带有 nil 值的 Swift 闭包捕获列表

Question

我正在捕获一个委托(delegate)引用，该引用最终设置为某个值，但最初为零。但是，即使设置了委托(delegate)，捕获的引用仍为零。

DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2)) { [weak delegate] in
    delegate?.something() // delegate is nil
}

DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2)) { [weak self] in
    self?.delegate?.something() // delegate is not nil
}

这是怎么回事？

Answer 1

在中使用像[weak delegate]这样的捕获列表将在闭包初始化时复制委托(delegate)的值。因此，当委托(delegate)在那一刻为 nil 时，无论您是否修改 self.delegate ， if 都会在闭包内保留 nil稍后的时间。这也适用于 [weak self]，除了 self 在 Swift 中通常不会改变。

检查这个例子:

class Delegate {

}

class A {
    var delegate:Delegate?

    func foo() {
        print ("in foo ---------------------")
        delegate = nil
        print ("delegate before: \(delegate)")  // prints: "nil"
        var closure = { print ("in closure: \(self.delegate)")}
        closure()   // prints: "in closure: nil"
        delegate = Delegate()
        print ("delegate after: \(delegate)") // prints "Optional(SwiftPlayground.Delegate)"
        closure() // prints "in closure: Optional(SwiftPlayground.Delegate)"
    }
    func bar() {
        print ("in bar ---------------------")
        delegate = nil  // prints "nil"
        print ("delegate before: \(delegate)")
        var closure = { [weak delegate] in print ("in closure: \(delegate)")}
        closure() // prints: "in closure: nil"
        delegate = Delegate()   // prints "Optional(SwiftPlayground.Delegate)"
        print ("delegate after: \(delegate)")
        closure() // prints "nil"
    }
}

let a = A()
a.foo()
a.bar()

这里，func bar 中的最后一个 closure() 调用将打印 nil，因为 delegate 是闭包初始化时nil。