GO context之WithTimeout的使用
使用方法
func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc)
//
func Background() Context
//Background返回一个非空的Context。 它永远不会被取消，没有价值，也没有期限。 它通常由主要功能，初始化和测试使用，并用作传入请求的顶级上下文。
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{}
}
1. context包的WithTimeout()函数接受一个 Context 和超时时间作为参数，返回其子Context和取消函数cancel
2. 新创建协程中传入子Context做参数，且需监控子Context的Done通道，若收到消息，则退出
3. 需要新协程结束时，在外面调用 cancel 函数，即会往子Context的Done通道发送消息
4. 若不调用cancel函数，到了原先创建Contetx时的超时时间，它也会自动调用cancel()函数，即会往子Context的Done通道发送消息
package main
import (
"context"
"fmt"
"time"
)
func main() {
// 创建一个子节点的context,3秒后自动超时
// 利用根Context创建一个父Context，使用父Context创建2个协程，超时时间设为3秒
ctx, cancel := context.WithTimeout(context.Background(), time.Second*3)
go watch(ctx, "监控1")
go watch(ctx, "监控2")
fmt.Println("现在开始等待8秒,time=", time.Now().Unix())
time.Sleep(8 * time.Second)
//等待8秒钟，再调用cancel函数，其实这个时候会发现在3秒钟的时候两个协程已经收到退出信号了
fmt.Println("等待8秒结束,准备调用cancel()函数，发现两个子协程已经结束了，time=", time.Now().Unix())
cancel()
}
// 单独的监控协程
func watch(ctx context.Context, name string) {
for {
select {
case <-ctx.Done():
fmt.Println(name, "收到信号，监控退出,time=", time.Now().Unix())
return
default:
fmt.Println(name, "goroutine监控中,time=", time.Now().Unix())
time.Sleep(1 * time.Second)
}
}
}
