DroidKaigi: Android UI: Patterns, Practices, Pitfalls

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Android UI Patterns, Practices, Pitfalls @chris_h_codes

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https:/ /github.com/aosp-mirror/.../View.java

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CSS IS AWESOME

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ViewGroup View

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override fun onLayout( changed: Boolean, left: Int, top: Int, right: Int, bottom: Int ) ViewGroup override fun onMeasure( widthSpec: Int, heightSpec: Int )\

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FrameLayout AbsoluteLayout LinearLayout RelativeLayout ListView

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GridLayout ConstraintLayout CoordinatorLayout NestedScrollView RecyclerView

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ConstraintLayout

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Text label This tag and its children can be replaced by one and a compound drawable

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override fun onMeasure( widthSpec: Int, heightSpec: Int )/ 1x 1x 1x 1x 1x 1x 1x 1x

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override fun onMeasure( widthSpec: Int, heightSpec: Int )/ 1x 2x 1x 2x 2x 1x 1x 1x

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override fun onMeasure( widthSpec: Int, heightSpec: Int )/ 2x 4x 2x 4x 4x 2x 2x 2x

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ConstraintLayout

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ConstraintLayout ConstraintLayout

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ConstraintLayout ConstraintLayout

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Main container Screen container Screen root Recycler List children

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Be mindful of your ConstraintLayouts

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Be mindful of your MotionLayouts

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Animation Complexity

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky android:animateLayoutChanges="true"

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky android:animateLayoutChanges="true" viewGroup .layoutTransition .enableTransitionType(LayoutTransition.CHANGING)

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky ObjectAnimator.ofFloat(0f, 1f)\ android:animateLayoutChanges="true"

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky ObjectAnimator.ofFloat(object : Property ...)\ android:animateLayoutChanges="true"

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky android:animateLayoutChanges="true" ObjectAnimator.ofFloat(...)\

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Animation Complexity view.animate() .translationX(100f) .alpha(0.5f) .rotation(45f) Easy Pretty tricky android:animateLayoutChanges="true" ObjectAnimator.ofFloat(...)\ TransitionManager .beginDelayedTransition(viewGroup)

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MotionLayout Easy to use, declarative syntax Seekable Can be driven by touch

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MotionLayout Easy to use, declarative syntax Seekable Can be driven by touch All animated views have to be direct children Can’t easily perform shared element transitions

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Transition Tips

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Transition Tips 1. Make them quick, debug them slow

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Transition Tips 1. Make them quick, debug them slow 2. Choreograph customisations with Kotlin

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class SomeFragment : Fragment() { sharedElementEnterTransition = TransitionInflater .from(context) .inflateTransition(android.R.transition.move) }

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sharedElementEnterTransition = TransitionInflater .from(context) .inflateTransition(android.R.transition.move)

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Transitions can… Target specific views Exclude specific views Have their duration, interpolation, and pathing modified

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inline fun transitionSet( block: TransitionSet.() -> Unit ): TransitionSet { return TransitionSet().apply(block) } inline fun TransitionSet.addSet( block: TransitionSet.() -> Unit ) { addTransition(TransitionSet().apply(block)) } inline fun TransitionSet.addTransition( transition: Transition, block: Transition.() -> Unit ) { transition.apply(block) addTransition(transition) }

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val transition = transitionSet { duration = 250 addSet { addTransition(ChangeBounds()) addTransition(ChangeTransform()) addTarget(currentFeelingTransitionName) interpolator = FastOutSlowInInterpolator() } addSet { addTransition(ChangeBounds()) { pathMotion = ArcMotion() } addTransition(ChangeTransform()) addTransition(TextResize()) addTarget(titleTransitionName) interpolator = FastOutLinearInInterpolator() } }

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Transition Tips 1. Make them quick, debug them slow 2. Choreograph customisations with Kotlin 3. Respect your parents

