IllegalStateException: Can not perform this action after onSaveInstanceState

There is one thing we all agree about Fragments -- it is easy to fall into problems with them. They were created for a noble reason, but using them involves effort because they can behave in unexpected ways.

When our team got a chance to refactor a legacy codebase, we decided to drop the traditional Android navigation pattern, in favour of a ViewGroup-driven UI stack with a single Activity. This talk is an exploration of our journey so far.

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1. IllegalStateException Saket Narayan uncommon.is Can not perform this action after onSaveInstanceState
2. @saketme Fragments ❤ T-6
3. Introduced in Android 3.0 “An important goal for Android 3.0 is to make it easier for developers to write applications that can scale across a variety of screen sizes, beyond the facilities already available in the platform” - 2011
4. Credits: Engadget
5. developer.android.com
6. Fragments > ActivityGroup
7. @saketme Fragments T-5
8. java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState at android.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1109) at android.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:399) at android.app.Activity.onBackPressed(Activity.java:2066) at android.app.Activity.onKeyDown(Activity.java:1962) at android.view.KeyEvent.dispatch(KeyEvent.java:2482) at android.app.Activity.dispatchKeyEvent(Activity.java:2274) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1668) at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1112) at android.app.Activity.dispatchKeyEvent(Activity.java:2269) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1668) at android.view.ViewRoot.deliverKeyEventPostIme(ViewRoot.java:2851) at android.view.ViewRoot.handleFinishedEvent(ViewRoot.java:2824) at android.view.ViewRoot.handleMessage(ViewRoot.java:2011) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:132) at android.app.ActivityThread.main(ActivityThread.java:4025) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:491) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) at dalvik.system.NativeStart.main(Native Method)
11. It’s too easy to fail Race conditions are difficult to avoid with asynchronous lifecycle
12. It’s too easy to fail FragmentTransaction#commit() { Handler#post(...) }
13. Too many gotchas
14. Too many gotchas @Override void onActivityResult(int requestCode, int resultCode, Intent data) { if (resultCode == RESULT_OK) { SuccessDialog.show(); } }
15. java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState at android.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1109) at android.app.FragmentManagerImpl.popBackStackImmediate(FragmentManager.java:399) at android.app.Activity.onBackPressed(Activity.java:2066) at android.app.Activity.onKeyDown(Activity.java:1962) at android.view.KeyEvent.dispatch(KeyEvent.java:2482) at android.app.Activity.dispatchKeyEvent(Activity.java:2274) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1668) at android.view.ViewGroup.dispatchKeyEvent(ViewGroup.java:1112) at android.app.Activity.dispatchKeyEvent(Activity.java:2269) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:1668) at android.view.ViewRoot.deliverKeyEventPostIme(ViewRoot.java:2851) at android.view.ViewRoot.handleFinishedEvent(ViewRoot.java:2824) at android.view.ViewRoot.handleMessage(ViewRoot.java:2011) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:132) at android.app.ActivityThread.main(ActivityThread.java:4025) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:491) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599) at dalvik.system.NativeStart.main(Native Method)
16. Too many gotchas @Override void onActivityResult(int requestCode, int resultCode, Intent data) { if (resultCode == RESULT_OK) { pendingResumeRunnable = () -> { SuccessDialog.show(); }; } } @Override void onResume() { pendingResumeRunnable.run(); }
17. Too many gotchas @Override void onActivityResult(int requestCode, int resultCode, Intent data) { if (resultCode == RESULT_OK) { lifecycle().onResume() .take(1) .subscribe(o -> { SuccessDialog.show(); }); } }
18. Too many gotchas Fragments get recreated before Activity#onCreate() returns
19. Too many gotchas class Activity { @BindView(R.id.text) TextView textView; @Override void onCreate(Bundle s) { super.onCreate(s); setContentView(...); ButterKnife.bind(this); showFragment(...); } void setText(String text) { textView.setText(text); } } class Fragment { @Override void onAttach(Activity a) { a.setText("Blrdroid"); } }
20. Too many gotchas class Activity { @BindView(R.id.text) TextView textView; @Override void onCreate(Bundle s) { super.onCreate(s); setContentView(...); ButterKnife.bind(this); showFragment(...); } void setText(String text) { textView.setText(text); } } class Fragment { @Override void onAttach(Activity a) { a.setText(“Blrdroid"); } }
21. Too many gotchas class Activity { @BindView(R.id.text) TextView textView; @Override void onCreate(Bundle s) { super.onCreate(s); setContentView(...); ButterKnife.bind(this); showFragment(...); } void setText(String text) { textView.setText(text); } } class Fragment { @Override void onAttach(Activity a) { a.setText("Blrdroid"); } }
22. Too many gotchas class Activity { @BindView(R.id.text) TextView textView; @Override void onCreate(Bundle s) { super.onCreate(s); setContentView(...); ButterKnife.bind(this); showFragment(...); } void setText(String text) { textView.setText(text); } } class Fragment { @Override void onAttach(Activity a) { a.setText(“Blrdroid"); } }
23. Too many gotchas class Activity { @BindView(R.id.text) TextView textView; @Override void onCreate(Bundle s) { super.onCreate(s); setContentView(...); ButterKnife.bind(this); showFragment(...); } void setText(String text) { textView.setText(text); } } class Fragment { @Override void onAttach(Activity a) { a.setText(“Blrdroid"); } } NullPointerException!
