Activities, Intents, Tasks, and the Back Stack (360|AnDev, July 2017)

The Android operating system is unique in the way that it manages the series of screens with which a user can interact. Generally, we know these “screens” as activities, though we can make exceptions for smaller components like fragments and views. As application developers, we create activities and move between them with intents. However, the way that Android uses intents to create, destroy, and maintain a “stack” of Activities is mostly hidden from us — for good reason! In this talk, we’ll work towards a solid understanding of activities, intents, tasks, and the back stack. We may also take a quick look into the mechanisms by which the Android operating system orchestrates all of these things internally.

Transcript

1. Activities, Intents, Tasks,
   and the Back Stack
   Matt Logan
   

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2. Activities
   class HomeActivity : AppCompatActivity() {

   

   override fun onCreate(savedInstanceState: Bundle?) {

   super.onCreate(savedInstanceState)

   setContentView(R.layout.activity_home)

   }

   }
   

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3. Activities
   class HomeActivity : AppCompatActivity() {

   

   override fun onCreate(savedInstanceState: Bundle?) {

   super.onCreate(savedInstanceState)

   setContentView(R.layout.activity_home)

   }

   }
   Entry point
   

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4. Activities
   class HomeActivity : AppCompatActivity() {

   

   override fun onCreate(savedInstanceState: Bundle?) {

   super.onCreate(savedInstanceState)

   setContentView(R.layout.activity_home)

   }

   }
   UI
   

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5. Activities
   class HomeActivity : AppCompatActivity() {

   

   override fun onCreate(savedInstanceState: Bundle?) {

   super.onCreate(savedInstanceState)

   setContentView(R.layout.activity_main)

   }

   

   @OnClick(R.id.view_trip_button)
   fun onViewTripButtonClicked() {

   val tripActivity =
   Intent(this, TripActivity!::class.java)

   startActivity(tripActivity)

   }

   }

   

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6. Activities
   class HomeActivity : AppCompatActivity() {

   

   override fun onCreate(savedInstanceState: Bundle?) {

   super.onCreate(savedInstanceState)

   setContentView(R.layout.activity_main)

   }

   

   @OnClick(R.id.view_trip_button)
   fun onViewTripButtonClicked() {

   val tripActivity =
   Intent(this, TripActivity!::class.java)

   startActivity(tripActivity)

   }

   }

   Navigation
   

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7. Activities
   • Entry point to your app
   • UI building block
   • Navigation
   

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8. How do activities handle navigation?
   

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9. Intents
   @OnClick(R.id.view_trip_button)

   fun onViewTripButtonClicked() {

   val tripActivity =
   Intent(this, TripActivity!::class.java)

   startActivity(tripActivity)

   }
   

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10. Intents
    @OnClick(R.id.view_trip_button)

    fun onViewTripButtonClicked() {

    val tripActivity =
    Intent(this, TripActivity!::class.java)

    startActivity(tripActivity)

    }
    

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11. Intents
    Intent(Context packageContext, Class> cls)
    Package context
    (where to look)
    Class of activity to start
    

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12. Intents
    class TripActivity : AppCompatActivity() {

    

    override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState)

    setContentView(R.layout.activity_trip)

    }

    

    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)

    startActivity(cameraIntent)

    }

    }
    

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13. Intents
    class TripActivity : AppCompatActivity() {

    

    override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState)

    setContentView(R.layout.activity_trip)

    }

    

    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)

    startActivity(cameraIntent)

    }

    }
    

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14. Intents
    Intent(String action)
    Action to fulfill, like capturing an image
    

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15. Intents
    • Navigation within your app
    • Navigation between apps
    • Can also pass data (we didn’t cover this)
    

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16. What if we want to navigate backward?
    

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17. Tasks
    Home Trip details Camera
    

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18. Tasks
    Home
    Trip details
    Camera
    

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19. Tasks
    HomeActivity
    TripActivity
    WeatherService
    TripProvider
    CameraActivity
    UploadService
    DownloadService
    GalleryActivity
    Travel app APK Camera app APK
    

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20. Tasks
    HomeActivity
    TripActivity
    WeatherService
    TripProvider
    CameraActivity
    UploadService
    DownloadService
    GalleryActivity
    Travel app APK Camera app APK
    

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21. Tasks
    • Collection of activities
    • Associated with a “back stack” of UI history
    • Can span multiple processes & APKs
    • Customized with intent flags & manifest attributes
    

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22. Launch modes
    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)
    

    startActivity(cameraIntent)

    }
    

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23. Launch modes
    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)

    cameraIntent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK)

    startActivity(cameraIntent)

