Activity Lifecycle in Android

Activity Lifecycle:  Activity is one of the building blocks of Android OS. In simple words. Activity is a screen that user interact with. For example, an email app might have one activity that show a list of  new emails, another activity to compose an email, and another activity for composing emails. Every Activity in android has lifecycle like created, started, resumed, paused, stopped or destroyed. These different states are known as Activity Lifecycle. In other words we can say Activity is a class pre-written in Java Programming.

An activity facilitates following key interactions between system and app:

• Keeping track of what the user currently caresabout to ensure that the system keeps running the process that is hosting the activity.
• Knowing that previous used processess contain things that the user may return to. And thus more highly prioritize keeping those processes around.
• Helping the app handle having its process killed so the user can return to activities with their previous state restored.
• Providing the way for the apps to implement user flows between each other, and for the system to coordinate these flows.

 

Imp: We implement the Activity as the subclass of Activity class

 

Below is Activity Lifecycle :

onCreate() – Called when the activity is first created

onStart() – Called just after it’s creation or by restart method after onStop(). Here Activity start

becoming visible to user

onResume() – Called when Activity is visible to user and user can interact with it

onPause() – Called when Activity content is not visible because user resume previous activity

onStop() – Called when activity is not visible to user because some other activity takes place of it

onRestart() – Called when user comes on screen or resume the activity which was stopped

onDestroy() – Called when Activity is not in background

 

 

LIST OF ACTIVITY LIFECYCLE METHODS OR STATES:

Activity Created: onCreate(Bundle savedInstanceState):

onCreate() method is called when activity gets memory in the OS. To use create state we need to override onCreate(Bundle savedInstanceState) method. Now there will be question in mind what is Bundle here, so Bundle is a data repository object that can store any kind of primitive data and this object will be null until some data isn’t saved in that. When an Activity first call or launched then onCreate(Bundle savedInstanceState) method is responsible to create the activity. When ever orientation(i.e. from horizontal to vertical or vertical to horizontal) of activity gets changed or when an Activity gets forcefully terminated by any Operating System then savedInstanceState i.e. object of Bundle Class will save the state of an Activity. It is best place to put initialization code.

Activity Started: onStart(): 

onStart() method is called just after it’s creation. In other case Activity can also be started by calling restart method i.e after activity stop. So this means onStart() gets called by Android OS when user switch between applications. For example, if a user was using Application A and then a notification comes and user clicked on notification and moved to Application B, in this case Application A will be paused. And again if a user again click on app icon of Application A then Application A which was stopped will again gets started.

Activity Resumed:.onResume():

Activity resumed is that situation when it is actually visible to user means the data displayed in the activity is visible to user. In lifecycle it always gets called after activity start and in most use case after activity paused (onPause).

Activity Paused: onPause():

Activity is called paused when it’s content is not visible to user, in most case onPause() method called by Android OS when user press Home button (Center Button on Device) to make hide. Activity also gets paused before stop called in case user press the back navigation button. The activity will go in paused state for these reasons also if a notification or some other dialog is overlaying any part (top or bottom) of the activity (screen). Similarly, if the other screen or dialog is transparent then user can see the screen but cannot interact with it. For example, if a call or notification comes in, the user will get the opportunity to take the call or ignore it.

 

Activity Stopped: onStop():

Activity is called stopped when it’s not visible to user. Any activity gets stopped in case some other activity takes place of it. For example, if a user was on screen 1 and click on some button and moves to screen 2. In this case Activity displaying content for screen 1 will be stopped. Every activity gets stopped before destroy in case of when user press back navigation button. So Activity will be in stopped state when hidden or replaced by other activities that have been launched or switched by user. In this case application will not present anything useful to the user directly as it’s going to stop.

Activity Restarted: onRestart():

Activity is called in restart state after stop state. So activity’s onRestart() function gets called when user comes on screen or resume the activity which was stopped. In other words, when Operating System starts the activity for the first time onRestart() never gets called. It gets called only in case when activity is resumes after stopped state.

Activity Destroyed: onDestroy():

Any activity is known as in destroyed state when it’s not in background. There can different cases at what time activity get destroyed. First is if user pressed the back navigation button then activity will be destroyed after completing the lifecycle of pause and stop. In case if user press the home button and app moves to background. User is not using it no more and it’s being shown in recent apps list. So in this case if system required resources need to use somewhere else then OS can destroy the Activity. After the Activity is destroyed if user again click the app icon, in this case activity will be recreated and follow the same lifecycle again. Another use case is with Splash Screens if there is call to finish() method from onCreate() of an activity then OS can directly call onDestroy() with calling onPause() and onStop().