24. Too many gotchas Fragments are not resumed when Activity#onResume() is called. Use Activity#onResumeFragments() instead.
25. Too many gotchas getFragmentManager() .beginTransaction() .replace(R.id.fragment_container, SomeFragment.newInstance()) .setCustomAnimations(R.anim.enter_animation, R.anim.exit_animation) .commit();
26. Too many gotchas getFragmentManager() .beginTransaction() .replace(R.id.fragment_container, SomeFragment.newInstance()) .setCustomAnimations(R.anim.enter_animation, R.anim.exit_animation) .commit();
27. Too many gotchas getFragmentManager() .beginTransaction() .setCustomAnimations(R.anim.enter_animation, R.anim.exit_animation) .replace(R.id.fragment_container, SomeFragment.newInstance()) .commit();
28. It’s possible to solve these problems
29. But it takes experience
30. Fragments are like Bhagwan An extra layer of uncertainty in life isn’t worth it.
31. Fragments are like Bhagwan An extra layer of uncertainty in life isn’t worth it. - Saket Narayan
32. Further reading bit.ly/square-fragments
33. @saketme Views >> Fragments T-4
34. @saketme Views >> Fragments T-4
35. Basic necessities • Lifecycle • Backstack • State persistence • Start a screen for result • Screen change animations • Nested screens • Dialog lifecycle and callbacks
36. Options • Conductor (Blueline) • Flow (Square) • Scoop (Lyft) • Magellan (Wealthfront) • RIBs (Uber)
37. Why not Conductor Designed as a drop-in replacement for fragments. public class HomeFragment extends Fragment { @Override View onCreateView(LayoutInflater inflater, ViewGroup container) { View view = inflater.inflate(R.layout.home, container,...); return view; } }
38. Why not Conductor Designed as a drop-in replacement for fragments. public class HomeController extends Controller { @Override View onCreateView(LayoutInflater inflater, ViewGroup container) { View view = inflater.inflate(R.layout.home, container,...); return view; } }
39. Why not Conductor Designed as a drop-in replacement for fragments. void startActivityForResult(...) void requestPermissions(...) void setHasOptionsMenu(...) void onCreateOptionsMenu(...)
40. Why not Conductor It is not simple. It feels as exhaustive as Fragments.
41. Why not Conductor It is not simple. It feels as exhaustive as Fragments. void onContextAvailable() void onContextUnavailable() void onSaveViewState(...) void onSaveInstanceState(...)
42. Why Flow Is a library for navigating between scopes (and not screens)
43. Why Flow Is a library for navigating between scopes (and not screens) Billing Dashboard User preferences
44. Why Flow Scopes can be nested Logged in Billing Dashboard User preferences Search
45. Why Flow Scopes may or may not have a visual existence
46. Why Flow Favors composition over inheritance
47. Why Flow Pro: Extremely simple. Flow is just a back-stack for scopes. Con: No batteries included. Flow does not even inflate Views.
