Broker

Broker is a Publish-Subscribe (a.k.a Pub/Sub, EventBus) library for Android and JVM built with Coroutines.

class MyActivity : AppCompatActivity(), GlobalBroker.Subscriber {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        subscribe<MyEvent>(this) { event ->
            // Handle event
        }
    }
}

class MyViewModel : ViewModel(), GlobalBroker.Publisher {

    fun doSomething() {
        publish(MyEvent(payload))
    }
}

Features

• Helps to decouple your code: publishers are loosely coupled to subscribers, and don't even need to know of their existence
• Works great with Activity, Fragment, Service, Custom View, ViewModel...
• Provides a global instance by default and lets you create your own instances
• Also provides useful extension functions to avoid boilerplate code
• Android Lifecycle-aware: unsubscribe to events automatically
• Retained event: cache the last published events
• Thread-safe: you can publish/subscribe from any thread
• Fast: all work is done outside the main thread and the events are delivered through a Coroutines Flow
• Small: ~30kb

Usage

Take a look at the sample app for working examples.

Creating events

Events (a.k.a Topic, Message) can be represented as object (without payload) and data class (with payload).

object EventA
data class EventB(val message: String)

You can also group your events inside a sealed class, this way you can organize events by module, feature, scope, or similar.

sealed class MyEvent {
    object EventA : MyEvent()
    data class EventB(val message: String) : MyEvent()
}

Global Pub/Sub

Broker provides a global instance by default with some useful extension functions.

Call GlobalBroker.subscribe<YourEvent>() to subscribe to an event and GlobalBroker.unsubscribe() to unsubscribe to all events.

To subscribe, you should pass as parameters:

• The subscriber (usually the current class but can be a String, Int, object...)
• A CoroutineScope (tip: use the built-in lifecycleScope and viewModelScope)
• An optional CoroutineContext to run your lambda (default is Dispatchers.Main)
• A suspend lambda used to handle the incoming events

Call subscribe() in onStart() (for Activity and Fragment) and onAttachedToWindow() (for Custom View), and call unsubscribe() in onStop() (for Activity and Fragment) and onDetachedFromWindow() (for Custom View).

class MyActivity : AppCompatActivity() {

    override fun onStart() {
        super.onStart()
        GlobalBroker.subscribe<MyEvent>(this, lifecycleScope) { event ->
            // Handle event
        }
    }

    override fun onStop() {
        GlobalBroker.unsubscribe(this)
        super.onStop()
    }
}

To publish events just call GlobalBroker.publish() passing the event as parameter. It can be called from any thread.

class MyViewModel : ViewModel() {

    fun doSomething() {
        GlobalBroker.publish(MyEvent)
    }
}

GlobalBroker.Publisher & GlobalBroker.Subscriber

You can avoid some boilerplate code by implementing the GlobalBroker.Publisher and GlobalBroker.Subscriber interfaces. This also helps to identify the role of your class: is it a Publisher or Subscriber?

class MyActivity : AppCompatActivity(), GlobalBroker.Subscriber {

    override fun onStart() {
        super.onStart()
        subscribe<MyEvent>(lifecycleScope) { event ->
            // Handle event
        }
    }

    override fun onStop() {
        unsubscribe()
        super.onStop()
    }
}

class MyViewModel : ViewModel(), GlobalBroker.Publisher {

    fun doSomething() {
        publish(MyEvent)
    }
}

Local Pub/Sub

In some situations a global instance is not a good option, because of that you can also create your own Broker instance.

In the example below, we use Koin to inject a Broker instance in the MyActivity scope.

val myModule = module {

    scope<MyActivity> {
        scoped { Broker() }

        viewModel { MyViewModel(broker = get()) }
    }
}

And now we can inject a local Broker instance:

class MyActivity : AppCompatActivity() {

    private val broker by instance<Broker>()

    override fun onStart() {
        super.onStart()
        broker.subscribe<MyEvent>(this, lifecycleScope) { event ->
            // Handle event
        }
    }

    override fun onStop() {
        broker.unsubscribe(this)
        super.onStop()
    }
}

class MyViewModel(broker: Broker) : ViewModel() {

    fun doSomething() {
        broker.publish(MyEvent)
    }
}

BrokerPublisher & BrokerSubscriber

Broker class implements two interfaces: BrokerPublisher and BrokerSubscriber. You can use this to inject only the necessary behavior into your class.

Let's back to the previous example. Instead of provide a Broker instance directly we can provide two injections, one for publishers and another for subscribers.

val myModule = module {

    scope<MyActivity> {
        val broker = Broker()

        scoped<BrokerPublisher> { broker }

        scoped<BrokerSubscriber> { broker }

        viewModel { MyViewModel(broker = get<BrokerPublisher>()) }
    }
}

Now we can inject only what our class needs:

class MyActivity : AppCompatActivity() {

    private val broker by instance<BrokerSubscriber>()
}


class MyViewModel(broker: BrokerPublisher) : ViewModel()

Android Lifecycle-aware

Broker's subscribers can be lifecycle-aware! Works for global and local instances.

Instead of subscribe in onStart() and unsubscribe in onStop() just subscribe in onCreate() and pass the lifecycleOnwer as parameter. Your events will now be automatically unsubscribed!

class MyActivity : AppCompatActivity(), GlobalBroker.Subscriber {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        subscribe<MyEvent>(owner = this) { event ->
            // Handle event
        }
    }
}

Retained Events

It's possible to retain the last published event of a given type. Every time you subscribe to a retained event it will be emitted immediately.

Just use the flags emitRetained when subscribing and retain when publishing:

subscribe<MyEvent>(this, emitRetained = true) { event ->
    // Handle event
}

publish(MyEvent, retain = true)

At any moment you can query for a retained event (will return null if there are none):

val lastEvent = getRetained<SampleEvent>()

// removeRetained() will also return the last retained event
val lastEvent = removeRetained<SampleEvent>()

Error handling

If the subscriber's lambda throws an error, Broker will catch it and publish the event BrokerExceptionEvent. Just subscribe to it if you want to handle the exceptions.

subscribe<BrokerExceptionEvent>(lifecycleScope) { event ->
    // Handle error
}

Import to your project

1. Add the JitPack repository in your root build.gradle at the end of repositories:

allprojects {
    repositories {
        maven { url 'https://jitpack.io' }
    }
}

2. Next, add the desired dependencies to your module:

dependencies {
    // Core
    implementation "com.github.adrielcafe.broker:broker-core:$currentVersion"

    // Android Lifecycle support
    implementation "com.github.adrielcafe.broker:broker-lifecycle:$currentVersion"
}

Current version:

Platform compatibility

	broker-core	broker-lifecycle
Android	✓	✓
JVM	✓