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Android Puzzlers: Traps, Pitfalls and Corner Cases

Android Puzzlers: Traps, Pitfalls and Corner Cases

Android is a marvellous platform that has been evolving for more than 10 years. Having such a large framework, so many methods and classes while maintaining legacy generally ends up with a price to pay: some parts of the SDK comes with their questions and puzzlers.

In this talk, we will present some of the most head-scratching API namings/usages/results and give an explanation for each of them.

Cyril Mottier

April 24, 2019
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Transcript

  1. Small applications with curious behavior Android puzzlers 1. What does

    it print? 2. The solution revealed 3. The moral
  2. A simple setup apply plugin: 'kotlin-android' apply plugin: 'kotlin-android-extensions' android

    { compileSdkVersion 28 defaultConfig { minSdkVersion 28 targetSdkVersion 28 versionCode 1 versionName "1.0" } buildTypes { release { minifyEnabled false proguardFiles getDefaultProguardFile('proguard-android-optimize.txt') } } } dependencies { implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:1.3.21" } common.gradle
  3. <?xml version="1.0" encoding="utf-8"?> <manifest xmlns:android="http://schemas.android.com/apk/res/android" xmlns:tools="http://schemas.android.com/tools" package="com.cyrilmottier.android.puzzlers" tools:ignore="AllowBackup,GoogleAppIndexingWarning"> <application android:allowBackup=“false"

    android:label=“@string/app_name“ android:supportsRtl="true" android:theme="@style/android:Theme.Material.Light.DarkActionBar"> <activity android:name=".MainActivity"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application> </manifest> A simple setup AndroidManifest.xml
  4. A simple setup minSdkVersion 28 targetSdkVersion 28 compileSdkVersion 28 Running

    on Android 9 (Pie) / API 28 Warnings/errors/lint turned off Focusing on vanilla-Android
  5. <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt

    layout/view_inflate.xml class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } }
  6. <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt

    layout/view_inflate.xml class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } }
  7. A child and child class MainActivity : Activity() { override

    fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt layout/view_inflate.xml
  8. B parent and child A child and child class MainActivity

    : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt layout/view_inflate.xml
  9. B parent and child C throws NullPointerException A child and

    child class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt layout/view_inflate.xml
  10. B parent and child C throws NullPointerException D Answer D

    A child and child class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt layout/view_inflate.xml
  11. B parent and child C throws NullPointerException D Answer D

    A child and child class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val parent = FrameLayout(this).apply { tag = "parent" } val result1 = layoutInflater.inflate(R.layout.view_inflate, parent) val result2 = layoutInflater.inflate(R.layout.view_inflate, null) trace("${result1.tag} and ${result2.tag}") } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent" android:layout_height="match_parent" android:tag="child" /> MainActivity.kt layout/view_inflate.xml
  12. attaches child to parent & returns parent null null parent

    parent returns child root returns child attaches child to parent & returns parent Output
  13. Favor the 3-args inflate method instead fun LayoutInflater.inflate( resource: Int,

    root: ViewGroup, attachToRoot: Boolean ): View Be explicit
  14. null null parent parent false true false true returns child

    root attachToRoot returns child returns child attaches child to parent & returns parent Output
  15. null null parent parent false true false true returns child

    root attachToRoot returns child returns child attaches child to parent & returns parent Output
  16. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) trace( "start=${text.paddingStart} top=${text.paddingTop} " + "end=${text.paddingEnd} bottom=${text.paddingBottom}" ) } }
  17. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) trace( "start=${text.paddingStart} top=${text.paddingTop} " + "end=${text.paddingEnd} bottom=${text.paddingBottom}" ) } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/padded_rect" android:padding="5px" android:paddingBottom="6px" android:paddingEnd="7px" android:paddingHorizontal="8px" android:paddingLeft="9px" android:paddingRight="10px" android:paddingStart="11px"/> layout/activity_main.xml
  18. MainActivity.kt <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/padded_rect"