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val transition = transitionSet { duration = 250 addSet { addTransition(ChangeBounds()) addTarget(viewA) interpolator = FastOutSlowInInterpolator() }/ addSet { addTransition(ChangeBounds()) { pathMotion = ArcMotion() } addTransition(TextResize()) addTarget(viewB) interpolator = FastOutLinearInInterpolator() }/ }/

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val transition = transitionSet { duration = 250 addSet { addTransition(ChangeBounds()) addTransition(ChangeTransform()) addTarget(viewA) interpolator = FastOutSlowInInterpolator() }/ addSet { addTransition(ChangeBounds()) { pathMotion = ArcMotion() } addTransition(ChangeTransform()) addTransition(TextResize()) addTarget(viewB) interpolator = FastOutLinearInInterpolator() }/ }/

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Transition Tips 1. Make them quick, debug them slow 2. Choreograph customisations with Kotlin 3. Respect your parents 4. Remember that arc motion is a thing

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addTransition(ChangeBounds()) { pathMotion = ArcMotion() }/

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addTransition(ChangeBounds()) { pathMotion = ArcMotionPlus() }/

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https:/ /github.com/neild001/ArcMotionPlus Using the ArcMotionPlus Using the ArcMotionPlus is straightforward. There are a just two settings • Arc Angle • Reﬂect Neil Davies neild001

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https:/ /gist.github.com/chris-horner/80837fd3f3ac54052b766648d80ddbd2 class ArcMotionPlus(private val arcAngle: Float = 90f, private val reflected: Boolean = false) : PathMotion() { override fun getPath(startX: Float, startY: Float, endX: Float, endY: Float): Path { val start = PointF(startX, startY) val end = PointF(endX, endY) require(!(start.x == end.x && start.y == end.y)) { "Start and end points cannot be the same." } require(arcAngle in 1.0..179.0) { "Arc angle must be between 1 and 179 degrees." } val angleRadians: Double = Math.toRadians(arcAngle.toDouble()) val deltaX: Float = start.x - end.x val deltaY: Float = start.y - end.y val halfChordLength: Float = sqrt((deltaX * deltaX + deltaY * deltaY).toDouble()).toFloat() / 2f val radius: Float = halfChordLength / sin(angleRadians / 2f).toFloat() // The length of the line from the start or end point to the control point. chris-horner / ArcMotionPlus.kt

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Transition Tips 1. Make them quick, debug them slow 2. Choreograph customisations with Kotlin 3. Respect your parents 4. Remember that arc motion is a thing

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ScrollView Shenanigans

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ScrollView Shenanigans

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Forget ActionBar

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AndroidManifest.xml activity_main.xml MainActivity.kt setSupportActionBar(toolbar)

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Toolbar Fragment A

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Title A Fragment A Fragment B

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Title B Fragment A Fragment B

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Title B Fragment B override fun onViewCreated() { setHasOptionsMenu(true) } override fun onCreateOptionsMenu() { super.onCreateOptionsMenu(menu, inflater) menu.clear() inflater.inflate(R.menu.screen_b, menu) }

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Title B

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Title B

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Don’t

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transitionSet { addTransition(ChangeBounds()) { addTarget(toolbar) } addTransition(Slide()) { addTarget(fromView) addTarget(toView) } }

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Styling Toolbars

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<item name="colorPrimary">@color/blue_700</item> <item name="colorPrimaryVariant">@color/blue_700_variant</item> <item name="colorPrimaryDark">@color/blue_700_variant</item> <item name="colorOnPrimary">@android:color/white</item> <item name="colorOnPrimarySurface">@android:color/white</item>

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Basically, never do this

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Respect Themes

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android:background="@color/brandPrimary"\

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android:background="?attr/colorPrimary"\

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Required Watching https://www.youtube.com/watch?v=Owkf8DhAOSo

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Inset Issues

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Inset Issues https://www.youtube.com/watch?v=_mGDMVRO3iE BECOMING A MASTER WINDOW FITTER @chrisbanes

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Is one of my parent ViewGroups “insets aware”? How far down do I need to put fitsSystemWindows="true"? How do I cope with cumulative padding? Questions I Always Had

