Swift override all setters and getters of a subclass - swift

I would like to override a setter/getter one time for but for all the properties for a class in swift
This my class. I want to call Realm each time I am adding a new value
class House : Object
{
var a:String
{
set {
do {
let realm = try Realm()
try realm.write {
a = newValue
}
}
catch {
}
}
}
var b:String
{
set {
do {
let realm = try Realm()
try realm.write {
b = newValue
}
}
catch {
}
}
}
}

There is no way in Swift how you can overwrite setters for all properties at once.
What you could generally do though is use:
overwritten setters per property
abstract computed properties wrapping low-level properties
intercept getters and setters by KVC accessor methods (e.g. is<Key>, get<Key>, …) and rely only on untyped dynamic KVC-based access via valueForKey(Path):, if you want to apply the decorated behavior (which you might want to avoid for this reason)
But Realm is using custom getters and setters under the hood, which are dynamically overwritten in an dynamically inserted intermediate class at runtime and relies on the presence of those. So the only approach, which is really feasible is having dynamic stored properties declared and adding for each of those an extra property, based on those.
var storedPropertyA: String = ""
var computedPropertyA: String {
get {
// add your extra behavior here
return storedPropertyA
}
set {
// add your extra behavior here
self.storedPropertyA = newValue
}
}
Beside that there is an alternative way of using the decorator pattern and decorate your whole object with extra behavior. In Swift, you could have your object and your decorator implement a common protocol, which defines your properties.
protocol HousingProperties {
var a: String { get set }
}
class House: HousingProperties {
var a: String = ""
}
class HouseDecorator: HousingProperties {
internal var house: House
init(house: House) { self.house = house }
var a: String {
// add your extra behavior here
self.house.a = a
}
}
Still I would NOT recommend to intercept property setters and getters for the purpose you intend here. Instead I'd advise to structure your application's architecture in a way, that allows you to be aware whether there is a write transaction or not and let the responsibility of making a write transaction in the hands of the code, which tries to modify objects.
Let me explain why:
Realm is using a multiversion concurrency control algorithm to manage persisted data and achieve thread-safety. This makes sure that different threads can read data at any point in time without having to read-lock and trying to synchronize these. Instead when a write is happening, all accessors are notified that there is new data and try to move on to the newest transaction. Until that has happened, all versions between the oldest data version, which is still used by a thread and the one written have to be retained. They can be first released when all threads advanced their commit pointers. If you do a lot of small transactions, you risk that your file size will blew up to unnecessary high values. For that reason, we recommend to batch write transactions to large changesets.

There is one hack to kind of attain what the poster is looking for, however possibly not advisable... Anyway; you can can create your own assignment operators that does whatever you want to do in realm prior to assigning the values
class MyType {
var myInt : Int = 0
var myString : String = ""
init(int: Int, string: String) {
myInt = int
myString = string
}
}
infix operator === {}
func ===<T>(lhs: T, rhs: T) -> T? {
Realm() // replace with whatever Realm()-specific stuff you want to do
return rhs
}
protocol MyAddableTypes {
func + (lhs: Self, rhs: Self) -> Self
}
extension String : MyAddableTypes {}
extension Int : MyAddableTypes {}
infix operator +== {} // ... -== similarily
func +==<T: MyAddableTypes>(lhs: T, rhs: T) -> T? {
Realm() // replace with whatever Realm()-specific stuff you want to do
return lhs+rhs
}
func Realm() {
// ...
print("Called realm")
}
var a = MyType(int: 1, string: "foo")
a.myString === "bar" // calls Realm(). After operation: a.myString = "bar"
a.myInt +== 1 // calls Realm(). After operation: a.myInt = 2
I thought I'd also mention that if you only want to do "Realm stuff" when a value is set (from your example: prior to setting a value, specifically), then the willSet method, used with stored properties, doesn't need to look so messy (nested closures), and personally, I would prefer this method
func Realm() {
print("Called realm")
}
class MyType {
// This isn't so messy, is it?
var myInt : Int = 0 { willSet { priorToSetValue(newValue) } }
var myString : String = "" { willSet { priorToSetValue(newValue) } }
var myDouble : Double = 0.0 { willSet { priorToSetValue(newValue) } }
private func priorToSetValue<T> (myVar: T) {
// replace with whatever Realm()-specific stuff you want to do,
// possibly including doing something with your new value
Realm()
}
init(int: Int, double: Double, string: String) {
myInt = int
myDouble = double
myString = string
}
}
var a = MyType(int: 1, double: 1.0, string: "foo")
a.myString = "bar"
print(a.myString) // calls Realm(). After operation: a.myString = "bar"
a.myInt += 1 // calls Realm(). After operation: a.myInt = 2

Related

PropertyWrapper subscript is not called. WHY?

