for-loop - swift 3 : how to write this for(;;) loop

Question

看起来 Apple 不喜欢 C 循环，但没有提供解决它的好方法(或者我找不到它)。我有这样的循环，从某个 View 转到 UI 层次结构中的根:

for var parentView = view; parentView != nil; parentView = parentView.parent {
    ...
}

如何用 Swift 3 的方式写这个？

Answer 1

这将是在 Swift 3 中实现的一种方式:

var parentView: View! = view
while parentView != nil {
    // Do stuff
    parentView = parentView.parent
}

如果你想把循环进程的东西放在 while 旁边而不是 block 的末尾，你可以使用 defer，像这样:

var parentView: View! = view
while parentView != nil {
    defer { parentView = parentView.parent }        
    // Do stuff
}

如果你想限制parentView的范围，你可以把所有的东西封装在一个do block 中:

do {
    var parentView: View! = view
    while parentView != nil {
        defer { parentView = parentView.parent }        
        // Do stuff
    }
}

但它非常冗长，因此您可以为类似的循环定义一个新的通用函数，如下所示:

func kindaCStyleLoop<T>(first: T, obtainNext: T -> T?, action: T -> ()) {
    var current: T! = first
    repeat {
        action(current)
        current = obtainNext(current)
    } while current != nil
}

kindaCStyleLoop(view, obtainNext: { $0.parent }) {
    // Do stuff with $0
}

最后一个依赖于 GeneratorType 和 SequenceType 来启用 for-in-loop 语法:

struct CStyleGenerator<T> : GeneratorType, SequenceType {
    let getNext: T -> T?
    var current: T!

    init(first: T, getNext: T -> T?) {
        self.getNext = getNext
        self.current = first
    }

    mutating func next() -> T? {
        defer {
            if current != nil {
                current = getNext(current)
            }
        }
        return current
    }
}

for parentView in CStyleGenerator(first: view, getNext: { $0.parent }) {
    // Do stuff with parentView
}