Explain the Android Activity Lifecycle

When developing Android applications, understanding the Activity Lifecycle is crucial. An Activity is one of the fundamental components of an Android app, representing a single screen with a user interface. Whether it’s a login screen, a settings page, or a photo gallery, every screen in an Android app is an Activity. The Activity Lifecycle is a set of states and callback methods that define how an Activity behaves from the moment it’s launched until it’s destroyed. In this article, we’ll explore the Android Activity Lifecycle in detail, explaining its states, callback methods, and how developers can use them to create seamless user experiences.

What is the Android Activity Lifecycle?

The Activity Lifecycle is a series of states that an Activity goes through during its existence. These states are managed by the Android operating system, which triggers specific callback methods at each stage. By overriding these methods, developers can control how their app behaves when the user navigates between screens, switches apps, or interacts with the device (e.g., rotating the screen or pressing the home button).

The Activity Lifecycle ensures that resources are managed efficiently, and the app responds appropriately to user actions and system events. For example, if the user receives a phone call while using your app, the system will pause the Activity to allow the call to take precedence. Once the call ends, the Activity resumes.

Key States of the Activity Lifecycle

The Activity Lifecycle consists of six primary states:

1. Created
2. Started
3. Resumed
4. Paused
5. Stopped
6. Destroyed

These states are managed through seven callback methods:

1. onCreate()
2. onStart()
3. onResume()
4. onPause()
5. onStop()
6. onRestart()
7. onDestroy()

Let’s break down each state and its corresponding callback method.

1. Created State (onCreate())

The Created state is the first state in the Activity Lifecycle. It is triggered when the Activity is first created. The onCreate() method is called, and this is where developers typically perform essential setup tasks, such as:

• Initializing variables.
• Binding data to views.
• Setting up the user interface using layouts.

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main); // Set the layout for the Activity
    // Initialize variables and setup UI
}        

The onCreate() method receives a Bundle object, which can be used to restore the Activity’s state if it was previously destroyed (e.g., during a configuration change like screen rotation).

2. Started State (onStart())

After the Activity is created, it enters the Started state. The onStart() method is called, and the Activity becomes visible to the user. However, it is not yet interactive at this stage. This is a good place to initialize components that need to be started when the Activity becomes visible, such as animations or background tasks.

@Override
protected void onStart() {
    super.onStart();
    // Start animations or background tasks
}        

3. Resumed State (onResume())

The Resumed state is where the Activity is fully visible and interactive. The onResume() method is called, and the user can now interact with the app. This is the state where the Activity is considered to be "in the foreground."

@Override
protected void onResume() {
    super.onResume();
    // Resume paused operations, such as animations or sensors
}        

Developers often use onResume() to restart processes that were paused when the Activity lost focus, such as refreshing data or restarting animations.

4. Paused State (onPause())

The Paused state occurs when the Activity is partially obscured by another Activity (e.g., a dialog or a transparent Activity). The onPause() method is called, and the Activity is no longer in the foreground. However, it is still visible.

@Override
protected void onPause() {
    super.onPause();
    // Pause ongoing operations, such as animations or sensors
}        

Developers should use onPause() to save critical data, release system resources, or pause operations that shouldn’t run while the Activity is not in focus.

5. Stopped State (onStop())

The Stopped state occurs when the Activity is no longer visible to the user. This happens when another Activity takes over the entire screen. The onStop() method is called, and the Activity is no longer interacting with the user.

@Override
protected void onStop() {
    super.onStop();
    // Stop heavy operations, such as database queries or network calls
}        

In this state, the Activity is still in memory, but it is not visible. Developers can use onStop() to release resources that are not needed while the Activity is hidden.

6. Destroyed State (onDestroy())

The Destroyed state is the final state in the Activity Lifecycle. The onDestroy() method is called when the Activity is being removed from memory. This can happen for several reasons, such as the user pressing the back button or the system reclaiming resources.

@Override
protected void onDestroy() {
    super.onDestroy();
    // Clean up resources, such as closing database connections
}        

Developers should use onDestroy() to release all remaining resources and ensure that no memory leaks occur.

Additional Callback: onRestart()

The onRestart() method is called when an Activity is returning to the foreground from the Stopped state. It precedes the onStart() method and is a good place to reinitialize components that were stopped in onStop().

@Override
protected void onRestart() {
    super.onRestart();
    // Reinitialize components that were stopped
}        

Visualizing the Activity Lifecycle

To better understand the Activity Lifecycle, here’s a simplified diagram of the states and callback methods:

1. Created → onCreate()
2. Started → onStart()
3. Resumed → onResume()
4. Paused → onPause()
5. Stopped → onStop()
6. Destroyed → onDestroy()

The onRestart() method bridges the gap between Stopped and Started.

Practical Use Cases for Lifecycle Methods

Understanding the Activity Lifecycle is essential for building robust and efficient apps. Here are some practical use cases for lifecycle methods:

1. Saving and Restoring State:
2. Managing Resources:
3. Handling Configuration Changes:
4. Preventing Memory Leaks:

Common Pitfalls to Avoid

• Ignoring Lifecycle Methods: Failing to override lifecycle methods can lead to crashes, memory leaks, or poor user experiences.
• Performing Heavy Operations in onCreate(): Heavy operations can slow down the app’s launch time. Consider using background threads for such tasks.
• Not Saving State: If the Activity is destroyed and recreated (e.g., during screen rotation), unsaved data will be lost.

Conclusion

The Android Activity Lifecycle is a critical concept for every Android developer to master. By understanding the lifecycle states and callback methods, developers can create apps that are efficient, responsive, and user-friendly. Properly managing resources, saving state, and handling configuration changes are just a few of the ways lifecycle methods can enhance your app’s performance and reliability. As you continue to build Android applications, keep the Activity Lifecycle in mind—it’s the foundation of a great user experience.