    android:padding="5px" android:paddingBottom="6px" android:paddingEnd="7px" android:paddingHorizontal="8px" android:paddingLeft="9px" android:paddingRight="10px" android:paddingStart="11px"/> <?xml version="1.0" encoding="utf-8"?> <shape xmlns:android="http://schemas.android.com/apk/res/android"> <padding android:bottom="1px" android:left="2px" android:right="3px" android:top="4px" /> </shape> layout/activity_main.xml drawable/padded_rect.xml class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) trace( "start=${text.paddingStart} top=${text.paddingTop} " + "end=${text.paddingEnd} bottom=${text.paddingBottom}" ) } }
  19. class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) trace( "start=${text.paddingStart} top=${text.paddingTop} " + "end=${text.paddingEnd} bottom=${text.paddingBottom}" ) } } B start=9 top=5 end=10 bottom=6 C start=2 top=4 end=3 bottom=1 D start=11 top=5 end=7 bottom=5 A start=5 top=5 end=5 bottom=5 MainActivity.kt <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/padded_rect" android:padding="5px" android:paddingBottom="6px" android:paddingEnd="7px" android:paddingHorizontal="8px" android:paddingLeft="9px" android:paddingRight="10px" android:paddingStart="11px"/> layout/activity_main.xml drawable/padded_rect.xml <?xml version="1.0" encoding="utf-8"?> <shape xmlns:android="http://schemas.android.com/apk/res/android"> <padding android:bottom="1px" android:left="2px" android:right="3px" android:top="4px" /> </shape>
  20. <?xml version="1.0" encoding="utf-8"?> <shape xmlns:android="http://schemas.android.com/apk/res/android"> <padding android:bottom="1px" android:left="2px" android:right="3px" android:top="4px"

    /> </shape> B start=9 top=5 end=10 bottom=6 C start=2 top=4 end=3 bottom=1 D start=11 top=5 end=7 bottom=5 A start=5 top=5 end=5 bottom=5 MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) trace( "start=${text.paddingStart} top=${text.paddingTop} " + "end=${text.paddingEnd} bottom=${text.paddingBottom}" ) } } <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text" android:layout_width="match_parent" android:layout_height="match_parent" android:background="@drawable/padded_rect" android:padding="5px" android:paddingBottom="6px" android:paddingEnd="7px" android:paddingHorizontal="8px" android:paddingLeft="9px" android:paddingRight="10px" android:paddingStart="11px"/> layout/activity_main.xml drawable/padded_rect.xml
  21. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) trace("onCreate") finish() trace("finish") } override fun onStart() { super.onStart() trace("onStart") } override fun onResume() { super.onResume() trace("onResume") } override fun onPause() { trace("onPause") super.onPause() } override fun onStop() { trace("onStop") super.onStop() } override fun onDestroy() { trace("onDestroy") super.onDestroy() } }
  22. B onCreate, finish, onStart, onResume, onPause, onStop, onDestroy C onCreate,

    onStart, onResume, onPause, onStop, onDestroy D onCreate, finish, onDestroy A onCreate MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) trace("onCreate") finish() trace("finish") } override fun onStart() { super.onStart() trace("onStart") } override fun onResume() { super.onResume() trace("onResume") } override fun onPause() { trace("onPause") super.onPause() } override fun onStop() { trace("onStop") super.onStop() } override fun onDestroy() { trace("onDestroy") super.onDestroy() } }
  23. B onCreate, finish, onStart, onResume, onPause, onStop, onDestroy C onCreate,

    onStart, onResume, onPause, onStop, onDestroy D onCreate, finish, onDestroy A onCreate MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) trace("onCreate") finish() trace("finish") } override fun onStart() { super.onStart() trace("onStart") } override fun onResume() { super.onResume() trace("onResume") } override fun onPause() { trace("onPause") super.onPause() } override fun onStop() { trace("onStop") super.onStop() } override fun onDestroy() { trace("onDestroy") super.onDestroy() } }
  24. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  25. MainActivity.kt drawable/level_list.xml <?xml version="1.0" encoding="utf-8"?> <level-list /> class MainActivity :

    Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  26. MainActivity.kt drawable/level_list.xml <?xml version="1.0" encoding="utf-8"?> <level-list /> drawable/layer_list.xml <?xml version="1.0"

    encoding="utf-8"?> <layer-list /> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  27. MainActivity.kt drawable/level_list.xml <?xml version="1.0" encoding="utf-8"?> <level-list /> drawable/layer_list.xml <?xml version="1.0"

    encoding="utf-8"?> <layer-list /> drawable/shape.xml <?xml version="1.0" encoding="utf-8"?> <shape /> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  28. B LevelDrawable, LayerListDrawable, ShapeDrawable C LevelListDrawable, LayerDrawable, GradientDrawable D throws

    InflateException A LevelListDrawable, LayerListDrawable, ShapeDrawable MainActivity.kt drawable/level_list.xml <?xml version="1.0" encoding="utf-8"?> <level-list /> drawable/layer_list.xml <?xml version="1.0" encoding="utf-8"?> <layer-list /> drawable/shape.xml <?xml version="1.0" encoding="utf-8"?> <shape /> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  29. B LevelDrawable, LayerListDrawable, ShapeDrawable C LevelListDrawable, LayerDrawable, GradientDrawable D throws

    InflateException A LevelListDrawable, LayerListDrawable, ShapeDrawable MainActivity.kt drawable/level_list.xml <?xml version="1.0" encoding="utf-8"?> <level-list /> drawable/layer_list.xml <?xml version="1.0" encoding="utf-8"?> <layer-list /> drawable/shape.xml <?xml version="1.0" encoding="utf-8"?> <shape /> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val classNames = arrayOf( R.drawable.level_list, R.drawable.layer_list, R.drawable.shape ).map { getDrawable(it)::class.java.simpleName } trace(classNames.joinToString(", ")) } }
  30. The general rule <level-list /> <transition /> <ripple /> <adaptive-icon

    /> <color /> <vector /> <animated-vector /> <scale /> <clip /> <rotate /> <inset /> <bitmap /> <nine-patch /> LevelListDrawable TransitionDrawable RippleDrawable AdaptiveIconDrawable ColorDrawable VectorDrawable AnimatedVectorDrawable ScaleDrawable ClipDrawable RotateDrawable InsetDrawable BitmapDrawable NinePatchDrawable <animated-image /> AnimatedImageDrawable xceptions
  31. Exceptions <selector /> <animated-selector /> <layer-list /> <animation-list /> <shape

    /> StateListDrawable AnimatedStateListDrawable LayerDrawable AnimationDrawable GradientDrawable
  32. Exceptions <selector /> <animated-selector /> <layer-list /> <animation-list /> <shape

    /> StateListDrawable AnimatedStateListDrawable LayerDrawable AnimationDrawable GradientDrawable ShapeDrawable PaintDrawable PictureDrawable
  33. Exceptions <selector /> <animated-selector /> <layer-list /> <animation-list /> <shape

    /> <animated-rotate /> StateListDrawable AnimatedStateListDrawable LayerDrawable AnimationDrawable GradientDrawable ShapeDrawable PaintDrawable PictureDrawable
  34. Exceptions <selector /> <animated-selector /> <layer-list /> <animation-list /> <shape

    /> <animated-rotate /> <com.cyrilmottier.YourDrawable /> StateListDrawable AnimatedStateListDrawable LayerDrawable AnimationDrawable GradientDrawable ShapeDrawable PaintDrawable PictureDrawable YourDrawable
  35. TracingFragment.kt class TracingFragment : Fragment() { override fun onAttach(context: Context?)

    { super.onAttach(context) trace("onAttach") } override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) trace("onCreate") } override fun onCreateView( inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle? ): View? { trace("onCreateView") return super.onCreateView(inflater, container, savedInstanceState) } override fun onDestroyView() { trace("onDestroyView") super.onDestroyView() } override fun onDestroy() { trace("onDestroy") super.onDestroy() } override fun onDetach() { trace("onDetach") super.onDetach() } }
  36. MainActivity.kt class MainActivity : FragmentActivity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) val fragment = TracingFragment() supportFragmentManager.beginTransaction() .add(android.R.id.content, fragment, "fragment:puzzler") .commit() supportFragmentManager.beginTransaction() .detach(fragment) .commit() } override fun onSaveInstanceState(outState: Bundle?) { // For the simplicity of this puzzler, // let’s consider state is never ever saved } }
  37. B onAttach, onCreate, onCreateView, onDestroyView C onCreate, onDestroy D Nothing

