
dispatchQueue.async
{
defer { waitSems[groupIndex].signal() }
var pathFinder = Self(for: graph)
pathLists[groupIndex] =
pathFinder.makePathLists(for: pointGroups[groupIndex])
}
}
// Need to signal each semaphore after waiting or will crash on return.
// See Apple documentation for DispatchSemaphore
waitSems.forEach { $0.wait(); $0.signal() }
var result = [Path]()
result.reserveCapacity(pathLists.reduce(0) { $0 + $1.count })
pathLists.forEach { result.append(contentsOf: $0) }
return result
}
private static func split<Value>(
_ values: [Value],
numberOfGroups: Int) -> [[Value]]
{
var groups = [[Value]]()
groups.reserveCapacity(numberOfGroups)
let groupSize = values.count / numberOfGroups
+ (values.count % numberOfGroups == 0 ? 0 : 1)
var valueIndex = values.startIndex
while valueIndex < values.endIndex
{
var group = [Value]()
group.reserveCapacity(groupSize)
let valueEnd = min(valueIndex + groupSize, values.endIndex)
while valueIndex < valueEnd
{
group.append(values[valueIndex])
valueIndex += 1
}
groups.append(group)
}
return groups
}
private mutating func makePathLists(for points: [Point]) -> [Path]
{
for src in points
{
for edge in graph[src]!
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
return pathList
}
private mutating func appendAllPaths()
{
assert(currentPath.count > 0, "currentPath must not be empty on entry")
guard let src = currentPath.last?.1 else { return }
guard let edges = graph[src] else
{
if currentPath.count > 1 {
pathList.append(currentPath)
}
return
}
for edge in edges
{
assert(edge.0 == src, "sanity check failed")
currentPath.append(edge)
appendAllPaths()
currentPath.removeLast()
}
}
}

`#Published var name: ClassType` doesn't work _outside_ of SwiftUI / manual trigger?

I found a lot of SwiftUI-related topics about this which didn't help (eg Why an ObservedObject array is not updated in my SwiftUI application?)
This doesn't work with Combine in Swift (specifically not using SwiftUI):
class SomeTask {
#Published var progress = Progress(totalUnitCount: 5) // Progress is a Class
[...]
}
var task = SomeTask()
let cancellable = task.$progress.sink { print($0.fractionCompleted) }
task.progress.completedUnitCount = 2
This is not SwiftUI-related so no ObservableObject inheritance to get objectWillChange, but even if I try to use ObservableObject and task.objectWillChange.send() it doesn't do anything, also trying to add extension Progress: ObservableObject {} doesn't help.
Since the publisher emits values through the var's willSet and since Progress is itself class-type nothing happens.
Looks like there is no real decent way to manually trigger it?
Only solution I found is to just re-assign itself which is quite awkward:
let pr = progress
progress = pr
(writing progress = progress is a compile-time error).
Only other way which might be working is probably by using Key-value-observing/KVO and/or writing a new #PublishedClassType property wrapper?
I was able to implement this using KVO, wrapped by a #propertyWrapper, with a CurrentValueSubject as the publisher:
#propertyWrapper
class PublishedClass<T : NSObject> {
private let subject: CurrentValueSubject<T, Never>
private var observation: NSKeyValueObservation? = nil
init<U>(wrappedValue: T, keyPath: ReferenceWritableKeyPath<T, U>) {
self.wrappedValue = wrappedValue
subject = CurrentValueSubject(wrappedValue)
observation = wrappedValue.observe(keyPath, options: [.new]) { (wrapped, change) in
self.subject.send(wrapped)
}
}
var wrappedValue: T
var projectedValue: CurrentValueSubject<T, Never> {
subject
}
deinit {
observation.invalidate()
}
}
Usage:
class Bar : NSObject {
#objc dynamic var a: Int
init(a: Int) {
self.a = a
}
}
class Foo {
#PublishedClass(keyPath: \.a)
var bar = Bar(a: 0)
}
let f = Foo()
let c = f.$bar.sink(receiveValue: { x in print(x.a) })
f.bar.a = 2
f.bar.a = 3
f.bar.a = 4
Output:
0
2
3
4
The disadvantage of using KVO is, of course, that the key path you pass in must be #objc dynamic and the root of the keypath must be an NSObject subclass. :(
I haven't tried, but it should be possible to extend this to observe on multiple key paths if you want.
