golang中context的作用详解

当一个goroutine可以启动其他goroutine，而这些goroutine可以启动其他goroutine，依此类推，则第一个goroutine应该能够向所有其它goroutine发送取消信号。

上下文包的唯一目的是在goroutine之间执行取消信号，而不管它们如何生成。上下文的接口定义为：

type Context interface {
 Deadline() (deadline time.Time, ok bool)
 Done() <- chan struct{}
 Err() error
 Value(key interface{}) interface{}
}

• Deadline：第一个值是截止日期，此时上下文将自动触发“取消”操作。第二个值是布尔值，true表示设置了截止日期，false表示未设置截止时间。如果没有设置截止日期，则必须手动调用cancel函数来取消上下文。
• Done：返回一个只读通道（仅在取消后），键入struct {}，当该通道可读时，表示父上下文已经发起了取消请求，根据此信号，开发人员可以执行一些清除操作，退出goroutine
• Err：返回取消上下文的原因
• Value：返回绑定到上下文的值，它是一个键值对，因此您需要传递一个Key来获取相应的值，此值是线程安全的

要创建上下文，必须指定父上下文。两个内置上下文（背景和待办事项）用作顶级父上下文：

var (
 background = new(emptyCtx)
 todo = new(emptyCtx)
)
func Background() Context {
 return background
}
func TODO() Context {
 return todo
}

背景，主要ü在主函数，初始化和测试代码的sed，是树结构中，根上下文，这是不能被取消的顶层语境。TODO，当您不知道要使用什么上下文时，可以使用它。它们本质上都是emptyCtx类型，都是不可取消的，没有固定的期限，也没有为Context赋任何值：键入emptyCtx int

type emptyCtx int
func (_ *emptyCtx) Deadline() (deadline time.Time, ok bool) {
 return
}
func (_ *emptyCtx) Done() <- chan struct{} {
 return nil
}
func (_ *emptyCtx) Err() error {
 return nil
}
func (*emptyCtx) Value(key interface{}) interface{} {
 return nil
}

上下文包还具有几个常用功能：func WithCancel（父上下文）（ctx上下文，取消CancelFunc）func WithDeadline（父上下文，截止时间.Time）（上下文，CancelFunc）func WithTimeout（父上下文，超时时间。持续时间）（上下文，CancelFunc）func WithValue（父上下文，键，val接口{}）上下文

请注意，这些方法意味着可以一次继承上下文以实现其他功能，例如，使用WithCancel函数传入根上下文，它会创建一个子上下文，该子上下文具有取消上下文的附加功能，然后使用此方法将context（context01）作为父上下文，并将其作为第一个参数传递给WithDeadline函数，与子context（context01）相比，获得子context（context02），它具有一个附加功能，可在之后自动取消上下文最后期限。

WithCancel

对于通道，尽管通道也可以通知许多嵌套的goroutine退出，但通道不是线程安全的，而上下文是线程安全的。

例如：

package main
import (
 "runtime"
 "fmt"
 "time"
 "context"
)
func monitor2(ch chan bool, index int) {
 for {
  select {
  case v := <- ch:
   fmt.Printf("monitor2: %v, the received channel value is: %v, ending\n", index, v)
   return
  default:
   fmt.Printf("monitor2: %v in progress...\n", index)
   time.Sleep(2 * time.Second)
  }
 }
}
func monitor1(ch chan bool, index int) {
 for {
  go monitor2(ch, index)
  select {
  case v := <- ch:
   // this branch is only reached when the ch channel is closed, or when data is sent(either true or false)
   fmt.Printf("monitor1: %v, the received channel value is: %v, ending\n", index, v)
   return
  default:
   fmt.Printf("monitor1: %v in progress...\n", index)
   time.Sleep(2 * time.Second)
  }
 }
}
func main() {
 var stopSingal chan bool = make(chan bool, 0)
 for i := 1; i <= 5; i = i + 1 {
  go monitor1(stopSingal, i)
 }
 time.Sleep(1 * time.Second)
 // close all gourtines
 cancel()
 // waiting 10 seconds, if the screen does not display <monitorX: xxxx in progress...>, all goroutines have been shut down
 time.Sleep(10 * time.Second)
 println(runtime.NumGoroutine())
 println("main program exit!!!!")
}

执行的结果是：

monitor1: 5 in progress...
monitor2: 5 in progress...
monitor1: 2 in progress...
monitor2: 2 in progress...
monitor2: 1 in progress...
monitor1: 1 in progress...
monitor1: 4 in progress...
monitor1: 3 in progress...
monitor2: 4 in progress...
monitor2: 3 in progress...
monitor1: 4, the received channel value is: false, ending
monitor1: 3, the received channel value is: false, ending
monitor2: 2, the received channel value is: false, ending
monitor2: 1, the received channel value is: false, ending
monitor1: 1, the received channel value is: false, ending
monitor2: 5, the received channel value is: false, ending
monitor2: 3, the received channel value is: false, ending
monitor2: 3, the received channel value is: false, ending
monitor2: 4, the received channel value is: false, ending
monitor2: 5, the received channel value is: false, ending
monitor2: 1, the received channel value is: false, ending
monitor1: 5, the received channel value is: false, ending
monitor1: 2, the received channel value is: false, ending
monitor2: 2, the received channel value is: false, ending
monitor2: 4, the received channel value is: false, ending
1
main program exit!!!!

