go筆記--幾個例子理解context的作用
經常在http框架里面看到一個context參數，它是做什么的呢，先簡單看看它的定義。
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{}
}
可以看到它一個接口類型、主要包含4個成員，我們暫時不知道它們的意義。
Deadline() (deadline time.Time, ok bool)
Done() <-chan struct{}
Err() error
Value(key interface{}) interface{}
先看一個簡單的例程
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------")
}
控制台會輸出
我們可以看到，for循環執行了5次 context 的派生函數gen()后，通過cancel()函數退出了gen里面的協程。
那么它在這里的作用也就清楚了:
context的作用
設置截止日期，超時或調用取消函數來通知所有使用任何派生 context 的函數來停止運行並返回。
咱們這是取消函數
contxt相關函數
withcancel
此函數創建從傳入的父 context 派生的新 context。父 context 可以是后台 context 或傳遞給函數的 context。
返回派生 context 和取消函數。只有創建它的函數才能調用取消函數來取消此 context。
deadline
WithDeadline()返回其父項的派生 context，當截止日期超過或取消函數被調用時，該 context 將被取消。例如，您可以創建一個將在以后的某個時間自動取消的 context，並在子函數中傳遞它。當因為截止日期耗盡而取消該 context 時，獲此 context 的所有函數都會收到通知去停止運行並返回。
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()類似，不同之處在於它將持續時間作為參數輸入而不是時間對象。
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
現在再看看這context的interface,就能簡單明了它們的意義了。
// 返回超時期限。
Deadline() (deadline time.Time, ok bool)
// 輔助cancel
Done() <-chan struct{}
// 獲取返回的錯誤
Err() error
Value(key interface{}) interface{}
可是還有個Value， 那我們再看看。
還有一個withValue() 函數 、
此函數接收 context 並返回派生 context，其中值 val 與 key 關聯，並通過 context 樹與 context 一起傳遞。這意味着一旦獲得帶有值的 context，從中派生的任何 context 都會獲得此值。不建議使用 context 值傳遞關鍵參數，而是函數應接收簽名中的那些值，使其顯式化。
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
那么明顯了 Value(key interface{}) interface{} 返回的即是context關聯的鍵值。