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.

Transcript

1. Android
   Puzzlers
   Traps, Pitfalls and Corner Cases
   @cyrilmottier
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2. It all began with…
   

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3. View Slide

4. Small applications with
   curious behavior
   Android puzzlers
   

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5. Small applications with
   curious behavior
   Android puzzlers
   1. What does it print?
   

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6. Small applications with
   curious behavior
   Android puzzlers
   1. What does it print?
   2. The solution revealed
   

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7. Small applications with
   curious behavior
   Android puzzlers
   1. What does it print?
   2. The solution revealed
   3. The moral
   

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8. 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
   

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9. 
   xmlns:tools="http://schemas.android.com/tools"
   package="com.cyrilmottier.android.puzzlers"
   tools:ignore="AllowBackup,GoogleAppIndexingWarning">
   android:allowBackup=“false"
   android:label=“@string/app_name“
   android:supportsRtl="true"
   android:theme="@style/android:Theme.Material.Light.DarkActionBar">
   
   
   
   
   
   
   
   
   A simple
   setup
   AndroidManifest.xml
   

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10. 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
    

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11. Inflation result
    

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12. 
    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}")
    }
    }
    

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13. 
    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}")
    }
    }
    

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14. 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}")
    }
    }
    
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:tag="child" />
    MainActivity.kt
    layout/view_inflate.xml
    

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15. 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}")
    }
    }
    
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:tag="child" />
    MainActivity.kt
    layout/view_inflate.xml
    

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16. 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}")
    }
    }
    
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:tag="child" />
    MainActivity.kt
    layout/view_inflate.xml
    

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17. 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}")
    }
    }
    
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:tag="child" />
    MainActivity.kt
    layout/view_inflate.xml
    

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18. 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}")
    }
    }
    
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:tag="child" />
    MainActivity.kt
    layout/view_inflate.xml
    

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19. attaches child to parent
    & returns parent
    null
    null
    parent
    parent
    returns child
    root
    returns child
    attaches child to parent
    & returns parent
    Output
    

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20. Favor the 3-args inflate
    method instead
    fun LayoutInflater.inflate(
    resource: Int,
    root: ViewGroup,
    attachToRoot: Boolean
    ): View
    Be explicit
    

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

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

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23. Padding precedence
    

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24. 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}"
    )
    }
    }
    

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25. 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}"
    )
    }
    }
    
    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
    

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26. MainActivity.kt
    
    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"/>
    
    
    android:right="3px" android:top="4px" />
    
    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}"
    )
    }
    }
    

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27. 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
    
    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
    
    
    android:right="3px" android:top="4px" />
    
    

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28. 
    
    android:right="3px" android:top="4px" />
    
    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}"
    )
    }
    }
    
    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
    

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29. background Drawable padding
    supportsRtl = false
    Lower
    Lower
    Higher
    

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30. background Drawable padding
    padding[Start|End]
    Lower
    Lower
    Higher
    supportsRtl = false
    

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31. background Drawable padding
    padding[Start|End]
    Lower
    Lower
    Higher
    padding[Left|Top|Right|Bottom]
    supportsRtl = false
    

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32. padding[Left|Top|Right|Bottom]
    padding[Horizontal|Vertical]
    background Drawable padding
    padding[Start|End]
    Lower
    Lower
    Higher
    supportsRtl = false
    

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33. padding[Left|Top|Right|Bottom]
    padding[Horizontal|Vertical]
    padding
    background Drawable padding
    padding[Start|End]
    Lower
    Lower
    Higher
    supportsRtl = false
    

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34. padding[Left|Top|Right|Bottom]
    padding[Horizontal|Vertical]
    padding
    background Drawable padding
    padding[Start|End] padding[Left|Top|Right|Bottom]
    padding[Horizontal|Vertical]
    padding
    padding[Start|End]
    background Drawable padding
    supportsRtl = true
    Lower
    Higher
    supportsRtl = false
    

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35. Activity lifecycle
    

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36. 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()
    }
    }
    

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37. 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()
    }
    }
    

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38. 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()
    }
    }
    

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40. “The following
    diagram shows
    the important
    state paths of
    an Activity.
    – d.android.com
    

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41. “The following
    diagram shows
    the important
    state paths of
    an Activity.
    – d.android.com
    

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42. Drawable tags
    

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43. 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(", "))
    }
    }
    

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44. MainActivity.kt
    drawable/level_list.xml
    
    
    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(", "))
    }
    }
    

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45. MainActivity.kt
    drawable/level_list.xml
    
    
    drawable/layer_list.xml
    
    
    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(", "))
    }
    }
    

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46. MainActivity.kt
    drawable/level_list.xml
    
    
    drawable/layer_list.xml
    
    
    drawable/shape.xml
    
    
    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(", "))
    }
    }
    

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47. B LevelDrawable,
    LayerListDrawable,
    ShapeDrawable
    C LevelListDrawable,
    LayerDrawable,
    GradientDrawable
    D throws InflateException
    A LevelListDrawable,
    LayerListDrawable,
    ShapeDrawable
    MainActivity.kt
    drawable/level_list.xml
    
    
    drawable/layer_list.xml
    
    
    drawable/shape.xml
    
    
    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(", "))
    }
    }
    

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48. B LevelDrawable,
    LayerListDrawable,
    ShapeDrawable
    C LevelListDrawable,
    LayerDrawable,
    GradientDrawable
    D throws InflateException
    A LevelListDrawable,
    LayerListDrawable,
    ShapeDrawable
    MainActivity.kt
    drawable/level_list.xml
    
    
    drawable/layer_list.xml
    
    
    drawable/shape.xml
    
    
    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(", "))
    }
    }
    

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49. The general rule
    ValarMorghulisDrawable
    

