Context | Go

Go 1.17中提出，主要提供子孙goroutine的通知关闭的统一解决方法

• Context接口

  type Context interface {
    Deadline() // 截止时间
    Done() // 手动Cancel时接收到值
    Err() // 错误
    Value() // 存储值
  }
  

• Context接口的两个默认实现

  • Background

    顶层Context

  • TODO

    暂时不知道具体使用场景时使用

• 三种实现方式

  • WithCancel

    // 父goroutine中调用cancel()时，子goroutine的ctx.Done()接收到值，即：关闭的通知信号
    var c = 1
    
    func doSome(i int) error {
      c++
      fmt.Println(c)
      if c > 3 {
        return errors.New("err occur")
      }
      return nil
    }
    
    func speakMemo(ctx context.Context, cancelFunc context.CancelFunc) {
      for {
        select {
        case <-ctx.Done():
          fmt.Printf("ctx.Done")
          return
        default:
          fmt.Println("exec default func")
          err := doSome(3)
          if err != nil {
            fmt.Printf("cancelFunc()")
            cancelFunc()
          }
        }
      }
    }
    
    func main() {
      rootContext := context.Background()
      ctx, cancelFunc := context.WithCancel(rootContext)
      go speakMemo(ctx, cancelFunc)
      time.Sleep(time.Second * 5)
    }
    

  • WithDeadline

    // 指定时间终止，规定还是最终需要主动调用cancel
    const shortDuration = 1 * time.Millisecond
    
    func main() {
        d := time.Now().Add(shortDuration)
        ctx, cancel := context.WithDeadline(context.Background(), d)
    
        // Even though ctx will be expired, it is good practice to call its
        // cancellation function in any case. Failure to do so may keep the
        // context and its parent alive longer than necessary.
        defer cancel()
    
        select {
        case <-time.After(1 * time.Second):
            fmt.Println("overslept")
        case <-ctx.Done():
            fmt.Println(ctx.Err())
        }
    }
    

  • WithValue

    // goroutine直接传值
    func main() {
      type favContextKey string
    
      f := func(ctx context.Context, k favContextKey) {
          if v := ctx.Value(k); v != nil {
              fmt.Println("found value:", v)
              return
          }
          fmt.Println("key not found:", k)
      }
    
      k := favContextKey("language")
      ctx := context.WithValue(context.Background(), k, "Go")
    
      f(ctx, k)
      f(ctx, favContextKey("color"))
    }