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Pods/Kingfisher/Sources/Utility/Delegate.swift

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+//
+//  Delegate.swift
+//  Kingfisher
+//
+//  Created by onevcat on 2018/10/10.
+//
+//  Copyright (c) 2019 Wei Wang <onevcat@gmail.com>
+//
+//  Permission is hereby granted, free of charge, to any person obtaining a copy
+//  of this software and associated documentation files (the "Software"), to deal
+//  in the Software without restriction, including without limitation the rights
+//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+//  copies of the Software, and to permit persons to whom the Software is
+//  furnished to do so, subject to the following conditions:
+//
+//  The above copyright notice and this permission notice shall be included in
+//  all copies or substantial portions of the Software.
+//
+//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+//  THE SOFTWARE.
+
+import Foundation
+/// A class that keeps a weakly reference for `self` when implementing `onXXX` behaviors.
+/// Instead of remembering to keep `self` as weak in a stored closure:
+///
+/// ```swift
+/// // MyClass.swift
+/// var onDone: (() -> Void)?
+/// func done() {
+///     onDone?()
+/// }
+///
+/// // ViewController.swift
+/// var obj: MyClass?
+///
+/// func doSomething() {
+///     obj = MyClass()
+///     obj!.onDone = { [weak self] in
+///         self?.reportDone()
+///     }
+/// }
+/// ```
+///
+/// You can create a `Delegate` and observe on `self`. Now, there is no retain cycle inside:
+///
+/// ```swift
+/// // MyClass.swift
+/// let onDone = Delegate<(), Void>()
+/// func done() {
+///     onDone.call()
+/// }
+///
+/// // ViewController.swift
+/// var obj: MyClass?
+///
+/// func doSomething() {
+///     obj = MyClass()
+///     obj!.onDone.delegate(on: self) { (self, _)
+///         // `self` here is shadowed and does not keep a strong ref.
+///         // So you can release both `MyClass` instance and `ViewController` instance.
+///         self.reportDone()
+///     }
+/// }
+/// ```
+///
+public class Delegate<Input, Output> {
+    public init() {}
+
+    private var block: ((Input) -> Output?)?
+    public func delegate<T: AnyObject>(on target: T, block: ((T, Input) -> Output)?) {
+        self.block = { [weak target] input in
+            guard let target = target else { return nil }
+            return block?(target, input)
+        }
+    }
+
+    public func call(_ input: Input) -> Output? {
+        return block?(input)
+    }
+
+    public func callAsFunction(_ input: Input) -> Output? {
+        return call(input)
+    }
+}
+
+extension Delegate where Input == Void {
+    public func call() -> Output? {
+        return call(())
+    }
+
+    public func callAsFunction() -> Output? {
+        return call()
+    }
+}
+
+extension Delegate where Input == Void, Output: OptionalProtocol {
+    public func call() -> Output {
+        return call(())
+    }
+
+    public func callAsFunction() -> Output {
+        return call()
+    }
+}
+
+extension Delegate where Output: OptionalProtocol {
+    public func call(_ input: Input) -> Output {
+        if let result = block?(input) {
+            return result
+        } else {
+            return Output._createNil
+        }
+    }
+
+    public func callAsFunction(_ input: Input) -> Output {
+        return call(input)
+    }
+}
+
+public protocol OptionalProtocol {
+    static var _createNil: Self { get }
+}
+extension Optional : OptionalProtocol {
+    public static var _createNil: Optional<Wrapped> {
+         return nil
+    }
+}