I am implementing my own AtomicDictionary property wrapper as follows:
#propertyWrapper
public class AtomicDictionary<Key: Hashable, Value>: CustomDebugStringConvertible {
public var wrappedValue = [Key: Value]()
private let queue = DispatchQueue(label: "atomicDictionary.\(UUID().uuidString)",
attributes: .concurrent)
public init() {}
public subscript(key: Key) -> Value? {
get {
queue.sync {
wrappedValue[key]
}
}
set {
queue.async(flags: .barrier) { [weak self] in
self?.wrappedValue[key] = newValue
}
}
}
public var debugDescription: String {
return wrappedValue.debugDescription
}
}
now, when I use it as follows:
class ViewController: UIViewController {
#AtomicDictionary var a: [String: Int]
override func viewDidLoad() {
super.viewDidLoad()
self.a["key"] = 5
}
}
The subscript function of the AtomicDicationary is not called!!
Does anybody have any explanation as to why that is?
Property wrappers merely provide an interface for the basic accessor methods, but that’s it. It’s not going to intercept subscripts or other methods.
The original property wrapper proposal SE-0258 shows us what is going on behind the scenes. It contemplates a hypothetical property wrapper, Lazy, in which:
The property declaration
#Lazy var foo = 1738
translates to:
private var _foo: Lazy<Int> = Lazy<Int>(wrappedValue: 1738)
var foo: Int {
get { return _foo.wrappedValue }
set { _foo.wrappedValue = newValue }
}
Note that foo is just an Int computed property. The _foo is the Lazy<Int>.
So, in your a["key"] = 5 example, it will not use your property wrapper’s subscript operator. It will get the value associated with a, use the dictionary’s own subscript operator to update that value (not the property wrapper’s subscript operator), and then it will set the value associated with a.
That’s all the property wrapper is doing, providing the get and set accessors. E.g., the declaration:
#AtomicDictionary var a: [String: Int]
translates to:
private var _a: AtomicDictionary<String, Int> = AtomicDictionary<String, Int>(wrappedValue: [:])
var a: [String: Int] {
get { return _a.wrappedValue }
set { _a.wrappedValue = newValue }
}
Any other methods you define are only accessible through _a in this example, not a (which is just a computed property that gets and sets the wrappedValue of _a).
So, you’re better off just defining a proper type for your “atomic dictionary”:
public class AtomicDictionary<Key: Hashable, Value> {
private var wrappedValue: [Key: Value]
private let queue = DispatchQueue(label: "atomicDictionary.\(UUID().uuidString)", attributes: .concurrent)
init(_ wrappedValue: [Key: Value] = [:]) {
self.wrappedValue = wrappedValue
}
public subscript(key: Key) -> Value? {
get {
queue.sync {
wrappedValue[key]
}
}
set {
queue.async(flags: .barrier) {
self.wrappedValue[key] = newValue
}
}
}
}
And
let a = AtomicDictionary<String, Int>()
That gives you the behavior you want.
And if you are going to supply CustomDebugStringConvertible conformance, make sure to use your synchronization mechanism there, too:
extension AtomicDictionary: CustomDebugStringConvertible {
public var debugDescription: String {
queue.sync { wrappedValue.debugDescription }
}
}
All interaction with the wrapped value must be synchronized.
Obviously you can use this general pattern with whatever synchronization mechanism you want, e.g., the above reader-writer pattern, GCD serial queue, locks, actors, etc. (The reader-writer pattern has a natural appeal, but, in practice, there are generally better mechanisms.)
Needless to say, the above presumes that subscript-level atomicity is sufficient. One should always be wary about general purpose thread-safe collections as often the correctness of our code relies on a higher-level of synchronization.

