19. Activity Lifecycle in Android

Understanding the activity lifecycle in Android is a critical aspect of developing robust and responsive applications. The activity lifecycle is a set of states and callbacks that dictate how an activity behaves as it transitions between different states during its existence. This knowledge is essential for managing the resources efficiently and ensuring a seamless user experience.

In Android, an activity represents a single screen with a user interface. As users interact with the app, activities are created, paused, resumed, and destroyed. The Android system manages these transitions, invoking specific lifecycle callback methods at each stage. By overriding these methods, developers can implement custom behavior to handle state changes effectively.

The lifecycle of an activity can be visualized as a sequence of method calls that the Android system makes in response to changes in the activity's state. The primary lifecycle methods include:

• onCreate(): This is the first method called when an activity is created. It's where you should perform one-time initialization like setting up the user interface, binding data to lists, and other setup tasks. The method receives a Bundle object containing the activity's previously saved state, if there is one.
• onStart(): This method is called when the activity becomes visible to the user. At this point, the activity is not yet in the foreground and not interacting with the user.
• onResume(): This method is invoked when the activity starts interacting with the user. At this point, the activity is at the top of the activity stack and captures all user input.
• onPause(): When an activity loses focus but is still partially visible, the system calls this method. This is where you should save any changes that should persist beyond the current user session, such as unsaved data or changes to the UI.
• onStop(): This method is called when the activity is no longer visible to the user. It might happen because another activity has been resumed and is covering it. At this point, the activity is no longer in the foreground.
• onDestroy(): This method is called before the activity is destroyed. It is the final call that the activity receives. This is where you should clean up resources, such as threads or database connections, that are not needed once the activity is gone.
• onRestart(): This method is called after the activity has been stopped, just before it is started again. It is usually followed by onStart().

Each of these methods provides a specific opportunity to perform tasks that are appropriate for that stage of the activity's lifecycle. Let's delve deeper into each of these methods and explore their significance in the lifecycle.

onCreate()

The onCreate() method is the first entry point into an activity's lifecycle. It is where you should perform all one-time initializations. This includes inflating the activity's UI using setContentView(), initializing variables, and setting up any necessary components such as ViewModels or database connections.

It's important to note that the onCreate() method receives a Bundle object, which contains any saved state information. This is particularly useful for restoring the state of the activity when it is recreated after being destroyed, such as during a configuration change.

onStart()

When the onStart() method is called, the activity is becoming visible to the user. This is the point where the activity is transitioning from the "created" state to the "started" state. At this point, the activity is visible but not yet in the foreground.

In onStart(), you should perform tasks that are necessary to prepare the activity for the user. This might include starting animations, refreshing UI elements, or initializing resources that are needed to display the activity's content.

onResume()

The onResume() method is called when the activity enters the "resumed" state and begins interacting with the user. At this point, the activity is at the top of the activity stack and captures all user input. This is where the activity is fully running and visible.

In onResume(), you should start any processes that need to run while the activity is in the foreground, such as animations or audio playback. This is also the place to acquire any resources that you need while the activity is active, such as registering listeners or sensors.

onPause()

The onPause() method is called when the activity is losing focus and is about to enter the "paused" state. This can happen when another activity is launched in the foreground, partially obscuring the current activity, or when the user navigates away from the activity.

In onPause(), you should save any changes that need to persist beyond the current user session, such as unsaved data or changes to the user interface. This is also the place to release any resources that are not needed while the activity is not in the foreground, such as stopping animations or audio playback.

onStop()

The onStop() method is called when the activity is no longer visible to the user. This can happen when another activity is resumed and fully obscures the current activity, or when the activity is being destroyed.

In onStop(), you should perform tasks that are necessary to release resources that are not needed while the activity is not visible. This might include saving data to a database, stopping background threads, or releasing resources that are tied to the activity's UI.

onDestroy()

The onDestroy() method is called before the activity is destroyed. This is the final call that the activity receives. It is important to clean up any resources that are not needed once the activity is gone, such as threads or database connections.

It's worth noting that the onDestroy() method is not always called when the activity is destroyed. For example, if the system needs to recover memory urgently, it may kill the activity without calling onDestroy(). Therefore, it is essential to save any critical data in onPause() or onStop().

onRestart()

The onRestart() method is called after the activity has been stopped, just before it is started again. This method is usually followed by onStart(). It is an opportunity to perform any necessary actions to prepare the activity to be displayed again.

In onRestart(), you can refresh the activity's UI or reload any data that might have changed while the activity was stopped. This ensures that the activity is up-to-date when it becomes visible to the user again.

Managing Configuration Changes

One of the challenges of managing the activity lifecycle is handling configuration changes, such as screen rotations or changes in language settings. By default, the Android system destroys and recreates the activity when a configuration change occurs. This means that the activity goes through the entire lifecycle again, from onDestroy() to onCreate().

To manage configuration changes effectively, you can override the onSaveInstanceState() method to save the activity's state before it is destroyed. This method receives a Bundle object where you can store any data that needs to be preserved. When the activity is recreated, the saved state is passed to the onCreate() method, allowing you to restore the activity's state.

Alternatively, you can declare certain configuration changes in the manifest file, using the android:configChanges attribute. This prevents the system from destroying and recreating the activity, allowing you to handle the changes manually in the onConfigurationChanged() method.

Conclusion

The activity lifecycle in Android is a fundamental concept that every developer should understand. By mastering the lifecycle methods and knowing when to use them, you can create applications that are efficient, responsive, and resilient to changes. Proper lifecycle management ensures that your app can handle interruptions gracefully, manage resources effectively, and provide a seamless user experience.

By carefully managing the transitions between different states of an activity, you can ensure that your application remains robust and performs well under various conditions. Whether you're handling configuration changes, saving user data, or managing resources, understanding the activity lifecycle is key to building successful Android applications.