39. An Android Jetpack Lifecycle Awareness Tutorial

The previous chapter provided an overview of lifecycle awareness and outlined the key classes and interfaces that make this possible within an Android app project. This chapter will build on this knowledge base by building an Android Studio project designed to highlight lifecycle awareness in action.

39.1 Creating the Example Lifecycle Project

Select the Create New Project quick start option from the welcome screen and, within the resulting new project dialog, choose the Fragment + ViewModel template before clicking on the Next button.

Enter LifecycleDemo into the Name field and specify com.ebookfrenzy.lifecycledemo as the package name. Before clicking on the Finish button, change the Minimum API level setting to API 26: Android 8.0 (Oreo) and the Language menu to Java.

39.2 Creating a Lifecycle Observer

As previously discussed, activities and fragments already implement the LifecycleOwner interface and are ready to be observed by other objects. To see this in practice, the next step in this tutorial is to add a new class to the project that will be able to observe the MainFragment instance.

To add the new class, right-click on app -> java -> com.ebookfrenzy.lifecycledemo in the Project tool window and select New -> Java Class... from the resulting menu. In the New Class dialog, name the class DemoObserver and press the keyboard Return key to create the DemoObserver.java file. The new file should automatically open in the editor where it will read as follows:

package com.ebookfrenzy.lifecycledemo;

 

public class DemoObserver {

}

Remaining in the editor, modify the class file to declare that it will be implementing the LifecycleObserver interface:

package com.ebookfrenzy.lifecycledemo;

 

import androidx...

39.3 Adding the Observer

Observers are added to lifecycle owners via calls to the addObserver() method of the owner’s Lifecycle object, a reference to which is obtained via a call to the getLifecycle() method. Edit the MainFragment.java class file and add code to the onActivityCreated() method to add the observer:

.

.

import com.ebookfrenzy.lifecycledemo.DemoObserver;

.

.

@Override

public void onActivityCreated(@Nullable Bundle savedInstanceState) {

    super.onActivityCreated(savedInstanceState);

    mViewModel = new ViewModelProvider(this).get(MainViewModel.class);

 

    getLifecycle().addObserver(new DemoObserver());

}

With the observer class created and added to the lifecycle owner’s Lifecycle object, the app is ready to be tested.

39.4 Testing the Observer

Since the DemoObserver class outputs diagnostic information to the Logcat console, it will be easier to see the output if a filter is configured to display only the DemoObserver messages. Using the steps outlined previously in “Android Activity State Changes by Example”, configure a filter for messages associated with the DemoObserver tag before running the app on a device or emulator.

On successful launch of the app, the Logcat output should indicate the following lifecycle state changes and events:

onCreate

CREATED

onStart

STARTED

onResume

RESUMED

With the app still running, perform a device rotation to trigger the destruction and recreation of the fragment, generating the following additional output:

onPause

STARTED

onStop

CREATED

onDestroy

DESTROYED

onCreate

CREATED

onStart

STARTED

onResume

RESUMED

Before moving to the next section in this chapter, take some time to compare the...

39.5 Creating a Lifecycle Owner

The final task in this chapter is to create a custom lifecycle owner class and demonstrate how to trigger events and modify the lifecycle state from within that class.

Add a new class by right-clicking on the app -> java -> com.ebookfrenzy.lifecycledemo entry in the Project tool window and selecting the New -> Java Class... menu option. Name the class DemoOwner in the Create Class dialog before tapping the keyboard Return key. With the new DemoOwner.java file loaded into the code editor, modify it as follows:

package com.ebookfrenzy.lifecycledemo;

 

import androidx.lifecycle.Lifecycle;

import androidx.lifecycle.LifecycleOwner;

import androidx.lifecycle.LifecycleRegistry;

 

import org.jetbrains.annotations.NotNull;

 

public class DemoOwner implements LifecycleOwner {

}

The class is going to need a LifecycleRegistry instance initialized with a reference to itself, and a getLifecycle() method configured...

39.6 Testing the Custom Lifecycle Owner

Build and run the app one final time, refer to the Logcat tool window and confirm that the observer detected the create, start and stop lifecycle events in the following order:

onCreate

CREATED

onStart

STARTED

onStop

CREATED

Note that the “created” state changes were triggered even though code was not added to the DemoOwner class to do this manually. In fact, these were triggered automatically both when the owner instance was first created and subsequently when the ON_STOP event was handled.

39.7 Summary

This chapter has provided a practical demonstration of the steps involved in implementing lifecycle awareness within an Android app. This included the creation of a lifecycle observer and the design and implementation of a basic lifecycle owner class.