1.前言
一个请求，可能涉及多个API调用，多个goroutine，如何在多个API 之间，以及多个goroutine之间协作和传递信息，就是一个问题。
比如一个网络请求Request，需要开启一些goroutine去访问后端资源(比如，数据库，RPC服务等)，这些goroutine又可能会开启其他的goroutine，如何跟踪和控制这些goroutine呢？
golang定义了 context包，用于解决这个问题。
context可以在多个API和进程之间，实现deadlines截止日期操作、cancelation signals取消操作以及传递request-scoped的值(即传递参数)。
context一个最大的特点是可以继承。父context，可以派生子context，子context有又可以派生子context。如果父context被取消，子context及其子context都会被取消。
常用派生的方法有:WithCancel WithDeadline WithTimeout WithValue.
其中，
WithCancel用于主动取消自身以及子context。WithDeadline和WithTimeout用于超时情况下的取消自身以及子context。WithTimeout实现上直接调用WithDeadline。WithValue用于context传递request-scoped的参数，而不是向函数传递的可选参数。
2.例子
下面例子演示如何控制多个goroutine的退出。
首先，启动3个goroutine，分别命名为"1"、"2"、"3"，每个goroutine又启动一个goroutine。也就是有6个goroutine。它们之间的关系如下：
- 1
- 11
- 2
- 21
- 3
- 31
最后，在main中调用取消操作cancel，这样所有的goroutine都被取消并退出。
具体代码如下：
package main
import (
"context"
"fmt"
"time"
)
func PrintTask(ctx context.Context, taskName string) {
for {
select {
case <- ctx.Done():
fmt.Println("task:", taskName, " exit...")
return
default:
time.Sleep(1*time.Second)
fmt.Println("task:", taskName, " doing something...")
}
}
}
func mainTask(ctx context.Context, taskName string) {
ctx1, cancel := context.WithCancel(ctx)
defer cancel()
// create a new task
newTaskName := taskName + "1"
go PrintTask(ctx1, newTaskName)
for {
select {
case <- ctx.Done():
fmt.Println("task:", taskName, " exit...")
return
default:
time.Sleep(1*time.Second)
fmt.Println("task:", taskName, " doing something...")
}
}
}
func main() {
ctx := context.Background()
ctx, cancel := context.WithCancel(ctx)
go mainTask(ctx, "1")
go mainTask(ctx, "2")
go mainTask(ctx, "3")
time.Sleep(3*time.Second)
cancel()
fmt.Println("main exit...")
time.Sleep(3*time.Second)
}输出
task: 1 doing something...
task: 21 doing something...
task: 11 doing something...
task: 3 doing something...
task: 2 doing something...
task: 31 doing something...
task: 3 doing something...
task: 2 doing something...
task: 11 doing something...
task: 21 doing something...
task: 31 doing something...
task: 1 doing something...
task: 11 doing something...
task: 3 doing something...
task: 1 doing something...
task: 21 doing something...
task: 2 doing something...
task: 31 doing something...
main exit...
task: 11 doing something...
task: 11 exit...
task: 21 doing something...
task: 21 exit...
task: 3 doing something...
task: 3 exit...
task: 31 doing something...
task: 31 exit...
task: 1 doing something...
task: 1 exit...
task: 2 doing something...
task: 2 exit...