Android - lifecycle_states

Lifecycle

Lifecycle-aware components perform actions in response to a change in the lifecycle status of another component, such as activities and fragments. These components help you produce better-organized, and often lighter-weight code, that is easier to maintain.

A common pattern is to implement the actions of the dependent components in the lifecycle methods of activities and fragments. However, this pattern leads to a poor organization of the code and to the proliferation of errors. By using lifecycle-aware components, you can move the code of dependent components out of the lifecycle methods and into the components themselves.

The androidx.lifecycle package provides classes and interfaces that let you build lifecycle-aware components—which are components that can automatically adjust their behavior based on the current lifecycle state of an activity or fragment.

Lifecycle

Lifecycle is a class that holds the information about the lifecycle state of a component (like an activity or a fragment) and allows other objects to observe this state.

Lifecycle uses two main enumerations to track the lifecycle status for its associated component:

• Event. The lifecycle events that are dispatched from the framework and the Lifecycle class. These events map to the callback events in activities and fragments.
• State. The current state of the component tracked by the Lifecycle object.

A class can monitor the component’s lifecycle status by adding annotations to its methods. Then you can add an observer by calling the addObserver() method of the Lifecycle class and passing an instance of your observer, as shown in the following example:

class MyObserver : LifecycleObserver {

    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    fun connectListener() {
        ...
    }

    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    fun disconnectListener() {
        ...
    }
}

myLifecycleOwner.getLifecycle().addObserver(MyObserver())

LifecycleOwner

LifecycleOwner is a single method interface that denotes that the class has a Lifecycle. It has one method, getLifecycle(), which must be implemented by the class. If you’re trying to manage the lifecycle of a whole application process instead, see ProcessLifecycleOwner.

This interface abstracts the ownership of a Lifecycle from individual classes, such as Fragment and AppCompatActivity, and allows writing components that work with them. Any custom application class can implement the LifecycleOwner interface.

Components that implement LifecycleObserver work seamlessly with components that implement LifecycleOwner because an owner can provide a lifecycle, which an observer can register to watch.

For the location tracking example, we can make the MyLocationListener class implement LifecycleObserver and then initialize it with the activity’s Lifecycle in the onCreate() method. This allows the MyLocationListener class to be self-sufficient, meaning that the logic to react to changes in lifecycle status is declared in MyLocationListener instead of the activity. Having the individual components store their own logic makes the activities and fragments logic easier to manage.

class MyActivity : AppCompatActivity() {
    private lateinit var myLocationListener: MyLocationListener

    override fun onCreate(...) {
        myLocationListener = MyLocationListener(this, lifecycle) { location ->
            // update UI
        }
        Util.checkUserStatus { result ->
            if (result) {
                myLocationListener.enable()
            }
        }
    }
}

A common use case is to avoid invoking certain callbacks if the Lifecycle isn’t in a good state right now. For example, if the callback runs a fragment transaction after the activity state is saved, it would trigger a crash, so we would never want to invoke that callback.

To make this use case easy, the Lifecycle class allows other objects to query the current state.

fun enable() {
        enabled = true
        if (lifecycle.currentState.isAtLeast(Lifecycle.State.STARTED)) {
            // connect if not connected
        }
    }

With this implementation, our LocationListener class is completely lifecycle-aware. If we need to use our LocationListener from another activity or fragment, we just need to initialize it. All of the setup and teardown operations are managed by the class itself.

Use cases for lifecycle-aware components

• Switching between coarse and fine-grained location updates. Use lifecycle-aware components to enable fine-grained location updates while your location app is visible and switch to coarse-grained updates when the app is in the background;
• Stopping and starting video buffering. Use lifecycle-aware components to start video buffering as soon as possible, but defer playback until app is fully started. You can also use lifecycle-aware components to terminate buffering when your app is destroyed;
• Starting and stopping network connectivity. Use lifecycle-aware components to enable live updating (streaming) of network data while an app is in the foreground and also to automatically pause when the app goes into the background;
• Pausing and resuming animated drawables. Use lifecycle-aware components to handle pausing animated drawables when while app is in the background and resume drawables after the app is in the foreground.

Links

https://developer.android.com/topic/libraries/architecture/lifecycle

credit goes to @swayangjit