Activities in the Wild: Exploring the Activity Lifecycle (360|AnDev, July 2016)

[Check out the video of this presentation from 360AnDev 2016: https://realm.io/news/activities-in-the-wild-exploring-the-activity-lifecycle-android/]

Running, or paused, or stopped, or destroyed? Oh my! As Android developers we have little control over what state our activities are in. But we do have the ability to hook into transitions between these states to respond appropriately and ensure a pleasant user experience.

Through concrete examples you will learn how the creation and destruction of your activity instances relate to what the user is doing, as well as to what the system is doing. We will explore how state is persisted (or not) in these various scenarios and also explore some little known facts about what happens to your activity in low memory situations.

Finally, we will discuss how knowing when these transitions happen has helped me deal with tricky situations, like continuing video playback while the user interacts with a full-screen overlay.

Transcript

1. Activities in the Wild
   Exploring the Activity Lifecycle
   Kristin Marsicano @kristinmars Big Nerd Ranch
   

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2. Expectation setting
   Hello, World!
   

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3. Demo: Ticker Tally
   Photo Credit: https://www.amazon.com/GOGO-Counter-Carnival-Manual-Mechanical/dp/B001KX1VW2
   

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4. Goal for Today
   Empower you to reason about the Activity lifecycle
   so you can provide a smooth user experience
   

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5. What is the Activity
   Lifecycle?
   

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6. Activity
   public class TickerActivity extends AppCompatActivity {
   private int mCount = 0;

   ...

   @Override

   protected void onCreate(Bundle savedInstanceState) {

   super.onCreate(savedInstanceState);

   setContentView(R.layout.activity_ticker);

   setupUI();

   updateUI();

   }

   

   ...
   }

   TickerActivity.java:
   

   

   

   

   

   

   

   

   

   

   activity_ticker.xml:
   

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7. Activity Manager
   

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8. Activity Manager
   User clicks
   Ticker icon on
   Launch Screen
   

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9. Activity Manager
   User clicks
   Ticker icon on
   Launch Screen
   

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10. Activity Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    

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11. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    

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12. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    

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13. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    Spin up app
    process
    

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14. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    Spin up app
    process
    Create TickerActivity
    instance
    

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15. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    Spin up app
    process
    Create TickerActivity
    instance
    Move TickerActivity
    to Running state
    

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16. Activity Manager
    Activity
    Manager
    User clicks
    Ticker icon on
    Launch Screen
    Intent:
    TickerActivity
    Spin up app
    process
    Create TickerActivity
    instance
    Move TickerActivity
    to Running state
    

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17. Android OS
    App Processes
    Launcher
    Contacts
    

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18. Android OS
    TerrificTicker
    App Processes
    Launcher
    Contacts
    

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19. Android OS
    TerrificTicker
    App Processes
    Launcher
    Contacts
    

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20. Android OS
    TerrificTicker
    One Foreground Activity
    Launcher
    Contacts
    Running
    Activity:
    What
    user
    sees
    

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21. Activity Lifecycle
    

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22. Activity Lifecycle
    Non-
    existent
    Stopped Paused Running
    Set of states Activity can be in
    

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23. Activity States
    

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24. Activity States
    State
    

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25. Activity States
    State In memory?
    

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26. Activity States
    State In memory? Visible to user?
    

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27. Activity States
    State In memory? Visible to user? In foreground?
    

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28. Activity States
    State In memory? Visible to user? In foreground?
    

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29. Activity States
    State In memory? Visible to user? In foreground?
    Non-Existent No No No
    

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30. Activity States
    State In memory? Visible to user? In foreground?
    Non-Existent No No No
    Stopped Yes No No
    

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31. Activity States
    State In memory? Visible to user? In foreground?
    Non-Existent No No No
    Stopped Yes No No
    Paused Yes Yes/Partially No
    

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32. Activity States
    State In memory? Visible to user? In foreground?
    Non-Existent No No No
    Stopped Yes No No
    Paused Yes Yes/Partially No
    Running
    (aka Resumed)
    Yes Yes Yes
    

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33. Activity in the Wild
    

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34. Activity Lifecycle
    

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35. Activity Lifecycle
    Non-
    existent
    Stopped Paused Running
    Set of states Activity can be in
    

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36. Activity Lifecycle
    Non-
    existent
    Stopped Paused Running
    Set of states Activity can be in
    Rules about when/how state changes
    

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37. Activity Lifecycle
    Non-
    existent
    Stopped Paused Running
    Set of states Activity can be in
    Rules about when/how state changes
    onDestroy()
    onCreate()
    onPause()
    onResume()
    onStart()
    onStop()
    Set of callback methods
    

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38. User Launches
    Activity
    Lifecycle by Example 1 of 5: “Build-up”
    

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39. Example: Launch
    TickerActivity
    

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40. Example: Launch
    TickerActivity
    Press Icon
    

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41. Example: Launch
    TickerActivity
    

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42. Laser Focus: Ticker Activity
    Android OS
    

