Swift:$0 在 Array.forEach 中是如何工作的? [英] Swift : How $0 works in Array.forEach?

问题描述

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

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]) }

推荐答案

简短回答

看看这段代码

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

这里是forEach

我的意思是这部分 { print($0) }

执行 4 次(对数组中的每个元素执行一次).每次执行时，$0 都包含 nums 数组的 n-th 元素的副本.

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.

所以第一次包含1，然后是2，依此类推...

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

这是输出

1
2
3
4

比较 forEach 与 for-in 结构

那么我们可以说 $0 就像下面代码中的 n 值吗?

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)
}

是的，它的含义几乎相同.

Yes, it has pretty much the same meaning.

forEach 方法接受一个闭包.闭包有这个签名.

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

(Self.Generator.Element) throws -> Void

当您使用 forEach 时，它会与您签订合同".

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

1. 您向 forEach 提供一个闭包，该闭包接受输入中的单个参数，其中该参数具有与数组相同的类型
2. forEach 会将该闭包应用于数组的每个元素.

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.

示例

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

但是在 Swift 中，您可以省略闭包参数的显式名称.在这个闭包里面，你可以使用 $0 作为第一个参数，$1 作为第二个参数，以此类推.

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.

所以前面的代码片段也可以写成下面这样

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

nums.forEach {
    print($0)
}

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