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Go中的Channel——range和select

2020-06-10 04:17:14 admin 903

译自[Channels in Go - range and
select](https://link.jianshu.com?t=http://golangtutorials.blogspot.com/2011/06/channels-
in-go-range-and-
select.html)，该文章分为两部分，第一部分的翻译见[Go中的Channel](https://www.jianshu.com/p/15c94893124c)

数据接受者总是面临这样的问题：何时停止等待数据？还会有更多的数据么，还是所有内容都完成了？我应该继续等待还是该做别的了？

对于该问题，一个可选的方式是，持续的访问数据源并检查channel是否已经关闭，但是这并不是高效的解决方式。Go提供了`range`关键字，将其使用在channel上时，会自动等待channel的动作一直到channel被关闭

示例代码 1
    package main                                                                                             
    import (
        "fmt"
        "time"
        "strconv"
    )
    
    func makeCakeAndSend(cs chan string, count int) {
        for i := 1; i <= count; i++ {
            cakeName := "Strawberry Cake " + strconv.Itoa(i)
            cs <- cakeName //send a strawberry cake
        }   
    }
    
    func receiveCakeAndPack(cs chan string) {
        for s := range cs {
            fmt.Println("Packing received cake: ", s)
        }
    }
    
    func main() {
        cs := make(chan string)
        go makeCakeAndSend(cs, 5)
        go receiveCakeAndPack(cs)
    
        //sleep for a while so that the program doesn’t exit immediately
        time.Sleep(3 * 1e9)
    }

输出结果
    Packing received cake: Strawberry Cake 1
    Packing received cake: Strawberry Cake 2
    Packing received cake: Strawberry Cake 3
    Packing received cake: Strawberry Cake 4
    Packing received cake: Strawberry Cake 5

我们告诉了蛋糕制作器我们需要5个蛋糕，但是蛋糕装箱器并不知道数目，而在之前版本的代码中，我们写死了具体的接收数目。上面的代码中，通过对channel使用`range`关键字，我们避免了给接收者写明要接收的数据个数这种不合理的需求——当channel被关闭时，接收者的`for`循环也被自动停止了

### Channel and select

`select`关键字用于多个channel的结合，这些channel会通过类似于 **are-you-ready polling**
的机制来工作。`select`中会有`case`代码块，用于发送或接收数据——不论通过`<-`操作符指定的发送还是接收操作准备好时，channel也就准备好了。在`select`中也可以有一个`default`代码块，其一直是准备好的。那么，在`select`中，哪一个代码块被执行的算法大致如下：

* 检查每个`case`代码块
  * 如果任意一个`case`代码块准备好发送或接收，执行对应内容
  * 如果多余一个`case`代码块准备好发送或接收，随机选取一个并执行对应内容
  * 如果任何一个`case`代码块都没有准备好，等待
  * 如果有`default`代码块，并且没有任何`case`代码块准备好，执行`default`代码块对应内容

在下面的程序中，我们扩展蛋糕制作工厂来模拟多于一种口味的蛋糕生产的情况——现在有草莓和巧克力两种口味！但是装箱机制还是同以前一样的。由于蛋糕来自不同的channel，而装箱器不知道确切的何时会有何种蛋糕放置到某个或多个channel上，这就可以用`select`语句来处理所有这些情况——一旦某一个channel准备好接收蛋糕/数据，`select`就会完成该对应的代码块内容

注意，我们这里使用的多个返回值`case cakeName, strbry_ok :=
<-strbry_cs`，第二个返回值是一个`bool`类型，当其为`false`时说明channel被关闭了。如果是`true`，说明有一个值被成功传递了。我们使用这个值来判断是否应该停止等待

示例代码 2
    package main
    
    import (
        "fmt"
        "time"
        "strconv"
    )
    
    func makeCakeAndSend(cs chan string, flavor string, count int) {
        for i := 1; i <= count; i++ {
            cakeName := flavor + " Cake " + strconv.Itoa(i)
            cs <- cakeName //send a strawberry cake
        }   
        close(cs)
    }
    
    func receiveCakeAndPack(strbry_cs chan string, choco_cs chan string) {
        strbry_closed, choco_closed := false, false
    
        for {
            //if both channels are closed then we can stop
            if (strbry_closed && choco_closed) { return }
            fmt.Println("Waiting for a new cake ...")
            select {
            case cakeName, strbry_ok := <-strbry_cs:
                if (!strbry_ok) {
                    strbry_closed = true
                    fmt.Println(" ... Strawberry channel closed!")
                } else {
                    fmt.Println("Received from Strawberry channel.  Now packing", cakeName)
                }   
            case cakeName, choco_ok := <-choco_cs:
                if (!choco_ok) {
                    choco_closed = true
                    fmt.Println(" ... Chocolate channel closed!")
                } else {
                    fmt.Println("Received from Chocolate channel.  Now packing", cakeName)
                }   
            }   
        }   
    }
    
    func main() {
        strbry_cs := make(chan string)
        choco_cs := make(chan string)
    
