Lifecycle Extensions

Api

One-time suspend functions

Examples

import dispatch.android.*

// This could be any LifecycleOwner -- Fragments, Activities, Services...
class SomeScreen : Fragment() {

  init {

    // auto-created MainImmediateCoroutineScope which is lifecycle-aware
    dispatchLifecycleScope //...

    // active only when "resumed".  starts a fresh coroutine each time
    // this is a rough proxy for LiveData behavior
    dispatchLifecycleScope.launchEveryResume {  }

    // active only when "started".  starts a fresh coroutine each time
    dispatchLifecycleScope.launchEveryStart {  }

    // launch when created, automatically stop on destroy
    dispatchLifecycleScope.launchEveryCreate {  }

    // it works as a normal CoroutineScope as well (because it is)
    dispatchLifecycleScope.launchMain {  }

  }
}

class SomeApplication : Application() {
  override fun onCreate() {
    super.onCreate()
    // A custom factory can be set to add elements to the CoroutineContext
    LifecycleScopeFactory.set { MainImmediateContext() + SomeCustomElement() }
  }
}

class SomeEspressoTest {
  @Before
  fun setUp() {
    // This custom factory can be used to use custom scopes for testing,
    // such as an idling dispatcher
    LifecycleScopeFactory.set { MainImmediateIdlingCoroutineScope().coroutineContext }
  }

  @After
  fun tearDown() {
    // The factory can also be reset to default
    LifecycleScopeFactory.reset()
  }
}

Difference from AndroidX

This module is really just a slightly different version of androidx-lifecycle-runtime-ktx — the library which gives us the lifecycleScope property.

Why not just use AndroidX? Because we need two things it doesn't offer.

Custom CoroutineScope factories

The way androidx-lifecycle-runtime constructs its CoroutineScope is hard-coded, which eliminates the possibility of using a custom CoroutineContext such as a DispatcherProvider or IdlingDispatcher. With dispatch.android.lifecycle, we can set a custom factory.

class SomeFragmentEspressoTest {

  // Not part of this artifact.  see dispatch-android-espresso
  @JvmField @Rule val idlingRule = IdlingDispatcherProviderRule()

  @Before
  fun setUp() {
    // set a custom factory which is applied to all newly created lifecycleScopes
    LifecycleScopeFactory.set {
      MainImmediateContext() + idlingRule.dispatcherProvider
    }

    // now SomeFragment will use an IdlingDispatcher in its CoroutineScope
  }
}

Automatic lifecycle jobs

Structured concurrency relies upon cancellation, but androidx-lifecycle-runtime.ktx doesn't cancel. It uses a special PausingDispatcher. This pausing behavior then leaks upstream, creating backpressure and potentially deadlocks.

There's a bug filed in their issue tracker, but 2.2.0 got released anyway.

This library's API surface is the same as that within the AndroidX version, but has a different strategy for handling lifecycle events. When a lifecycle state enters the desired range, such as at ON_RESUME, a new coroutine is created. When the state exists the range, that coroutine is cancelled. If the lifecycle state enters the desired range again, a new coroutine is created.

import dispatch.android.*

class SomeFragment : Fragment() {

  val viewModel: SomeViewModel by viewModels()

  init {
    // automatically created CoroutineScope using the factory described above
    dispatchLifecycleScope.launchWhenResumed {
      viewModel.someFlow.consume {  }
    }
  }

}

This has the desired effect of not leaking backpressure upstream (which in this example is the viewModel).

Minimum Gradle Config

Add to your module's build.gradle.kts:

repositories {
  mavenCentral()
}

dependencies {

  implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.5.2")
  implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.5.2")
  implementation("com.rickbusarow.dispatch:dispatch-android-lifecycle-extensions:1.0.0-beta10-SNAPSHOT")

  implementation("androidx.lifecycle:lifecycle-common:2.2.0")
}