内部闭包的捕获列表是否需要将“ self”重新声明为“ weak”或“ unowned”？

Question

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[weak self]在someFunctionWithTrailingClosure的捕获列表中声明self为，而没有weak在的捕获列表中再次声明自身，则它anotherFunctionWithTrailingClosure
self已经成为一种Optional类型，但它也同样成为weak引用吗？

谢谢！

Answer 1

该[weak self]中anotherFunctionWithTrailingClosure是没有必要的。

您可以凭经验进行测试：

class Experiment {

    func someFunctionWithTrailingClosure(closure: () -> ()) {
        print("starting someFunctionWithTrailingClosure")
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {
            closure()
            print("finishing someFunctionWithTrailingClosure")
        }
    }

    func anotherFunctionWithTrailingClosure(closure: () -> ()) {
        print("starting anotherFunctionWithTrailingClosure")
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(1 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()) {
            closure()
            print("finishing anotherFunctionWithTrailingClosure")
        }
    }

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

    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")
    }
}

然后：

func performExperiment() {
    dispatch_async(dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)) { 
        let obj = Experiment()

        obj.testCompletionHandlers()

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

        NSThread.sleepForTimeInterval(1.5) 
    }
}

如果执行此操作，您将看到doSomething永不被调用，并且deinit在anotherFunctionWithTrailingClosure调用其闭包之前被调用。

验证了此行为后，我个人仍然倾向于使用[weak self]语法anotherFunctionWithTrailingClosure使我的意图明确，因为所有这些远非显而易见。