go-ethereum: github.com/ethereum/go-ethereum/event Index | Examples | Files

package event

import "github.com/ethereum/go-ethereum/event"

Package event deals with subscriptions to real-time events.



Package Files

event.go feed.go subscription.go


var ErrMuxClosed = errors.New("event: mux closed")

ErrMuxClosed is returned when Posting on a closed TypeMux.

type Feed Uses

type Feed struct {
    // contains filtered or unexported fields

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.

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.

The zero value is ready to use.


// This example shows how the return value of Send can be used for request/reply
// interaction between event consumers and producers.
var feed event.Feed
type ackedEvent struct {
    i   int
    ack chan<- struct{}

// Consumers wait for events on the feed and acknowledge processing.
done := make(chan struct{})
defer close(done)
for i := 0; i < 3; i++ {
    ch := make(chan ackedEvent, 100)
    sub := feed.Subscribe(ch)
    go func() {
        defer sub.Unsubscribe()
        for {
            select {
            case ev := <-ch:
                fmt.Println(ev.i) // "process" the event
                ev.ack <- struct{}{}
            case <-done:

// The producer sends values of type ackedEvent with increasing values of i.
// It waits for all consumers to acknowledge before sending the next event.
for i := 0; i < 3; i++ {
    acksignal := make(chan struct{})
    n := feed.Send(ackedEvent{i, acksignal})
    for ack := 0; ack < n; ack++ {



func (*Feed) Send Uses

func (f *Feed) Send(value interface{}) (nsent int)

Send delivers to all subscribed channels simultaneously. It returns the number of subscribers that the value was sent to.

func (*Feed) Subscribe Uses

func (f *Feed) Subscribe(channel interface{}) Subscription

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.

type ResubscribeErrFunc Uses

type ResubscribeErrFunc func(context.Context, error) (Subscription, error)

A ResubscribeErrFunc attempts to establish a subscription. For every call but the first, the second argument to this function is the error that occurred with the previous subscription.

type ResubscribeFunc Uses

type ResubscribeFunc func(context.Context) (Subscription, error)

A ResubscribeFunc attempts to establish a subscription.

type Subscription Uses

type Subscription interface {
    Err() <-chan error // returns the error channel
    Unsubscribe()      // cancels sending of events, closing the error channel

Subscription represents a stream of events. The carrier of the events is typically a channel, but isn't part of the interface.

Subscriptions can fail while established. Failures are reported through an error channel. It receives a value if there is an issue with the subscription (e.g. the network connection delivering the events has been closed). Only one value will ever be sent.

The error channel is closed when the subscription ends successfully (i.e. when the source of events is closed). It is also closed when Unsubscribe is called.

The Unsubscribe method cancels the sending of events. You must call Unsubscribe in all cases to ensure that resources related to the subscription are released. It can be called any number of times.

func NewSubscription Uses

func NewSubscription(producer func(<-chan struct{}) error) Subscription

NewSubscription runs a producer function as a subscription in a new goroutine. The channel given to the producer is closed when Unsubscribe is called. If fn returns an error, it is sent on the subscription's error channel.


// Create a subscription that sends 10 integers on ch.
ch := make(chan int)
sub := event.NewSubscription(func(quit <-chan struct{}) error {
    for i := 0; i < 10; i++ {
        select {
        case ch <- i:
        case <-quit:
            return nil
    return nil

// This is the consumer. It reads 5 integers, then aborts the subscription.
// Note that Unsubscribe waits until the producer has shut down.
for i := range ch {
    if i == 4 {



func Resubscribe Uses

func Resubscribe(backoffMax time.Duration, fn ResubscribeFunc) Subscription

Resubscribe calls fn repeatedly to keep a subscription established. When the subscription is established, Resubscribe waits for it to fail and calls fn again. This process repeats until Unsubscribe is called or the active subscription ends successfully.

Resubscribe applies backoff between calls to fn. The time between calls is adapted based on the error rate, but will never exceed backoffMax.

func ResubscribeErr Uses

func ResubscribeErr(backoffMax time.Duration, fn ResubscribeErrFunc) Subscription

ResubscribeErr calls fn repeatedly to keep a subscription established. When the subscription is established, ResubscribeErr waits for it to fail and calls fn again. This process repeats until Unsubscribe is called or the active subscription ends successfully.

The difference between Resubscribe and ResubscribeErr is that with ResubscribeErr, the error of the failing subscription is available to the callback for logging purposes.

