Android Activity Lifecycle

An activity represents a single screen with a user interface. For example, an email application might have one activity that shows a list of new emails, another activity to compose an email, and another activity for reading emails. If an application has more than one activity, then one of them should be marked as the activity that is presented when the application is launched.

If you have worked with C, C++ or Java programming language then you must have seen that your program starts from main() function. Very similar way, Android system initiates its program with in anActivity starting with a call on onCreate() callback method. There is a sequence of callback methods that start up an activity and a sequence of callback methods that tear down an activity as shown in the below Activity lifecycle diagram: (image courtesy : android.com )

activity_lifecycle

 

The Activity class defines the following callbacks i.e. events. You don’t need to implement all the callbacks methods. However, it’s important that you understand each one and implement those that ensure your app behaves the way users expect.

CallbackDescription
onCreate()This is the first callback and called when the activity is first created.
onStart() This callback is called when the activity becomes visible to the user.
onResume()This is called when the user starts interacting with the application.
onPause()The paused activity does not receive user input and cannot execute any code and called when the current activity is being paused and the previous activity is being resumed.
onStop()This callback is called when the activity is no longer visible.
onDestroy()This callback is called before the activity is destroyed by the system.
onRestart()This callback is called when the activity restarts after stopping it.

Example

This example will take you through simple steps to show Android application activity life cycle. Follow the following steps to modify the Android application we created in Hello World Example chapter:

StepDescription
1You will use eclipse IDE to create an Android application and name it as HelloWorld under a package com.example.helloworld as explained in the Hello World Example chapter.
2Modify main activity file MainActivity.java as explained below. Keep rest of the files unchanged.
3Run the application to launch Android emulator and verify the result of the changes done in the application.

Following is the content of the modified main activity filesrc/com.example.helloworld/MainActivity.java. This file includes each of the fundamental lifecycle methods. The Log.d() method has been used to generate log messages:

 

An activity class loads all the UI component using the XML file available in res/layout folder of the project. Following statement loads UI components from res/layout/activity_main.xml file:

 

An application can have one or more activities without any restrictions. Every activity you define for your application must be declared in your AndroidManifest.xml file and the main activity for your app must be declared in the manifest with an <intent-filter> that includes the MAIN action and LAUNCHER category as follows:

 

If either the MAIN action or LAUNCHER category are not declared for one of your activities, then your app icon will not appear in the Home screen’s list of apps.

Let’s try to run our modified Hello World! application we just modified. I assume you had created yourAVD while doing environment setup. To run the app from Eclipse, open one of your project’s activity files and click Run icon from the toolbar. Eclipse installs the app on your AVD and starts it and if everything is fine with your setup and application, it will display Emulator window and you should see following log messages in LogCat window in Eclipse IDE:

 

android_logcat_window

Let us try to click Red button on the Android emulator and it will generate following events messages in LogCat window in Eclipse IDE:

Let us again try to click Menu button on the Android emulator and it will generate following events messages in LogCat window in Eclipse IDE:
Next, let us again try to click Back button on the Android emulator and it will generate following events messages in LogCat window in Eclipse IDE and this completes the Acitivity Life Cycle for an Android Application.

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