        //two cake makers
        go makeCakeAndSend(choco_cs, "Chocolate", 3)  //make 3 chocolate cakes and send
        go makeCakeAndSend(strbry_cs, "Strawberry", 3)  //make 3 strawberry cakes and send
    
        //one cake receiver and packer
        go receiveCakeAndPack(strbry_cs, choco_cs)  //pack all cakes received on these cake channels
    
        //sleep for a while so that the program doesn’t exit immediately
        time.Sleep(2 * 1e9)
    }

输出结果 Waiting for a new cake ... 
    Received from Strawberry channel. Now packing Strawberry Cake 1 
    Waiting for a new cake ... 
    Received from Chocolate channel. Now packing Chocolate Cake 1 
    Waiting for a new cake ... 
    Received from Chocolate channel. Now packing Chocolate Cake 2 
    Waiting for a new cake ... 
    Received from Strawberry channel. Now packing Strawberry Cake 2 
    Waiting for a new cake ... 
    Received from Strawberry channel. Now packing Strawberry Cake 3 
    Waiting for a new cake ... 
    Received from Chocolate channel. Now packing Chocolate Cake 3 
    Waiting for a new cake ... 
    ... Strawberry channel closed! 
    Waiting for a new cake ... 
    ... Chocolate channel closed!

### 写在最后

实际上，有经验的Gopher一眼就能发现，`示例代码1`中的channel是没有正确关闭的，在`for range`语句的执行 **一直没有停止**
因为channel一直存在而没有被关闭，只不过随着`time.Sleep()`结束，main函数退出，所有的goroutine被关闭，该语句也被结束了而已

正确的解决步骤：  
a)发送器一旦停止发送数据后立即关闭channel  
b)接收器一旦停止接收内容，终止程序  
c)移除`time.Sleep`语句

修改后代码：

package main
    
    import (
        "fmt"
        "strconv"
    )
    
    func makeCakeAndSend(cs chan string, count int) {
        for i := 1; i <= count; i++ {
            cakeName := "Strawberry Cake " + strconv.Itoa(i)
            cs <- cakeName //send a strawberry cake
        }
        close(cs)
    }
    
    func receiveCakeAndPack(cs chan string) {
        for s := range cs {
            fmt.Println("Packing received cake: ", s)
        }
    }
    
    func main() {
        cs := make(chan string)
        go makeCakeAndSend(cs, 5)
        receiveCakeAndPack(cs)
    }

这样才是对channel使用`range`进行处理的优雅方法

同样的，第二个例子中，`time.Sleep()`语句可以去除掉，我们只需要让`receiveCakeAndPack`函数执行完毕后退出程序即可

修改后代码：

package main
    
    import (
        "fmt"
        "strconv"
    )
    
    func makeCakeAndSend(cs chan string, flavor string, count int) {
        for i := 1; i <= count; i++ {
            cakeName := flavor + " Cake " + strconv.Itoa(i)
            cs <- cakeName //send a strawberry cake
        }
        close(cs)
    }
    
    func receiveCakeAndPack(strbry_cs chan string, choco_cs chan string) {
        strbry_closed, choco_closed := false, false
    
        for {
            //if both channels are closed then we can stop
            if strbry_closed && choco_closed {
                return
            }
            fmt.Println("Waiting for a new cake ...")
            select {
            case cakeName, strbry_ok := <-strbry_cs:
                if !strbry_ok {
                    strbry_closed = true
                    fmt.Println(" ... Strawberry channel closed!")
                } else {
                    fmt.Println("Received from Strawberry channel.  Now packing", cakeName)
                }
            case cakeName, choco_ok := <-choco_cs:
                if !choco_ok {
                    choco_closed = true
                    fmt.Println(" ... Chocolate channel closed!")
                } else {
                    fmt.Println("Received from Chocolate channel.  Now packing", cakeName)
                }
            }
        }
    }
    
    func main() {
        strbry_cs := make(chan string)
        choco_cs := make(chan string)
    
        //two cake makers
        go makeCakeAndSend(choco_cs, "Chocolate", 3)   //make 3 chocolate cakes and send
        go makeCakeAndSend(strbry_cs, "Strawberry", 4) //make 3 strawberry cakes and send
    
        //one cake receiver and packer
        receiveCakeAndPack(strbry_cs, choco_cs) //pack all cakes received on these cake channels
    }

转载文章，原文链接： Go中的Channel——range和select

关键字词go Channelrange

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网站首页 返回列表像“草根”一样，紧贴着地面，低调的存在，冬去春来，枯荣无恙。

Go中的Channel——range和select

zhou 2年前

zhou 2年前

zhou 2年前

zhou 2年前

武云 2年前

网站首页 返回列表 像“草根”一样，紧贴着地面，低调的存在，冬去春来，枯荣无恙。

Go中的Channel——range和select

zhou 2年前

zhou 2年前

zhou 2年前

zhou 2年前

武云 2年前

网站首页返回列表像“草根”一样，紧贴着地面，低调的存在，冬去春来，枯荣无恙。