Marking CacheManager with 'class' solved my problem.
Here's the case: a simple cacher, the mutating get is not what I want, so how should do for the reference types or just class type?
protocol Cacher {
associatedtype K
associatedtype V
subscript (key: K) -> V? {get set}
}
protocol MemoryCacher: Cacher {}
protocol FileCacher: Cacher {}
updated. add PrimayCacher | SecondaryCacher associatedtypes
protocol CacheManager {
associatedtype Key
associatedtype Value
associatedtype PrimaryCacher: MemoryCacher where PrimaryCacher.K == Key, PrimaryCacher.V == Value
associatedtype SecondaryCacher: FileCacher where SecondaryCacher.K == Key, SecondaryCacher.V == Value
var primaryCacher: PrimaryCacher { get set }
var secondaryCacher: SecondaryCacher{ get set }
subscript(key: Key) -> Value? { get set }
}
//try to provide a default subscript conformance, but actually not for the `mutating` get
extension CacheManager {
subscript(key: Key) -> Value? {
mutating get {
guard let result = primaryCacher[key] else {
if let value = secondaryCacher?[key] {
primaryCacher[key] = value // the mutating is required for this
return value
}
return nil
}
return result
}
set {
primaryCacher[key] = newValue
secondaryCacher?[key] = newValue
}
}
}
You just need to add a type restriction to AnyObject, which all classes conform to.
extension CacherManager where Self: AnyObject {
subscript(key: Key) -> Value? {
get {
guard let result = primaryCacher[key] else {
if let value = secondaryCacher?[key] {
primaryCacher[key] = value // the mutating is required for this
return value
}
return nil
}
return result
}
set {
primaryCacher[key] = newValue
secondaryCacher?[key] = newValue
}
}
}
I also had to slightly modify your CacheManager declaration for the code to compile and add the required protocol methods to MemoryCacher and FileCacher that you omitted from the question.
protocol Cacher {
associatedtype K
associatedtype V
subscript (key: K) -> V? {get set}
}
class MemoryCacher<K, V>: Cacher {
private var value: V?
subscript(key: K) -> V? {
get {
return value
}
set {
value = newValue
}
}
}
class FileCacher<K, V>: Cacher {
private var value: V?
subscript(key: K) -> V? {
get {
return value
}
set {
value = newValue
}
}
}
protocol CacheManager {
associatedtype Key
associatedtype Value
var primaryCacher: MemoryCacher<Key,Value> { get set }
var secondaryCacher: FileCacher<Key,Value>? { get set }
subscript(key: Key) -> Value? { get set }
}
Related
I wanted to create a "where_non_null" operation that works on any swift sequence - which is easy if you return an array, but obviously that is potentially bad performance wise - because you are forcing the entire sequence to resolve in memory - so I created the following that just goes line by line:
//
// this iterates through the underlying sequence, and returns only the values that are not null
//
public class Not_null_iterator<T> : IteratorProtocol
{
public typealias Element = T
private let next_function : () -> T?
init<T_iterator: IteratorProtocol>( _ source: T_iterator ) where T_iterator.Element == Optional<T>
{
var iterator = source
next_function =
{
while (true)
{
if let next_value = iterator.next()
{
if let not_null_value = next_value
{
return not_null_value
}
}
else
{
return nil
}
}
}
}
public func next() -> T? {
next_function()
}
}
//
// a sequence wrapping an underlying sequence, that removes any nulls as we go through
//
public class Not_null_sequence<T > : Sequence
{
private var iterator_creator : () -> Not_null_iterator<T>
init<T_source_sequence : Sequence >( _ source : T_source_sequence ) where T_source_sequence.Element == Optional<T>
{
iterator_creator =
{
Not_null_iterator(source.makeIterator())
}
}
public func makeIterator() -> Not_null_iterator<T>
{
iterator_creator()
}
}
extension Sequence
{
//
// return only the not null values in the sequence without ever resolving more than one item in memory at one time and remove the optionality on the type
//
func where_not_null<T>() -> Not_null_sequence<T> where Element == Optional<T>
{
return Not_null_sequence( self)
}
}
class Where_not_null_tests : XCTestCase
{
public func test_where_not_null()
{
let source = [1, 2, 3, nil, 4]
let checked : [Int] = Array(source.where_not_null())
XCTAssertEqual([1,2,3,4],checked)
}
}
which works great - however I had to define the next() and make_iterator() functions in the constructor, because I couldn't find any type safe way of putting the source into a class level variable.
