Swift：$ 0如何在Array.forEach中工作？ [英] Swift : How $0 works in Array.forEach?

问题描述

我已经看到大多数swift开发人员正在开始使用.forEach，了解了另一种迭代数组的方法。但是，$ 0的含义是什么，它是如何工作的？如果它是一个索引，那么它应该增加0,1,2 ...

$ p $ @IBOutlet var headingLabels：[UILabel]！
....

headingLabels.forEach {$ 0.attributedText = NSAttributedString（string：$ 0.text !, attributes：[NSKernAttributeName：1]）}

$ b

解决方案

简短的回答

此代码

  let nums = [1,2,3,4] 
 nums.forEach {print（$ 0） } 
  

在 forEach

我的意思是这部分 {print（$ 0）}

被执行4次（对于数组内的每个元素都执行一次）。每次执行 $ 0 都包含您的第n 元素的副本 nums 数组。

所以第一次包含 1 ，然后 2 等等...

以下是输出：

  1 
 2 
 3 
 4 
  





比较 forEach 与 for-在构建

所以我们可以说 $ 0 就像 $ pre $ n code $ n $ {
print $ （n）
}


是的，它的含义几乎相同。

它是如何工作的？

forEach 方法接受关闭。关闭有这个签名。

 （Self.Generator.Element）throws  - >无效
  

使用 forEach 与您签订合同。

1. 您向 forEach 在输入中接受一个参数，其中param具有相同类型的数组


2. forEach 会将该闭包应用于
   
   
   

   示例

   
   
   

     nums.forEach {（n）in 
    print（n）
   } 
     

   然而，在Swift中，可以省略闭包参数的显式名称。在这个闭包内部，你可以使用 $ 0 作为第一个参数，第二个参数是 $ 1

   
   
   

   所以前面的代码片段也可以写成如下

     nums.forEach {
    print（$ 0）
   } 
     

   
   

   I have seen most of the swift developer are starting using .forEach, understood its another way to iterate array. But what is the meaning of '$0' and how it works? If it's an index then it should increment 0,1,2...

   @IBOutlet var headingLabels: [UILabel]!
   ....
   
   headingLabels.forEach { $0.attributedText = NSAttributedString(string: $0.text!, attributes: [NSKernAttributeName: 1]) }
   

   解决方案

   Short answer

   Look at this code

   let nums = [1,2,3,4]
   nums.forEach { print($0) }
   

   Here the closure following forEach

   I mean this part { print($0) }

   is executed 4 times (once for every element inside the array). Each time it is executed $0 contains a copy of the n-th element of your thenums array.

   So the first time contains 1, then 2 and so on...

   Here's the output

   1
   2
   3
   4
   

   Comparing forEach with the for-in construct

   So can we say tha $0 is like the n value in the following code?

   for n in nums {
       print(n)
   }
   

   Yes, it has pretty much the same meaning.

   How does it work?

   The forEach method accept a closure. The closure has this signature.

   (Self.Generator.Element) throws -> Void
   

   When you use the forEach it makes a "contract" with you.

   1. You provide to the forEach a closure that accept a single param in input where the param has the same type of the Array
   2. The forEach will apply that closure to each element of the array.

   Example

   nums.forEach { (n) in
       print(n)
   }
   

   However in Swift you can omit explicit names for the parameters of a closure. In this inside the closure you can refer that params using $0 for the first param, $1 for the second one and so on.

   So the previous snippet of code can be also be written like below

   nums.forEach {
       print($0)
   }
   

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