Swift语法——Swift Sequences 探究

今天看到Array 的API中有这么一个声明的函数：

    mutating func extend<S : SequenceType where T == T>(newElements: S)

官方对SequenceType的声明API如下：

protocol SequenceType : _Sequence_Type {

    /// A type that provides the *sequence* 's iteration interface and
    /// encapsulates its iteration state.
    typealias Generator : GeneratorType

    /// Return a *generator* over the elements of this *sequence*.
    ///
    /// Complexity: O(1)
    func generate() -> Generator
}

protocol GeneratorType {

    /// The type of element generated by `self`.
    typealias Element

    /// Advance to the next element and return it,or `nil` if no next
    /// element exists.
    ///
    /// Requires: `next()` has not been applied to a copy of `self`
    /// since the copy was made,and no preceding call to `self.next()`
    /// has returned `nil`.  Specific implementations of this protocol
    /// are encouraged to respond to violations of this requirement by
    /// calling `preconditionFailure("...")`.
    mutating func next() -> Element?
}

具体怎么用呢，咱们从for...in...来了解下：

首先让我们先来看一下常见的这个遍历方法：

for x in mySequence {
    // iterations here
}

而Swift实际上是按照下面这个方法来实现的：

var __g: Generator = mySequence.generate()
while let x = __g.next() {
    // iterations here
}

这是什么意思呢？

1）Swift调用generate()来生成了一个Generator，这个对象是 一个私有的变量即__g;

2) __g调用了next()函数，返回了一个optional类型对象element？。这个对象element被解包后赋值给了x;

3)通过next()函数一直来获取下一个元素，直到为nil停止。

所以，要实现Sequence必须要先有一个Generator。如下面的例子是对数组进行倒序索引：

1) 先声明继承与GeneratorType的类，实现Element和next()

///GeneratorType
class CountdownGenerator: GeneratorType {
    typealias Element = Int
    
    var element: Element
    
    init<T>(array: [T]){
        self.element = array.count - 1
    }
    
    func next() -> Element? {
        return self.element < 0 ? nil : element--
    }
}

class ReverseSequence<T>: SequenceType {
    typealias Generator = CountdownGenerator
    
    var array: [T]
    
    init(array: [T]){
        self.array = array
    }
    
    func generate() -> Generator {
        return CountdownGenerator(array: array)
    }
}

        let array = ["a","b","c"]
        
        for i in ReverseSequence(array: array){
            println("索引(i)的值是：(array[i])")
        }

索引2的值是：c
索引1的值是：b
索引0的值是：a

Generator

（编辑：李大同）

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