Is there a way of doing that?
[and yes, I'm aware swift people prefer camel case]
Rather than just using one generic parameter, you'd need two generic parameters. You can't just constrain one generic parameter to say that it has to be some sequence with an element of some Optional. You need another generic parameter to say what the optional's type is:
class NotNilIterator<T: IteratorProtocol, U>: IteratorProtocol where T.Element == U? {
typealias Element = U
var iterator: T
init(_ source: T) {
iterator = source
}
func next() -> Element? {
// I feel this is clearer what is going on
while true {
switch iterator.next() {
case .some(.none):
continue
case .none:
return nil
case .some(.some(let element)):
return element
}
}
}
}
class NotNilSequence<T: Sequence, U> : Sequence where T.Element == U?
{
let sequence: T
init(_ source : T)
{
sequence = source
}
public func makeIterator() -> NotNilIterator<T.Iterator, U>
{
.init(sequence.makeIterator())
}
}
whereNotNil would then be declared like this:
func whereNotNil<T>() -> NotNilSequence<Self, T> where Self.Element == T?
{
return .init(self)
}
Note the use of self types. The first parameter is the type of the underlying sequence, the second is the non-optional type.
Note that this sort of "lazily computed sequence" is already built into Swift. To lazily filter out the nils, do:
let array = [1, 2, 3, nil, 4]
let arrayWithoutNil = array.lazy.compactMap { $0 }
The downside is that the type names are quite long. arrayWithoutNil is of type
LazyMapSequence<LazyFilterSequence<LazyMapSequence<LazySequence<[Int?]>.Elements, Int?>>, Int>
But you can indeed get non-optional Ints out of it, so it does work.
The way swift generics work can sometimes be very confusing (but has it's advantages). Instead of declaring that a variable is of a generic protocol (resp. a protocol with associated types), you instead declare another generic type which itself conforms to your protocol. Here's your iterator as an example (I have taken the liberty to clean up the code a bit):
public class Not_null_iterator<T, T_iterator> : IteratorProtocol where
T_iterator: IteratorProtocol,
T_iterator.Element == Optional<T>
{
private var source: T_iterator
init(_ source: T_iterator) {
self.source = source
}
public func next() -> T? {
while let next_value = source.next()
{
if let not_null_value = next_value
{
return not_null_value
}
}
return nil
}
}
The non-null sequence works analogous:
public class Not_null_sequence<T, Source>: Sequence where
Source: Sequence,
Source.Element == Optional<T>
{
private var source: Source
init(_ source: Source) {
self.source = source
}
public func makeIterator() -> Not_null_iterator<T, Source.Iterator> {
Not_null_iterator(self.source.makeIterator())
}
}
Using this some IteratorProtocol is just a nice way to let the compiler figure out the type. It is equivalent to saying Not_null_iterator<T, Source.Iterator>
As a (potentially) interesting side-note, to clean up the generic mess even more, you can nest the iterator class inside the Not_null_sequence:
public class Not_null_sequence<T, Source>: Sequence where
Source: Sequence,
Source.Element == Optional<T>
{
private var source: Source
init(_ source: Source) {
self.source = source
}
public func makeIterator() -> Iterator{
Iterator(self.source.makeIterator())
}
public class Iterator: IteratorProtocol {
private var source: Source.Iterator
init(_ source: Source.Iterator) {
self.source = source
}
public func next() -> T? {
while let next_value = source.next()
{
if let not_null_value = next_value
{
return not_null_value
}
}
return nil
}
}
}
I'm trying to remove a generic item from a linked list. The problem I'm having is that when I'm checking head?.value == value, it requires "T" to conform to Equatable.
I'm having trouble making that work. When I try to make NNode<T> Equatable by adding the Equatable to my generic class NNode<T: Equatable>, my typealias Node has an error of not conforming to protocol Equatable. What do I do?
class NNode<T: Equatable> {
var value: T
var next: NNode?
init(value: T, next: NNode?) {
self.value = value
self.next = next
}
}
class LinkedList<T> {
public typealias Node = NNode<T>
func delete(value: T) {
if head?.value == value {
head = head?.next
}
var prev: Node?
var current = head
while current != nil && current?.value != value {
prev = current
current = current?.next
}
prev?.next = current?.next
}
}
You need to conform T to Equatable in the LinkedList as well:
class NNode<T: Equatable> { ... }
class LinkedList<T: Equatable> {
public typealias Node = NNode<T>
...