You can try using CurrentValueSubject<Progress, Never>:
class SomeTask: ObservableObject {
var progress = CurrentValueSubject<Progress, Never>(Progress(totalUnitCount: 5))
func setProgress(_ value: Int) {
progress.value.completedUnitCount = value
progress.send(progress.value)
}
}
var task = SomeTask()
let cancellable = task.progress.sink { print($0.fractionCompleted) }
task.setProgress(3)
task.setProgress(1)
This way your Progress can still be a class.
Based on the ideas I did implement a #PublishedKVO property wrapper and put it up on github as a small swift package, supporting multiple key paths.
https://github.com/matis-schotte/PublishedKVO
Usable as:
class Example {
#PublishedKVO(\.completedUnitCount)
var progress = Progress(totalUnitCount: 2)
#Published
var textualRepresentation = "text"
}
let ex = Example()
// Set up the publishers
let c1 = ex.$progress.sink { print("\($0.fractionCompleted) completed") }
let c1 = ex.$textualRepresentation.sink { print("\($0)") }
// Interact with the class as usual
ex.progress.completedUnitCount += 1
// outputs "0.5 completed"
// And compare with Combines #Published (almost°) same behaviour
ex.textualRepresentation = "string"
// outputs "string"
ex.$progress.emit() // Re-emits the current value
ex.$progress.send(ex.progress) // Emits given value

How to Unit-Test with global structs?

It is clear for me that in a UnitTest you
generate an input property
pass this property to the method you want to test
Compare the results with your expected results
However, what if you have a global struct with e.g. the game xp and game level which has private setters and can't be modified. I automatically load this data from the UserDefaults when the app starts. How can you test methods that access that global struct, when you can not alter the input?
Example:
import UIKit
//Global struct with private data
struct GameStatus {
private(set) static var xp: Int = 0
private(set) static var level: Int = 0
/// Holds all winning states
enum MyGameStatus {
case hasNotYetWon
case hasWon
}
/// Today's game state of the user against ISH
static var todaysGameStatus: MyGameStatus {
if xp >= 100 {
return .hasWon
} else {
return .hasNotYetWon
}
}
func restoreXpAndLevel() {
// reads UserData value
}
func increaseXp(for: Int) {
//...
}
}
// class with methods to test
class LevelView: UIView {
enum LevelState {
case showStart
case showCountdown
case showFinalCuontdown
}
var state: LevelState {
if GameStatus.xp > 95 {
return .showFinalCuontdown
} else if GameStatus.xp > 90 {
return .showCountdown
}
return .showStart
}
//...configurations depending on the level
}
First, LevelView looks like it has too much logic in it. The point of a view is to display model data. It's not to include business logic like GameStatus.xp > 95. That should be done elsewhere and set into the view.
Next, why is GameStatus static? This is just complicating this. Pass the GameStatus to the view when it changes. That's the job of the view controller. Views just draw stuff. If anything is really unit-testable in your view, it probably shouldn't be in a view.
