21

如果我有一个闭包传递给这样的函数：

 someFunctionWithTrailingClosure { [weak self] in
     anotherFunctionWithTrailingClosure { [weak self] in 
         self?.doSomething()
     }
 }

如果我在捕获列表中声明 self而没有[weak self]在捕获列表中再次someFunctionWithTrailingClosure重新声明它已经成为一种类型，但它是否也成为引用？weakanotherFunctionWithTrailingClosure selfOptionalweak

谢谢！

4

3 回答 3

24

不需要输入[weak self]。anotherFunctionWithTrailingClosure

您可以凭经验对此进行测试：

class Experiment {
    func someFunctionWithTrailingClosure(closure: @escaping () -> Void) {
        print("starting", #function)
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing", #function)
        }
    }

    func anotherFunctionWithTrailingClosure(closure: @escaping () -> Void) {
        print("starting", #function)
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing", #function)
        }
    }

    func doSomething() {
        print(#function)
    }

    func testCompletionHandlers() {
        someFunctionWithTrailingClosure { [weak self] in
            self?.anotherFunctionWithTrailingClosure { // [weak self] in
                self?.doSomething()
            }
        }
    }

    // go ahead and add `deinit`, so I can see when this is deallocated

    deinit {
        print("deinit")
    }
}

接着：

func performExperiment() {
    DispatchQueue.global().async {
        let obj = Experiment()

        obj.testCompletionHandlers()

        // sleep long enough for `anotherFunctionWithTrailingClosure` to start, but not yet call its completion handler

        Thread.sleep(forTimeInterval: 1.5)
    }
}

如果您这样做，您将看到doSomething从未调用过并且在调用其闭包deinit之前调用过。anotherFunctionWithTrailingClosure

话虽如此，我可能仍然倾向于使用[weak self]on 语法anotherFunctionWithTrailingClosure来明确表达我的意图。

于 2016-08-03T18:34:30.373 回答
7

TL;博士

虽然[weak self]在外部块中使用一次很好（EX1），但如果您将此引用更改为strong（例如guard let self = self），您也需要[weak self]在内部块中使用a（EX3）。

同样在内部[weak self]块上只使用一次通常是一个错误（EX2_B）。不幸的是，这是重构代码时常犯的错误，而且在发生时很难发现。

一个好的经验法则是，weak如果对象在闭包之外很强大，则始终使用。

不保留的示例self（即通常这些是“好”场景）：

// EX1
fn { [weak self] in
  self?.foo() 
}

// EX2
fn { [weak self] in 
  fn2 {
    self?.foo()
  }
}
// self is weak inside fn, thus adding an extra `[weak self]` inside fn2 is unnecessary

// EX3
fn { [weak self] in 
  guard let self = self else { return }
  fn2 { [weak self] in
    self?.foo()
  }
}

保留的示例self（即通常是“坏”场景）：

// EX1_B
fn {
  self.foo()
}
// fn retains self

// EX2_B
fn {
  fn2 { [weak self] in
    self.foo()
  }
}
// fn retains self (this is a common, hard-to-spot mistake)

// EX3_B
fn { [weak self] in 
  guard let self = self else { return }
  fn2 {
    self.foo()
  }
}
// fn2 retains self

正如Hamish 所暗示的，有两个主要原因weak是有用的：

  1. 防止保留循环。
  2. 以防止对象的寿命超过应有的寿命。

更多关于#2（防止长寿命物体）

在Rob 的示例中，该函数没有保留闭包（除了 dispatch_async 之外，几乎可以保证在将来的某个时间点触发闭包），因此您永远不会以保留周期结束。因此，weak在这种情况下使用是为了防止 #2 发生。

正如 Hamish 提到的，在此示例中实际上不需要弱来防止保留循环，因为没有保留循环。weak，在这种情况下，用于防止对象比需要的寿命更长。这完全取决于您的用例何时考虑一个对象的寿命超过需要的时间。因此，有时您只想使用weak外部（EX2），而有时您想使用weak外部，strong内部，weak内部舞蹈（EX3）。