Require calling method M after setting property P

I have a property and a method:
class C {
var myProperty = ""
func myFunc() {
}
}
I would like to have a rule that when you set myProperty you must then proceed to call myFunc. Of course I can have a mental rule that requires this, but a mental contract is no contract at all. I want to enforce this contract through the class's API.
(NOTE All of this is purely about what goes on inside C. Both myProperty and myFunc can be private as far as visibility outside C is concerned; that doesn't affect the problem.)
Obviously we can do this with a setter observer:
class C1 {
var myProperty = "" {
didSet {
self.myFunc()
}
}
func myFunc() {
}
}
However, there's something I don't like about that solution: it relies upon what amounts to a hidden side effect. It is up to me to know that saying self.myProperty = "..." also triggers a call to myFunc. I don't like that kind of hidden side effect; it makes my code harder to understand, and violates one of my key programming principles, Say What You Mean.
What do I mean? I mean something like this:
self.setMyPropertyAndCallMyFunc("...")
I like that, because it tells me exactly what this call does every time it occurs in my code. So we could write this:
class C2 {
var myProperty = ""
func myFunc() {
}
func setMyPropertyAndCallMyFunc(_ s:String) {
self.myProperty = s
self.myFunc()
}
}
But there's a problem with that, namely that it is now possible to set myProperty without calling myFunc, breaking the rule that we set out to enforce in the first place! The only way I've found to enforce that rule is to have another class that defends the property with privacy. For example:
class C3 {
class Inner {
private(set) var myProperty = ""
private func myFunc() {
}
func setMyPropertyAndCallMyFunc(_ s:String) {
self.myProperty = s
self.myFunc()
}
}
let inner = Inner()
}
That is in fact what I'm doing, but it seems a bit nutty to have a second class just to enforce these privacy and naming rules. My question is: Is there another way?
Yes, though I don't know if it's actually any better. You can hide myProperty inside of a closure rather than inside of a class/struct. For example:
class C {
public var myProperty: String { return _myPropertyGetter() }
public func setMyPropertyAndCallMyFunc(value: String) {
_myPropertySetter(self, value)
}
func myFunc() {
print("myFunc")
}
private let (_myPropertyGetter, _myPropertySetter): (() -> String, (C, String) -> ()) = {
var property = ""
return ({ property }, { property = $1; $0.myFunc() })
}()
}
let c = C()
c.myProperty
c.setMyPropertyAndCallMyFunc(value: "x")
c.myProperty
There's a lot of complexity here, like passing self to _setMyPropertyAndCallMyFunc to allow myFunc to live outside the closure, and maybe some of that could be dispensed with. But the basic idea is to generate two functions that are the only functions that have access to the property storage, making it "super-private" without creating an inner class.
If you don't need myFunc() to be public (and from your Inner example, I think you don't), then you can do it a little simpler, and this almost might be better than Inner.
class C {
public var myProperty: String { return _myPropertyGetter() }
public func setMyPropertyAndCallMyFunc(value: String) { _myPropertySetter(value) }
private let (_myPropertyGetter, _myPropertySetter): (() -> String, (String) -> ()) = {
var property = ""
func myFunc() {
print("myFunc")
}
return ({ property }, { property = $0; myFunc() })
}()
}