    }
    

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24. Launch modes
    • FLAG_ACTIVITY_NEW_TASK
    • Starts activity in a new task
    • If activity is already running in a task, will
    reuse existing task
    • Unless paired with
    FLAG_ACTIVITY_MULTIPLE_TASK
    

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25. Launch modes
    Home Trip details
    Camera
    Task A
    Task B
    

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26. Launch modes
    Home
    Trip Details
    Camera
    Task A Task B
    

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27. Launch modes
    intent.addFlags(

    FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK)
    Kotlin:
    intent.addFlags(

    FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_MULTIPLE_TASK);
    Java:
    

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28. Launch modes
    intent.addFlags(

    FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK)
    Kotlin:
    intent.addFlags(

    FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_MULTIPLE_TASK);
    Java: Bitwise “or” operator
    

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29. Launch modes
    public static final int FLAG_ACTIVITY_NEW_TASK = 0x10000000;
    public static final int FLAG_ACTIVITY_MULTIPLE_TASK = 0x08000000;
    val flags = FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK
    

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30. Launch modes
    = 0000 1000 0000 0000 0000 0000 0000 0000
    = 0001 0000 0000 0000 0000 0000 0000 0000
    public static final int FLAG_ACTIVITY_NEW_TASK = 0x10000000;
    public static final int FLAG_ACTIVITY_MULTIPLE_TASK = 0x08000000;
    val flags = FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK
    

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31. Launch modes
    public static final int FLAG_ACTIVITY_NEW_TASK = 0x10000000;
    = 0000 1000 0000 0000 0000 0000 0000 0000
    = 0001 0000 0000 0000 0000 0000 0000 0000
    public static final int FLAG_ACTIVITY_MULTIPLE_TASK = 0x08000000;
    val flags = FLAG_ACTIVITY_NEW_TASK or FLAG_ACTIVITY_MULTIPLE_TASK
    = 0001 1000 0000 0000 0000 0000 0000 0000
    = 402,653,184
    Result
    Result
    

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32. Launch modes
    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)

    cameraIntent.addFlags(FLAG_ACTIVITY_NEW_TASK or
    FLAG_ACTIVITY_MULTIPLE_TASK)

    startActivity(cameraIntent)

    }
    

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33. Launch modes
    @OnClick(R.id.open_camera_button)

    fun onOpenCameraButtonClicked() {

    val cameraIntent =
    Intent(MediaStore.ACTION_IMAGE_CAPTURE)

    cameraIntent.addFlags(402_653_184)
    

    startActivity(cameraIntent)

    }
    Don't do this!
    

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34. Launch modes
    • FLAG_ACTIVITY_SINGLE_TOP
    • Activity won’t be launched if already at top
    • Intent will be delivered to existing activity
    via onNewIntent(Intent) callback
    

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35. Launch modes
    Home Trip details
    onNewIntent()
    FLAG_ACTIVITY_SINGLE_TOP
    

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36. Launch modes
    override fun onNewIntent(intent: Intent) {

    super.onNewIntent(intent)

    

    !// Get data from new intent 

    val newTripName =
    intent.extras.getString(EXTRA_TRIP_NAME)

    

    !// Clear existing UI and repopulate with new data

    refreshUi(newTripName)

    }
    

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37. Launch modes
    • FLAG_ACTIVITY_CLEAR_TOP
    • Destroys everything on top if it already exists
    • Can be used with FLAG_ACTIVITY_NEW_TASK
    • Locates activity in any task
    • Destroys all activities on top
    • Activity is brought to foreground
    

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38. Launch modes
    Home Trip details
    FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_NEW_TASK
    Weather home Weather details Weather forecast
    

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39. Launch modes
    Home Trip details
    Weather home Weather details
    onNewIntent()
    FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_NEW_TASK
    

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40. Launch modes
    android:name=".TripActivity"

    android:launchMode="standard"

    !/>
    

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41. Launch modes
    • "standard" — new instance in current task
    • "singleTop" — won’t be launched if already at top
    • "singleTask" — creates new task for activity
    • "singleInstance" — no other activities can be
    launched into same task
    • Can be overridden with intent flags
    

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42. Task affinity
    android:name=".TripActivity"

    android:taskAffinity="tripTask"

    !/>
    

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43. Task affinity
    af·fin·i·ty
    noun
    a spontaneous or natural liking or sympathy for
    someone or something.
    “Matt has an affinity for pizza.”
    

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44. Task affinity
    af·fin·i·ty
    noun
    an indication of which task an activity prefers to
    belong to.
    

