Android activity life cycle - what are all these methods for

Question

What is the life cycle of an Android activity  Why are so many similar sounding methods  onCreate    onStart    onResume    called during initialization  and so many others  onPause    onStop    onDestroy    called at the end   When are these methods called  and how should they be used properly

User · Answer

I like this question and the answers to it, but so far there isn't coverage of less frequently used callbacks like onPostCreate() or onPostResume(). Steve Pomeroy has attempted a diagram including these and how they relate to Android's Fragment life cycle, at https://github.com/xxv/android-lifecycle. I revised Steve's large diagram to include only the Activity portion and formatted it for letter size one-page printout. I've posted it as a text PDF at https://github.com/code-read/android-lifecycle/blob/master/AndroidActivityLifecycle1.pdf and below is its image:

User · Answer

The entire confusion is caused since Google chose non-intuivitive names instead of something as follows   onCreateAndPrepareToDisplay      instead of onCreate     onPrepareToDisplay               instead of onRestart     onVisible                        instead of onStart     onBeginInteraction               instead of onResume     onPauseInteraction               instead of onPause     onInvisible                      instead of onStop  onDestroy                        no change     The Activity Diagram can be interpreted as

User · Answer

Adding some more info on top of highly rated answer  Added additional section of KILLABLE and next set of methods  which are going to be called in the life cycle    Source  developer android com    Note the  Killable  column in the above table -- for those methods that are marked as being killable  after that method returns the process hosting the activity may be killed by the system at any time without another line of its code being executed   Because of this  you should use the onPause   method to write any persistent data  such as user edits  to storage  In addition  the method onSaveInstanceState Bundle  is called before placing the activity in such a background state  allowing you to save away any dynamic instance state in your activity into the given Bundle  to be later received in onCreate Bundle  if the activity needs to be re-created    Note that it is important to save persistent data in onPause   instead of onSaveInstanceState Bundle  because the latter is not part of the lifecycle callbacks  so will not be called in every situation as described in its documentation   I would like to add few more methods  These are not listed as life cycle methods but they will be called during life cycle depending on some conditions  Depending on your requirement  you may have to implement these methods in your application for proper handling of state    onPostCreate Bundle savedInstanceState       Called when activity start-up is complete  after onStart   and onRestoreInstanceState Bundle  have been called     onPostResume        Called when activity resume is complete  after onResume   has been called     onSaveInstanceState Bundle outState       Called to retrieve per-instance state from an activity before being killed so that the state can be restored in onCreate Bundle  or onRestoreInstanceState Bundle   the Bundle populated by this method will be passed to both     onRestoreInstanceState Bundle savedInstanceState       This method is called after onStart   when the activity is being re-initialized from a previously saved state  given here in savedInstanceState    My application code using all these methods   public class MainActivity extends AppCompatActivity implements View OnClickListener       private EditText txtUserName      private EditText txtPassword      Button  loginButton       Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity main           Log d  Ravi   Main OnCreate            txtUserName  EditText  findViewById R id username           txtPassword  EditText  findViewById R id password           loginButton     Button   findViewById R id login           loginButton setOnClickListener this                Override     public void onClick View view            Log d  Ravi    Login processing initiated            Intent intent   new Intent this LoginActivity class           Bundle bundle   new Bundle            bundle putString  userName  txtUserName getText   toString             bundle putString  password  txtPassword getText   toString             intent putExtras bundle           startActivityForResult intent 1             IntentFilter           public void onActivityResult int requestCode  int resultCode  Intent resIntent           Log d  Ravi back result     start            String result   resIntent getStringExtra  result            Log d  Ravi back result    result           TextView txtView    TextView findViewById R id txtView           txtView setText result            Intent sendIntent   new Intent              sendIntent setPackage  com whatsapp            sendIntent setAction Intent ACTION SEND           sendIntent putExtra Intent EXTRA TEXT   Message               sendIntent setType  text plain            startActivity sendIntent               Override     protected void onStart             super onStart            Log d  Ravi   Main Start                Override     protected void onRestart             super onRestart            Log d  Ravi   Main ReStart                Override     protected void onPause             super onPause            Log d  Ravi   Main Pause                Override     protected void onResume             super onResume            Log d  Ravi   Main Resume                Override     protected void onStop             super onStop            Log d  Ravi   Main Stop                Override     protected void onDestroy             super onDestroy            Log d  Ravi   Main OnDestroy                Override     public void onPostCreate Bundle savedInstanceState  PersistableBundle persistentState            super onPostCreate savedInstanceState  persistentState           Log d  Ravi   Main onPostCreate                Override     protected void onPostResume             super onPostResume            Log d  Ravi   Main PostResume                Override     public void onSaveInstanceState Bundle outState  PersistableBundle outPersistentState            super onSaveInstanceState outState  outPersistentState               Override     protected void onRestoreInstanceState Bundle savedInstanceState            super onRestoreInstanceState savedInstanceState             Login Activity   public class LoginActivity extends AppCompatActivity        private TextView txtView       Override     protected void onCreate Bundle savedInstanceState            super onCreate savedInstanceState           setContentView R layout activity login           txtView    TextView  findViewById R id Result           Log d  Ravi   Login OnCreate            Bundle bundle   getIntent   getExtras            txtView setText bundle getString  userName       bundle getString  password               Intent  intent   new Intent this MainActivity class           Intent  intent   new Intent            intent putExtra  result   Success            setResult 1 intent             finish              output    Before pause   D Ravi  Main OnCreate D Ravi  Main Start D Ravi  Main Resume D Ravi  Main PostResume   output    After resume from pause   D Ravi  Main ReStart D Ravi  Main Start D Ravi  Main Resume D Ravi  Main PostResume   Note that onPostResume   is invoked even though it s not quoted as life cycle method