}
How can I perform conformance check against protocol with AssociatedType. Xcode shows error:
Protocol 'MyListener' can only be used as a generic constraint because
it has Self or associated type requirements
My ultimate goal is to extract "MyListener.section" from an array of weakObjects, where the handler matches the function argument.
Note. The NSPointerArray of weakObjects is suppose to capture different types of MyListeners.
public class MyHandler<O,E> {
var source = [O]()
var dest = [E]()
}
public protocol MyListener:class {
var section: Int {get}
associatedtype O
associatedtype E
var handler: MyHandler<O,E>? { get }
}
public class MyAnnouncer {
private let mapWeakObjects: NSPointerArray = NSPointerArray.weakObjects()
public func add<L: MyListener>(listener: L) {
let pointer = Unmanaged.passUnretained(listener).toOpaque()
mapWeakObjects.addPointer(pointer)
}
public func search<O, E> (h:MyHandler<O,E>) -> [Int] {
_ = mapWeakObjects.allObjects.filter { listener in
if listener is MyListener { // Compilation failed
}
if let _ = listener as? MyListener { //Compilation error
}
if listener is MyListener.Type { //Compilation failed
}
}
return [] // ultimate goal is to extract corresponding [MyListener.section].
}
}
Unfortunately, Swift doesn't support protocols with AssociatedType to conformance.
You should try to use Type Erasure. One of the way is to implement type erasure by creating new AnyType class.
Here is another way to release type erasure (example from the internet)
protocol SpecialValue { /* some code*/ }
protocol TypeErasedSpecialController {
var typeErasedCurrentValue: SpecialValue? { get }
}
protocol SpecialController : TypeErasedSpecialController {
associatedtype SpecialValueType : SpecialValue
var currentValue: SpecialValueType? { get }
}
extension SpecialController {
var typeErasedCurrentValue: SpecialValue? { return currentValue }
}
extension String : SpecialValue {}
struct S : SpecialController {
var currentValue: String?
}
var x: Any = S(currentValue: "Hello World!")
if let sc = x as? TypeErasedSpecialController { // Now we can perform conformance
print(sc.typeErasedCurrentValue)
}
Is it possible to have a subscript in Swift that has different signatures for the getter and setter?
For example, I want the getter to return a Set<Int> and the setter to take an Int (not a Set<Int>).
This code won't compile but it gives you an idea of what I'm trying to do:
struct Foo{
subscript(index: Int)->String{
get{
return "bar" // returns a String
}
set(newValue: String?){ // takes a String? instead of a String
print(newValue)
}
}
}
How can I do this?
This is very ugly, and I strongly discourage you from doing so, but technically this is possible:
struct Foo {
subscript(concreteValueFor index: Int) -> String {
return "Get concrete \(index)"
}
subscript(optionalValueFor index: Int) -> String? {
get { return nil }
set { print("Set optional \(index)") }
}
}
var foo = Foo()
foo[concreteValueFor: 1] // Returns "Get concrete 1"
foo[optionalValueFor: 2] = "" // Prints "Set optional 2"
For a multimap some time ago I made something like this:
public struct Multimap<Key: Hashable, Value: Hashable>: CollectionType {
public typealias _Element = Set<Value>
public typealias Element = (Key, _Element)
public typealias Index = DictionaryIndex<Key, _Element>
public typealias Generator = DictionaryGenerator<Key, _Element>
private typealias Storage = [Key: _Element]
private var storage = Storage()
public var startIndex: Index { return storage.startIndex }
public var endIndex: Index { return storage.endIndex }
public subscript(position: Index) -> _Element { return storage[position].1 }
public subscript(position: Index) -> Element { return storage[position] }
subscript(key: Key) -> Set<Value> {
get { return storage[key] ?? Set<Value>() }
set { storage[key] = newValue.isEmpty ? nil : newValue }
}
public func generate() -> Generator { return storage.generate() }
}
Usage:
var foo = Multimap<Int, String>()
foo[0] // Returns an emtpy Set<String>
foo[0].insert("Ook") // Inserts a value at index 0
foo[0].insert("Eek")
foo[0] // Now this returns a set { "Ook", "Eek" }
foo[1].insert("Banana")
foo[1].insert("Book")
foo[0].unionInPlace(foo[1])
foo[0] // Returns a set { "Banana", "Ook", "Eek", "Book" }
So far I have only been able to achieve this using a global function. I am not sure if it is possible but I was hoping to write an extension to a generic class that would hopefully achieve the same thing.
