ContextBase
Context的API和主要的两种函数
type Context
A Context carries a deadline, a cancellation signal, and other values across API boundaries.
Context's methods may be called by multiple goroutines simultaneously.
Context's methods may be called by multiple goroutines simultaneously.
上下文跨API边界携带期限、取消信号和其他值。Context的方法可以被多个goroutines同时调用。
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.
// The close of the Done channel may happen asynchronously,
// after the cancel function returns.
//
// 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 cancellation.
Done() <-chan struct{}
// If Done is not yet closed, Err returns nil.
// If Done is closed, Err returns a non-nil error explaining why:
// Canceled if the context was canceled
// or DeadlineExceeded if the context's deadline passed.
// After Err returns a non-nil error, successive calls to Err return the same error.
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
//
// // 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{}
}
func BackGround
Background returns a non-nil, empty Context. It is never canceled, has no values, and has no deadline. It is typically used by the main function, initialization, and tests, and as the top-level Context for incoming requests.
返回一个非nil的空上下文。它永远不会被取消，没有值，也没有截止日期。它通常用于主函数、初始化和测试，并作为传入请求的顶级上下文。也就是说，最上级和main.go常用
func Background() Context
func TODO
TODO returns a non-nil, empty Context. Code should use context.TODO when it's unclear which Context to use or it is not yet available (because the surrounding function has not yet been extended to accept a Context parameter).
TODO返回一个非nil的空上下文。代码应该使用上下文。当不清楚要使用哪个上下文或者上下文还不可用(因为周围的函数还没有扩展到接受上下文参数)时，可以使用TODO。
func TODO() Context
func WithCancel
WithCancel returns a copy of parent with a new Done channel. The returned context's Done channel is closed when the returned cancel function is called or when the parent context's Done channel is closed, whichever happens first.
Canceling this context releases resources associated with it, so code should call cancel as soon as the operations running in this Context complete.
WithCancel返回带有新Done通道的父节点的副本。当返回的cancel函数被调用时，或当父上下文的Done通道被关闭时，返回的上下文的Done通道被关闭(无论哪个先发生)。取消这个上下文会释放与其关联的资源，因此代码应该在这个上下文中运行的操作完成后立即调用cancel。
func WithCancel(parent Context) (ctx Context, cancel CancelFunc)
demo
package main
import (
"context"
"fmt"
)
func main() {
// 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():
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
}
}
}
# 输出
1
2
3
4
5
func WithDeadline
WithDeadline returns a copy of the parent context with the deadline adjusted to be no later than d. If the parent's deadline is already earlier than d, WithDeadline(parent, d) is semantically equivalent to parent. The returned context's Done channel is closed when the deadline expires, when the returned cancel function is called, or when the parent context's Done channel is closed, whichever happens first.
Canceling this context releases resources associated with it, so code should call cancel as soon as the operations running in this Context complete.
WithDeadline返回父上下文的一个副本，其截止日期调整为不迟于d。如果父上下文的截止日期已经早于d，那么WithDeadline(parent, d)在语义上等同于parent。当截止日期到期时，当返回的cancel函数被调用时，或者当父上下文的Done通道被关闭时(以最先发生的方式)，返回上下文的Done通道被关闭。取消这个上下文会释放与其关联的资源，因此代码应该在这个上下文中运行的操作完成后立即调用cancel。
func WithDeadline(parent Context, d time.Time) (Context, CancelFunc)
demo
// 和cancel相比 多了个过期时间
package main
import (
"context"
"fmt"
"time"
)
const shortDuration = 1 * time.Millisecond
func main() {
d := time.Now().Add(shortDuration)
ctx, cancel := context.WithDeadline(context.Background(), d)
// Even though ctx will be expired, it is good practice to call its
// cancellation 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())
}
}
# 输出
context deadline exceeded