这里使用一个通道向所有goroutine发送结束通知，但是这里的情况相对简单，如果在一个复杂的项目中，假设多个goroutine有某种错误并重复执行，则可以重复关闭或关闭该通道通道，然后向其写入值，从而触发运行时恐慌。这就是为什么我们使用上下文来避免这些问题的原因，以WithCancel为例：

package main
import (
 "runtime"
 "fmt"
 "time"
 "context"
)
func monitor2(ctx context.Context, number int) {
 for {
  select {
  case v := <- ctx.Done():
   fmt.Printf("monitor: %v, the received channel value is: %v, ending\n", number,v)
   return
  default:
   fmt.Printf("monitor: %v in progress...\n", number)
   time.Sleep(2 * time.Second)
  }
 }
}
func monitor1(ctx context.Context, number int) {
 for {
  go monitor2(ctx, number)
  select {
  case v := <- ctx.Done():
   // this branch is only reached when the ch channel is closed, or when data is sent(either true or false)
   fmt.Printf("monitor: %v, the received channel value is: %v, ending\n", number, v)
   return
  default:
   fmt.Printf("monitor: %v in progress...\n", number)
   time.Sleep(2 * time.Second)
  }
 }
}
func main() {
 var ctx context.Context = nil
 var cancel context.CancelFunc = nil
 ctx, cancel = context.WithCancel(context.Background())
 for i := 1; i <= 5; i = i + 1 {
  go monitor1(ctx, i)
 }
 time.Sleep(1 * time.Second)
 // close all gourtines
 cancel()
 // waiting 10 seconds, if the screen does not display <monitor: xxxx in progress>, all goroutines have been shut down
 time.Sleep(10 * time.Second)
 println(runtime.NumGoroutine())
 println("main program exit!!!!")
}

WithTimeout和WithDeadline

WithTimeout和WithDeadline在用法和功能上基本相同，它们都表示上下文将在一定时间后自动取消，唯一的区别可以从函数的定义中看出，传递给WithDeadline的第二个参数是类型time.Duration类型，它是一个相对时间，表示取消超时后的时间。例：

package main
import (
 "runtime"
 "fmt"
 "time"
 "context"
)
func monitor2(ctx context.Context, index int) {
 for {
  select {
  case v := <- ctx.Done():
   fmt.Printf("monitor2: %v, the received channel value is: %v, ending\n", index, v)
   return
  default:
   fmt.Printf("monitor2: %v in progress...\n", index)
   time.Sleep(2 * time.Second)
  }
 }
}
func monitor1(ctx context.Context, index int) {
 for {
  go monitor2(ctx, index)
  select {
  case v := <- ctx.Done():
   // this branch is only reached when the ch channel is closed, or when data is sent(either true or false)
   fmt.Printf("monitor1: %v, the received channel value is: %v, ending\n", index, v)
   return
  default:
   fmt.Printf("monitor1: %v in progress...\n", index)
   time.Sleep(2 * time.Second)
  }
 }
}
func main() {
 var ctx01 context.Context = nil
 var ctx02 context.Context = nil
 var cancel context.CancelFunc = nil
 ctx01, cancel = context.WithCancel(context.Background())
 ctx02, cancel = context.WithDeadline(ctx01, time.Now().Add(1 * time.Second)) // If it's WithTimeout, just change this line to "ctx02, cancel = context.WithTimeout(ctx01, 1 * time.Second)"
 defer cancel()
 for i := 1; i <= 5; i = i + 1 {
  go monitor1(ctx02, i)
 }
 time.Sleep(5 * time.Second)
 if ctx02.Err() != nil {
  fmt.Println("the cause of cancel is: ", ctx02.Err())
 }
 println(runtime.NumGoroutine())
 println("main program exit!!!!")
}

WithValue

一些必需的元数据也可以通过上下文传递，该上下文将附加到上下文中以供使用。元数据作为键值传递，但请注意，键必须具有可比性，并且值必须是线程安全的。

package main
import (
 "runtime"
 "fmt"
 "time"
 "context"
)
func monitor(ctx context.Context, index int) {
 for {
  select {
  case <- ctx.Done():
   // this branch is only reached when the ch channel is closed, or when data is sent(either true or false)
   fmt.Printf("monitor %v, end of monitoring. \n", index)
   return
  default:
   var value interface{} = ctx.Value("Nets")
   fmt.Printf("monitor %v, is monitoring %v\n", index, value)
   time.Sleep(2 * time.Second)
  }
 }
}
func main() {
 var ctx01 context.Context = nil
 var ctx02 context.Context = nil
 var cancel context.CancelFunc = nil
 ctx01, cancel = context.WithCancel(context.Background())
 ctx02, cancel = context.WithTimeout(ctx01, 1 * time.Second)
 var ctx03 context.Context = context.WithValue(ctx02, "Nets", "Champion") // key: "Nets", value: "Champion"

 defer cancel()
 for i := 1; i <= 5; i = i + 1 {
  go monitor(ctx03, i)
 }
 time.Sleep(5 * time.Second)
 if ctx02.Err() != nil {
  fmt.Println("the cause of cancel is: ", ctx02.Err())
 }
 println(runtime.NumGoroutine())
 println("main program exit!!!!")
}

关于上下文，还有一些注意事项：不要将Context存储在结构类型中，而是将Context明确传递给需要它的每个函数，并且Context应该是第一个参数。

即使函数允许，也不要传递nil Context，或者如果您不确定要使用哪个Context，请传递context。不要将可能作为函数参数传递给上下文值的变量传递。

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