Get all key paths from a struct in Swift 4

Let's say I have that struct:
struct MyStruct {
let x: Bool
let y: Bool
}
In Swift 4 we can now access it's properties with the myStruct[keyPath: \MyStruct.x] interface.
What I need is a way to access all it's key paths, something like:
extension MyStruct {
static func getAllKeyPaths() -> [WritableKeyPath<MyStruct, Bool>] {
return [
\MyStruct.x,
\MyStruct.y
]
}
}
But, obviously, without me having to manually declare every property in an array.
How can I achieve that?
DISCLAIMER:
Please note that the following code is for educational purpose only and it should not be used in a real application, and might contains a lot of bugs/strange behaviors if KeyPath are used this way.
Answer:
I don't know if your question is still relevant today, but the challenge was fun :)
This is actually possible using the mirroring API.
The KeyPath API currently doesn't allow us to initialize a new KeyPath from a string, but it does support dictionary "parsing".
The idea here is to build a dictionary that will describe the struct using the mirroring API, then iterate over the key to build the KeyPath array.
Swift 4.2 playground:
protocol KeyPathListable {
// require empty init as the implementation use the mirroring API, which require
// to be used on an instance. So we need to be able to create a new instance of the
// type.
init()
var _keyPathReadableFormat: [String: Any] { get }
static var allKeyPaths: [KeyPath<Foo, Any?>] { get }
}
extension KeyPathListable {
var _keyPathReadableFormat: [String: Any] {
let mirror = Mirror(reflecting: self)
var description: [String: Any] = [:]
for case let (label?, value) in mirror.children {
description[label] = value
}
return description
}
static var allKeyPaths: [KeyPath<Self, Any?>] {
var keyPaths: [KeyPath<Self, Any?>] = []
let instance = Self()
for (key, _) in instance._keyPathReadableFormat {
keyPaths.append(\Self._keyPathReadableFormat[key])
}
return keyPaths
}
}
struct Foo: KeyPathListable {
var x: Int
var y: Int
}
extension Foo {
// Custom init inside an extension to keep auto generated `init(x:, y:)`
init() {
x = 0
y = 0
}
}
let xKey = Foo.allKeyPaths[0]
let yKey = Foo.allKeyPaths[1]
var foo = Foo(x: 10, y: 20)
let x = foo[keyPath: xKey]!
let y = foo[keyPath: yKey]!
print(x)
print(y)
Note that the printed output is not always in the same order (probably because of the mirroring API, but not so sure about that).
After modifying rraphael's answer I asked about this on the Swift forums.
It is possible, discussion here:
Getting KeyPaths to members automatically using Mirror
Also, the Swift for TensorFlow team has this already built in to Swift for TensorFlow, which may make its way to pure swift:
Dynamic property iteration using key paths
I propose my solution. It has the advantage of dealing correctly with #Published values when using the Combine framework.
For the sake of clarity, it is a simplified version of what I have really. In the full version, I pass some options to the Mirror.allKeyPaths() function to change behaviour ( To enumerate structs and/or classes properties in sub-dictionaries for example ).
The first Mirror extension propose some functions to simplify properties enumeration.
The second extension implements the keyPaths dictionaries creation, replacing
#Published properties by correct name and value
The last part is the KeyPathIterable protocol, that add enumeration
capability to associated object
swift
// MARK: - Convenience extensions
extension String {
/// Returns string without first character
var byRemovingFirstCharacter: String {
guard count > 1 else { return "" }
return String(suffix(count-1))
}
}
// MARK: - Mirror convenience extension
extension Mirror {
/// Iterates through all children
static func forEachProperty(of object: Any, doClosure: (String, Any)->Void) {
for (property, value) in Mirror(reflecting: object).children where property != nil {
doClosure(property!, value)
}
}
/// Executes closure if property named 'property' is found
///
/// Returns true if property was found
#discardableResult static func withProperty(_ property: String, of object: Any, doClosure: (String, Any)->Void) -> Bool {
for (property, value) in Mirror(reflecting: object).children where property == property {
doClosure(property!, value)
return true
}
return false
}
/// Utility function to determine if a value is marked #Published
static func isValuePublished(_ value: Any) -> Bool {
let valueTypeAsString = String(describing: type(of: value))
let prefix = valueTypeAsString.prefix { $0 != "<" }
return prefix == "Published"
}
}
// MARK: - Mirror extension to return any object properties as [Property, Value] dictionary
extension Mirror {
/// Returns objects properties as a dictionary [property: value]
static func allKeyPaths(for object: Any) -> [String: Any] {
var out = [String: Any]()
Mirror.forEachProperty(of: object) { property, value in
// If value is of type Published<Some>, we transform to 'regular' property label and value
if Self.isValuePublished(value) {
Mirror.withProperty("value", of: value) { _, subValue in
out[property.byRemovingFirstCharacter] = subValue
}
} else {
out[property] = value
}
}
return out
}
}
// MARK: - KeyPathIterable protocol
protocol KeyPathIterable {
}
extension KeyPathIterable {
/// Returns all object properties
var allKeyPaths: [String: Any] {
return Mirror.allKeyPaths(for: self)
}
}

