event包实现了同一个进程内部的事件发布和订阅模式。
## event.go
目前这部分代码被标记为Deprecated，告知用户使用Feed这个对象。 不过在代码中任然有使用。 而且这部分的代码也不多。 就简单介绍一下。
数据结构
TypeMux是主要的使用。 subm记录了所有的订阅者。 可以看到每中类型都可以有很多的订阅者。
// TypeMuxEvent is a time-tagged notification pushed to subscribers.
type TypeMuxEvent struct {
Time time.Time
Data interface{}
}
// A TypeMux dispatches events to registered receivers. Receivers can be
// registered to handle events of certain type. Any operation
// called after mux is stopped will return ErrMuxClosed.
//
// The zero value is ready to use.
//
// Deprecated: use Feed
type TypeMux struct {
mutex sync.RWMutex
subm map[reflect.Type][]*TypeMuxSubscription
stopped bool
}
创建一个订阅,可以同时订阅多种类型。
// Subscribe creates a subscription for events of the given types. The
// subscription's channel is closed when it is unsubscribed
// or the mux is closed.
func (mux *TypeMux) Subscribe(types ...interface{}) *TypeMuxSubscription {
sub := newsub(mux)
mux.mutex.Lock()
defer mux.mutex.Unlock()
if mux.stopped {
// set the status to closed so that calling Unsubscribe after this
// call will short circuit.
sub.closed = true
close(sub.postC)
} else {
if mux.subm == nil {
mux.subm = make(map[reflect.Type][]*TypeMuxSubscription)
}
for _, t := range types {
rtyp := reflect.TypeOf(t)
oldsubs := mux.subm[rtyp]
if find(oldsubs, sub) != -1 {
panic(fmt.Sprintf("event: duplicate type %s in Subscribe", rtyp))
}
subs := make([]*TypeMuxSubscription, len(oldsubs)+1)
copy(subs, oldsubs)
subs[len(oldsubs)] = sub
mux.subm[rtyp] = subs
}
}
return sub
}
// TypeMuxSubscription is a subscription established through TypeMux.
type TypeMuxSubscription struct {
mux *TypeMux
created time.Time
closeMu sync.Mutex
closing chan struct{}
closed bool
// these two are the same channel. they are stored separately so
// postC can be set to nil without affecting the return value of
// Chan.
postMu sync.RWMutex
// readC 和 postC 其实是同一个channel。 不过一个是从channel读 一个只从channel写
// 单方向的channel
readC <-chan *TypeMuxEvent
postC chan<- *TypeMuxEvent
}
func newsub(mux *TypeMux) *TypeMuxSubscription {
c := make(chan *TypeMuxEvent)
return &TypeMuxSubscription{
mux: mux,
created: time.Now(),
readC: c,
postC: c,
closing: make(chan struct{}),
}
}
发布一个event到TypeMux上面，这个时候所有订阅了这个类型的都会收到这个消息。
// Post sends an event to all receivers registered for the given type.
// It returns ErrMuxClosed if the mux has been stopped.
func (mux *TypeMux) Post(ev interface{}) error {
event := &TypeMuxEvent{
Time: time.Now(),
Data: ev,
}
rtyp := reflect.TypeOf(ev)
mux.mutex.RLock()
if mux.stopped {
mux.mutex.RUnlock()
return ErrMuxClosed
}
subs := mux.subm[rtyp]
mux.mutex.RUnlock()
for _, sub := range subs {
// 阻塞式的投递。
sub.deliver(event)
}
return nil
}
func (s *TypeMuxSubscription) deliver(event *TypeMuxEvent) {
// Short circuit delivery if stale event
if s.created.After(event.Time) {
return
}
// Otherwise deliver the event
s.postMu.RLock()
defer s.postMu.RUnlock()
select { //阻塞方式的方法
case s.postC <- event:
case <-s.closing:
}
}
## feed.go
目前主要使用的对象。取代了前面说的event.go内部的TypeMux
feed数据结构
// Feed implements one-to-many subscriptions where the carrier of events is a channel.
// Values sent to a Feed are delivered to all subscribed channels simultaneously.
// Feed 实现了 1对多的订阅模式，使用了channel来传递事件。 发送给Feed的值会同时被传递给所有订阅的channel。
// Feeds can only be used with a single type. The type is determined by the first Send or
// Subscribe operation. Subsequent calls to these methods panic if the type does not
// match.