更多关于#1（防止保留周期）

为了检查保留周期问题，假设一个函数正在存储对块（即堆）的引用，而不是直接引用该函数（即堆栈）。很多时候我们不知道类/函数的内部结构，所以假设函数保留了块会更安全。

现在您可以使用外部轻松创建保留循环weak，并且仅使用strong内部 (EX3_B)：

public class CaptureListExperiment {

    public init() {

    }

    var _someFunctionWithTrailingClosure: (() -> ())?
    var _anotherFunctionWithTrailingClosure: (() -> ())?

    func someFunctionWithTrailingClosure(closure: @escaping () -> ()) {
        print("starting someFunctionWithTrailingClosure")
        _someFunctionWithTrailingClosure = closure

        DispatchQueue.global().asyncAfter(deadline: .now() + 1) { [weak self] in
            self?._someFunctionWithTrailingClosure!()
            print("finishing someFunctionWithTrailingClosure")
        }
    }

    func anotherFunctionWithTrailingClosure(closure: @escaping () -> ()) {
        print("starting anotherFunctionWithTrailingClosure")
        _anotherFunctionWithTrailingClosure = closure

        DispatchQueue.global().asyncAfter(deadline: .now() + 1) { [weak self] in
            self?._anotherFunctionWithTrailingClosure!()
            print("finishing anotherFunctionWithTrailingClosure")
        }
    }

    func doSomething() {
        print("doSomething")
    }

    public func testCompletionHandlers() {
        someFunctionWithTrailingClosure { [weak self] in
            guard let self = self else { return }
            self.anotherFunctionWithTrailingClosure { // [weak self] in
                self.doSomething()
            }
        }
    }

    // go ahead and add `deinit`, so I can see when this is deallocated

    deinit {
        print("deinit")
    }
}

func performExperiment() {

    let obj = CaptureListExperiment()

    obj.testCompletionHandlers()
    Thread.sleep(forTimeInterval: 1.3)
}

performExperiment()

/* Output:

starting someFunctionWithTrailingClosure
starting anotherFunctionWithTrailingClosure
finishing someFunctionWithTrailingClosure
doSomething
finishing anotherFunctionWithTrailingClosure
*/

请注意，deinit它没有被调用，因为创建了一个保留循环。

这可以通过删除strong参考（EX2）来解决：

public func testCompletionHandlers() {
    someFunctionWithTrailingClosure { [weak self] in
        //guard let self = self else { return }
        self?.anotherFunctionWithTrailingClosure { // [weak self] in
            self?.doSomething()
        }
    }
}

或者使用弱/强/弱的舞蹈（EX3）：

public func testCompletionHandlers() {
    someFunctionWithTrailingClosure { [weak self] in
        guard let self = self else { return }
        self.anotherFunctionWithTrailingClosure { [weak self] in
            self?.doSomething()
        }
    }
}
于 2020-06-12T21:19:41.940 回答
2

为 Swift 4.2 更新：

public class CaptureListExperiment {

    public init() {

    }

    func someFunctionWithTrailingClosure(closure: @escaping () -> ()) {
        print("starting someFunctionWithTrailingClosure")

        DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing someFunctionWithTrailingClosure")
        }
    }

    func anotherFunctionWithTrailingClosure(closure: @escaping () -> ()) {
        print("starting anotherFunctionWithTrailingClosure")

        DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
            closure()
            print("finishing anotherFunctionWithTrailingClosure")
        }
    }

    func doSomething() {
        print("doSomething")
    }

    public func testCompletionHandlers() {
        someFunctionWithTrailingClosure { [weak self] in
            guard let self = self else { return }
            self.anotherFunctionWithTrailingClosure { // [weak self] in
                self.doSomething()
            }
        }
    }

    // go ahead and add `deinit`, so I can see when this is deallocated

    deinit {
        print("deinit")
    }
}

尝试一下 Playgorund：

func performExperiment() {

    let obj = CaptureListExperiment()

    obj.testCompletionHandlers()
    Thread.sleep(forTimeInterval: 1.3)
}

performExperiment()
于 2018-07-16T05:15:18.800 回答