Below is the working global function it is using SignalProducer class from ReactiveCocoa but the principle should be the same for any generic class.
func ignoreNilValues <Value,Error> (producer: SignalProducer<Value?,Error>) -> SignalProducer<Value, Error> {
return producer.filter { return $0 != nil }.map { $0! }
}
Update:
I have made progress but have still fallen short of a complete solution
Given any class with some generic property
class GenericClass<SomeType> {
var someProperty: [SomeType] = []
}
How can I write an extension that will filter any optional values and return the value using the Wrapped type?
The following will filter any nil values but still return it as the Optional type.
protocol AnOptional {
var isNil: Bool {get}
}
extension Optional : AnOptional {
var isNil: Bool {
get {
guard let hasValue = self.map({ (value: Wrapped) -> Bool in
return true
}) else {
return true
}
return !hasValue
}
}
}
extension GenericClass where SomeType : AnOptional {
func filterNilValuesOfSomeProperty() -> [SomeType] {
return someProperty.filter({ (anOptional: AnOptional) -> Bool in
return !anOptional.isNil
})
}
}
As can be seen
let aClass = GenericClass<Int?>()
aClass.someProperty = [3,5,6,nil,4,3,6, nil]
let x = aClass.someProperty
//x = [Some(3),Some(5),Some(6),nil,Some(4),Some(3),Some(6), nil]
let y = aClass.filterNilValuesOfSomeProperty()
//y = [Some(3),Some(5),Some(6),Some(4),Some(3),Some(6)]
Is it possible to write a class extension that would return the wrapped type? In the example above it would be [Int] instead of [Int?].
I rewrote the global function solution for this example.
func ignoreNilValues <Value> (aClass: GenericClass<Value?>) -> GenericClass<Value> {
let aNewClass = GenericClass<Value>()
aNewClass.someProperty = aClass.someProperty.filter({ (v: Value?) -> Bool in
v != nil
}).map { (oldValue: Value?) -> Value in
return oldValue!
}
return aNewClass
}
let z = ignoreNilValues(aClass).someProperty
//z = [3, 5, 6, 4, 3, 6]
The "trick" is to define a protocol to which all optionals conform
(this is from Creating an extension to filter nils from an Array in Swift
with a minor simplification; the idea goes back to this Apple Forum Thread):
protocol OptionalType {
typealias Wrapped
func intoOptional() -> Wrapped?
}
extension Optional : OptionalType {
func intoOptional() -> Wrapped? {
return self
}
}
You can use that in your case as:
class GenericClass<SomeType> {
var someProperty: [SomeType] = []
}
extension GenericClass where SomeType : OptionalType {
func filterNilValuesOfSomeProperty() -> [SomeType.Wrapped] {
return someProperty.flatMap { $0.intoOptional() }
}
}
which uses the flatMap() method from SequenceType:
extension SequenceType {
/// Return an `Array` containing the non-nil results of mapping
/// `transform` over `self`.
///
/// - Complexity: O(*M* + *N*), where *M* is the length of `self`
/// and *N* is the length of the result.
#warn_unused_result
public func flatMap<T>(#noescape transform: (Self.Generator.Element) throws -> T?) rethrows -> [T]
}
Example:
let aClass = GenericClass<Int?>()
aClass.someProperty = [3,5,6,nil,4,3,6, nil]
let x = aClass.someProperty
print(x) // [Optional(3), Optional(5), Optional(6), nil, Optional(4), Optional(3), Optional(6), nil]
let y = aClass.filterNilValuesOfSomeProperty()
print(y) // [3, 5, 6, 4, 3, 6]
In Swift 3 and later the protocol has to be defined as
protocol OptionalType {
associatedtype Wrapped
func intoOptional() -> Wrapped?
}
I have this solution using in my app, create a protocol, and added an extension to Optional.
protocol OptionalUnwrap {
associatedtype Wrapped
func unwrap(default defaultValue: #autoclosure () -> Wrapped) -> Wrapped
}
extension Optional: OptionalUnwrap {
func unwrap(default defaultValue: #autoclosure () -> Wrapped) -> Wrapped {
if let value = self {
return value
}
return defaultValue()
}
}
You can use it like this, you have to provide a default value, so if optional is nil it will return the default value. It works with all types.
struct StructName {
var name: String
var age: Int
}
var structName3: StructName?
let unwrapped = structName3.unwrap(default: StructName(name: "", age: 2345))
print(unwrapped.age)
var version: Int?
version.unwrap(default: 5)
var subject: String? = "iOS"
subject.unwrap(default: "")