User · Answer

From the Android Developers page   onPause        Called when the system is about to start resuming a previous activity    This is typically used to commit unsaved changes to persistent data    stop animations and other things that may be consuming CPU  etc    Implementations of this method must be very quick because the next   activity will not be resumed until this method returns  Followed by   either onResume   if the activity returns back to the front  or   onStop   if it becomes invisible to the user    onStop        Called when the activity is no longer visible to the user  because   another activity has been resumed and is covering this one  This may   happen either because a new activity is being started  an existing one   is being brought in front of this one  or this one is being destroyed    Followed by either onRestart   if this activity is coming back to   interact with the user  or onDestroy   if this activity is going away    Now suppose there are three Activities and you go from A to B  then onPause of A will be called now from B to C  then onPause of B and onStop of A will be called   The paused Activity gets a Resume and Stopped gets Restarted   When you call this finish    onPause-onStop-onDestroy will be called  The main thing to remember is  paused Activities get Stopped and a Stopped activity gets Destroyed whenever Android requires memory for other operations   I hope it s clear enough

User · Answer

See it in Activity Lifecycle  at Android Developers      onCreate        Called when the activity is first created  This is where you should do   all of your normal static set up  create views  bind data to lists    etc  This method also provides you with a Bundle containing the   activity s previously frozen state  if there was one  Always followed   by onStart      onRestart        Called after your activity has been stopped  prior to it being started   again  Always followed by onStart     onStart        Called when the activity is becoming visible to the user  Followed by   onResume   if the activity comes to the foreground    onResume        Called when the activity will start interacting with the user  At this   point your activity is at the top of the activity stack  with user   input going to it  Always followed by onPause      onPause         Called as part of the activity lifecycle when an activity is going   into the background    but has not  yet  been killed  The counterpart to onResume      When activity B is launched in front of activity A  this callback will be invoked on A    B will not be created until A s onPause   returns  so be sure to not   do anything lengthy here    onStop        Called when you are no longer visible to the user  You will next   receive either onRestart    onDestroy    or nothing  depending on   later user activity    Note that this method may never be called  in low memory situations   where the system does not have enough memory to keep your activity s   process running after its onPause   method is called    onDestroy        The final call you receive before your activity is destroyed  This   can happen either because the activity is finishing  someone called   finish   on it  or because the system is temporarily destroying this   instance of the activity to save space  You can distinguish between  these two scenarios with the isFinishing   method    When the Activity first time loads the events are called as below   onCreate   onStart   onResume     When you click on Phone button the Activity goes to the background and the below events are called   onPause   onStop     Exit the phone dialer and the below events will be called   onRestart   onStart   onResume     When you click the back button OR try to finish   the activity the events are called as below   onPause   onStop   onDestroy       Activity States  The Android OS uses a priority queue to assist in managing activities running on the device  Based on the state a particular Android activity is in  it will be assigned a certain priority within the OS  This priority system helps Android identify activities that are no longer in use  allowing the OS to reclaim memory and resources  The following diagram illustrates the states an activity can go through  during its lifetime   These states can be broken into three main groups as follows   Active or Running - Activities are considered active or running if they are in the foreground  also known as the top of the activity stack  This is considered the highest priority activity in the Android Activity stack  and as such will only be killed by the OS in extreme situations  such as if the activity tries to use more memory than is available on the device as this could cause the UI to become unresponsive   Paused - When the device goes to sleep  or an activity is still visible but partially hidden by a new  non-full-sized or transparent activity  the activity is considered paused  Paused activities are still alive  that is  they maintain all state and member information  and remain attached to the window manager  This is considered to be the second highest priority activity in the Android Activity stack and  as such  will only be killed by the OS if killing this activity will satisfy the resource requirements needed to keep the Active Running Activity stable and responsive   Stopped - Activities that are completely obscured by another activity are considered stopped or in the background  Stopped activities still try to retain their state and member information for as long as possible  but stopped activities are considered to be the lowest priority of the three states and  as such  the OS will kill activities in this state first to satisfy the resource requirements of higher priority activities    Sample activity to understand the life cycle    import android app Activity  import android os Bundle  import android util Log  public class MainActivity extends Activity       String tag    LifeCycleEvents           Called when the activity is first created          Override     public void onCreate Bundle savedInstanceState           super onCreate savedInstanceState          setContentView R layout main          Log d tag   In the onCreate   event              public void onStart                super onStart           Log d tag   In the onStart   event              public void onRestart                super onRestart           Log d tag   In the onRestart   event              public void onResume                super onResume           Log d tag   In the onResume   event              public void onPause                super onPause           Log d tag   In the onPause   event              public void onStop                super onStop           Log d tag   In the onStop   event              public void onDestroy                super onDestroy           Log d tag   In the onDestroy   event

