# context

### 簡介：

當我們使用 goroutine 時需要一個地方來儲存互相會共用的變數或是讓 goroutine 在特定情況下停止執行，此時可以使用 context。

包含以下方法

```go
WithCancel
WithDeadline
WithTimeout
WithValue
```

<https://golang.org/pkg/context/>

使用前需要把 Background 傳進去

```
context.Background()
```

之後放到 WithDeadline 或 WithTimeout 或 WithCancel，最後如果要傳值再放進 WithValue。

在 goroutine 裡面使用 select，當 context timeout 或 cancel 後會觸發 select 裡面的

&#x20;`case <-ctx.Done():`

範例：

```go
func someHandler() {
    ctx, cancel := context.WithCancel(context.Background()) // 利用context產生cancel方法
    valueCtx := context.WithValue(ctx, key, 1)
    go doStuff(valueCtx)
    time.Sleep(10 * time.Second)
    cancel()
}

func doStuff(ctx context.Context) {
	for {
	  select {
		case <-ctx.Done(): // cancel 時會觸發
			fmt.Println(ctx.Value(key))
			return
		default:
			//取出值
			var value int = ctx.Value(key).(int)
		}
	}
}
```

### Context vs Channel

1. Use `context` to cancel

   ```go
   type Worker struct {
     Ctx context.Context
     Cancel context.CancelFunc
     Jobs chan Job
   }

   func (w *Worker) Run() {
     for {
       select {
       case <-w.Ctx.Done():
         return
       case j := <-w.Jobs:
         // Do some work
     }
   }

   go w.Run()
   w.Cancel()
   ```

   2\. Use a kill channel to signal cancellation and a done channel to indicate that goroutine has been terminated.

   ```go
   type Worker struct {
     Done chan struct{}
     Kill chan struct{}
     Jobs chan Job
   }

   func (w *Worker) Run() {
     defer func() {
       w.Done <- struct{}{}
     }
     for {
       select {
       case <-w.Kill:
         return
       case j := <-w.Jobs:
         // Do some work
     }
   }

   go w.Run()
   w.Kill <- struct{}{}
   ```

   <https://stackoverflow.com/q/54335907/4622645>


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