github.com/ethereum/go-ethereum/event包实现了一个事件发布订阅的库,使用接口主要是event.Feed 类型，以前还有event.TypeMux 类型，看代码注释，说过时了，目前主要使用Feed 类型。
package main
import (
"fmt"
"sync"
"github.com/ethereum/go-ethereum/event"
)
func main() {
type someEvent struct{ I int }
var feed event.Feed
var wg sync.WaitGroup
ch := make(chan someEvent)
sub := feed.Subscribe(ch)
wg.Add(1)
go func() {
defer wg.Done()
for event := range ch {
fmt.Printf("Received: %#v\n", event.I)
}
sub.Unsubscribe()
fmt.Println("done")
}()
feed.Send(someEvent{5})
feed.Send(someEvent{10})
feed.Send(someEvent{7})
feed.Send(someEvent{14})
close(ch)
wg.Wait()
}
通过调用event.Feed 类型的Subscrible方法订阅事件通知，需要使用者提前指定接收事件的channel，Subscribe返回Subscription对象，是一个接口类型:
type Subscription interface {
Err() <-chan error // returns the error channel
Unsubscribe() // cancels sending of events, closing the error channel
}
Err() 返回获取error 的channel，调用Unsubscribe()取消事件订阅。事件的发布者调用 Send() 方法，发送事件。
可以使用同一个channel实例，多次调用Feed 的Subscrible()方法:
package main
import (
"fmt"
"sync"
"github.com/ethereum/go-ethereum/event"
)
func main() {
var (
feed event.Feed
recv sync.WaitGroup
sender sync.WaitGroup
)
ch := make(chan int)
feed.Subscribe(ch)
feed.Subscribe(ch)
feed.Subscribe(ch)
expectSends := func(value, n int) {
defer sender.Done()
if nsent := feed.Send(value); nsent != n {
fmt.Printf("send delivered %d times, want %d\n", nsent, n)
}
}
expectRecv := func(wantValue, n int) {
defer recv.Done()
for v := range ch {
if v != wantValue {
fmt.Printf("received %d, want %d\n", v, wantValue)
} else {
fmt.Printf("recv v = %d\n", v)
}
}
}
sender.Add(3)
for i := 0; i < 3; i++ {
go expectSends(1, 3)
}
go func() {
sender.Wait()
close(ch)
}()
recv.Add(1)
go expectRecv(1, 3)
recv.Wait()
}
这个例子中， 有三个订阅者， 有三个发送者， 每个发送者发送三次1， 同一个channel ch 里面被推送了9个1.
ethereum event 库还提供了一些高级别的方便接口， 比如event.NewSubscription函数，接收一个函数类型，作为数据的生产者， producer本身在后台一个单独的goroutine内执行， 后台goroutine往用户的channel 发送数据:
package main
import (
"fmt"
"github.com/ethereum/go-ethereum/event"
)
func main() {
ch := make(chan int)
sub := event.NewSubscription(func(quit <-chan struct{}) error {
for i := 0; i < 10; i++ {
select {
case ch <- i:
case <-quit:
fmt.Println("unsubscribed")
return nil
}
}
return nil
})
for i := range ch {
fmt.Println(i)
if i == 4 {
sub.Unsubscribe()
break
}
}
}
库也提供了event.SubscriptionScope类型用于追踪多个订阅者，提供集中的取消订阅功能:
package main
import (
"fmt"
"sync"
"github.com/ethereum/go-ethereum/event"
)
// This example demonstrates how SubscriptionScope can be used to control the lifetime of
// subscriptions.
//
// Our example program consists of two servers, each of which performs a calculation when
// requested. The servers also allow subscribing to results of all computations.
type divServer struct{ results event.Feed }
type mulServer struct{ results event.Feed }
func (s *divServer) do(a, b int) int {
r := a / b
s.results.Send(r)
return r
}
func (s *mulServer) do(a, b int) int {
r := a * b
s.results.Send(r)
return r
}
// The servers are contained in an App. The app controls the servers and exposes them
// through its API.
type App struct {
divServer
mulServer
scope event.SubscriptionScope
}
func (s *App) Calc(op byte, a, b int) int {
switch op {
case '/':
return s.divServer.do(a, b)
case '*':
return s.mulServer.do(a, b)
default:
panic("invalid op")
}
}
// The app's SubscribeResults method starts sending calculation results to the given
// channel. Subscriptions created through this method are tied to the lifetime of the App
// because they are registered in the scope.
func (s *App) SubscribeResults(op byte, ch chan<- int) event.Subscription {
switch op {
case '/':
return s.scope.Track(s.divServer.results.Subscribe(ch))
case '*':
return s.scope.Track(s.mulServer.results.Subscribe(ch))
default:
panic("invalid op")
}
}
// Stop stops the App, closing all subscriptions created through SubscribeResults.
func (s *App) Stop() {
s.scope.Close()
}
func main() {
var (
app App
wg sync.WaitGroup
divs = make(chan int)
muls = make(chan int)
)
divsub := app.SubscribeResults('/', divs)
mulsub := app.SubscribeResults('*', muls)
wg.Add(1)
go func() {
defer wg.Done()
defer fmt.Println("subscriber exited")
for {
select {
case result := <-divs:
fmt.Println("division happened:", result)
case result := <-muls:
fmt.Println("multiplication happened:", result)
case divErr := <-divsub.Err():
fmt.Println("divsub.Err() :", divErr)
return
case mulErr := <-mulsub.Err():
fmt.Println("mulsub.Err() :", mulErr)
return
}
}
}()
app.Calc('/', 22, 11)
app.Calc('*', 3, 4)
app.Stop()
wg.Wait()
}
SubscriptionScope的Close() 方法接收Track方法的返回值 ， Track 方法负责追踪订阅者。
本文由 Copernicus团队 喻建写作，转载无需授权。
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