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Programming with Android: Activities

Programming with Android: Activities. Luca Bedogni Marco Di Felice Dipartimento di Informatica: Scienza e Ingegneria Università di Bologna. Activity. Outline : What is started by the device It contains the application's informations Has methoda to answer certain events

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Programming with Android: Activities

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  1. Programming with Android: Activities Luca Bedogni Marco Di Felice Dipartimento di Informatica: Scienza e Ingegneria Università di Bologna

  2. Activity 2 • Outline: • Whatisstarted by the device • Itcontains the application'sinformations • Hasmethoda to answercertainevents • An applicationcould be composed of multiple activities

  3. Creating an activity 3 • Create a class that is a subclass of Activity • Implement callback methods • OnCreate(): • Initialize • SetContentView()

  4. Activity lifecycle 4

  5. Activity lifecycle 5

  6. Activities 6 • Need to implementevery single method? No! • Itdepends on the applicationcomplexity • Whyitisimportant to understand the activitylifecycle? • So yourapplicationdoesnot crash (or do funnythings) while the userisrunningsomething else on the smartphone • So yourapplicationdoesnotconsumeunnecessaryresources • So the user can safely stop yourapplication and return to itlater

  7. Activitiesstates 7 • Resumed • The activityis in the foreground, and the user can interact. • Paused • The activityispartiallyoverlayed by anotheractivity. Cannotexecuteany code norreceiveinputs. • Stopped • Activity ishidden, in the background. Itcannotexecuteany code.

  8. Activity lifecycle 8 • OnCreate() • Called when the activity is created • Should contain the initialization operations • Has a Bundle parameter • If onCreate() succesfull terminates, it calls onStart()

  9. Activity lifecycle 9 • OnStart() • Called when onCreate() terminates • Called right before it is visible to user • If it has the focus, then onResume() is called • If not, onStop() is called

  10. Activity lifecycle 10 • OnResume() • Called when the activity is ready to get input from users • Called when the activity is resumed too • If it succesfully terminates, then the Activity is RUNNING

  11. Activity lifecycle 11 • OnPause() • Called when another activity comes to the foreground, or when someone presses back • Commit unsaved changes to persistent data • Stop cpu-consuming processes • Make it fast

  12. Activity lifecycle 12 • OnRestart() • Similar to onCreate() • We have an activity that was previously stopped

  13. Activity lifecycle 13 • OnStop() • Activity is no longer visible to the user • Could be called because: • the activity is about to be destroyed • another activity comes to the foreground

  14. 14 Activity lifecycle OnDestroy() The activity is about to be destroyed Could happen because: The systems need some stack space Someone called finish() method on this activity Could check with isFinishing()

  15. Activity loops 15 • Mainly 3 different loops • Entire lifetime • Between onCreate() and onDestroy(). • Setup of global state in onCreate() • Release remaining resources in onDestroy() • Visible lifetime • Between onStart() and onStop(). • Maintain resources that has to be shown to the user. • Foreground lifetime • Between onResume() and onPause(). • Code should be light.

  16. Activitiesin the manifest 16 • Declare them before running them • <activityandroid:name=".MainActivity"android:label="@string/app_name"> • <intent-filter> • <actionandroid:name="android.intent.action.MAIN"/> • <categoryandroid:name="android.intent.category.LAUNCHER"/> • </intent-filter> • </activity> • Why “MAIN” and “LAUNCHER”? • To show the application in the menu

  17. RecreatingActivities 17

  18. RecreatingActivities 18 • Android keeps the state of each view • Remember to assign unique Ids to them • So, no code is needed for the “basic” behavior • What if I want to save more data? • Override onSaveInstanceState() and onRestoreInstanceState() • staticfinalString STATE_SCORE ="playerScore"; • @Override • publicvoid onSaveInstanceState(Bundle savedInstanceState){ •     savedInstanceState.putInt(STATE_SCORE, mCurrentScore); • super.onSaveInstanceState(savedInstanceState); • }

  19. RecreatingActivities 19 • @Override • protectedvoid onCreate(Bundle savedInstanceState){ • super.onCreate(savedInstanceState);// Always call the superclass first • if(savedInstanceState !=null){ • // Restore value of members from saved state •         mCurrentScore = savedInstanceState.getInt(STATE_SCORE); • }else{ • // Probably initialize members with default values for a new instance • } • } • publicvoid onRestoreInstanceState(Bundle savedInstanceState){ • super.onRestoreInstanceState(savedInstanceState); •     mCurrentScore = savedInstanceState.getInt(STATE_SCORE); • }

  20. Activity: Conclusions 20 • Activities should be declared in the Manifest • Extend the Activity class • Code wisely • Put your code in the right place • Optimize it • Test even on low-end devices

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