    A onAttach, onCreate, onCreateView, onDestroyView, onDestroy, onDetach MainActivity.kt class MainActivity : FragmentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val fragment = TracingFragment() supportFragmentManager.beginTransaction() .add(android.R.id.content, fragment, "fragment:puzzler") .commit() supportFragmentManager.beginTransaction() .detach(fragment) .commit() } override fun onSaveInstanceState(outState: Bundle?) { // For the simplicity of this puzzler, // let’s consider state is never ever saved } }
  38. B onAttach, onCreate, onCreateView, onDestroyView C onCreate, onDestroy D Nothing

    A onAttach, onCreate, onCreateView, onDestroyView, onDestroy, onDetach MainActivity.kt class MainActivity : FragmentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val fragment = TracingFragment() supportFragmentManager.beginTransaction() .add(android.R.id.content, fragment, "fragment:puzzler") .commit() supportFragmentManager.beginTransaction() .detach(fragment) .commit() } override fun onSaveInstanceState(outState: Bundle?) { // For the simplicity of this puzzler, // let’s consider state is never ever saved } }
  39. add remove/replace attach/detach the Fragment and more… creates/destroys the Fragment’s

    associated View turns Fragment’s View visibility to GONE/VISIBLE Fragment actions attach detach show hide FragmentTransaction commands
  40. MainActivity.kt private const val CHANNEL_ID = "channel" class MainActivity :

    Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val manager = getSystemService(NotificationManager::class.java) val aChannel = NotificationChannel( CHANNEL_ID, // id "A name", // name NotificationManager.IMPORTANCE_DEFAULT // importance (3) ) aChannel.lightColor = Color.RED manager.createNotificationChannel(aChannel) manager.deleteNotificationChannel(CHANNEL_ID) val anotherChannel = NotificationChannel( CHANNEL_ID, // id "Another name", // name NotificationManager.IMPORTANCE_HIGH // importance (4) ) anotherChannel.lightColor = Color.GREEN manager.createNotificationChannel(anotherChannel) with(manager.getNotificationChannel(CHANNEL_ID)) { trace("$name, $importance, 0x${Integer.toHexString(lightColor)}") } } }
  41. MainActivity.kt private const val CHANNEL_ID = "channel" class MainActivity :

    Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val manager = getSystemService(NotificationManager::class.java) val aChannel = NotificationChannel( CHANNEL_ID, // id "A name", // name NotificationManager.IMPORTANCE_DEFAULT // importance (3) ) aChannel.lightColor = Color.RED manager.createNotificationChannel(aChannel) manager.deleteNotificationChannel(CHANNEL_ID) val anotherChannel = NotificationChannel( CHANNEL_ID, // id "Another name", // name NotificationManager.IMPORTANCE_HIGH // importance (4) ) anotherChannel.lightColor = Color.GREEN manager.createNotificationChannel(anotherChannel) with(manager.getNotificationChannel(CHANNEL_ID)) { trace("$name, $importance, 0x${Integer.toHexString(lightColor)}") } } } B Another name, 4, 0xff00ff00 C Another name, 3, 0xff00ff00 D Another name, 3, 0xffff0000 A A name, 3, 0xffff0000
  42. MainActivity.kt B Another name, 4, 0xff00ff00 C Another name, 3,

    0xff00ff00 D Another name, 3, 0xffff0000 A A name, 3, 0xffff0000 private const val CHANNEL_ID = "channel" class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val manager = getSystemService(NotificationManager::class.java) val aChannel = NotificationChannel( CHANNEL_ID, // id "A name", // name NotificationManager.IMPORTANCE_DEFAULT // importance (3) ) aChannel.lightColor = Color.RED manager.createNotificationChannel(aChannel) manager.deleteNotificationChannel(CHANNEL_ID) val anotherChannel = NotificationChannel( CHANNEL_ID, // id "Another name", // name NotificationManager.IMPORTANCE_HIGH // importance (4) ) anotherChannel.lightColor = Color.GREEN manager.createNotificationChannel(anotherChannel) with(manager.getNotificationChannel(CHANNEL_ID)) { trace("$name, $importance, 0x${Integer.toHexString(lightColor)}") } } }
  43. It rather “hides”: the only possible deletion is app uninstall

