Difficulty: Beginner | Easy | Normal | ChallengingThis article has been developed using Xcode 11.4.1, and Swift 5.2.2

难度：初学者| 容易 | 普通| 具有挑战性本文是使用Xcode 11.4.1和Swift 5.2.2开发的

This article is tested on Xcode 12

本文已在Xcode 12上经过测试

先决条件： (Prerequisites:)

• You will be expected to be aware how to make a Single View Application in Swift

  您应该知道如何在Swift中制作Single View应用程序

• You will need to have some knowledge of closures, although I’ll do my best to have you covered in this article

  您将需要对闭包有所了解，尽管我会尽力在本文中进行介绍。

• By definition this article relies on some knowledge of error handling in Swift

  根据定义，本文依赖于Swift中的错误处理知识

• You should be aware of @escaping

  您应该注意@转义

术语 (Terminology)

Closures: A combination of functions and references to the surrounding context

闭包：功能和对周围环境的引用的组合

动机 (Motivation)

One of the important things in swift is using closures to make functions return and good practice for Swift coders.

swift的重要一环是使用闭包使函数返回并成为Swift编码人员的良好实践。

闭包的普通用法 (The ordinary use of closures)

One of the uses of closures is to do something, and then something else (clear, right?)

closures的用途之一是先做某件事，然后再做别的事情(清楚，对吧？)

the output of this (when called with doThis(a: "test", thenThis: completion)) is (can you guess?)

这个的输出(当用doThis(a: "test", thenThis: completion) )是(您能猜到吗？)

this then this test

this then this test

now because the function that is passed can be changed, we can do just that by instead passing throught the completionFunction by using doThis(a: "test", thenThis: completionFunction) which then gives the following output:

现在，因为传递的功能是可以改变的，我们能做到这一点的，而不是通过throught的completionFunction使用doThis(a: "test", thenThis: completionFunction)然后给出了以下的输出：

this then this function test

this then this function test

Now notice that the first function completion is assigned to a constant (through let) but the second is simply a function - no matter, Swift can cope with both of these functions.

现在注意，第一个函数completion已分配给一个常量(通过let )，但是第二个仅是一个function -不管怎样， Swift可以处理这两个函数。

Awesome — we are passing a closure to a function as a function- Most excellent news.

太棒了-我们正在将闭包作为函数传递给我们-最出色的消息。

We’ll expand this second example as we move forwards in this tutorial.

在本教程中，我们将继续扩展第二个示例 。

异步闭包的使用 (The use of async closures)

Network calls can return at any time after they are called. I’m pretending to have a network call here, but you could actually do it with the following network manager if you’d like).

网络呼叫在被调用后可以随时返回。 我假装在这里打个网络电话，但是如果您愿意的话，实际上您可以与以下网络管理员联系 。

So imagine that you need to need to go to the network to do some work. You make a network call to increment a number (it’s an example, right?) but goes and makes a network call to do just that. Well, we can model that:

因此，想象您需要去网络做一些工作。 您进行网络呼叫以增加一个数字(这是一个例子，对吗？)，但是去进行网络呼叫来做到这一点。 好吧，我们可以建模：

I know there are some tricky parts to this — we are using a trailing closure which can be tricky. Assuming you’re OK with all that let us move on.

我知道这有一些棘手的部分-我们正在使用结尾的闭包，这可能很棘手。 假设您对所有让我们继续进行的事情都感到满意。

But there is a problem (in this example, and not just that it is contrived) — incNum will produce an error if 13 is returned.

但是存在一个问题(在此示例中，不仅是人为的)，如果返回13， incNum将产生错误。

错误 (The Error)

So what about producing an Error when 13 is returned from incNum?.

那么从incNum返回13时产生一个Error incNum办？

Let us say that 13 is a bad number (in some cultures it is considered to be unlucky). So we shall set up a rather basic function incNum that will call a completion handler to return a number increased by one.

让我们说13是一个坏数字(在某些文化中，它被认为是不幸的)。 因此，我们将建立一个相当基本的函数incNum ，该函数将调用completion处理程序以返回加一的数字。

The Error will indicate if we have returned an 'unlucky' number from incNum - but see underneath the code snippet for the (rather large) error:

该Error将指示我们是否从incNum返回了一个“不幸”数字-但是请参见代码段下方的(相当大的)错误：

Invalid conversion from throwing function of type '(Int) throws -> Void' to non-throwing function type '(Int) -> Void'

Invalid conversion from throwing function of type '(Int) throws -> Void' to non-throwing function type '(Int) -> Void'

Oh dear!

噢亲爱的！

What Swift is telling us, is that when we throw BadLuckError.unlucky we are actually throwing from the closure.

Swift告诉我们的是，当我们throw BadLuckError.unlucky ，实际上是从闭包中抛出。

从拖尾闭合处抛出 (Throwing from a trailing closure)

The solution will come first, followed by the explaination

解决方案将首先出现，然后进行解释

We have marked callingFunction as throws which means that it can throw an error by placing the throws keyword at the end of the function signature. This meant that we needed to call the calling function suing a do-try-catch block:

我们已经标记callingFunction为throws ，这意味着它可以throw通过将错误throws的函数签名结束关键字。 这意味着我们需要使用do-try-catch块来调用调用函数：

do { try callingFunction(num: 12, closure: incNum) } catch { print ("DONE") }

do { try callingFunction(num: 12, closure: incNum) } catch { print ("DONE") }

The incNum function will not throw an error on it's own, so is marked with rethrows as it throws from the completion - which therefore must be marked with try. Since completion does indeed throw it has it's signature changed to (Int) throws -> Void).

incNum函数不会自行抛出错误，因此从completion时rethrows时将被标记为rethrows -因此必须使用try进行标记。 由于completion确实会throw ，因此其签名已更改为(Int) throws -> Void) 。

This modified signature is copied back to the signature of the callingFunction which marks the closure as the same (Int) throws -> Void), but since rethrows can only be used in function declarations throws is used instead.

将此修改后的签名复制回给callingFunction的签名，该签名将闭包标记为相同的(Int) throws -> Void) callingFunction (Int) throws -> Void) ，但是由于rethrows只能在函数声明中使用，因此使用throws代替。

Fantastico!

幻想曲！

注意事项 (The caveats)

You can’t throw an error from async closures — which means that you either force the closure to be synchronous using semaphores (not recommended due to performance) or use Swift’s result type.

您不能从异步闭包中引发错误-这意味着您可以使用信号量(由于性能不建议使用)强制闭包同步，或者使用Swift的结果类型 。

结论 (Conclusion)

I hope this article has helped you out!

希望本文对您有所帮助！

It can be tricky to work through this type of article — we are using throws and rethrows and it can feel a little complex. However, I hope this article has helped you out and you can see how to progress in your coding journey.

处理此类文章可能很棘手-我们使用的是throws和rethrows ，感觉有些复杂。 但是，希望本文对您有所帮助，并且您可以看到如何在编码过程中取得进步。

If you’ve any questions, comments or suggestions please hit me up on Twitter

如果您有任何疑问，意见或建议，请在Twitter上打我

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