What Is the Android Activity Lifecycle?

Before you start with Android, you need to know a thing called the Android Activity Lifecycle. This is important to know as you begin your journey as an Android dev.

In this post I'll try to make it easier to understand activity and how they go through various states.

The Android documentation is a bit complicated. To give you a better understanding we first need to feel comfortable with what an activity is.

An activity in Android is basically every screen in which users can interact with and perform actions. Most of the time, an app will contain multiple screens (activities) that users can use to navigate an app as they listen to music, play games, watch videos, check out pics, etc.

When developing any kind of application, it’s our responsibility to make sure our users have a great experience. And for that we need to manage our applications state responsibly.

Dive into the Technical

Here's a diagram of Android's activity lifecycle,

Each stage is responsible for a certain task. When you open an app or click on a button that takes you to another screen, then onCreate() will be the first activity cycle that gets executed. It is followed by onStart() and onResume(). And voila! We can see the interface of the app we’re creating.
When another activity is called, onPause() and onStop() will be also called on the first activity.
The onDestroy() removes the app altogether. It is invoked when,

• You call finish() on the current activity
• Your phone is short in resources (e.g., battery, memory).

What Happens at Each Stage

onCreate(Bundle)

This method is the first to be called when an activity is first created. This is the place where we can create and setup views and accumulate the app's state.

 

If onCreate() includes a bundle, we can restore the app's state from the data in the bundle.

onStart()

This is called immediately after onCreate(Bundle). It will make the app visible to the user.

 

onStart() can be called as often as needed during an activity lifecycle

onResume()

This is the stage where your users can finally interact with your application.

 

Yay! Our activity finally running.

onPause()

This is called when another activity comes into the foreground.

 

Our other activity may still be visible in the background, however we won’t be able to interact with it. An example could be when we open a dialog.

onStop()

When this gets executed you won't be able to see your activity or interact with it.

 

It’s important to note if an activity is completely obscured by another activity, it is stopped. It still retains all state and member information. But it is no longer visible to the user so its window is hidden.

onDestroy()

This is called where we reach the final stage of an activity. There's no going back.

 

At this point the system is free to garbage collect the activity data to free up space.

Maintenance Activities

As you can see in the diagram, above, there are three key loops when maintaining activities,

What Can We Learn From This?

It’s our responsibility as developers to make sure that we handle each of these lifecycle changes gracefully.

• Terminate activity to free up valuable resources
• Make efficient use of resources
• Handle unexpected changes