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// Copyright 2016 The go-ethereum Authors
// This file is part of the go-ethereum library.
//
// The go-ethereum library is free software: you can redistribute it and/or modify
// it under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// The go-ethereum library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU Lesser General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.

package event_test

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 ExampleSubscriptionScope() {
    // 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)
    wg.Add(1)
    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():
                return
            case <-mulsub.Err():
                return
            }
        }
    }()

    // 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.
    app.Stop()
    wg.Wait()

    // Output:
    // division happened: 2
    // multiplication happened: 12
    // subscriber exited
}