延迟中的快速完成

Question

我喜欢defer，因为在复杂的多case场景下，我不能忘记在函数结束前调用completion。

但是当完成传递给另一个函数时，它不应该在第一个函数的 defer 中运行。

这是我通常做的：

func doSomething(completion: ()->()) {
    /// Set this to false, before passing the completion to another function.
    var runCompletionInDefer = true
    defer {
        if runCompletionInDefer { completion() }
    }
    runCompletionInDefer = false
    doSomethingElse { completion() }
}

func doSomethingElse(completion: ()->()) {
    completion()
}

有没有更好的解决方案？

使用这种方法，在将完成传递给另一个函数之前，您仍然必须记住：

runCompletionInDefer = false

Answer 1

我认为您不应该将代码放入defer，直到您确定每次到达函数末尾时都确实想要执行该代码。

所以让我建议，也许你真正想要defer 的并不是完成闭包的字面执行，而是一个检查，让你确信完成闭包已被某人在某处调用到达时间defer。各种“执行收据”。

为此，可以将闭包包装到一个跟踪其执行的结构中。下面是这样一个包装器的示例：

protocol CompletionReceiptProtocol {
    associatedtype Closure: Any
    var receipt: Bool { get }
    func execute()
}

class SomethingCompletion: CompletionReceiptProtocol {
    typealias Closure = (()->Void)

    var receipt = false
    private var closure: Closure

    init(_ closure: @escaping Closure) {
        self.closure = closure
    }

    func execute() {
        self.closure()
        self.receipt = true
    }

}

下面是你将如何使用包装器：

func doSomething(completion nakedCompletion: @escaping ()->Void) {
    let completion = SomethingCompletion(nakedCompletion)
    defer {
        assert(completion.receipt , "Oops! End of function reached without completion being called!") // or whatever error handling or logging you want to do in that case
    }

    // Scenario A: `doSomething` executes the completion itself
    completion.execute()

    // Scenario B: `doSomething` delegates the completion to `doSomethingElse`...
    doSomethingElse(completion: completion)
}

func doSomethingElse(completion: SomethingCompletion) {
    completion.execute() //... and it's called here instead. So it's all good!
}