Read once and then set to nil

I'd like to implement such property, that it's value is available for reading only one time, and then the property should be set to nil.
I've implemented it in such way:
private var _readOnce: String?
var readOnce: String? {
get {
let value = _readOnce
_readOnce = nil
return value
}
set {
_readOnce = newValue
}
}
readOnce = "Andrej"
print("read once = \(readOnce)") // prints read once = Optional("Andrej")\n"
print("read once = \(readOnce)") // prints read once = nil\n"
But I'feel like using a separate property _readOnce is not the "swifty" / "most elegant" way to do it.
Does anyone know of a different way, that wouldn't require to use a separate property?
I can confirm that the above code works, it's only that I feel it could be more elegant with less lines to achieve the same behaviour.
I don't know that there's a way to avoid having a backing property, but what I'd probably do is to make a helper type to wrap up the behavior. Something like this:
struct OneTimeValue<T>
{
private var isUnread = true
private let value : T
init(_ value: T)
{
self.value = value
}
func get() -> T?
{
guard isUnread else {
return nil
}
self.isUnread = false
return self.value
}
}
You could also write this a little differently if you prefer, by nilling out value inside of get(), for example, but the general plan holds.
Then your class becomes:
class Miser
{
var readOnce : String?
{
return self._readOnce.get()
}
private let _readOnce = OneTimeValue("Can't touch this (twice)")
}
I've also used this pattern for a UserDefaultsValue (storage to/from user defaults) and a SynchronizedValue (read-write lock on a property) and I think it works well.
As far as I know it is not possible without a second variable. This is because computed properties do not store any data for the variable they represent:
In addition to stored properties, classes, structures, and
enumerations can define computed properties, which do not actually
store a value.
For non-computed properties, the only observers you can have are based upon the setting of the variable, not the getting (i.e. willSet and didSet)
Hope that helps!
EDIT:
It can be done with closures and property observers if you're careful:
This requires no other variables (instead the value is captured by the closure), but it is rather unclear — I wouldn't recommend it.
var readOnce: () -> String? = {nil} {
didSet{
readOnce = { [weak self, readOnce] in
self?.readOnce = {nil}
return readOnce()
}
}
}
readOnce() // returns nil
readOnce = {"Hi"}
readOnce() // returns optional wrapped "Hi"
readOnce() // returns nil
A more 'Swifty' answer for you :D
After Swift 5.1, We can use Property Wrapper
#propertyWrapper
struct ReturnAndFree<T> {
private var value: T?
init(wrappedValue: T?) {
value = wrappedValue
}
var wrappedValue: T? {
mutating get {
defer { value = nil }
return value
}
set {
value = newValue
}
}
}

What is a good alternative for static stored properties of generic types in swift?

