go笔记--几个例子理解context的作用

go笔记--几个例子理解context的作用
- context interface

经常在http框架里面看到一个context参数，它是做什么的呢，先简单看看它的定义。

context interface

type Context interface {

    // Deadline returns the time when work done on behalf of this context

    // should be canceled. Deadline returns ok==false when no deadline is

    // set. Successive calls to Deadline return the same results.

    Deadline() (deadline time.Time, ok bool)

    // Done returns a channel that's closed when work done on behalf of this

    // context should be canceled. Done may return nil if this context can

    // never be canceled. Successive calls to Done return the same value.

    //

    // WithCancel arranges for Done to be closed when cancel is called;

    // WithDeadline arranges for Done to be closed when the deadline

    // expires; WithTimeout arranges for Done to be closed when the timeout

    // elapses.

    //

    // Done is provided for use in select statements:

    //

    //  // Stream generates values with DoSomething and sends them to out

    //  // until DoSomething returns an error or ctx.Done is closed.

    //  func Stream(ctx context.Context, out chan<- Value) error {

    //  	for {

    //  		v, err := DoSomething(ctx)

    //  		if err != nil {

    //  			return err

    //  		}

    //  		select {

    //  		case <-ctx.Done():

    //  			return ctx.Err()

    //  		case out <- v:

    //  		}

    //  	}

    //  }

    //

    // See https://blog.golang.org/pipelines for more examples of how to use

    // a Done channel for cancelation.

    Done() <-chan struct{}

    // Err returns a non-nil error value after Done is closed. Err returns

    // Canceled if the context was canceled or DeadlineExceeded if the

    // context's deadline passed. No other values for Err are defined.

    // After Done is closed, successive calls to Err return the same value.

    Err() error

    // Value returns the value associated with this context for key, or nil

    // if no value is associated with key. Successive calls to Value with

    // the same key returns the same result.

    //

    // Use context values only for request-scoped data that transits

    // processes and API boundaries, not for passing optional parameters to

    // functions.

    //

    // A key identifies a specific value in a Context. Functions that wish

    // to store values in Context typically allocate a key in a global

    // variable then use that key as the argument to context.WithValue and

    // Context.Value. A key can be any type that supports equality;

    // packages should define keys as an unexported type to avoid

    // collisions.

    //

    // Packages that define a Context key should provide type-safe accessors

    // for the values stored using that key:

    //

    // 	// Package user defines a User type that's stored in Contexts.

    // 	package user

    //

    // 	import "context"

    //

    // 	// User is the type of value stored in the Contexts.

    // 	type User struct {...}

    //

    // 	// key is an unexported type for keys defined in this package.

    // 	// This prevents collisions with keys defined in other packages.

    // 	type key int

    //

    // 	// userKey is the key for user.User values in Contexts. It is

    // 	// unexported; clients use user.NewContext and user.FromContext

    // 	// instead of using this key directly.

    // 	var userKey key = 0

    //

    // 	// NewContext returns a new Context that carries value u.

    // 	func NewContext(ctx context.Context, u *User) context.Context {

    // 		return context.WithValue(ctx, userKey, u)

    // 	}

    //

    // 	// FromContext returns the User value stored in ctx, if any.

    // 	func FromContext(ctx context.Context) (*User, bool) {

    // 		u, ok := ctx.Value(userKey).(*User)

    // 		return u, ok

    // 	}

    Value(key interface{}) interface{}

}

可以看到它一个接口类型、主要包含4个成员，我们暂时不知道它们的意义。

    Deadline() (deadline time.Time, ok bool)

    Done() <-chan struct{}

    Err() error

    Value(key interface{}) interface{}

先看一个简单的例程

package main

import (

	"context"

	"fmt"

	"time"

)

func cancel1() {

		// gen generates integers in a separate goroutine and

	// sends them to the returned channel.

	// The callers of gen need to cancel the context once

	// they are done consuming generated integers not to leak

	// the internal goroutine started by gen.

	gen := func(ctx context.Context) <-chan int {

		dst := make(chan int)

		n := 1

		go func() {

			for {

				select {

				case <-ctx.Done():

					fmt.Println("ctx.Done()")

					return // returning not to leak the goroutine

				case dst <- n:

					n++

				}

			}

		}()

		return dst

	}

	ctx, cancel := context.WithCancel(context.Background())

	defer cancel() // cancel when we are finished consuming integers

	for n := range gen(ctx) {

		fmt.Println(n)

		if n == 5 {

			break

		}

	}

}

func main() {

	fmt.Println("-----start-----")

	fmt.Println("call cancel1()")

	go cancel1()

	fmt.Println("cancel1() end")

	time.Sleep(time.Second)

	fmt.Println("----end------")

}

控制台会输出

我们可以看到，for循环执行了5次 context 的派生函数gen()后，通过cancel()函数退出了gen里面的协程。

那么它在这里的作用也就清楚了:

context的作用

设置截止日期，超时或调用取消函数来通知所有使用任何派生 context 的函数来停止运行并返回。

咱们这是取消函数

contxt相关函数

withcancel

此函数创建从传入的父 context 派生的新 context。父 context 可以是后台 context 或传递给函数的 context。

返回派生 context 和取消函数。只有创建它的函数才能调用取消函数来取消此 context。

deadline

WithDeadline()返回其父项的派生 context，当截止日期超过或取消函数被调用时，该 context 将被取消。例如，您可以创建一个将在以后的某个时间自动取消的 context，并在子函数中传递它。当因为截止日期耗尽而取消该 context 时，获此 context 的所有函数都会收到通知去停止运行并返回。

小例：

func deadline1() {

	d := time.Now().Add(1200 * time.Millisecond)

	ctx, cancel := context.WithDeadline(context.Background(), d)

	// Even though ctx will be expired, it is good practice to call its

	// cancelation 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())

	}

}

func main() {

	fmt.Println("-----start-----")

	fmt.Println("call deadline1()")

	go deadline1()

	fmt.Println("deadline1() end")

	time.Sleep(3*time.Second)

	fmt.Println("----end------")

}

withtimeout()和WithDeadline()类似，不同之处在于它将持续时间作为参数输入而不是时间对象。

timeout

// Pass a context with a timeout to tell a blocking function that it

// should abandon its work after the timeout elapses.

ctx, cancel := context.WithTimeout(context.Background(), 50*time.Millisecond)

defer cancel()

select {

case <-time.After(1 * time.Second):

    fmt.Println("overslept")

case <-ctx.Done():

    fmt.Println(ctx.Err()) // prints "context deadline exceeded"

}

Output:

context deadline exceeded

现在再看看这context的interface,就能简单明了它们的意义了。

	// 返回超时期限。

    Deadline() (deadline time.Time, ok bool)

	// 辅助cancel

    Done() <-chan struct{}

	// 获取返回的错误

    Err() error

    Value(key interface{}) interface{}

可是还有个Value，那我们再看看。

还有一个withValue() 函数、

此函数接收 context 并返回派生 context，其中值 val 与 key 关联，并通过 context 树与 context 一起传递。这意味着一旦获得带有值的 context，从中派生的任何 context 都会获得此值。不建议使用 context 值传递关键参数，而是函数应接收签名中的那些值，使其显式化。

Code:

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"))

Output:

found value: Go

key not found: color

那么明显了 Value(key interface{}) interface{} 返回的即是context关联的键值。