An activity is usually a single screen that the user sees on the device at one time. An application typically has multiple activities, and the user flips back and forth among them. As such, activities are the most visible part of your application.Activity Manager is responsible for creating, destroying, and managing activities. For example, when the user starts an application for the first time, the Activity Manager will create its activity and put it onto the screen. Later, when the user switches screens, the Activity Manager will move that previous activity to a holding place. This way, if the user wants to go back to an older activity, it can be started more quickly. Older activities that the user hasn’t used in a while will be destroyed in order to free more space for the currently active one. This mechanism is designed to help improve the speed of the user interface and thus improve the overall user experience.
An Activity in Android can exist in four states as described below:
1. Active/Running state
This is a state when an activity is in the front and has focus in it. It is completely visible and active to the user.
2. Paused state
In paused state, the activity is partially visible to the user but not active and lost focus. This occurs when some another Activity is on top of this one which doesnot cover the entire screen or having some transparancy so that the underlying Activity is partially visible. A paused activity is completely alive and maintains its state but it can be killed by system under low memory when memory can be freed by no other ways.
3. Stopped state
This is when the Activity is no longer visible in the screen. Another activity is on top of it and completely obscures its view. In this state also the activity is alive and preserves its state, but more likely to be killed by the system to free resources whenever necessary
4. Destroyed/Dead state
An Activity is said to be dead or destroyed when it no longer exists in the memory. Either the Activity hasn’t been started yet or once it was started and killed by the system in Paused or Stopped state to free resources.
- When we launch an Activity in Android, it first calls the onCreate() method. This is where we do User Interface creation and initialization of data elements. This method is provided with a Bundle object as parameter to restore the UI state.
- onStart() method is called before the Activity is being visible to the User. Remember that Activity is still not Active.
- With the onResume() method, the Activity become visible and Active for the user to interact with. The Activity will be at the top of the Activity stack at this point. Now the Activity is in running /active state and is able to receive user inputs.
- In the Active state, onPause() method will be called when the system is about to resume another Activity on top of this one or when the user is about to navigate to some other other parts of the system. It is the last guaranteed call to a method before the Activity can get killed by the system. That is, there’s a possibility that your activity may be killed by the system at the paused state without executing any further method calls. Therefore it is important to save the user interface configuration and critical data at this method.
- By default, an Activity can remain in the paused state if:
- The user has pressed the home button
- Another activity or notification which is on top of it doesn’t completely obscures the visibility of underlying Activity.
- The device goes to sleep.
- There are three possibilities for an Activity under paused state:
In all other cases it goes to stopped state by executing onStop() method. This is the default action when the user has pressed the back button, or a new activity which completely covers it resumes on top.
- The user resumes the Activity by closing the new Activity or notification and the paused Activity gets Active/Running by calling onResume() method.
- It gets killed by the system under extremely low memory conditions. In this case there will be no further method calls before the destruction of the Activity and it needs to be re-run from the beginning by calling onCreate() and restoring the previous configuration from bundle object.
- An Activity under stopped state also has three different scenarios to happen:
- System kills it to free resources. An activity under stopped sate is more likely to be killed by the system than one in the paused state. It needs to start the cycle again with onCreate().
- It get restarted by calling onRestart() , onStart() and onResume() methods in the order if the user navigates back to the Activity again. In this case, the User Interface is intact and no need to be restored.
- onDestroy() method is called and the Activity is destroyed. This is the final method we can call before the Activity is destroyed. This occurs either because the Activity is finishing the operation or the system is temporarily destroying it to save space.
Android Activity Lifecycle Loops
By analyzing the Android Activity lifecycle diagram we can see there are three lifecycle loops exist for every Activity and are defined by those callback methods.
Entire Lifetime: This is the lifetime between the first call to the onCreate() and the final call to onDestroy() method. We create all global resources such as screen layout, global variables etc in onCreate() and release all resources with onDestroy() call.
Visible Lifetime: It is the lifetime of an Activity between onStart() and onStop() method calls. In this the Activity is visible to the user and he may or may not be able to interact with it. During the visible lifetime, an Activity maintains its state intact.
Foreground Lifetime: Foreground lifetime starts with onResume() and ends with onPause() method calls. During this, the Activity is completely visible to the user and is on top of all other Activities so that user can interact with it.