    Doesn’t really delete fun NotificationManager .deleteNotificationChannel( channelId: String )
  44. Name/description Importance Always modifiable Mainly for i18n purposes & user-defined

    names/descriptions. Modifiable under certain conditions Only if new importance is lower than the current one & none of the channel’s settings have been altered by the user.
  45. Name/description Importance Group Always modifiable Mainly for i18n purposes &

    user-defined names/descriptions. Modifiable under certain conditions Modifiable under certain conditions Only if the channel has no group yet. Only if new importance is lower than the current one & none of the channel’s settings have been altered by the user.
  46. Name/description Importance Group Other fields Always modifiable Mainly for i18n

    purposes & user-defined names/descriptions. Modifiable under certain conditions Only if new importance is lower than the current one & none of the channel’s settings have been altered by the user. Modifiable under certain conditions Only if the channel has no group yet. Never Changing seamlessly user-alterable settings would result in a bad UX.
  47. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) val prefs = getSharedPreferences("test", Context.MODE_PRIVATE) prefs.edit() .putString("key", "hell") .apply() prefs.edit() .putString("key", null) .apply() val value = prefs.getString("key", "heaven") trace("Go to $value!") } }
  48. B Go to hell! C Go to null! D Either

    A, B or C A Go to heaven! MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val prefs = getSharedPreferences("test", Context.MODE_PRIVATE) prefs.edit() .putString("key", "hell") .apply() prefs.edit() .putString("key", null) .apply() val value = prefs.getString("key", "heaven") trace("Go to $value!") } }
  49. B Go to hell! C Go to null! D Either

    A, B or C A Go to heaven! MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val prefs = getSharedPreferences("test", Context.MODE_PRIVATE) prefs.edit() .putString("key", "hell") .apply() prefs.edit() .putString("key", null) .apply() val value = prefs.getString("key", "heaven") trace("Go to $value!") } }
  50. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) linear_layout.addOnLayoutChangeListener { v, _, _, _, _, _, _, _, _ -> trace( "LL: ${linear_layout.width}px, " + "TV1: ${text_view_1.width}px, " + "TV2: ${text_view_2.width}px" ) } } }
  51. layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/linear_layout" android:layout_width="400px" android:layout_height="match_parent"> <TextView

    android:id="@+id/text_view_1" android:layout_width="300px" android:layout_height="match_parent" android:layout_weight="1" /> <TextView android:id="@+id/text_view_2" android:layout_width="200px" android:layout_height="match_parent" android:layout_weight="2" /> </LinearLayout> MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) linear_layout.addOnLayoutChangeListener { v, _, _, _, _, _, _, _, _ -> trace( "LL: ${linear_layout.width}px, " + "TV1: ${text_view_1.width}px, " + "TV2: ${text_view_2.width}px" ) } } }
  52. class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) linear_layout.addOnLayoutChangeListener { v, _, _, _, _, _, _, _, _ -> trace( "LL: ${linear_layout.width}px, " + "TV1: ${text_view_1.width}px, " + "TV2: ${text_view_2.width}px" ) } } } B LL: 500px, TV1: 300px, TV2: 200px C LL: 400px, TV1: 233px, TV2: 167px D LL: 400px, TV1: 300px, TV2: 200px A LL: 400px, TV1: 267px, TV2: 133px layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/linear_layout" android:layout_width="400px" android:layout_height="match_parent"> <TextView android:id="@+id/text_view_1" android:layout_width="300px" android:layout_height="match_parent" android:layout_weight="1" /> <TextView android:id="@+id/text_view_2" android:layout_width="200px" android:layout_height="match_parent" android:layout_weight="2" /> </LinearLayout> MainActivity.kt
  53. class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) {

