浇铸闭合/块 [英] Cast closures/blocks

问题描述

在Objective-C中，我经常传递块。我经常使用它们来实现模式，这有助于避免存储东西到实例变量，从而避免线程/时序问题。

In Objective-C, I often pass around blocks. I use them very often to implement patterns that help avoid storing stuff into instance variables, thus avoiding threading/timing issues.

例如，我将它们分配给一个 CAAnimation via - [CAAnimation setValue：forKey：] 所以我可以在动画完成时执行块。 （Objective-C可以把块当作对象;你也可以做 [someBlock copy] 和 [someBlock release] 。 ）

For example, I assign them to a CAAnimation via -[CAAnimation setValue:forKey:] so I can execute the block when the animation is finished. (Objective-C can treat blocks as objects; you also can do [someBlock copy] and [someBlock release].)

但是，试图在Swift中使用这些模式和Objective-C似乎是非常困难的。 （修改），我们可以看到该语言仍在不断变化：已修改代码，使其适用于Xcode6-beta2，以前的版本适用于Xcode6-beta1。）

However, trying to use these patterns in Swift together with Objective-C seems to be very difficult. ( and we can see that the language is still in flux: have adapted the code so it works on Xcode6-beta2, previous version worked on Xcode6-beta1.)

例如，我不能将 AnyObject 转换回块/闭包。下面的代码从编译器产生错误：

For example, I can't convert AnyObject back to a block/closure. The following yields an error from the compiler:

override func animationDidStop(anim: CAAnimation!, finished flag: Bool)
{
    let completion : AnyObject! = anim.valueForKey("completionClosure")
    (completion as (@objc_block ()->Void))()
    // Cannot convert the expression's type 'Void' to type '@objc_block () -> Void'
}

我找到了一个解决方法，但是很丑陋我的桥接头，我有：

I have found a workaround, but it's pretty ugly, IMHO: in my bridging header, I have:

static inline id blockToObject(void(^block)())
{
    return block;
}

static inline void callBlockAsObject(id block)
{
    ((void(^)())block)();
}

现在我可以在Swift中执行：

And now I can do this in Swift:

func someFunc(completion: (@objc_block ()->Void))
{
    let animation = CAKeyframeAnimation(keyPath: "position")
    animation.delegate = self
    animation.setValue(blockToObject(completion), forKey: "completionClosure")
    …
}

override func animationDidStop(anim: CAAnimation!, finished flag: Bool)
{
    let completion : AnyObject! = anim.valueForKey("completionClosure")
    callBlockAsObject(completion)
}

它工作，但我需要一个新的功能，我想使用的每个块类型，我在编译器，这是不好的。

It works, but I'd need a new function for every block type that I'd like to use and I'm hacking around the compiler which can't be good either.

那么有没有办法以纯粹的Swift方式解决这个问题？

So is there a way to solve this in a pure Swift way?

推荐答案

$ c> Block 参数化的函数类型？

How about a generic Block parameterized with the function type?

class Block<T> {
  let f : T
  init (_ f: T) { self.f = f }
}

分配其中一个;它将是 AnyObject 的子类型，因此可以分配到字典和数组中。这看起来不太繁重，尤其是使用尾随闭包语法。正在使用中：

Allocate one of these; it will be a subtype of AnyObject and thus be assignable into dictionaries and arrays. This doesn't seem too onerous especially with the trailing closure syntax. In use:

  5> var b1 = Block<() -> ()> { print ("Blocked b1") }
b1: Block<() -> ()> = {
  f = ...
}
  6> b1.f()
Blocked b1

，另一个例子是阻止类型被推断：

and another example where the Block type is inferred:

 11> var ar = [Block { (x:Int) in print ("Block: \(x)") }]
ar: [Block<(Int) -> ()>] = 1 value {
  [0] = {
    f = ...
  }
}
 12> ar[0].f(111)
Block: 111

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