Finally, the piece that you're struggling with is the user defaults. So extract that piece into a generic GameStorage.
protocol GameStorage {
var xp: Int { get set }
var level: Int { get set }
}
Now make UserDefaults a GameStorage:
extension UserDefaults: GameStorage {
var xp: Int {
get { /* Read from UserDefaults */ return ... }
set { /* Write to UserDefaults */ }
}
var level: Int {
get { /* Read from UserDefaults */ return ... }
set { /* Write to UserDefaults */ }
}
}
And for testing, create a static one:
struct StaticGameStorage: GameStorage {
var xp: Int
var level: Int
}
Now when you create a GameStatus, pass it storage. But you can give that a default value, so you don't have to pass it all the time
class GameStatus {
private var storage: GameStorage
// A default parameter means you don't have to pass it normally, but you can
init(storage: GameStorage = UserDefaults.standard) {
self.storage = storage
}
With that, xp and level can just pass through to storage. No need for a special "load the storage now" step.
private(set) var xp: Int {
get { return storage.xp }
set { storage.xp = newValue }
}
private(set) var level: Int {
get { return storage.level }
set { storage.level = newValue }
}
EDIT: I made a change here from GameStatus being a struct to a class. That's because GameStatus lacks value semantics. If there are two copies of GameStatus, and you modify one of them, the other may change, too (because they both write to UserDefaults). A struct without value semantics is dangerous.
It's possible to regain value semantics, and it's worth considering. For example, instead of passing through xp and level to the storage, you could go back to your original design that has an explicit "restore" step that loads from storage (and I assume a "save" step that writes to storage). Then GameStatus would be an appropriate struct.
I'd also extract LevelState so that you can more easily test it and it captures the business logic outside of the view.
enum LevelState {
case showStart
case showCountdown
case showFinalCountDown
init(xp: Int) {
if xp > 95 {
self = .showFinalCountDown
} else if xp > 90 {
self = .showCountdown
}
self = .showStart
}
}
If this is only ever used by this one view, it's fine to nest it. Just don't make it private. You can test LevelView.LevelState without having to do anything with LevelView itself.
And then you can update the view's GameStatus as you need to:
class LevelView: UIView {
var gameStatus: GameStatus? {
didSet {
// Refresh the view with the new status
}
}
var state: LevelState {
guard let xp = gameStatus?.xp else { return .showStart }
return LevelState(xp: xp)
}
//...configurations depending on the level
}
Now the view itself doesn't need logic testing. You might do image-based testing to make sure it draws correctly given different inputs, but that's completely end-to-end. All the logic is simple and testable. You can test GameStatus and LevelState without UIKit at all by passing a StaticGameStorage to GameStatus.
The solution is Dependency Injection!
You can create a Persisting protocol and a facade class to interact with the user defaults
protocol Persisting {
func getObject(key: String) -> Any?
func persist(value: Any, key: String)
}
final class Persist: Persisting {
func getObject(key: String) -> Any? {
return UserDefaults.standard.object(forKey: key)
}
func persist(object: Any, key: String) {
UserDefaults.standard.set(value: object, forKey: key)
}
}
class MockPersist: Persisting {
// this is set from the test
var mockObjectToReturn: Any?
func getObject(key: String) -> Any? {
return mockObjectToReturn
}
var didCallPersistObject: (Any?, String)
func persist(object: Any, key: String) {
didCallPersistObject.0 = object
didCallPersistObject.1 = key
}
}
And now on you struct, you gonna need to inject this a var of type Persisting.
When testing you gonna need to inject the MockPersist and assert against the vars defined on the MockPersist class.
Hope this helps

How to implement a Thread Safe HashTable (PhoneBook) Data Structure in Swift?

I am trying to implement a Thread-Safe PhoneBook object. The phone book should be able to add a person, and look up a person based on their name and phoneNumber. From an implementation perspective this simply involves two hash tables, one associating name -> Person and another associating phone# -> Person.
