Activity Life Cycle in Android

What is Activity :

• Activity is a single screen
• Activity is a single, individual focused screen that’s comes in front to the user.
• Activity doesn’t contain anything of its own.
• It loads the layout that contain activity elements called views like Buttons, EditText, TextViews, Spinner, Checkboxes, RadioButtons etc.
• Users can perform actions by interacting  with the visual components within an activity.

Four States of Activity:

• Active/Running State
• Paused State
• Stopped State
• Destroyed State

Activity Life Cycle:

Active/Running State:

State when the activity is in foreground or active or in focus to the user mobile device.

Paused State:

Activity is partially visible, or not in the focus somehow another activity has taken place. Activity in paused state does not cover the entire screen.

Stopped State:

Activity is no longer visible in the screen. Another activity is on top of it.

Destroyed State:

Dead or destroyed when it no longer exists in the memory.

Details Description:

onCreate()-

• Whenever Activity launches, It calls the onCreate() method of Activity class.
• Every class is treated as Activity when it extends Activit class of Android.
• It must override the onCreate() method.
• This method defines the UI or the layout the activity class loads.
• It has Bundle object to store UI state.

onStart()-

• This method is called before any activity is visible to the user.
• This spot of time activity is still not active.

onResume()-

• Activity is visible and interactive to the user.
• It able to receive inputs.
• It is on the top of the activity stack

onPause()-

• Whenever another activity comes into the front original activity goes into partial background.
• There is a possibility of being killed by the system at the activities paused state.
• If the user has pressed paused button, notification comes in front, device in sleep mode in all these cases the activity is in paused state

onDestroy()-

• Called when the activity is destroyed.
• This is the last method which is being called before the activity is destroyed.
• By this method system will able to release the activity from the memory and get some free space. 

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