CompletionHandler和闭包

Question

我有几个问题要问

1)什么是CompletionHandler和闭包，什么时候使用它? 2)闭包与CompletionHandler

这对我来说有点困惑。

Answer 1

完成处理程序和闭包是同义词。它们在Objective-C中称为块。

您可以将它们视为在被调用时执行代码块的对象(非常类似于函数)。

// 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操作的闭包:一旦您完成了某项操作，您就可以调用完成处理程序来执行代码来完成该操作。

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