The caveat is I want this object to be threadSafe. This means I would like to be able to support concurrent lookups in the PhoneBook while ensuring only one thread can add a Person to the PhoneBook at a time. This is the basic reader-writers problem, and I am trying to solve this using GrandCentralDispatch and dispatch barriers. I am struggling to solve this though as I am running into issues.. Below is my Swift playground code:
//: Playground - noun: a place where people can play
import UIKit
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
public class Person: CustomStringConvertible {
public var description: String {
get {
return "Person: \(name), \(phoneNumber)"
}
}
public var name: String
public var phoneNumber: String
private var readLock = ReaderWriterLock()
public init(name: String, phoneNumber: String) {
self.name = name
self.phoneNumber = phoneNumber
}
public func uniquePerson() -> Person {
let randomID = UUID().uuidString
return Person(name: randomID, phoneNumber: randomID)
}
}
public enum Qos {
case threadSafe, none
}
public class PhoneBook {
private var qualityOfService: Qos = .none
public var nameToPersonMap = [String: Person]()
public var phoneNumberToPersonMap = [String: Person]()
private var readWriteLock = ReaderWriterLock()
public init(_ qos: Qos) {
self.qualityOfService = qos
}
public func personByName(_ name: String) -> Person? {
var person: Person? = nil
if qualityOfService == .threadSafe {
readWriteLock.concurrentlyRead { [weak self] in
guard let strongSelf = self else { return }
person = strongSelf.nameToPersonMap[name]
}
} else {
person = nameToPersonMap[name]
}
return person
}
public func personByPhoneNumber( _ phoneNumber: String) -> Person? {
var person: Person? = nil
if qualityOfService == .threadSafe {
readWriteLock.concurrentlyRead { [weak self] in
guard let strongSelf = self else { return }
person = strongSelf.phoneNumberToPersonMap[phoneNumber]
}
} else {
person = phoneNumberToPersonMap[phoneNumber]
}
return person
}
public func addPerson(_ person: Person) {
if qualityOfService == .threadSafe {
readWriteLock.exclusivelyWrite { [weak self] in
guard let strongSelf = self else { return }
strongSelf.nameToPersonMap[person.name] = person
strongSelf.phoneNumberToPersonMap[person.phoneNumber] = person
}
} else {
nameToPersonMap[person.name] = person
phoneNumberToPersonMap[person.phoneNumber] = person
}
}
}
// A ReaderWriterLock implemented using GCD and OS Barriers.
public class ReaderWriterLock {
private let concurrentQueue = DispatchQueue(label: "com.ReaderWriterLock.Queue", attributes: DispatchQueue.Attributes.concurrent)
private var writeClosure: (() -> Void)!
public func concurrentlyRead(_ readClosure: (() -> Void)) {
concurrentQueue.sync {
readClosure()
}
}
public func exclusivelyWrite(_ writeClosure: #escaping (() -> Void)) {
self.writeClosure = writeClosure
concurrentQueue.async(flags: .barrier) { [weak self] in
guard let strongSelf = self else { return }
strongSelf.writeClosure()
}
}
}
// MARK: Testing the synchronization and thread-safety
for _ in 0..<5 {
let iterations = 1000
let phoneBook = PhoneBook(.none)
let concurrentTestQueue = DispatchQueue(label: "com.PhoneBookTest.Queue", attributes: DispatchQueue.Attributes.concurrent)
for _ in 0..<iterations {
let person = Person(name: "", phoneNumber: "").uniquePerson()
concurrentTestQueue.async {
phoneBook.addPerson(person)
}
}
sleep(10)
print(phoneBook.nameToPersonMap.count)
}
To test my code I run 1000 concurrent threads that simply add a new Person to the PhoneBook. Each Person is unique so after the 1000 threads complete I am expecting the PhoneBook to contain a count of 1000. Everytime I perform a write I perform a dispatch_barrier call, update the hash tables, and return. To my knowledge this is all we need to do; however, after repeated runs of the 1000 threads I get the number of entries in the PhoneBook to be inconsistent and all over the place:
Phone Book Entries: 856
Phone Book Entries: 901
Phone Book Entries: 876
Phone Book Entries: 902
Phone Book Entries: 912
Can anyone please help me figure out what is going on? Is there something wrong with my locking code or even worse something wrong with how my test is constructed? I am very new to this multi-threaded problem space, thanks!