// Feed只能被单个类型使用。这个和之前的event不同，event可以使用多个类型。 类型被第一个Send调用或者是Subscribe调用决定。 后续的调用如果类型和其不一致会panic
// The zero value is ready to use.
type Feed struct {
once sync.Once // ensures that init only runs once
sendLock chan struct{} // sendLock has a one-element buffer and is empty when held.It protects sendCases.
removeSub chan interface{} // interrupts Send
sendCases caseList // the active set of select cases used by Send
// The inbox holds newly subscribed channels until they are added to sendCases.
mu sync.Mutex
inbox caseList
etype reflect.Type
closed bool
}
初始化 初始化会被once来保护保证只会被执行一次。
func (f *Feed) init() {
f.removeSub = make(chan interface{})
f.sendLock = make(chan struct{}, 1)
f.sendLock <- struct{}{}
f.sendCases = caseList{{Chan: reflect.ValueOf(f.removeSub), Dir: reflect.SelectRecv}}
}
订阅，订阅投递了一个channel。 相对与event的不同。event的订阅是传入了需要订阅的类型，然后channel是在event的订阅代码里面构建然后返回的。 这种直接投递channel的模式可能会更加灵活。
然后根据传入的channel生成了SelectCase。放入inbox。
// Subscribe adds a channel to the feed. Future sends will be delivered on the channel
// until the subscription is canceled. All channels added must have the same element type.
//
// The channel should have ample buffer space to avoid blocking other subscribers.
// Slow subscribers are not dropped.
func (f *Feed) Subscribe(channel interface{}) Subscription {
f.once.Do(f.init)
chanval := reflect.ValueOf(channel)
chantyp := chanval.Type()
if chantyp.Kind() != reflect.Chan || chantyp.ChanDir()&reflect.SendDir == 0 { // 如果类型不是channel。 或者是channel的方向不能发送数据。那么错误退出。
panic(errBadChannel)
}
sub := &feedSub{feed: f, channel: chanval, err: make(chan error, 1)}
f.mu.Lock()
defer f.mu.Unlock()
if !f.typecheck(chantyp.Elem()) {
panic(feedTypeError{op: "Subscribe", got: chantyp, want: reflect.ChanOf(reflect.SendDir, f.etype)})
}
// Add the select case to the inbox.
// The next Send will add it to f.sendCases.
cas := reflect.SelectCase{Dir: reflect.SelectSend, Chan: chanval}
f.inbox = append(f.inbox, cas)
return sub
}
Send方法,feed的Send方法不是遍历所有的channel然后阻塞方式的发送。这样可能导致慢的客户端影响快的客户端。 而是使用反射的方式使用SelectCase。 首先调用非阻塞方式的TrySend来尝试发送。这样如果没有慢的客户端。数据会直接全部发送完成。 如果TrySend部分客户端失败。 那么后续在循环Select的方式发送。 我猜测这也是feed会取代event的原因。
// Send delivers to all subscribed channels simultaneously.
// It returns the number of subscribers that the value was sent to.
func (f *Feed) Send(value interface{}) (nsent int) {
f.once.Do(f.init)
<-f.sendLock
// Add new cases from the inbox after taking the send lock.
f.mu.Lock()
f.sendCases = append(f.sendCases, f.inbox...)
f.inbox = nil
f.mu.Unlock()
// Set the sent value on all channels.
rvalue := reflect.ValueOf(value)
if !f.typecheck(rvalue.Type()) {
f.sendLock <- struct{}{}
panic(feedTypeError{op: "Send", got: rvalue.Type(), want: f.etype})
}
for i := firstSubSendCase; i < len(f.sendCases); i++ {
f.sendCases[i].Send = rvalue
}
// Send until all channels except removeSub have been chosen.
cases := f.sendCases
for {
// Fast path: try sending without blocking before adding to the select set.
// This should usually succeed if subscribers are fast enough and have free
// buffer space.
for i := firstSubSendCase; i < len(cases); i++ {
if cases[i].Chan.TrySend(rvalue) {
nsent++
cases = cases.deactivate(i)
i--
}
}
if len(cases) == firstSubSendCase {
break
}
// Select on all the receivers, waiting for them to unblock.
chosen, recv, _ := reflect.Select(cases)
if chosen == 0 /* <-f.removeSub */ {
index := f.sendCases.find(recv.Interface())
f.sendCases = f.sendCases.delete(index)
if index >= 0 && index < len(cases) {
cases = f.sendCases[:len(cases)-1]
}
} else {
cases = cases.deactivate(chosen)
nsent++
}
}
// Forget about the sent value and hand off the send lock.
for i := firstSubSendCase; i < len(f.sendCases); i++ {
f.sendCases[i].Send = reflect.Value{}
}
f.sendLock <- struct{}{}
return nsent
}
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