Android Activities: Process, Lifecycle, Management PDF

By Wissam H. Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 2 Android is based on Linux OS Android applications are Linux processes At launch time, an Android process is created It contains : A copy of the Dalvik...

By Wissam H. Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 2 Android is based on Linux OS Android applications are Linux processes At launch time, an Android process is created It contains : A copy of the Dalvik VM, shared among processes A copy of the Android framework classes A copy of the Application own classes, loaded out of your APK Any objects created by you or the framework classes, such as the instance of your Activity subclass The Back button : When pressed on the last activity, application appears as being destroyed, but the process is kept in memory for a little while (an optimization in case the user decide to re-launch the application) The Home button : The home screen appears as with “Back” button, but the main activity is not destroyed Termination : android can destroy an application to free up memory for others, the decision is made depend on many factors like : the occupation of memory, the state of the destroyed application (foreground/background)… Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 3 Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 4 Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 5 During the life of an activity, the system calls a set of lifecycle methods in a sequence similar to a step pyramid. Implementing lifecycle methods properly ensures the app behaves well in several ways (doesn't crash after a call, doesn't lose user progress, …) Only 3 states are static (exist for a long period) : – Resumed : activity is in the foreground and the user can interact with it – Paused : this happen in many scenarios : activity is partially obscured by another semi-transparent activity such as a dialog (As long as the activity is still partially visible but not in focus) user is in multi-window mode and taps the other window Some event interrupts app execution (phone call, device screen’s turning off,…) – Stopped : activity is completely hidden and not visible to the user; it is considered to be in the background. the activity instance and all its state information such as member variables is retained, but it cannot execute any code. This is a good place to stop refreshing or running visual things Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 6 onDestroy( ) : Most apps don't need to implement this method because local class references are destroyed with the activity and your activity should perform most cleanup during onPause() and onStop(). Can be used to destroy background Threads or to close a DB helper onPause( ) : Can be implemented to : stop ongoing actions that should not continue while paused (such as a video playback), stop ongoing actions that could consume CPU, or release system resources (as broadcast receivers, handles to sensors). It is recommended to avoid performing CPU-intensive work during onPause(), such as writing to a database, because it can slow the visible transition to the next activity. Remember to re-acquire current released resources when passing to onResume(). onStop( ) : When activity receives a call to the onStop() method, it's no longer visible and should release resources that aren't needed while the user is not using it. This method is also used to shut-down CPU intensive operations (as writing DB). onStart( ) : Use to allocate resources that has been released with onStop().  Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 7 Saving state : – Android allows activities to save information about their state by calling automatically the method onSaveInstanceState(Bundle b). – When this happen ?? When Activity go to background (Activity covered by another one) When a configuration change occurs (for example screen rotation) Does not happen on Exiting Activity normally with “BACK” button. Restoring state : – Android allows activities to restore their state by calling automatically the method : onRestoreInstanceState(Bundle b) – When this happen ?? In System Process kill scenario (process of application is killed by system because device is low on resources) On recreating application after a configuration change. Coming back naturally from background to screen by task switching does not invoke a restore state call – N.B: To test “System Process kill” scenario, developer must activate a specific option on the emulator/smartphone : Settings > Developer Options > Don’t Keep Activities Naturally, the system save/restore automatically the UI state of an activity (for all Views that have an ID property). But developer can add extra state data saving/restoring by overwriting methods onSaveInstanceState and onRestoreInstanceState (overwriting must start with a “super” call to preserve UI state save/recovery feature). Example: public void onSaveInstanceState(Bundle savedInstanceState) { super.onSaveInstanceState(savedInstanceState); savedInstanceState.putInt("game_level", my_currentLevel); } public void onRestoreInstanceState(Bundle savedInstanceState) { super.onRestoreInstanceState(savedInstanceState); my_currentLevel = savedInstanceState.getInt("game_level");} Activities : process, lifecycle, management by Wissam Hlayhel Fall 2020 Page 8 Any new activity must be declared in the manifest (without an intent filter) An activity can be explicit (defined by the app) or implicit (OS or other app) – Launching activity with explicit intent : startActivity(new Intent(this, OtherActivity.class)); – Passing data with intent: Setter : intent.putExtra(string key, string/int/…) Getter : getIntent().getStringExtra(string key) Getting result from a started activity – startActivityforResult( )