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@@ -1,5 +1,89 @@
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# 14.4 使用select切换协程
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+从不不同的并发执行的协程中获取值可以通过关键字`select`来完成，它和`switch`控制语句非常相似（章节5.3）也被称作通信开关；它的行为像是“你准备好了吗”的轮询机制；`select`监听进入通道的数据，也可以是用通道发送值的时候。
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+```go
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+select {
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+case u:= <- ch1:
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+ ...
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+case v:= <- ch2:
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+ ...
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+ ...
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+default: // no value ready to be received
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+ ...
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+}
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+```
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+`default`语句是可选的；fallthrough行为，和普通的switch相似，是不允许的。在任何一个case中执行`break`或者`return`，select就结束了。
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+
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+`select`做得就是：选择处理列出的多个通信情况中的一个。
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+* 如果都阻塞了，会等待直到其中一个可以处理
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+* 如果多个可以处理，随机选择一个
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+* 如果没有通道操作可以处理并且写了`default`语句，它就会执行：`default`永远是可运行的（这就是准备好了，可以执行）。
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+
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+在`select`中使用发送操作并且有`default`可以确保发送不被阻塞！如果没有cases，select就会一直阻塞。
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+
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+`select`语句实现了一种监听模式，通常用在（无限）循环中；在某种情况下，通过`break`语句使循环退出。
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+
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+在程序[goroutine_select.go](examples/chapter_14/goroutine_select.go)中有2个通道`ch1`和`ch2`，三个协程`pump1()`，`pump2()`和`suck()`。这是一个典型的生产者消费者模式。在无限循环中，`ch1`和`ch2`通过`pump1()`和`pump2()`填充整数；`suck()`也是在无限循环中轮询输入的，通过`select`语句获取`ch1`和`ch2`的整数并输出。选择哪一个case取决于哪一个通道收到了信息。程序在main执行1秒后结束。
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+
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+示例 14.10-[goroutine_select.go](examples/chapter_14/goroutine_select.go)
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+```go
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+package main
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+
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+import (
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+ "fmt"
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+ "time"
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+)
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+
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+func main() {
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+ ch1 := make(chan int)
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+ ch2 := make(chan int)
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+
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+ go pump1(ch1)
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+ go pump2(ch2)
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+ go suck(ch1, ch2)
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+
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+ time.Sleep(1e9)
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+}
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+
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+func pump1(ch chan int) {
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+ for i := 0; ; i++ {
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+ ch <- i * 2
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+ }
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+}
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+
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+func pump2(ch chan int) {
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+ for i := 0; ; i++ {
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+ ch <- i + 5
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+ }
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+}
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+
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+func suck(ch1, ch2 chan int) {
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+ for {
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+ select {
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+ case v := <-ch1:
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+ fmt.Printf("Received on channel 1: %d\n", v)
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+ case v := <-ch2:
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+ fmt.Printf("Received on channel 2: %d\n", v)
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+ }
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+ }
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+}
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+```
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+输出：
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+```
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+Received on channel 2: 5
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+Received on channel 2: 6
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+Received on channel 1: 0
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+Received on channel 2: 7
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+Received on channel 2: 8
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+Received on channel 2: 9
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+Received on channel 2: 10
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+Received on channel 1: 2
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+Received on channel 2: 11
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+...
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+Received on channel 2: 47404
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+Received on channel 1: 94346
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+Received on channel 1: 94348
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+```
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## 链接
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glight2000