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
2. Expectation setting Hello, World!
3. Demo: Ticker Tally Photo Credit: https://www.amazon.com/GOGO-Counter-Carnival-Manual-Mechanical/dp/B001KX1VW2
4. Goal for Today Empower you to reason about the Activity lifecycle so you can provide a smooth user experience
5. What is the Activity Lifecycle?
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: <?xml version="1.0" encoding="utf-8"?>
 <LinearLayout ...>
 
 <TextView ... />
 
 <TextView ... />
 
 <Button … />
 
 </LinearLayout>
 activity_ticker.xml:
7. Activity Manager
8. Activity Manager User clicks Ticker icon on Launch Screen
9. Activity Manager User clicks Ticker icon on Launch Screen
10. Activity Manager User clicks Ticker icon on Launch Screen Intent: TickerActivity
11. Activity Manager Activity Manager User clicks Ticker icon on Launch Screen Intent: TickerActivity
12. Activity Manager Activity Manager User clicks Ticker icon on Launch Screen Intent: TickerActivity
13. Activity Manager Activity Manager User clicks Ticker icon on Launch Screen Intent: TickerActivity Spin up app process
14. Activity Manager Activity Manager User clicks Ticker icon on Launch Screen Intent: TickerActivity Spin up app process Create TickerActivity instance
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
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
17. Android OS App Processes Launcher Contacts
18. Android OS TerrificTicker App Processes Launcher Contacts
19. Android OS TerrificTicker App Processes Launcher Contacts
20. Android OS TerrificTicker One Foreground Activity Launcher Contacts Running Activity: What user sees
21. Activity Lifecycle
22. Activity Lifecycle Non- existent Stopped Paused Running Set of states Activity can be in
23. Activity States
24. Activity States State
25. Activity States State In memory?
26. Activity States State In memory? Visible to user?
27. Activity States State In memory? Visible to user? In foreground?
28. Activity States State In memory? Visible to user? In foreground?
29. Activity States State In memory? Visible to user? In foreground? Non-Existent No No No
30. Activity States State In memory? Visible to user? In foreground? Non-Existent No No No Stopped Yes No No
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
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
33. Activity in the Wild
34. Activity Lifecycle
35. Activity Lifecycle Non- existent Stopped Paused Running Set of states Activity can be in
36. Activity Lifecycle Non- existent Stopped Paused Running Set of states Activity can be in Rules about when/how state changes
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
38. User Launches Activity Lifecycle by Example 1 of 5: “Build-up”
39. Example: Launch TickerActivity
40. Example: Launch TickerActivity Press Icon
41. Example: Launch TickerActivity
42. Laser Focus: Ticker Activity Android OS
43. Start in Non-Existent State Non- existent Android OS
44. User Launches App Non- existent Android OS User presses icon
45. onCreate() Non- existent Android OS TerrificTicker Process Instance Created Ticker Activity mCount = 0 Stopped Instance Created onCreate()
46. Android OS onStart() Paused onStart() Non- existent Stopped Instance Created onCreate() TerrificTicker Process Ticker Activity mCount = 0
47. onResume() Running onResume() Paused onStart() Non- existent Stopped Instance Created onCreate() Android OS TerrificTicker Process Ticker Activity mCount = 0
48. Pressing Back Lifecycle by Example 2 of 5: “Tear down”
49. User Increments Count Running onResume() Paused onStart() Non- existent Stopped Instance Created onCreate() Android OS TerrificTicker Process Ticker Activity mCount = 0 Press button
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
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
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
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
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
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
56. onDestroy() onDestroy() Instance Destroyed Non- existent Stopped Instance Created onCreate() Paused Running User presses back onPause() onResume() onStart() onStop() Android OS TerrificTicker Process
57. User Launches App Again Android OS User presses icon TerrificTicker Process Non- existent
58. User Launches App Again Android OS User presses icon TerrificTicker Process Non- existent What will the user see? 0 (0) 1 (1)
59. Answer: (0) Android OS TerrificTicker Process Ticker Activity mCount = 0 0 Non- existent Stopped onCreate() Paused Running onResume() onStart()
60. Pressing Home Lifecycle Example 3 of 5: “Switching Tasks”
61. Increment Count Twice
62. Increment Count Twice Press home
63. 1 Increment Count Twice Press home
64. 1 Increment Count Twice 2 Press home
65. Home Screen
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
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
68. onPause() onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() 2 Android OS TerrificTicker Process Ticker Activity mCount = 2
69. onStop() onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 2
70. User Launches App Again Android OS User presses icon TerrificTicker Process Ticker Activity mCount = 2 Stopped
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)
72. Answer: (2) onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 2 2
73. Launching Another Activity Lifecycle Example 4 of 5: “Fully occlude TickerActivity”
74. Launch TickerActivity Press Icon
75. 0 Increment Count Press three times
76. 0 1 Increment Count Press three times
77. 0 1 Increment Count 2 Press three times
78. 0 1 Increment Count 2 Press three times 3
79. Launch SaveActivity Press Save Count
80. SaveActivity is Running
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
82. Use Presses Save onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3
83. SaveActivity Launches Android OS TerrificTicker Process Ticker Activity mCount = 3 Save Activity
84. TickerActivity: Stopped onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3 Save Activity
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
86. SaveActivity is Destroyed Android OS TerrificTicker Process Ticker Activity mCount = 3 Save Activity
87. SaveActivity is Destroyed Android OS TerrificTicker Process Ticker Activity mCount = 3
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()
89. onStart() onStart() Android OS TerrificTicker Process Ticker Activity mCount = 3 onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStop()
90. onStart() onStart() Android OS TerrificTicker Process Ticker Activity mCount = 3 onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStop()
91. onStart() onResume() Android OS TerrificTicker Process Ticker Activity mCount = 3 onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStop()
92. Launching a Transparent Activity Lifecycle Example 5 of 5: “Partially occlude TickerActivity”
93. Launch SaveActivity Press Save Count
94. SaveActivity is Running
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
96. Use Presses Save onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3
97. SaveActivity Launches Android OS TerrificTicker Process Ticker Activity mCount = 3 Save Activity
98. TickerActivity: Paused onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3 Save Activity
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()
100. User Presses Back Android OS TerrificTicker Process Ticker Activity mCount = 3 onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop()
101. TickerActivity: Running onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3
102. Lifecycle Summary Exists? Visible? Foreground?
103. Activity Lifecycle onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop()
104. Entire Lifetime (Instance Lifetime) Instance Lifetime onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop()
105. Entire Lifetime Visible Lifetime Visible Lifetime onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop()
106. Entire Lifetime Foreground Lifetime Visible Lifetime onDestroy() Non- existent Stopped onCreate() Foreground Lifetime Paused Running onPause() onResume() onStart() onStop()
107. Going Home, Revisited A Cautionary Tale
108. Launch TickerActivity
109. Do Lots of Work 42
110. Press Home 42
111. Launch Pokemon Go
112. Play Pokemon GO
113. Press Home
114. Press Ticker Icon
115. Launch Ticker
116. Launch Ticker
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
118. onPause() onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() 3 Android OS TerrificTicker Process Ticker Activity mCount = 3
119. onStop() onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() Android OS TerrificTicker Process Ticker Activity mCount = 3
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
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
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
123. TickerActivity Jumps to Non- Existent onDestroy() Non- existent Stopped onCreate() Paused Running onPause() onResume() onStart() onStop() X Android OS VideoPlayer Process VideoActivity
124. Saving Transient State onSaveInstanceState to the rescue
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);
 } ...
 }

