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完成14.2
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glight2000
168
eBook/14.2.md
168
eBook/14.2.md
@@ -456,7 +456,175 @@ func (c *container) Iter () <- chan items {
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return ch
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}
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```
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在协程里，一个for循环迭代容器c中的元素（对于树或图的算法，这种简单的for循环可以替换为深度优先搜索）。
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调用这个方法的代码可以这样迭代容器：
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```go
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for x := range container.Iter() { ... }
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```
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可以运行在自己的协程中，所以上边的迭代用到了一个通道和两个协程（可能运行在两个线程上）。就有了一个特殊的生产者-消费者模式。如果程序在协程给通道写完值之前结束，协程不会被回收；设计如此。这种行为看起来是错误的，但是通道是一种线程安全的通信。在这种情况下，协程尝试写入一个通道，而这个通道永远不会被读取，这可能是个bug而并非期望它被静默的回收。
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习惯用法：生产者消费者模式
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假设你有`Produce()`函数来产生`Consume`函数需要的值。它们都可以运行在独立的协程中，生产者在通道中放入给消费者读取的值。整个处理过程可以替换为无限循环：
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```go
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for {
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Consume(Produce())
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}
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```
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## 14.2.11 通道的方向
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通道类型可以用注解来表示它只发送或者只接收：
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```go
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var send_only chan<- int // channel can only receive data
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var recv_only <-chan int // channel can onley send data
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```
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只接收的通道（<-chan T）无法关闭，因为关闭通道是发送者用来表示不再给通道发送值了，所以对只接收通道是没有意义的。通道创建的时候都是双向的，但也可以分配有方向的通道变量，就像以下代码：
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```go
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var c = make(chan int) // bidirectional
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go source(c)
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go sink(c)
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func source(ch chan<- int){
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for { ch <- 1 }
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}
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func sink(ch <-chan int) {
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for { <-ch }
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}
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```
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习惯用法：管道和选择器模式
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更具体的例子还有协程处理它从通道接收的数据并发送给输出通道：
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```go
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sendChan := make(chan int)
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reciveChan := make(chan string)
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go processChannel(sendChan, receiveChan)
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func processChannel(in <-chan int, out chan<- string) {
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for inValue := range in {
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result := ... /// processing inValue
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out <- result
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}
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}
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```
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通过使用方向注解来限制协程对通道的操作。
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这里有一个来自Go指导的很赞的例子，打印了输出的主要数字，使用选择器（‘筛’）作为它的算法。每个素数都有一个选择器，如下图：
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版本1： 示例 14.7-[sieve1.go](examples/chapter_14/sieve1.go)
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```go
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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.package main
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package main
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import "fmt"
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// Send the sequence 2, 3, 4, ... to channel 'ch'.
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func generate(ch chan int) {
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for i := 2; ; i++ {
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ch <- i // Send 'i' to channel 'ch'.
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}
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}
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// Copy the values from channel 'in' to channel 'out',
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// removing those divisible by 'prime'.
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func filter(in, out chan int, prime int) {
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for {
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i := <-in // Receive value of new variable 'i' from 'in'.
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if i%prime != 0 {
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out <- i // Send 'i' to channel 'out'.
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}
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}
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}
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// The prime sieve: Daisy-chain filter processes together.
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func main() {
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ch := make(chan int) // Create a new channel.
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go generate(ch) // Start generate() as a goroutine.
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for {
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prime := <-ch
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fmt.Print(prime, " ")
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ch1 := make(chan int)
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go filter(ch, ch1, prime)
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ch = ch1
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}
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}
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```
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协程`filter(in, out chan int, prime int)`拷贝整数到输出通道，丢弃掉可以被prime整除的数字。然后每个prime又开启了一个新的协程，生成器和选择器并发请求。
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```
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输出：2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101
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103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223
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227 229 233 239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349
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353 359 367 373 379 383 389 397 401 409 419 421 431 433 439 443 449 457 461 463 467 479
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487 491 499 503 509 521 523 541 547 557 563 569 571 577 587 593 599 601 607 613 617 619
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631 641 643 647 653 659 661 673 677 683 691 701 709 719 727 733 739 743 751 757 761 769
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773 787 797 809 811 821 823 827 829 839 853 857 859 863 877 881 883 887 907 911 919 929
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937 941 947 953 967 971 977 983 991 997 1009 1013...
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```
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第二个版本引入了上边的习惯用法：函数`sieve`,`generate`,和`filter`都是工厂；它们创建通道并返回，而且使用了协程的lambda函数。`main`函数现在短小清晰：它调用`sieve()`返回了包含素数的通道，然后通过`fmt.Println(<-primes)`打印出来。
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版本2：示例 14.8-[sieve2.go](examples/chapter_14/sieve2.go)
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```go
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// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package main
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import (
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"fmt"
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)
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// Send the sequence 2, 3, 4, ... to returned channel
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func generate() chan int {
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ch := make(chan int)
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go func() {
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for i := 2; ; i++ {
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ch <- i
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}
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}()
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return ch
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}
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// Filter out input values divisible by 'prime', send rest to returned channel
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func filter(in chan int, prime int) chan int {
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out := make(chan int)
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go func() {
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for {
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if i := <-in; i%prime != 0 {
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out <- i
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}
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}
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}()
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return out
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}
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func sieve() chan int {
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out := make(chan int)
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go func() {
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ch := generate()
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for {
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prime := <-ch
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ch = filter(ch, prime)
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out <- prime
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}
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}()
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return out
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}
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func main() {
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primes := sieve()
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for {
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fmt.Println(<-primes)
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}
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}
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```
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## 链接
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Reference in New Issue
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