The problem is your ReaderWriterLock. You are saving the writeClosure as a property, and then asynchronously dispatching a closure that calls that saved property. But if another exclusiveWrite came in during the intervening period of time, your writeClosure property would be replaced with the new closure.
In this case, it means that you can be adding the same Person multiple times. And because you're using a dictionary, those duplicates have the same key, and therefore don't result in you're seeing all 1000 entries.
You can actually simplify ReaderWriterLock, completely eliminating that property. I’d also make concurrentRead a generic, returning the value (just like sync does), and rethrowing any errors (if any).
public class ReaderWriterLock {
private let queue = DispatchQueue(label: "com.domain.app.rwLock", attributes: .concurrent)
public func concurrentlyRead<T>(_ block: (() throws -> T)) rethrows -> T {
return try queue.sync {
try block()
}
}
public func exclusivelyWrite(_ block: #escaping (() -> Void)) {
queue.async(flags: .barrier) {
block()
}
}
}
A couple of other, unrelated observations:
By the way, this simplified ReaderWriterLock happens to solves another concern. That writeClosure property, which we've now removed, could have easily introduced a strong reference cycle.
Yes, you were scrupulous about using [weak self], so there wasn't any strong reference cycle, but it was possible. I would advise that wherever you employ a closure property, that you set that closure property to nil when you're done with it, so any strong references that closure may have accidentally entailed will be resolved. That way a persistent strong reference cycle is never possible. (Plus, the closure itself and any local variables or other external references it has will be resolved.)
You're sleeping for 10 seconds. That should be more than enough, but I'd advise against just adding random sleep calls (because you never can be 100% sure). Fortunately, you have a concurrent queue, so you can use that:
concurrentTestQueue.async(flags: .barrier) {
print(phoneBook.count)
}
Because of that barrier, it will wait until everything else you put on that queue is done.
Note, I did not just print nameToPersonMap.count. This array has been carefully synchronized within PhoneBook, so you can't just let random, external classes access it directly without synchronization.
Whenever you have some property which you're synchronizing internally, it should be private and then create a thread-safe function/variable to retrieve whatever you need:
public class PhoneBook {
private var nameToPersonMap = [String: Person]()
private var phoneNumberToPersonMap = [String: Person]()
...
var count: Int {
return readWriteLock.concurrentlyRead {
nameToPersonMap.count
}
}
}
You say you're testing thread safety, but then created PhoneBook with .none option (achieving no thread-safety). In that scenario, I'd expect problems. You have to create your PhoneBook with the .threadSafe option.
You have a number of strongSelf patterns. That's rather unswifty. It is generally not needed in Swift as you can use [weak self] and then just do optional chaining.
Pulling all of this together, here is my final playground:
PlaygroundPage.current.needsIndefiniteExecution = true
public class Person {
public let name: String
public let phoneNumber: String
public init(name: String, phoneNumber: String) {
self.name = name
self.phoneNumber = phoneNumber
}
public static func uniquePerson() -> Person {
let randomID = UUID().uuidString
return Person(name: randomID, phoneNumber: randomID)
}
}
extension Person: CustomStringConvertible {
public var description: String {
return "Person: \(name), \(phoneNumber)"
}
}
public enum ThreadSafety { // Changed the name from Qos, because this has nothing to do with quality of service, but is just a question of thread safety
case threadSafe, none
}
public class PhoneBook {
private var threadSafety: ThreadSafety
private var nameToPersonMap = [String: Person]() // if you're synchronizing these, you really shouldn't expose them to the public
private var phoneNumberToPersonMap = [String: Person]() // if you're synchronizing these, you really shouldn't expose them to the public
private var readWriteLock = ReaderWriterLock()
public init(_ threadSafety: ThreadSafety) {
self.threadSafety = threadSafety
}
public func personByName(_ name: String) -> Person? {
if threadSafety == .threadSafe {
return readWriteLock.concurrentlyRead { [weak self] in
self?.nameToPersonMap[name]
}
} else {
return nameToPersonMap[name]
}
}
public func personByPhoneNumber(_ phoneNumber: String) -> Person? {
if threadSafety == .threadSafe {
return readWriteLock.concurrentlyRead { [weak self] in
self?.phoneNumberToPersonMap[phoneNumber]
}
} else {
return phoneNumberToPersonMap[phoneNumber]
}
}
public func addPerson(_ person: Person) {
if threadSafety == .threadSafe {
readWriteLock.exclusivelyWrite { [weak self] in
self?.nameToPersonMap[person.name] = person
self?.phoneNumberToPersonMap[person.phoneNumber] = person
}
} else {
nameToPersonMap[person.name] = person
phoneNumberToPersonMap[person.phoneNumber] = person
}
}
var count: Int {
return readWriteLock.concurrentlyRead {
nameToPersonMap.count
}
}
}
// A ReaderWriterLock implemented using GCD concurrent queue and barriers.