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50. The general rule
    

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51. The general rule
    
    
    
    
    
    
    
    
    
    
    
    
    
    LevelListDrawable
    TransitionDrawable
    RippleDrawable
    AdaptiveIconDrawable
    ColorDrawable
    VectorDrawable
    AnimatedVectorDrawable
    ScaleDrawable
    ClipDrawable
    RotateDrawable
    InsetDrawable
    BitmapDrawable
    NinePatchDrawable
    AnimatedImageDrawable
    xceptions
    

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52. Exceptions
    The general rule
    

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53. Exceptions
    
    
    
    
    
    StateListDrawable
    AnimatedStateListDrawable
    LayerDrawable
    AnimationDrawable
    GradientDrawable
    

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54. Exceptions
    
    
    
    
    
    StateListDrawable
    AnimatedStateListDrawable
    LayerDrawable
    AnimationDrawable
    GradientDrawable
    ShapeDrawable
    PaintDrawable
    PictureDrawable
    

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55. Exceptions
    
    
    
    
    
    
    StateListDrawable
    AnimatedStateListDrawable
    LayerDrawable
    AnimationDrawable
    GradientDrawable
    ShapeDrawable
    PaintDrawable
    PictureDrawable
    

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56. Exceptions
    
    
    
    
    
    
    
    StateListDrawable
    AnimatedStateListDrawable
    LayerDrawable
    AnimationDrawable
    GradientDrawable
    ShapeDrawable
    PaintDrawable
    PictureDrawable
    YourDrawable
    

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57. Fragment lifecycle
    

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58. 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()
    }
    }
    

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59. 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
    }
    }
    

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60. 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
    }
    }
    

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61. 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
    }
    }
    

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62. 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
    

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63. Notification channels
    

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64. 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)}")
    }
    }
    }
    

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65. 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
    

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66. 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)}")
    }
    }
    }
    

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67. fun NotificationManager
    .deleteNotificationChannel(
    channelId: String
    )
    

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68. It rather “hides”: the only
    possible deletion is app uninstall
    Doesn’t really delete
    fun NotificationManager
    .deleteNotificationChannel(
    channelId: String
    )
    

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69. Name/description Always modifiable
    Mainly for i18n purposes & user-defined names/descriptions.
    

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70. 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.
    

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71. 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.
    

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72. 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.
    

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73. Preferences read/write
    

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74. 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!")
    }
    }
    

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75. 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!")
    }
    }
    

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76. 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!")
    }
    }
    

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77. apply() is atomic
    async to disk
    

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78. async to disk
    apply() is
    atomic
    sync to memory
    

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79. sync to memory
    async to disk
    apply() is
    

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80. View Slide

81. “ defValue -
    String: Value to
    return if this
    preference does
    not exist.
    

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82. ¯\_(ツ)_/¯
    Well…
    not exactly
    

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83. Null values are
    always considered
    as “not existing” so
    expect “defValue”
    

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84. LinearLayout weight
    

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85. 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"
    )
    }
    }
    }
    

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86. layout/activity_main.xml
    
    android:id="@+id/linear_layout"
    android:layout_width="400px"
    android:layout_height="match_parent">
    android:id="@+id/text_view_1"
    android:layout_width="300px"
    android:layout_height="match_parent"
    android:layout_weight="1" />
    android:id="@+id/text_view_2"
    android:layout_width="200px"
    android:layout_height="match_parent"
    android:layout_weight="2" />
    
    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"
    )
    }
    }
    }
    

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87. 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
    
    android:id="@+id/linear_layout"
    android:layout_width="400px"
    android:layout_height="match_parent">
    android:id="@+id/text_view_1"
    android:layout_width="300px"
    android:layout_height="match_parent"
    android:layout_weight="1" />
    android:id="@+id/text_view_2"
    android:layout_width="200px"
    android:layout_height="match_parent"
    android:layout_weight="2" />
    
    MainActivity.kt
    

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88. 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
    
    android:id="@+id/linear_layout"
    android:layout_width="400px"
    android:layout_height="match_parent">
    android:id="@+id/text_view_1"
    android:layout_width="300px"
    android:layout_height="match_parent"
    android:layout_weight="1" />
    android:id="@+id/text_view_2"
    android:layout_width="200px"
    android:layout_height="match_parent"
    android:layout_weight="2" />
    
    MainActivity.kt
    

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89. View Slide

90. So we might end
    up in some very
    extreme & weird
    cases…
    

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91. Locale resolution
    

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92. build.gradle
    apply from: puzzler
    android {
    defaultConfig {
    resConfigs "es"
    }
    }
    

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93. 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}"
    )
    }
    }
    

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94. 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
    
    android:id="@+id/text_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:text="@string/hello_world" />
    

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95. 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
    
    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
    
    Bonjour monde !
    
    

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96. MainActivity.kt
    layout/activity_main.xml
    
    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
    
    Bonjour monde !
    
    values-es/strings.xml
    
    ¡Hola mundo!
    
    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}"
    )
    }
    }
    

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97. 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
    
    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
    
    Bonjour monde !
    
    values-es/strings.xml
    
    ¡Hola mundo!
    
    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}"
    )
    }
    }
    

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98. 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
    
    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
    
    Bonjour monde !
    
    values-es/strings.xml
    
    ¡Hola mundo!
    
    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}"
    )
    }
    }
    

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99. User settings
    de_DE
    fr_FR
    es_ES
    App resources
    default
    es
    

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100. 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
     

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101. 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
     

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102. 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
     

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103. 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
     

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104. 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
     

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

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106. 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
     

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107. 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
     

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108. 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
     

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109. That’s all folks!
     @cyrilmottier • cyrilmottier.com
     

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