是否可以在 Swift 中创建一个仅限于一个类的数组扩展? [英] Is it possible to make an Array extension in Swift that is restricted to one class?

问题描述

我可以制作一个仅适用于字符串的数组扩展吗?

Can I make an Array extension that applies to, for instance, just Strings?

推荐答案

从 Swift 2 开始，现在可以通过协议扩展来实现，为符合类型提供方法和属性实现(可选地受附加约束限制).

As of Swift 2, this can now be achieved with protocol extensions, which provide method and property implementations to conforming types (optionally restricted by additional constraints).

一个简单的例子:为所有符合的类型定义一个方法到 SequenceType(例如 Array)，其中序列元素是 String:

A simple example: Define a method for all types conforming to SequenceType (such as Array) where the sequence element is a String:

extension SequenceType where Generator.Element == String {
    func joined() -> String {
        return "".join(self)
    }
}

let a = ["foo", "bar"].joined()
print(a) // foobar

不能直接为struct Array定义扩展方法，只能为所有类型定义符合某些协议(带有可选约束).所以一个必须找到一个 Array 符合的协议并提供所有必要的方法.在上面的例子中，就是SequenceType.

The extension method cannot be defined for struct Array directly, but only for all types conforming to some protocol (with optional constraints). So one has to find a protocol to which Array conforms and which provides all the necessary methods. In the above example, that is SequenceType.

另一个例子(如何在 Swift 中将正确位置的元素插入到已排序的数组中?):

extension CollectionType where Generator.Element : Comparable, Index : RandomAccessIndexType {
    typealias T = Generator.Element
    func insertionIndexOf(elem: T) -> Index {
        var lo = self.startIndex
        var hi = self.endIndex
        while lo != hi {
            // mid = lo + (hi - 1 - lo)/2
            let mid = lo.advancedBy(lo.distanceTo(hi.predecessor())/2)
            if self[mid] < elem {
                lo = mid + 1
            } else if elem < self[mid] {
                hi = mid
            } else {
                return mid // found at position `mid`
            }
        }
        return lo // not found, would be inserted at position `lo`
    }
}

let ar = [1, 3, 5, 7]
let pos = ar.insertionIndexOf(6)
print(pos) // 3

这里的方法被定义为CollectionType的扩展，因为需要对元素进行下标访问，并且元素是要求是 Comparable.

Here the method is defined as an extension to CollectionType because subscript access to the elements is needed, and the elements are required to be Comparable.

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