CompletionHandler 和闭包

Question

我有一些问题，

1) 什么是 CompletionHandler 和 Closure 以及何时使用它？ 2) 闭包 vs CompletionHandler

这让我有点困惑。

Answer 1

完成处理程序和闭包是同义词。它们在 Objective-C 中被称为块。

您可以将它们视为在调用它们时执行一组代码的对象（很像一个函数）。

// My view controller has a property that is a closure
// It also has an instance method that calls the closure
class ViewController {

    // The closure takes a String as a parameter and returns nothing (Void)
    var myClosure: ((String) -> (Void))?
    let helloString = "hello"

    // When this method is triggered, it will call my closure
    func doStuff() {
        myClosure(helloString)?
    }
}

let vc = ViewController()

// Here we define what the closure will do when it gets called
// All it does is print the parameter we've given it
vc.myClosure = { helloString in
    print(helloString) // This will print "hello"
}

// We're calling the doStuff() instance method of our view controller
// This will trigger the print statement that we defined above
vc.doStuff()

完成处理程序只是一个用于完成某个动作的闭包：一旦你完成了某件事，你就调用你的完成处理程序，它执行代码来完成那个动作。

这只是一个基本的解释，更多细节你应该查看文档：https://docs.swift.org/swift-book/LanguageGuide/Closures.html