Since static stored properties are not (yet) supported for generic types in swift, I wonder what is a good alternative.
My specific use-case is that I want to build an ORM in swift. I have an Entity protocol which has an associatedtype for the primary key, since some entities will have an integer as their id and some will have a string etc. So that makes the Entity protocol generic.
Now I also have an EntityCollection<T: Entity> type, which manages collections of entities and as you can see it is also generic. The goal of EntityCollection is that it lets you use collections of entities as if they were normal arrays without having to be aware that there's a database behind it. EntityCollection will take care of querying and caching and being as optimized as possible.
I wanted to use static properties on the EntityCollection to store all the entities that have already been fetched from the database. So that if two separate instances of EntityCollection want to fetch the same entity from the database, the database will be queried only once.
Do you guys have any idea how else I could achieve that?
The reason that Swift doesn't currently support static stored properties on generic types is that separate property storage would be required for each specialisation of the generic placeholder(s) – there's more discussion of this in this Q&A.
We can however implement this ourselves with a global dictionary (remember that static properties are nothing more than global properties namespaced to a given type). There are a few obstacles to overcome in doing this though.
The first obstacle is that we need a key type. Ideally this would be the metatype value for the generic placeholder(s) of the type; however metatypes can't currently conform to protocols, and so therefore aren't Hashable. To fix this, we can build a wrapper:
/// Hashable wrapper for any metatype value.
struct AnyHashableMetatype : Hashable {
static func ==(lhs: AnyHashableMetatype, rhs: AnyHashableMetatype) -> Bool {
return lhs.base == rhs.base
}
let base: Any.Type
init(_ base: Any.Type) {
self.base = base
}
func hash(into hasher: inout Hasher) {
hasher.combine(ObjectIdentifier(base))
}
// Pre Swift 4.2:
// var hashValue: Int { return ObjectIdentifier(base).hashValue }
}
The second is that each value of the dictionary can be a different type; fortunately that can be easily solved by just erasing to Any and casting back when we need to.
So here's what that would look like:
protocol Entity {
associatedtype PrimaryKey
}
struct Foo : Entity {
typealias PrimaryKey = String
}
struct Bar : Entity {
typealias PrimaryKey = Int
}
// Make sure this is in a seperate file along with EntityCollection in order to
// maintain the invariant that the metatype used for the key describes the
// element type of the array value.
fileprivate var _loadedEntities = [AnyHashableMetatype: Any]()
struct EntityCollection<T : Entity> {
static var loadedEntities: [T] {
get {
return _loadedEntities[AnyHashableMetatype(T.self), default: []] as! [T]
}
set {
_loadedEntities[AnyHashableMetatype(T.self)] = newValue
}
}
// ...
}
EntityCollection<Foo>.loadedEntities += [Foo(), Foo()]
EntityCollection<Bar>.loadedEntities.append(Bar())
print(EntityCollection<Foo>.loadedEntities) // [Foo(), Foo()]
print(EntityCollection<Bar>.loadedEntities) // [Bar()]
We are able to maintain the invariant that the metatype used for the key describes the element type of the array value through the implementation of loadedEntities, as we only store a [T] value for a T.self key.
There is a potential performance issue here however from using a getter and setter; the array values will suffer from copying on mutation (mutating calls the getter to get a temporary array, that array is mutated and then the setter is called).
(hopefully we get generalised addressors soon...)
Depending on whether this is a performance concern, you could implement a static method to perform in-place mutation of the array values:
func with<T, R>(
_ value: inout T, _ mutations: (inout T) throws -> R
) rethrows -> R {
return try mutations(&value)
}
extension EntityCollection {
static func withLoadedEntities<R>(
_ body: (inout [T]) throws -> R
) rethrows -> R {
return try with(&_loadedEntities) { dict -> R in
let key = AnyHashableMetatype(T.self)
var entities = (dict.removeValue(forKey: key) ?? []) as! [T]
defer {
dict.updateValue(entities, forKey: key)
}
return try body(&entities)
}
}
}
EntityCollection<Foo>.withLoadedEntities { entities in
entities += [Foo(), Foo()] // in-place mutation of the array
}
There's quite a bit going on here, let's unpack it a bit:
We first remove the array from the dictionary (if it exists).
We then apply the mutations to the array. As it's now uniquely referenced (no longer present in the dictionary), it can be mutated in-place.
We then put the mutated array back in the dictionary (using defer so we can neatly return from body and then put the array back).
We're using with(_:_:) here in order to ensure we have write access to _loadedEntities throughout the entirety of withLoadedEntities(_:) to ensure that Swift catches exclusive access violations like this:
EntityCollection<Foo>.withLoadedEntities { entities in
entities += [Foo(), Foo()]
EntityCollection<Foo>.withLoadedEntities { print($0) } // crash!
}
I'm not sure if I like this yet or not, but I used a static computed property:
private extension Array where Element: String {
static var allIdentifiers: [String] {
get {
return ["String 1", "String 2"]
}
}
}
Thoughts?
