具有 nil 值的 Swift 闭包捕获列表

Question

我正在捕获最终设置为某个值但最初为零的委托引用。但是，即使设置了委托，捕获的引用仍然为零。

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

使用像[weak delegate] in 这样的捕获列表将在初始化闭包的那一刻复制委托的值。因此，当代理在那一刻为nil 时，无论您稍后是否修改self.delegate，if 都会留在闭包内的nil。这也适用于[weak self]，除了self 通常在 Swift 中不会改变。

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，因为在初始化 closure 时，delegate 是 nil。