48. Why Flow Designed by Square™
49. Options • Conductor (Blueline) • Flow (Square) • Scoop (Lyft) • Magellan (Wealthfront) • RIBs (Uber)
50. @saketme Our Flow setup T-3
51. Setup Fragments -> Views (called “Screens”) FragmentManager -> ScreenRouter Bundles -> ScreenKey
52. Setup interface ScreenKey extends Parcelable { @LayoutRes int layoutResId(); }
53. Setup public DashboardScreenKey implements ScreenKey { @LayoutRes int layoutResId() { return R.layout.dashboard; } }
54. Setup public PreferencesScreenKey implements ScreenKey { @LayoutRes int layoutResId() { return R.layout.preferences; } }
55. Setup class ScreenRouter { void push(ScreenKey key) { flow.push(key); } void pop() { flow.goBack(); } ... }
56. Setup class TheActivity { @Inject ScreenRouter screenRouter; @Override void onAttachBaseContext(Context c) { super.onAttachBaseContext(screenRouter.install(c)); } @Override void onBackPressed() { BackPressCallback callback = screenRouter.pop(); if (!callback.popped()) { super.onBackPressed(); } } }
57. Screen routing class DashboardScreen extends FrameLayout { public static final ScreenKey KEY = DashboardScreen.Key.create(); public DashboardScreen(Context context, AttributeSet attrs) { super(context, attrs); } }
58. Screen routing class DashboardScreen extends FrameLayout { public static final ScreenKey KEY = DashboardScreen.Key.create(); public DashboardScreen(Context context, AttributeSet attrs) { super(context, attrs); } }
59. Screen routing class LogInScreen extends FrameLayout { @Inject ScreenRouter screenRouter; void openDashboard() { screenRouter.push(DashboardScreen.KEY); } }
60. Screen routing class LogInScreen extends FrameLayout { @Inject ScreenRouter screenRouter; void openDashboard() { screenRouter.push(DashboardScreen.KEY); } }
61. Screen routing class LogInScreen extends FrameLayout { @Inject ScreenRouter screenRouter; void openDashboard() { screenRouter.push(DashboardScreen.KEY_BUILDER .theme(Theme.DARK) .key1(“value1”) .key2("value2") .build()); } }
62. Screen routing class DashboardScreen extends FrameLayout { @Inject ScreenRouter screenRouter; void onAttachedToWindow() { super.onAttachedToWindow(); DashboardScreen.Key key = screenRouter.key(this); applyTheme(key.theme()); } }
63. Screen routing class DashboardScreen extends FrameLayout { @Inject ScreenRouter screenRouter; void onAttachedToWindow() { super.onAttachedToWindow(); DashboardScreen.Key key = screenRouter.key(this); applyTheme(key.theme()); } }
64. Screen routing <?xml version="1.0" encoding="utf-8"?> <is.uncommon.DashboardScreen> android:layout_width="match_parent" android:layout_height="match_parent"> <!-- Child Views will go here.--> </is.uncommon.DashboardScreen>
65. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
66. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
67. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
68. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
69. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
70. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
71. Inflating screens @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
72. Screen routing FYI: changeKey() gets called on every app resume. Make sure you only change Views when needed. https://github.com/square/flow/issues/173
73. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> screenRouter.push(GifPickerScreen.KEY); ); screenRouter.streamResults() .ofType(GifPickerResult.class) .takeUntil(lifecycle.onDetach()) .subscribe(result -> { gifImageView.setImage(result.gif()); }); } }
74. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> screenRouter.push(GifPickerScreen.KEY); ); screenRouter.streamResults() .ofType(GifPickerResult.class) .takeUntil(lifecycle.onDetach()) .subscribe(result -> { gifImageView.setImage(result.gif()); }); } }
75. Sending screen results class GifPickerScreen extends FrameLayout { void onGifSelected(Gif gif) { GifPickerResult result = GifPickerResult.create(gif); screenRouter.sendResultAndPop(result); } }
76. Sending screen results class GifPickerScreen extends FrameLayout { void onGifSelected(Gif gif) { GifPickerResult result = GifPickerResult.create(gif); screenRouter.sendResultAndPop(result); } }
77. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> screenRouter.push(GifPickerScreen.KEY); ); screenRouter.streamResults() .ofType(GifPickerResult.class) .takeUntil(lifecycle.onDetach()) .subscribe(result -> { gifImageView.setImage(result.gif()); }); } }
78. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> screenRouter.push(GifPickerScreen.KEY); ); screenRouter.streamResults() .ofType(GifPickerResult.class) .takeUntil(lifecycle.onDetach()) .subscribe(result -> { gifImageView.setImage(result.gif()); }); } } Observable<Result>
79. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> startActivityForResult(GifPickerActivity.intent(), REQ_CODE_GIF); ); screenRouter.streamResults() .ofType(ActivityResult.class) .filter(result -> result.requestCode() == REQ_CODE_GIF) .flatMap(result -> GifPickerActivity.extractGif(result.intentData()) .takeUntil(lifecycle.onDetach()) .subscribe(gif -> { gifImageView.setImage(gif); }); } }
80. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> startActivityForResult(GifPickerActivity.intent(), REQ_CODE_GIF); ); screenRouter.streamResults() .ofType(ActivityResult.class) .filter(result -> result.requestCode() == REQ_CODE_GIF) .flatMap(result -> GifPickerActivity.extractGif(result.intentData()) .takeUntil(lifecycle.onDetach()) .subscribe(gif -> { gifImageView.setImage(gif); }); } }
81. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> startActivityForResult(GifPickerActivity.intent(), REQ_CODE_GIF); ); screenRouter.streamResults() .ofType(ActivityResult.class) .filter(result -> result.requestCode() == REQ_CODE_GIF) .flatMap(result -> GifPickerActivity.extractGif(result.intentData()) .takeUntil(lifecycle.onDetach()) .subscribe(gif -> { gifImageView.setImage(gif); }); } }
82. Sending screen results class TheActivity extends Activity { @Override void onActivityResult(int requestCode, int resultCode, Intent data) { screenRouter.sendResult(ActivityResult.create(requestCode, resultCode, data)); } }
83. Sending screen results class DashboardScreen extends FrameLayout { void onAttachedToWindow() { gifButton.setOnClickListener(o -> startActivityForResult(GifPickerActivity.intent(), REQ_CODE_GIF); ); screenRouter.streamResults() .ofType(ActivityResult.class) .filter(result -> result.requestCode() == REQ_CODE_GIF) .flatMap(result -> GifPickerActivity.extractGif(result.intentData()) .takeUntil(lifecycle.onDetach()) .subscribe(gif -> { gifImageView.setImage(gif); }); } }
84. Other stuff • Back-press interceptor • Dialogs • Nested screens
85. @saketme Obvious wins with Views T-2
86. Life is synchronous again DashboardScreen dashboardScreen = inflate(...); dashboardScreen.title.setText(...); class DashboardScreen { TextView title; @Override void onFinishInflate() { title = findViewById(...); } }
87. Life is synchronous again DashboardScreen dashboardScreen = inflate(...); dashboardScreen.title.setText(...); class DashboardScreen { TextView title; @Override void onFinishInflate() { title = findViewById(...); } }
88. Life is synchronous again DashboardScreen dashboardScreen = inflate(...); dashboardScreen.title.setText(...); class DashboardScreen { TextView title; @Override void onFinishInflate() { title = findViewById(...); } }
89. Life is synchronous again DashboardScreen dashboardScreen = inflate(...); dashboardScreen.title.setText(...); class DashboardScreen { TextView title; @Override void onFinishInflate() { title = findViewById(...); } }
90. Consistent Lifecycle class Activity { @Override void onCreate(Bundle b) { super.onCreate(); setContentView(...); } @Override void onRestoreInstanceState(Bundle b) { // Views get restored here. super.onRestoreInstanceState(b); } }
91. Intercepting View inflation LayoutInflater#inflate(R.layout..., parentView); -> LayoutInflater#createViewFromTag() -> LayoutInflater.Factory2#onCreateView()
92. Intercepting View inflation class DaggerLayoutFactory implements LayoutInflater.Factory2 { private DaggerComponent component; public View onCreateView(View parent, String name, ...) { View view = inflate(parent, name); if (view instanceOf ViewWithDaggerDependencies) { view.injectDependencies(component); } } }
93. Intercepting View inflation class DaggerLayoutFactory implements LayoutInflater.Factory2 { private DaggerComponent component; public View onCreateView(View parent, String name, ...) { View view = inflate(parent, name); if (view instanceOf ViewWithDaggerDependencies) { view.injectDependencies(component); } } }
94. Intercepting View inflation class DaggerLayoutFactory implements LayoutInflater.Factory2 { private DaggerComponent component; public View onCreateView(View parent, String name, ...) { View view = inflate(parent, name); if (view instanceOf ViewWithDaggerDependencies) { view.injectDependencies(component); } } }
95. @saketme Dealing with libraries that use Fragments T-1
96. Google Maps class MapsScreen extends FrameLayout { @Inject TheActivity activity; void onAttachedToWindow() { activity.getFragmentManager() .beginTransaction() .add(getId(), MapFragment.newInstance()) .commitNowAllowingStateLoss(); } void onDetachedFromWindow() { activity.getFragmentManager() .beginTransaction() .remove(mapFragment) .commitNowAllowingStateLoss(); } }
97. Google Maps class MapsScreen extends FrameLayout { @Inject TheActivity activity; void onAttachedToWindow() { activity.getFragmentManager() .beginTransaction() .add(getId(), MapFragment.newInstance()) .commitNowAllowingStateLoss(); } void onDetachedFromWindow() { activity.getFragmentManager() .beginTransaction() .remove(mapFragment) .commitNowAllowingStateLoss(); } } #YOLO
98. @saketme Designing a fallback T-0
99. If Flow doesn’t work out We swap Flow with Conductor/Scoop/Magellan in ScreenRouter
100. If Views don’t work out We can use Fragments along with Flow
101. If Views don’t work out @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
102. If Views don’t work out @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { if (frame.getChildCount() > 0) { View outgoingView = frame.getChildAt(0); if (outgoing != null) { outgoing.save(outgoingView); } frame.removeAllViews(); } View incomingView = inflate(layoutResIdForKey(incoming), ...); frame.addView(inView); incoming.restore(inView); }
103. If Views don’t work out @Override void changeKey(State outgoing, State incoming, Direction direction, ...) { Fragment incomingFrag = ScreenFragment.create(incoming.key()); fragmentManager .beginTransaction() .replace(frame.getId(), incomingFrag) .commitNow(); }
104. App stays unaffected • Views are dumb • Refactorability is favoured
105. Summary • Use Conductor for an existing app • Flow is recommended for a new app
106. @saketme onComplete() uncommon.is