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
127. Activity Record Stored 3 Activity Record Store Instance A
128. Activity Record Stored 3 Activity Record Store onSaveInstanceState ticker_count: 3 Instance A
129. Activity Record Stored 3 Activity Record Store onSaveInstanceState ticker_count: 3 TerrificTicker Instance A Record ticker_count: 3 Instance A
130. Process Gets Killed Activity Record Store TerrificTicker Instance A Record ticker_count: 3
131. User Launches App Activity Record Store TerrificTicker Instance A Record ticker_count: 3
132. User Launches App Activity Record Store TerrificTicker Instance A Record ticker_count: 3 onCreate ticker_count: 3
133. User Launches App Activity Record Store TerrificTicker Instance A Record ticker_count: 3 3 Recreated Instance A onCreate ticker_count: 3
134. User Launches App Activity Record Store 3 Recreated Instance A onCreate ticker_count: 3
135. User Launches App Activity Record Store 3 Recreated Instance A
136. Full Circle
137. Structuring your Activity What to put in each callback?
138. onCreate • call setContentView() • “hook up” UI widgets • check for saved state bundle
139. onPause • save vital data • typically stop animations or other things consuming CPU • should be quick
140. onStop • persist vital data • last guaranteed teardown method
141. onDestroy • last place to free resources • not guaranteed to be called
142. But Also… • It depends!
143. Reasoning About the Lifecycle What to put in each callback?
144. Displaying Location • Show user their location • Minimize battery usage • Maximize precision when user is actively looking at my map
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()
146. Video Playback
147. Video Playback • User starts video activity • User presses a transparent settings activity • video should keep playing!
148. Video Playback: Which Callbacks? • initialize player - onCreate • start playback - onStart • stop playback - onStop • release player resources - onDestroy
149. Capstone Challenge What state are these activities in?
150. Challenge: Multi-Window What state is each in?
151. Next Steps
152. @kristinmars github.com/kristinmars bignerdranch.com