The Android Lifecycle Cheat Sheet — Part II: Activity Lifecycle

Part I — Application Process Part I I — Activity Lifecycle 👉 You are here📍 Part I I I — Avoid This Mistakes (Activity Lifecycle) Part IV — Intro to Fragment Lifecycle Part V — Fragment View Lifecycle 👉 Upcoming ⏰ Part VI — View Model Lifecycle 👉 Upcoming ⏰

What is a Lifecycle?

It is the specification of an object’s creation and destruction times, along with a set of special callback methods that the system automatically invokes in response to specific events.

What is an Android Activity?

Among other functions it acts as a screen area controller. This implies that the screen is a shared resource among different applications, leading to various implications:

• The OS should be capable of transferring control over windows from one Activity to another.
• Activities need to be notified when the state of their control changes.
• Activities should be able to exchange data among themselves.

The Activity lifecycle supports these requirements ☝️

Create and Destroy

• Activity objects are instantiated by the system. To create an Activity, you request it, and the system instantiates it for you. You cannot call the constructor of the Activity yourself; they are under the management of the Android operating system.
• After instantiating these objects, Android will invoke the onCreate() method.
• The onDestroy() method is invoked to inform the Activity that it is no longer needed.
• Despite its name, even after onDestroy() returns, the actual destruction of the Activity object is not guaranteed.

Start and Stop

• onStart() is called when the Activity gains control of any visible area of the screen.
• onStop() is called when the Activity no longer has control of any visible part of the screen.
• ⚠️ Notice that an Activity can be in the background but still has not gone through onStop() because these two methods discussed earlier are related to the visibility of the screen.⚠️ For example:

1. Activity 1 is in the foreground.

2. Activity 2 comes to the foreground, and Activity 1 goes to the background, but Activity 1 is still visible because Activity 2 is transparent or does not fill the entire screen.

3. Therefore, onStop() for Activity 1 is NOT called, even though it is in the background.

Resume and Pause

• onResume() is called when the Activity becomes interactive to the user (this means when the Activity becomes the top of its respective Activity stack).
• onPause() is called when the Activity becomes non-interactive to the user (the Activity is not at the top of its respective Activity stack).

Activities Back Stack

1. Launch Activity 1: onCreate() , onStart() , onResume() executed for activity 1.
2. Launch Activity 2: onPause() executed for activity 1. onCreate() , onStart() , onResume() executed for activity 2. onStop() executed for activity 1.
3. Launch Activity 3: onPause() executed for activity 2. onCreate() , onStart() , onResume() executed for activity 3. onStop() executed for activity 2.
4. Go back: onPause() , onStop() , onDestroy() executed for activity 3. onStart() , onResume() executed for activity 2.
5. Launch Activity 4: onPause() executed for activity 2. onCreate() , onStart() , onResume() executed for activity 4 onStop() executed for activity 2.
6. Go back: onPause() , onStop() , onDestroy() executed for activity 4. onStart() , onResume() executed for activity 2.
7. Go back: On Android 11 and older, activity 1 would be destroyed on back navigation. On Android 12 and newer, it goes to background and can be brought to foreground later.

Android Lifecycle App

🚀 I am building an app to showcase and learn the hole Android lifecycle. Suggestions and contributions are welcome! https://github.com/waleska404/android-lifecycle

👉 Next chapter: Part I I I — Avoid This Mistakes (Activity Lifecycle) 👈 Previous chapter: Part I — Application Process