ResubscribeErr applies backoff between calls to fn. The time between calls is adapted based on the error rate, but will never exceed backoffMax.

type SubscriptionScope Uses

type SubscriptionScope struct {
    // contains filtered or unexported fields

SubscriptionScope provides a facility to unsubscribe multiple subscriptions at once.

For code that handle more than one subscription, a scope can be used to conveniently unsubscribe all of them with a single call. The example demonstrates a typical use in a larger program.

The zero value is ready to use.


package main

import (


// 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
    return r

func (s *mulServer) do(a, b int) int {
    r := a * b
    return r

// The servers are contained in an App. The app controls the servers and exposes them
// through its API.
type App struct {
    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)
        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))
        panic("invalid op")

// Stop stops the App, closing all subscriptions created through SubscribeResults.
func (s *App) Stop() {

func main() {
    // Create the app.
    var (
        app  App
        wg   sync.WaitGroup
        divs = make(chan int)
        muls = make(chan int)

    // Run a subscriber in the background.
    divsub := app.SubscribeResults('/', divs)
    mulsub := app.SubscribeResults('*', muls)
    go func() {
        defer wg.Done()
        defer fmt.Println("subscriber exited")
        defer divsub.Unsubscribe()
        defer mulsub.Unsubscribe()
        for {
            select {
            case result := <-divs:
                fmt.Println("division happened:", result)
            case result := <-muls:
                fmt.Println("multiplication happened:", result)
            case <-divsub.Err():
            case <-mulsub.Err():

    // Interact with the app.
    app.Calc('/', 22, 11)
    app.Calc('*', 3, 4)

    // Stop the app. This shuts down the subscriptions, causing the subscriber to exit.


func (*SubscriptionScope) Close Uses

func (sc *SubscriptionScope) Close()

Close calls Unsubscribe on all tracked subscriptions and prevents further additions to the tracked set. Calls to Track after Close return nil.

func (*SubscriptionScope) Count Uses

func (sc *SubscriptionScope) Count() int

Count returns the number of tracked subscriptions. It is meant to be used for debugging.

func (*SubscriptionScope) Track Uses

func (sc *SubscriptionScope) Track(s Subscription) Subscription

Track starts tracking a subscription. If the scope is closed, Track returns nil. The returned subscription is a wrapper. Unsubscribing the wrapper removes it from the scope.

type TypeMux Uses

type TypeMux struct {
    // contains filtered or unexported fields

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 someEvent struct{ I int }
type otherEvent struct{ S string }
type yetAnotherEvent struct{ X, Y int }

var mux TypeMux

// Start a subscriber.
done := make(chan struct{})
sub := mux.Subscribe(someEvent{}, otherEvent{})
go func() {
    for event := range sub.Chan() {
        fmt.Printf("Received: %#v\n", event.Data)

// Post some events.
mux.Post(yetAnotherEvent{X: 3, Y: 4})

// Stop closes all subscription channels.
// The subscriber goroutine will print "done"
// and exit.

// Wait for subscriber to return.


Received: event.someEvent{I:5}
Received: event.someEvent{I:6}
Received: event.otherEvent{S:"whoa"}

func (*TypeMux) Post Uses

func (mux *TypeMux) Post(ev interface{}) error

Post sends an event to all receivers registered for the given type. It returns ErrMuxClosed if the mux has been stopped.

func (*TypeMux) Stop Uses

func (mux *TypeMux) Stop()

Stop closes a mux. The mux can no longer be used. Future Post calls will fail with ErrMuxClosed. Stop blocks until all current deliveries have finished.

func (*TypeMux) Subscribe Uses

func (mux *TypeMux) Subscribe(types ...interface{}) *TypeMuxSubscription

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.

type TypeMuxEvent Uses

type TypeMuxEvent struct {
    Time time.Time
    Data interface{}

TypeMuxEvent is a time-tagged notification pushed to subscribers.

type TypeMuxSubscription Uses

type TypeMuxSubscription struct {
    // contains filtered or unexported fields

TypeMuxSubscription is a subscription established through TypeMux.

func (*TypeMuxSubscription) Chan Uses

func (s *TypeMuxSubscription) Chan() <-chan *TypeMuxEvent

func (*TypeMuxSubscription) Closed Uses

func (s *TypeMuxSubscription) Closed() bool

func (*TypeMuxSubscription) Unsubscribe Uses

func (s *TypeMuxSubscription) Unsubscribe()

Package event imports 7 packages (graph) and is imported by 3512 packages. Updated 2021-01-21. Refresh now. Tools for package owners.