public class ReaderWriterLock {
private let queue = DispatchQueue(label: "com.domain.app.rwLock", attributes: .concurrent)
public func concurrentlyRead<T>(_ block: (() throws -> T)) rethrows -> T {
return try queue.sync {
try block()
}
}
public func exclusivelyWrite(_ block: #escaping (() -> Void)) {
queue.async(flags: .barrier) {
block()
}
}
}
for _ in 0 ..< 5 {
let iterations = 1000
let phoneBook = PhoneBook(.threadSafe)
let concurrentTestQueue = DispatchQueue(label: "com.PhoneBookTest.Queue", attributes: .concurrent)
for _ in 0..<iterations {
let person = Person.uniquePerson()
concurrentTestQueue.async {
phoneBook.addPerson(person)
}
}
concurrentTestQueue.async(flags: .barrier) {
print(phoneBook.count)
}
}
Personally, I'd be inclined to take it a step further and
move the synchronization into a generic class; and
change the model to be an array of Person object, so that:
The model supports multiple people with the same or phone number; and
You can use value types if you want.
For example:
public struct Person {
public let name: String
public let phoneNumber: String
public static func uniquePerson() -> Person {
return Person(name: UUID().uuidString, phoneNumber: UUID().uuidString)
}
}
public struct PhoneBook {
private var synchronizedPeople = Synchronized([Person]())
public func people(name: String? = nil, phone: String? = nil) -> [Person]? {
return synchronizedPeople.value.filter {
(name == nil || $0.name == name) && (phone == nil || $0.phoneNumber == phone)
}
}
public func append(_ person: Person) {
synchronizedPeople.writer { people in
people.append(person)
}
}
public var count: Int {
return synchronizedPeople.reader { $0.count }
}
}
/// A structure to provide thread-safe access to some underlying object using reader-writer pattern.
public class Synchronized<T> {
/// Private value. Use `public` `value` computed property (or `reader` and `writer` methods)
/// for safe, thread-safe access to this underlying value.
private var _value: T
/// Private reader-write synchronization queue
private let queue = DispatchQueue(label: Bundle.main.bundleIdentifier! + ".synchronized", qos: .default, attributes: .concurrent)
/// Create `Synchronized` object
///
/// - Parameter value: The initial value to be synchronized.
public init(_ value: T) {
_value = value
}
/// A threadsafe variable to set and get the underlying object, as a convenience when higher level synchronization is not needed
public var value: T {
get { reader { $0 } }
set { writer { $0 = newValue } }
}
/// A "reader" method to allow thread-safe, read-only concurrent access to the underlying object.
///
/// - Warning: If the underlying object is a reference type, you are responsible for making sure you
/// do not mutating anything. If you stick with value types (`struct` or primitive types),
/// this will be enforced for you.
public func reader<U>(_ block: (T) throws -> U) rethrows -> U {
return try queue.sync { try block(_value) }
}
/// A "writer" method to allow thread-safe write with barrier to the underlying object
func writer(_ block: #escaping (inout T) -> Void) {
queue.async(flags: .barrier) {
block(&self._value)
}
}
}
In some cases you use might NSCache class. The documentation claims that it's thread safe:
You can add, remove, and query items in the cache from different threads without having to lock the cache yourself.