An hour ago i have a problem almost like yours. I also want to have a BaseService class and many other services inherited from this one with only one static instance. And the problem is all services use their own model (ex: UserService using UserModel..)
In short I tried following code. And it works!.
class BaseService<Model> where Model:BaseModel {
var models:[Model]?;
}
class UserService : BaseService<User> {
static let shared = UserService();
private init() {}
}
Hope it helps.
I think the trick was BaseService itself will not be used directly so NO NEED TO HAVE static stored property. (P.S. I wish swift supports abstract class, BaseService should be)
It turns out that, although properties are not allowed, methods and computed properties are. So you can do something like this:
class MyClass<T> {
static func myValue() -> String { return "MyValue" }
}
Or:
class MyClass<T> {
static var myValue: String { return "MyValue" }
}
This isn't ideal, but this is the solution I came up with to fit my needs.
I'm using a non-generic class to store the data. In my case, I'm using it to store singletons. I have the following class:
private class GenericStatic {
private static var singletons: [String:Any] = [:]
static func singleton<GenericInstance, SingletonType>(for generic: GenericInstance, _ newInstance: () -> SingletonType) -> SingletonType {
let key = "\(String(describing: GenericInstance.self)).\(String(describing: SingletonType.self))"
if singletons[key] == nil {
singletons[key] = newInstance()
}
return singletons[key] as! SingletonType
}
}
This is basically just a cache.
The function singleton takes the generic that is responsible for the singleton and a closure that returns a new instance of the singleton.
It generates a string key from the generic instance class name and checks the dictionary (singletons) to see if it already exists. If not, it calls the closure to create and store it, otherwise it returns it.
From a generic class, you can use a static property as described by Caleb. For example:
open class Something<G> {
open static var number: Int {
return GenericStatic.singleton(for: self) {
print("Creating singleton for \(String(describing: self))")
return 5
}
}
}
Testing the following, you can see that each singleton is only created once per generic type:
print(Something<Int>.number) // prints "Creating singleton for Something<Int>" followed by 5
print(Something<Int>.number) // prints 5
print(Something<String>.number) // prints "Creating singleton for Something<String>"
This solution may offer some insight into why this isn't handled automatically in Swift.
I chose to implement this by making the singleton static to each generic instance, but that may or may not be your intention or need.
Depending on how many types you need to support and whether inheritance is (not) an option for you, conditional conformance could also do the trick:
final class A<T> {}
final class B {}
final class C {}
extension A where T == B {
static var stored: [T] = []
}
extension A where T == C {
static var stored: [T] = []
}
let a1 = A<B>()
A<B>.stored = [B()]
A<B>.stored
let a2 = A<C>()
A<C>.stored = [C()]
A<C>.stored
Well I also ran into the same problem and was able to device a logical work around for it. I had to create a static instance of urlsession using a generic class as handler.
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
let neworkHandler = NetworkHandler<String>()
neworkHandler.download()
neworkHandler.download()
}
class SessionConfigurator: NSObject{
static var configuration:URLSessionConfiguration{
let sessionConfig = URLSessionConfiguration.background(withIdentifier: "com.bundle.id")
sessionConfig.isDiscretionary = true
sessionConfig.allowsCellularAccess = true
return sessionConfig
}
static var urlSession:URLSession?
class NetworkHandler<T> :NSObject, URLSessionDelegate{
func download(){
if SessionConfigurator.urlSession == nil{
SessionConfigurator.urlSession = URLSession(configuration:SessionConfigurator.configuration, delegate:self, delegateQueue: OperationQueue.main)
}
}
All I can come up with is to separate out the notion of source (where the collection comes from) and then collection itself. And then the make the source responsible for caching. At that point the source can actually be an instance, so it can keep whatever caches it wants/needs to and your EntityCollection is just responsible for maintaining a CollectionType and/or SequenceType protocol around the source.
Something like:
protocol Entity {
associatedtype IdType : Comparable
var id : IdType { get }
}
protocol Source {
associatedtype EntityType : Entity
func first() -> [EntityType]?
func next(_: EntityType) -> [EntityType]?
}
class WebEntityGenerator <EntityType:Entity, SourceType:Source where EntityType == SourceType.EntityType> : GeneratorType { ... }
class WebEntityCollection : SequenceType { ... }
would work if you have a typical paged web data interface. Then you could do something along the lines of:
class WebQuerySource<EntityType:Entity> : Source {
var cache : [EntityType]
...
func query(query:String) -> WebEntityCollection {
...
}
}
let source = WebQuerySource<MyEntityType>(some base url)
for result in source.query(some query argument) {
}
source.query(some query argument)
.map { ... }
.filter { ... }
Something like this?
protocol Entity {
}
class EntityCollection {
static var cachedResults = [Entity]()
func findById(id: Int) -> Entity? {
// Search cache for entity with id from table
// Return result if exists else...
// Query database
// If entry exists in the database append it to the cache and return it else...
// Return nil
}
}