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func main() {
	var wg sync.WaitGroup

	wg.Add(2)
	go func() {
		time.Sleep(2*time.Second)
		fmt.Println("1号完成")
		wg.Done()
	}()
	go func() {
		time.Sleep(2*time.Second)
		fmt.Println("2号完成")
		wg.Done()
	}()
	wg.Wait()
	fmt.Println("好了，大家都干完了，放工")
}

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func main() {
	stop := make(chan struct{})

	go func() {
		for {
			select {
			case <-stop:
				fmt.Println("监控退出，停止了...")
				return
			default:
				fmt.Println("goroutine监控中...")
				time.Sleep(2 * time.Second)
			}
		}
	}()

	time.Sleep(10 * time.Second)
	fmt.Println("可以了，通知监控停止")
	close(stop)
	//为了检测监控过是否停止，如果没有监控输出，就表示停止了
	time.Sleep(5 * time.Second)

}

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func main() {
	ctx, cancel := context.WithCancel(context.Background())
	go func(ctx context.Context) {
		for {
			select {
			case <-ctx.Done():
				fmt.Println("监控退出，停止了...")
				return
			default:
				fmt.Println("goroutine监控中...")
				time.Sleep(2 * time.Second)
			}
		}
	}(ctx)

	time.Sleep(10 * time.Second)
	fmt.Println("可以了，通知监控停止")
	cancel()
	//为了检测监控过是否停止，如果没有监控输出，就表示停止了
	time.Sleep(5 * time.Second)

}

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func main() {
	ctx, cancel := context.WithCancel(context.Background())
	go watch(ctx,"【监控1】")
	go watch(ctx,"【监控2】")
	go watch(ctx,"【监控3】")

	time.Sleep(10 * time.Second)
	fmt.Println("可以了，通知监控停止")
	cancel()
	//为了检测监控过是否停止，如果没有监控输出，就表示停止了
	time.Sleep(5 * time.Second)
}

func watch(ctx context.Context, name string) {
	for {
		select {
		case <-ctx.Done():
			fmt.Println(name,"监控退出，停止了...")
			return
		default:
			fmt.Println(name,"goroutine监控中...")
			time.Sleep(2 * time.Second)
		}
	}
}

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

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var (
	background = new(emptyCtx)
	todo       = new(emptyCtx)
)

func Background() Context {
	return background
}
func TODO() Context {
	return todo
}

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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
}

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func WithCancel(parent Context) (ctx Context, cancel CancelFunc) 

func WithDeadline(parent Context, d time.Time) (Context, CancelFunc)

func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)

func WithValue(parent Context, key, val interface{}) Context

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// newCancelCtx returns an initialized cancelCtx.
func newCancelCtx(parent Context) cancelCtx {
	return cancelCtx{Context: parent}
}

func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
	c := newCancelCtx(parent)
	propagateCancel(parent, &c)
	return &c, func() { c.cancel(true, Canceled) }
}

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//*cancelCtx 和 *timerCtx 都实现了canceler接口，实现该接口的类型都可以被直接canceled
type canceler interface {
	cancel(removeFromParent bool, err error)
	Done() <-chan struct{}
}

type cancelCtx struct {
	Context

	mu       sync.Mutex            // protects following fields
	done     chan struct{}         // created lazily, closed by first cancel call
	children map[canceler]struct{} // set to nil by the first cancel call
	err      error                 // set to non-nil by the first cancel call
}

func (c *cancelCtx) Done() <-chan struct{} {
	c.mu.Lock()
	if c.done == nil {
		c.done = make(chan struct{})
	}
	d := c.done
	c.mu.Unlock()
	return d
}

func (c *cancelCtx) Err() error {
	c.mu.Lock()
	err := c.err
	c.mu.Unlock()
	return err
}

type stringer interface {
	String() string
}

//核心是关闭c.done
//同时会设置c.err = err, c.children = nil
//依次遍历c.children，每个child分别cancel
//如果设置了removeFromParent，则将c从其parent的children中删除
func (c *cancelCtx) cancel(removeFromParent bool, err error) {
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
	c.mu.Lock()
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
	c.err = err
	if c.done == nil {
		c.done = closedchan
	} else {
		close(c.done)
	}
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	}
	c.children = nil
	c.mu.Unlock()

	if removeFromParent {
		removeChild(c.Context, c) // 从此处可以看到 cancelCtx的Context项是一个类似于parent的概念
	}
}

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// propagateCancel arranges for child to be canceled when parent is.
func propagateCancel(parent Context, child canceler) {
	if parent.Done() == nil {
		return // parent is never canceled
	}
	if p, ok := parentCancelCtx(parent); ok {
		p.mu.Lock()
		if p.err != nil {
			// parent has already been canceled
			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
		go func() {
			select {
			case <-parent.Done():
				child.cancel(false, parent.Err())
			case <-child.Done():
			}
		}()
	}
}

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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:
		}
	}
}

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func main() {
	ctx, cancel := context.WithCancel(context.Background())

	valueCtx := context.WithValue(ctx, key, "add value")

	go watch(valueCtx)
	time.Sleep(10 * time.Second)
	cancel()

	time.Sleep(5 * time.Second)
}

func watch(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			//get value
			fmt.Println(ctx.Value(key), "is cancel")

			return
		default:
			//get value
			fmt.Println(ctx.Value(key), "int goroutine")

			time.Sleep(2 * time.Second)
		}
	}
}

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package main

import (
	"fmt"
	"sync"
	"time"

	"golang.org/x/net/context"
)

var (
	wg sync.WaitGroup
)

func work(ctx context.Context) error {
	defer wg.Done()

	for i := 0; i < 1000; i++ {
		select {
		case <-time.After(2 * time.Second):
			fmt.Println("Doing some work ", i)

		// we received the signal of cancelation in this channel
		case <-ctx.Done():
			fmt.Println("Cancel the context ", i)
			return ctx.Err()
		}
	}
	return nil
}

func main() {
	ctx, cancel := context.WithTimeout(context.Background(), 4*time.Second)
	defer cancel()

	fmt.Println("Hey, I'm going to do some work")

	wg.Add(1)
	go work(ctx)
	wg.Wait()

	fmt.Println("Finished. I'm going home")
}

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package main

import (
	"context"
	"fmt"
	"time"
)

func main() {
	d := time.Now().Add(1 * time.Second)
	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(2 * time.Second):
		fmt.Println("oversleep")
	case <-ctx.Done():
		fmt.Println(ctx.Err())
	}
}