这是Channels in go的第二篇，这篇主要讲range and select ，第一篇可以打开这个链接https://xiequan.info/channels-in-go/。
接收Channle的数据的时候我们会遇到，什么时候需要停止等待数据。有更多数据，还是已经全部完成？我们是继续等待还是继续？一种方法就是不断的轮询和检查通道是否已经关闭，但是这种方法并不是特别有效。
Channels and range
Go提供了range关键词,当它与Channel 一起使用的时候他会等待channel的关闭。
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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) } |
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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 |
Channels and select
golang 的 select 的功能和 select, poll, epoll 相似， 就是监听 IO 操作，当 IO 操作发生时，触发相应的动作。注意到 select 的代码形式和 switch 非常相似， 不过 select 的 case 里的操作语句只能是 IO 操作
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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) } |
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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! |
select 会一直等待等到某个 case 语句完成， 也就是等到成功从 ch1 或者 ch2 中读到数据。 则 select 语句结束
Channels in Go range and select
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