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45. Task affinity
    • Useful with FLAG_ACTIVITY_NEW_TASK
    • Activity gets launched into task of activities
    with the same affinity
    

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46. Task affinity
    Home
    Trip details
    taskAffinity="tripTask"
    Weather details
    taskAffinity="tripTask"
    Weather home
    

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47. Task affinity
    • Relevant when
    allowTaskReparenting="true"
    • Activity can be reassigned tasks as needed
    

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48. Task affinity
    Home Trip details
    Weather home
    Weather details
    allowTaskReparenting="true"
    

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49. Task affinity
    Home Trip details
    Weather home Weather details
    allowTaskReparenting="true"
    

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50. Task affinity
    • One more use case
    • Two “apps” in one APK
    • Use affinities to separate activities into tasks
    

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51. Task affinity
    android:name=".HomeActivity"!/>
    

    android:name=".TripDetailsActivity"

    android:taskAffinity="tripTask"!/>
    android:name=".ExendedTripDetailsActivity"

    android:taskAffinity="tripTask"!/>
    

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52. Clearing the back stack
    • Default behavior:
    • After some amount of time, system clears all
    activities in task except the root
    • User has likely abandoned that task
    • Modified with more manifest attributes
    

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53. Clearing the back stack
    android:name=".TripActivity"

    android:alwaysRetainTaskState="true"

    !/>
    Prevents clearing of activities from task
    

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54. Clearing the back stack
    android:name=".TripActivity"

    android:clearTaskOnLaunch="true"

    !/>
    Removes all activities except root when
    user returns to task
    

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55. Clearing the back stack
    android:name=".TripActivity"

    android:finishOnTaskLaunch="true"

    !/>
    Same as clearTaskOnLaunch but only
    operates on a single activity
    

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56. Task helpers
    • NavUtils has some navigation helper methods
    • navigateUpTo(Activity, Intent)
    • navigateUpFromSameTask(Activity)
    • getParentActivityIntent(Activity)
    

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57. Task helpers
    • TaskStackBuilder lets you create a back
    stack manually
    • Useful when starting activity in a new task
    with no UI history
    • Lets build the typical UI history that would
    lead the user to that point
    

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58. Recents
    • Back button should never traverse tasks
    • User can switch tasks with “recents” system UI
    • Basically a view of all tasks
    • Special behavior added in Lollipop for
    “documents”
    

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59. How does all of this work?
    

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60. How’s it work?
    • Intents are handled by
    ActivityManagerService
    • One of several system services
    • Orchestrates activities, including tasks and
    their respective back stacks
    

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61. How’s it work?
    val activityManager =

    getSystemService(ACTIVITY_SERVICE) as ActivityManager
    What about this activity manager?
    

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62. How’s it work?
    • ActivityManagerService is a system service
    • ActivityManager is for app developers
    

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63. How’s it work?
    • ActivityManager
    • Some methods are broadly useful:
    • isLowRamDevice()
    • clearApplicationUserData()
    • Apps can interact with its task stack via:
    • ActivityManager.AppStack
    • ActivityManager.RecentTaskInfo
    

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64. Why does Android work this way?
    

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65. Design considerations
    • Android OS operates on modular components
    • Enforces separation of concerns at
    application level and at device level
    • Navigation is handled by the OS, not your app
    • Allows for navigation model involving
    activities in separate APKs
    

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66. What if we want to handle navigation
    without involving the Android OS?
    

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67. In-app navigation
    • Official, supported solution: fragments
    • FragmentManager & FragmentTransaction take
    the place of ActivityManagerService & intents
    • Lifecycle methods very similar to Activity
    • Fragment back stack similar to task back stack
    • Built in state restoration
    • Support for animated transitions
    

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68. In-app navigation
    • Non-fragment solutions
    • Can avoid some complexity of fragments
    • Manually add & remove views to & from a
    root view group
    • You have to maintain a navigation stack
    yourself!
    

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69. Resources
    Android “Activities — Tasks and Back Stack” developer guide
    https://developer.android.com/guide/components/activities/tasks-and-back-stack.html
    Dan Morrill’s 2008 Google I/O talk: Inside the Android Application Framework
    https://www.youtube.com/watch?v=TkPiXRNee7A
    Anatomy of Android: Activity Manager
    https://anatomyofandroid.com/2013/10/16/activity-manager/
    The Cheese Factory: Understand Android Activity’s launchMode
    https://inthecheesefactory.com/blog/understand-android-activity-launchmode/en
    Android Programming: The Big Nerd Ranch Guide
    https://www.bignerdranch.com/books/android-programming/
    Square Engineering Blog: Advocating Against Android Fragments
    https://medium.com/square-corner-blog/advocating-against-android-fragments-81fd0b462c97
    

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70. Thanks!
    @_mattlogan
    Twitter
    mattlogan.me
    Blog & links
    github.com/mattlogan
    Open source
    

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