User · Answer

Activity has six states   Created Started Resumed Paused Stopped Destroyed   Activity lifecycle has seven methods    onCreate   onStart   onResume   onPause   onStop   onRestart   onDestroy       Situations    When open the app  onCreate   -- gt  onStart   -- gt   onResume    When back button pressed and exit the app  onPaused   --  gt  onStop   -- gt  onDestory    When home button pressed   onPaused   -- gt  onStop    After pressed home button when again open app from recent task list or clicked on icon  onRestart   -- gt  onStart   -- gt  onResume    When open app another app from notification bar or open settings  onPaused   -- gt  onStop    Back button pressed from another app or settings then used can see our app  onRestart   -- gt  onStart   -- gt  onResume    When any dialog open on screen  onPause    After dismiss the dialog or back button from dialog   onResume    Any phone is ringing and user in the app  onPause   -- gt  onResume     When user pressed phone s answer button  onPause    After call end   onResume    When phone screen off  onPaused   -- gt  onStop    When screen is turned back on  onRestart   -- gt  onStart   -- gt  onResume

User · Answer

I run some logs as per answers above and here is the output   Starting Activity  On Activity Load  First Time                                                                                                                                                   D IndividualChatActivity  onCreate   D IndividualChatActivity  onStart   D IndividualChatActivity  onResume   D IndividualChatActivity  onPostResume    Reload After BackPressed                                                                                                                                                  D IndividualChatActivity  onCreate   D IndividualChatActivity  onStart   D IndividualChatActivity  onResume   D IndividualChatActivity  onPostResume    OnMaximize Circle Button                                                                                                                                                   D IndividualChatActivity  onRestart   D IndividualChatActivity  onStart   D IndividualChatActivity  onResume   D IndividualChatActivity  onPostResume    OnMaximize Square Button                                                                                                                                                   D IndividualChatActivity  onRestart   D IndividualChatActivity  onStart   D IndividualChatActivity  onResume   D IndividualChatActivity  onPostResume     Stopping The Activity  On BackPressed                                                                                                                                                  D IndividualChatActivity  onPause  D IndividualChatActivity  onStop   D IndividualChatActivity  onDestroy    OnMinimize  Circle Button                                                                                                                                                   D IndividualChatActivity  onPause   D IndividualChatActivity  onStop    OnMinimize  Square Button                                                                                                                                                   D IndividualChatActivity  onPause   D IndividualChatActivity  onStop    Going To Another Activity                                                                                                                                                  D IndividualChatActivity  onPause  D IndividualChatActivity  onStop    Close The App                                                                                                                                                  D IndividualChatActivity  onDestroy     In my personal opinion only two are required onStart and onStop     onResume seems to be in every instance of getting back  and onPause in every instance of leaving  except for closing the app

User · Answer

ANDROID LIFE-CYCLE  There are seven methods that manage the life cycle of an Android application    onCreate   onStart   onResume   onRestart   onPause   onStop   onDestroy       Answer for what are all these methods for   Let us take a simple scenario where knowing in what order these methods are called will help us give a clarity why they are used    Suppose you are using a calculator app  Three methods are called in succession to start the app    onCreate    - - -    onStart    - - -    onResume     When I am using the calculator app  suddenly a call comes the  The calculator activity goes to the background and another activity say  Dealing with the call comes to the foreground  and now two methods are called in succession    onPause    - - -    onStop     Now say I finish the conversation on the phone  the calculator activity comes to foreground from the background  so three methods are called in succession    onRestart    - - -    onStart    - - -    onResume     Finally  say I have finished all the tasks in calculator app  and I want to exit the app  Futher two methods are called in succession    onStop    - - -    onDestroy      There are four states an activity can possibly exist    Starting State Running State Paused State Stopped state   Starting state involves   Creating a new Linux process  allocating new memory for the new UI objects  and setting up the whole screen  So most of the work is involved here   Running state involves   It is the activity  state  that is currently on the screen  This state alone handles things such as typing on the screen  and touching  amp  clicking buttons   Paused state involves   When an activity is not in the foreground and instead it is in the background  then the activity is said to be in paused state   Stopped state involves   A stopped activity can only be bought into foreground by restarting it and also it can be destroyed at any point in time   The activity manager handles all these states in such a way that the user experience and performance is always at its best even in scenarios where the new activity is added to the existing activities

[android] Android activity life cycle - what are all these methods for?

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