Here is an article that describes quite useful tricks related to NSCache
I don’t think you are using it wrong :).
The original (on macos) generates:
0 swift 0x000000010c9c536a PrintStackTraceSignalHandler(void*) + 42
1 swift 0x000000010c9c47a6 SignalHandler(int) + 662
2 libsystem_platform.dylib 0x00007fffbbdadb3a _sigtramp + 26
3 libsystem_platform.dylib 000000000000000000 _sigtramp + 1143284960
4 libswiftCore.dylib 0x0000000112696944 _T0SSwcp + 36
5 libswiftCore.dylib 0x000000011245fa92 _T0s24_VariantDictionaryBufferO018ensureUniqueNativeC0Sb11reallocated_Sb15capacityChangedtSiF + 1634
6 libswiftCore.dylib 0x0000000112461fd2 _T0s24_VariantDictionaryBufferO17nativeUpdateValueq_Sgq__x6forKeytF + 1074
If you remove the ‘.concurrent’ from your ReaderWriter queue, "the problem disappears”.©
If you restore the .concurrent, but change the async invocation in the writer side to be sync:
swift(10504,0x70000896f000) malloc: *** error for object 0x7fcaa440cee8: incorrect checksum for freed object - object was probably modified after being freed.
Which would be a bit astonishing if it weren’t swift?
I dug in, replaced your ‘string’ based array with an Int one by interposing a hash function, replaced the sleep(10) with a barrier dispatch to flush any laggardly blocks through, and that made it more reproducibly crash with the somewhat more helpful:
x(10534,0x700000f01000) malloc: *** error for object 0x7f8c9ee00008: incorrect checksum for freed object - object was probably modified after being freed.
But when a search of the source revealed no malloc or free, perhaps the stack dump is more useful.
Anyways, best way to solve your problem: use go instead; it actually makes sense.

Weak reference becomes nil after dispatch into serial queue

I've toyed around with Swift Playground and noticed the following issue:
The code below describes a series of object connected to one another in the following way:
objectC --> ObjectB -- weak ref to another C --> another C --> Object B etc..
Each objectC consists of
- a ref to a object B
- a weak ref to a delegate => this one becomes nil!!
Each objectB consists of
- A var integer
- A weak ref to another object C
The code does the following:
objectC call a function, say run(), which will evaluate (objectB.weak_ref_to_another_C), and call objectB.weak_ref_to_another_C.run() in a serial Queue.
After calling .run() a couple of times, C's delegate mysteriously becomes nil....
Any idea what I'm doing wrong? To start the code, simply call test_recursive_serial() on Swift Playground.
let serialQueue = DispatchQueue(label: "myQueue");
public protocol my_protocol:class {
func do_something(ofValue:Int,completion:((Int) -> Void))
}
public class classA:my_protocol {
public let some_value:Int;
public init(value:Int){
self.some_value = value;
}
public func do_something(ofValue:Int,completion:((Int) -> Void)) {
print("A:\(some_value) in current thread \(Thread.current) is executing \(Thread.current.isExecuting)");
if self.some_value == ofValue {
completion(ofValue);
}
}
}
public class classB {
public weak var jump_to_C:classC?;
public var value:Int = 0;
}
public class classC {
weak var delegate:my_protocol?{
willSet {
if (newValue == nil) { print("target set to nil") }
else { print("target set to delegate") }
}
}
var someB:classB?
public func do_something_else() {
print(self.delegate!)