    super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) linear_layout.addOnLayoutChangeListener { v, _, _, _, _, _, _, _, _ -> trace( "LL: ${linear_layout.width}px, " + "TV1: ${text_view_1.width}px, " + "TV2: ${text_view_2.width}px" ) } } } B LL: 500px, TV1: 300px, TV2: 200px C LL: 400px, TV1: 233px, TV2: 167px D LL: 400px, TV1: 300px, TV2: 200px A LL: 400px, TV1: 267px, TV2: 133px layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/linear_layout" android:layout_width="400px" android:layout_height="match_parent"> <TextView android:id="@+id/text_view_1" android:layout_width="300px" android:layout_height="match_parent" android:layout_weight="1" /> <TextView android:id="@+id/text_view_2" android:layout_width="200px" android:layout_height="match_parent" android:layout_weight="2" /> </LinearLayout> MainActivity.kt
  54. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } }
  55. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } } layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text_view" android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" android:text="@string/hello_world" />
  56. MainActivity.kt class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?)

    { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } } layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text_view" android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" android:text="@string/hello_world" /> values/strings.xml <resources> <string name="hello_world">Bonjour monde !</string> </resources>
  57. MainActivity.kt layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text_view" android:layout_width="match_parent" android:layout_height="match_parent"

    android:gravity="center" android:text="@string/hello_world" /> values/strings.xml <resources> <string name="hello_world">Bonjour monde !</string> </resources> values-es/strings.xml <resources> <string name="hello_world">¡Hola mundo!</string> </resources> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } }
  58. B [en_US,fr_FR,es_ES], fr_FR, Bonjour monde ! C [en_US,fr_FR,es_ES], en_US, Bonjour

    monde ! D Resources.NotFoundException A [fr,es], fr, Bonjour monde ! MainActivity.kt layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text_view" android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" android:text="@string/hello_world" /> values/strings.xml <resources> <string name="hello_world">Bonjour monde !</string> </resources> values-es/strings.xml <resources> <string name="hello_world">¡Hola mundo!</string> </resources> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } }
  59. B [en_US,fr_FR,es_ES], fr_FR, Bonjour monde ! C [en_US,fr_FR,es_ES], en_US, Bonjour

    monde ! D Resources.NotFoundException A [fr,es], fr, Bonjour monde ! MainActivity.kt layout/activity_main.xml <?xml version="1.0" encoding="utf-8"?> <TextView xmlns:android="http://schemas.android.com/apk/res/android" android:id="@+id/text_view" android:layout_width="match_parent" android:layout_height="match_parent" android:gravity="center" android:text="@string/hello_world" /> values/strings.xml <resources> <string name="hello_world">Bonjour monde !</string> </resources> values-es/strings.xml <resources> <string name="hello_world">¡Hola mundo!</string> </resources> class MainActivity : Activity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) // User’s locale preferences are set to: en_US, fr_FR, es_ES trace("${LocaleList.getDefault()}, " + "${Locale.getDefault()}, ${text_view.text}" ) } }
  60. User settings de_DE fr_FR es_ES App resources default es Resolution

    path de de_DE Children of de fr_FR fr Children of fr es_ES es Children of es Default
  61. User settings de_DE fr_FR es_ES App resources default es Resolution

    path de de_DE Children of de fr_FR fr Children of fr es_ES es Children of es Default
  62. User settings de_DE fr_FR es_ES App resources default es Resolution

    path de de_DE Children of de fr_FR fr Children of fr es_ES es Children of es Default
  63. User settings de_DE fr_FR es_ES App resources default es Resolution

    path de de_DE Children of de fr_FR fr Children of fr es_ES es Children of es Default
  64. User settings de_DE fr_FR es_ES App resources default es Resolution

    path de de_DE Children of de fr_FR fr Children of fr es_ES es Children of es Default es
  65. User settings en_US fr_FR es_ES App resources default es Resolution

    path English is always considered as supported if in user settings
  66. User settings en_US fr_FR es_ES App resources default es Resolution

    path en en_US Children of en fr_FR fr Children of fr es_ES es Children of es Default English is always considered as supported if in user settings
  67. User settings en_US fr_FR es_ES App resources default es Resolution

    path en en_US Children of en fr_FR fr Children of fr es_ES es Children of es Default English is always considered as supported if in user settings
  68. User settings en_US fr_FR es_ES App resources default es Resolution

    path en en_US Children of en fr_FR fr Children of fr es_ES es Children of es Default English is always considered as supported if in user settings default