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43. Start in Non-Existent State
    Non-
    existent
    Android OS
    

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44. User Launches App
    Non-
    existent
    Android OS
    User presses icon
    

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45. onCreate()
    Non-
    existent
    Android OS
    TerrificTicker
    Process
    Instance Created
    Ticker
    Activity
    mCount = 0
    Stopped
    Instance Created
    onCreate()
    

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46. Android OS
    onStart()
    Paused
    onStart()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    

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47. onResume()
    Running
    onResume()
    Paused
    onStart()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    

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48. Pressing Back
    Lifecycle by Example 2 of 5: “Tear down”
    

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49. User Increments Count
    Running
    onResume()
    Paused
    onStart()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    Press button
    

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50. User Increments Count
    Running
    onResume()
    Paused
    onStart()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    Press button
    Ticker
    Activity
    mCount = 1
    

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51. User Increments Count
    Running
    onResume()
    Paused
    onStart()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    Press button
    1
    Ticker
    Activity
    mCount = 1
    

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52. User Presses Back
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Paused Running
    onResume()
    onStart()
    Press Back
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 1
    1
    

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53. onPause()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Paused Running
    User presses back
    onPause()
    onResume()
    onStart()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 1
    1
    

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54. onStop()
    onStop()
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Paused Running
    User presses back
    onPause()
    onResume()
    onStart()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 1
    

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55. onDestroy()
    onDestroy()
    Instance Destroyed
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Paused Running
    User presses back
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 1
    

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56. onDestroy()
    onDestroy()
    Instance Destroyed
    Non-
    existent
    Stopped
    Instance Created
    onCreate()
    Paused Running
    User presses back
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    

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57. User Launches App Again
    Android OS
    User presses icon
    TerrificTicker
    Process
    Non-
    existent
    

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58. User Launches App Again
    Android OS
    User presses icon
    TerrificTicker
    Process
    Non-
    existent
    What will the user see?
    0
    (0)
    1
    (1)
    

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59. Answer: (0)
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 0
    0
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onResume()
    onStart()
    

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60. Pressing Home
    Lifecycle Example 3 of 5: “Switching Tasks”
    

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61. Increment Count Twice
    

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62. Increment Count Twice
    Press home
    

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63. 1
    Increment Count Twice
    Press home
    

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64. 1
    Increment Count Twice
    2
    Press home
    

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65. Home Screen
    

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66. TickActivity is Running
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    0
    2
    

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67. User Presses Home
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    0
    2
    User presses
    Home
    

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68. onPause()
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    2
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    

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69. onStop()
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    

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70. User Launches App Again
    Android OS
    User presses icon
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    Stopped
    

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71. User Launches App Again
    Android OS
    User presses icon
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    Stopped
    What will the user see?
    0
    (0)
    2
    (2)
    

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72. Answer: (2)
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 2
    2
    

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73. Launching Another
    Activity
    Lifecycle Example 4 of 5: “Fully occlude TickerActivity”
    

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74. Launch TickerActivity
    Press Icon
    

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75. 0
    Increment Count
    Press three times
    

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76. 0
    1
    Increment Count
    Press three times
    

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77. 0
    1
    Increment Count
    2
    Press three times
    

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78. 0
    1
    Increment Count
    2
    Press three times
    3
    

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79. Launch SaveActivity
    Press Save Count
    

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80. SaveActivity is Running
    

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81. From the Start: TickerActivity
    is Running
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    

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82. Use Presses Save
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    

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83. SaveActivity Launches
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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84. TickerActivity: Stopped
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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85. User Presses Back
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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86. SaveActivity is Destroyed
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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87. SaveActivity is Destroyed
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    

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88. Ticker Activity Moves to
    Stopped from Running
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    

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89. onStart()
    onStart()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStop()
    

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90. onStart()
    onStart()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStop()
    

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91. onStart()
    onResume()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStop()
    

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92. Launching a
    Transparent Activity
    Lifecycle Example 5 of 5: “Partially occlude
    TickerActivity”
    

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93. Launch SaveActivity
    Press Save Count
    

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94. SaveActivity is Running
    

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95. From the Start: TickerActivity
    is Running
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    

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96. Use Presses Save
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    

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97. SaveActivity Launches
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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98. TickerActivity: Paused
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    

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99. User Presses Back
    Android OS
    TerrificTicker
    Process
    Ticker
    Activity
    mCount = 3
    Save
    Activity
    onDestroy()
    Non-
    existent
    Stopped
    onCreate()
    Paused Running
    onPause()
    onResume()
    onStart()
    onStop()
    

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100. User Presses Back
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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101. TickerActivity: Running
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     

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102. Lifecycle Summary
     Exists? Visible? Foreground?
     