}
public func do_another(withValue:Int,completion:((Int) -> Void)) {
}
public func run(completion:#escaping ((Int) -> Void)) {
print("\(self.someB?.value)");
assert(self.delegate != nil, "not here");
if let obj = someB?.jump_to_C, obj !== self {
someB?.value += 1;
print("\(someB!)")
usleep(10000);
if let value = someB?.value, value > 100 {
completion(someB!.value);
} else {
serialQueue.async {
print("lauching...")
obj.run(completion: completion);
}
}
}else{
print("pointing to self or nil...\(someB)")
}
}
}
public func test_recursive_serial() {
let my_a = classA(value:100);
let arrayC:[classC] = (0..<10).map { (i) -> classC in
let c = classC();
c.delegate = my_a;
return c;
}
let arrayB:[classB] = (0..<10).map { (i) -> classB in
let b = classB();
let ii = (i + 1 >= 10) ? 0 : i + 1;
b.jump_to_C = arrayC[ii]
return b;
}
arrayC.forEach { (cc) in
cc.someB = arrayB[Int(arc4random())%arrayB.count];
}
arrayC.first!.run() { (value) in
print("done!");
}
}
Important note: if test_recursive_serial() content is directly called from the playground, that is not through a function, the problem doesn't appear.
Edit: You'll need to add 'PlaygroundPage.current.needsIndefiniteExecution = true' to the playground code.
Edit: Ok, I feel I need to add this. Big mistake on my side, test_recursive_serial() doesn't keep a reference on any of the called objects, so obviously, they all become nil after the code leaves the function. Hence the problem. Thanks to Guy Kogus for pointing that out.
Final edit: Adding this, in the hope it might help. Swift playground are great to test-drive code, but can sometime become very busy. Within the current issue, the solution requires to set the variables first, and then pass them to test_recursive_serial() which in turn adds to the chatty appearance of the playground. Here's another option to keep your code tidy and self-contained, while dealing with async functions of various flavours...
If you have an async task - one that doesn't fit into URL fetch -, say:
myObject.myNonBlockingTask(){ print("I'm done!"}
First, include XCTest at the top of your file.
import XCTest
then add the following:
func waitForNotificationNamed(_ notificationName: String,timeout:TimeInterval = 5.0) -> Bool {
let expectation = XCTNSNotificationExpectation(name: notificationName)
let result = XCTWaiter().wait(for: [expectation], timeout: timeout)
return result == .completed
}
finally, change your completion block to:
myObject.myNonBlockingTask(){
print("I'm done!")
let name = NSNotification.Name(rawValue: "foobar");
NotificationCenter.default.post(name:name , object: nil)
}
XCTAssert(waitForNotificationNamed("foobar", timeout: 90));
the full playground code will look like:
public func my_function() {
let somevar:Int = 123
let myObject = MyClass(somevar);
myObject.myNonBlockingTask(){
print("I'm done!")
let name = NSNotification.Name(rawValue: "foobar");
NotificationCenter.default.post(name:name , object: nil)
}
XCTAssert(waitForNotificationNamed("foobar", timeout: 90));
}
Playground will wait on the notification before going any further, and also generate an exception if it times out. All locally created objects will remain valid until the execution completes.
Hope this helps.
The main issue is that you're testing this in Playgrounds, which doesn't necessarily play nicely with multithreading. Following from this SO question, change the test_recursive_serial function to:
arrayC.first!.run() { (value) in
print("done! \(value)")
XCPlaygroundPage.currentPage.needsIndefiniteExecution = false
}
XCPlaygroundPage.currentPage.needsIndefiniteExecution = true
while XCPlaygroundPage.currentPage.needsIndefiniteExecution {
}
(You'll need to add import XCPlayground at the top of the code to make it work.)
If you don't add that code change, then my_a is released after you leave that function, which is why delegate becomes nil on the second call to run.
I also found that in run, if you don't call the completion closure in the else case like so:
public func run(completion:#escaping ((Int) -> Void)) {
...
if let obj = someB?.jump_to_C, obj !== self {
...
}else{
print("pointing to self or nil...\(someB)")
completion(-1) // Added fallback
}
}
Then the program gets stuck. By adding that it runs to the end, although I haven't actually worked out why.
Also, please get rid of all your ;s, this isn't Objective-C 😜