捕獲強弱的參考並保留週期

class MyClass {
    func sayHi() { print("Hello") }
    deinit { print("Goodbye") }
}

當閉包捕獲引用型別（類例項）時，預設情況下它包含一個強引用：

let closure: () -> Void
do {
    let obj = MyClass()
    // Captures a strong reference to `obj`: the object will be kept alive
    // as long as the closure itself is alive.
    closure = { obj.sayHi() }
    closure()  // The object is still alive; prints "Hello"
} // obj goes out of scope
closure()  // The object is still alive; prints "Hello"

閉包的捕獲列表可用於指定弱引用或無主引用：

let closure: () -> Void
do {
    let obj = MyClass()
    // Captures a weak reference to `obj`: the closure will not keep the object alive;
    // the object becomes optional inside the closure.
    closure = { [weak obj] in obj?.sayHi() }
    closure()  // The object is still alive; prints "Hello"
} // obj goes out of scope and is deallocated; prints "Goodbye"
closure()  // `obj` is nil from inside the closure; this does not print anything.

let closure: () -> Void
do {
    let obj = MyClass()
    // Captures an unowned reference to `obj`: the closure will not keep the object alive;
    // the object is always assumed to be accessible while the closure is alive.
    closure = { [unowned obj] in obj.sayHi() }
    closure()  // The object is still alive; prints "Hello"
} // obj goes out of scope and is deallocated; prints "Goodbye"
closure()  // crash! obj is being accessed after it's deallocated.

有關更多資訊，請參閱“ 記憶體管理” 主題和 “Swift 程式語言” 的“ 自動引用計數” 部分。

保留週期

如果一個物件保持一個閉包，該閉包也保持對該物件的強引用，那麼這是一個保留週期 。除非迴圈被破壞，否則儲存物件和閉包的記憶體將被洩露（永不回收）。

class Game {
    var score = 0
    let controller: GCController
    init(controller: GCController) {
        self.controller = controller

        // BAD: the block captures self strongly, but self holds the controller
        // (and thus the block) strongly, which is a cycle.
        self.controller.controllerPausedHandler = {
            let curScore = self.score
            print("Pause button pressed; current score: \(curScore)")
        }

        // SOLUTION: use `weak self` to break the cycle.
        self.controller.controllerPausedHandler = { [weak self] in
            guard let strongSelf = self else { return }
            let curScore = strongSelf.score
            print("Pause button pressed; current score: \(curScore)")
        }
    }
}