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103. Activity Lifecycle
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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104. Entire Lifetime (Instance Lifetime)
     Instance Lifetime
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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105. Entire Lifetime
     Visible Lifetime
     Visible Lifetime
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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106. Entire Lifetime
     Foreground Lifetime
     Visible Lifetime
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Foreground Lifetime
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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107. Going Home,
     Revisited
     A Cautionary Tale
     

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108. Launch TickerActivity
     

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109. Do Lots of Work
     42
     

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110. Press Home
     42
     

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111. Launch Pokemon Go
     

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112. Play Pokemon GO
     

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113. Press Home
     

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114. Press Ticker Icon
     

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115. Launch Ticker
     

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116. Launch Ticker
     

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117. User Presses Home
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     User presses
     home
     3
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     

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118. onPause()
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     3
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     

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119. onStop()
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     

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120. User Plays Pokemon GO
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     Pokemon
     GO Process
     VideoActivity
     

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121. Uh oh…
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     VideoPlayer
     Process
     VideoActivity
     

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122. Ticker Process Gets Killed
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     X
     Android OS
     TerrificTicker
     Process
     Ticker
     Activity
     mCount = 3
     VideoPlayer
     Process
     VideoActivity
     X
     

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123. TickerActivity Jumps to Non-
     Existent
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     X
     Android OS
     VideoPlayer
     Process
     VideoActivity
     

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124. Saving Transient State
     onSaveInstanceState to the rescue
     

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125. onSaveInstanceState()
     public class TickerActivity extends AppCompatActivity {

     

     private static final String KEY_TICKER_COUNT = "ticker_count";

     ...

     

     @Override

     protected void onSaveInstanceState(Bundle outState) {

     super.onSaveInstanceState(outState);

     outState.putInt(KEY_TICKER_COUNT, mCount);

     }
     ...

     }

     

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126. public class TickerActivity extends AppCompatActivity {

     

     ...
     

     @Override

     protected void onCreate(Bundle savedInstanceState) {

     super.onCreate(savedInstanceState);

     setContentView(R.layout.activity_ticker);

     setupUI();

     

     if (savedInstanceState != null) {

     mCount = savedInstanceState.getInt(KEY_TICKER_COUNT, 0);

     }

     

     updateUI();

     }
     ...

     }

     Using State Bundle
     

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127. Activity Record Stored
     3
     Activity Record
     Store
     Instance A
     

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128. Activity Record Stored
     3
     Activity Record
     Store
     onSaveInstanceState
     ticker_count: 3
     Instance A
     

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129. Activity Record Stored
     3
     Activity Record
     Store
     onSaveInstanceState
     ticker_count: 3
     TerrificTicker Instance
     A Record
     ticker_count: 3
     Instance A
     

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130. Process Gets Killed
     Activity Record
     Store
     TerrificTicker Instance
     A Record
     ticker_count: 3
     

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131. User Launches App
     Activity Record
     Store
     TerrificTicker Instance
     A Record
     ticker_count: 3
     

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132. User Launches App
     Activity Record
     Store
     TerrificTicker Instance
     A Record
     ticker_count: 3
     onCreate
     ticker_count: 3
     

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133. User Launches App
     Activity Record
     Store
     TerrificTicker Instance
     A Record
     ticker_count: 3
     3
     Recreated
     Instance A
     onCreate
     ticker_count: 3
     

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134. User Launches App
     Activity Record
     Store
     3
     Recreated
     Instance A
     onCreate
     ticker_count: 3
     

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135. User Launches App
     Activity Record
     Store
     3
     Recreated
     Instance A
     

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136. Full Circle
     

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137. Structuring your
     Activity
     What to put in each callback?
     

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138. onCreate
     • call setContentView()
     • “hook up” UI widgets
     • check for saved state bundle
     

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139. onPause
     • save vital data
     • typically stop animations or other things consuming
     CPU
     • should be quick
     

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140. onStop
     • persist vital data
     • last guaranteed teardown method
     

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141. onDestroy
     • last place to free resources
     • not guaranteed to be called
     

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142. But Also…
     • It depends!
     

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143. Reasoning About the
     Lifecycle
     What to put in each callback?
     

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144. Displaying Location
     • Show user their location
     • Minimize battery usage
     • Maximize precision when user is actively looking at
     my map
     

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145. Displaying Location: Which
     Callbacks?
     • Start sampling - onStart
     • Set higher sampling rate/accuracy - onResume
     • Set lower sampling rate/accuracy - onPause
     • Stop sampling - onStop
     onDestroy()
     Non-
     existent
     Stopped
     onCreate()
     Paused Running
     onPause()
     onResume()
     onStart()
     onStop()
     

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146. Video Playback
     

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147. Video Playback
     • User starts video activity
     • User presses a transparent settings activity
     • video should keep playing!
     

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148. Video Playback: Which
     Callbacks?
     • initialize player - onCreate
     • start playback - onStart
     • stop playback - onStop
     • release player resources - onDestroy
     

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149. Capstone Challenge
     What state are these activities in?
     

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150. Challenge: Multi-Window
     What state is each in?
     

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151. Next Steps
     

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152. @kristinmars
     github.com/kristinmars
     bignerdranch.com
     

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