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Give up

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Always render edge-to-edge Give up

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override fun onCreate(savedInstanceState: Bundle?) { // Render under the status and navigation bars. window.decorView.systemUiVisibility = window.decorView.systemUiVisibility or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN or View.SYSTEM_UI_FLAG_LAYOUT_STABLE } Always render edge-to-edge

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toolbar.updatePaddingWithInsets(top = true) bottomNav.updatePaddingWithInsets(bottom = true)

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https:/ /gist.github.com/chris-horner/4718402eb6ebd4d89b63491245a359ff fun View.updatePaddingWithInsets(left: Boolean = false, top: Boolean = false, right: Boolean = false, bottom: Boolean = false) { doOnApplyWindowInsets { insets, padding -> updatePadding(left = if (left) padding.left + insets.systemWindowInsetLeft else padding.left, top = if (top) padding.top + insets.systemWindowInsetTop else padding.top, right = if (right) padding.right + insets.systemWindowInsetRight else padding.right, bottom = if (bottom) padding.bottom + insets.systemWindowInsetBottom else padding.bottom) } } inline fun View.doOnApplyWindowInsets(crossinline block: (insets: WindowInsets, padding: Rect) -> Unit) { // Create a snapshot of padding. val initialPadding = Rect(paddingLeft, paddingTop, paddingRight, paddingBottom) // Set an actual OnApplyWindowInsetsListener which proxies to the given lambda, also passing in the original padding. chris-horner / InsetUtils.kt

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https:/ /github.com/chrisbanes/insetter

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https:/ /github.com/material-components/material-components-android/releases

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Beware the Shadow Barber

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Drawable Digressions

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Dynamic?

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Dynamic?

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class CustomLayout : LinearLayout { fun display(items: List) { for (index in items.indices) { val view: View = inflate(R.layout.item) if (index != 0) { val params = view.layoutParams params.topMargin = dpToPx(16) } addView(view) } } }

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class CustomLayout : LinearLayout { fun display(items: List) { for (index in items.indices) { val view: View = inflate(R.layout.item) if (index != 0) { val space = Space(context) space.layoutParams.height = dpToPx(16) addView(space) } addView(view) } } }

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What if…?

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We do have…

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Recycler Wrongdoings

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Generally, as long as you give views an ID they’ll try and restore their state

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You’re in a race!

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fun onRestoreInstanceState(state: Parcelable) You’re in a race!

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dataStream .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe { items -> adapter.setItems(items) }\

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dataStream .replay(1) .refCount() .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe { items -> adapter.setItems(items) }\

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dataStream .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .replay(1) .refCount() .subscribe { items -> adapter.setItems(items) }\

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.subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .replay(1) .refCount() .subscribe { items -> adapter.setItems(items) }\ val dataStream: Flow>

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dataStream .onEach { items -> display(items) } .launchIn(scope)

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dataStream .onEach { items -> display(items) } .launchIn(scope) val scope = ???

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dataStream .onEach { items -> display(items) } .launchIn(scope) val scope = viewModelScope

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dataStream .onEach { items -> display(items) } .launchIn(scope) val scope = MainScope()

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/** * Creates the main [CoroutineScope] for UI components. */ val scope = MainScope()

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val scope = MainScope() ContextScope(SupervisorJob() + Dispatchers.Main)

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val scope = MainScope() val scope = viewModelScope ContextScope(SupervisorJob() + Dispatchers.Main)

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val scope = MainScope() val scope = viewModelScope ContextScope(SupervisorJob() + Dispatchers.Main) SupervisorJob() + Dispatchers.Main.immediate

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Attach and put data in your adapter right after your views are inﬂated

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What Takeaways?

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Takeaways ConstraintLayout and MotionLayout are great! But remember there are alternatives Widgets sometimes have surprising properties Handle Toolbars and window insets manually Understand and get the most out of the theming system Watch out for gotchas when it comes to state restoration and rendering

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chris_h_codes chris-horner chrishorner.codes Android UI Patterns, Practices, Pitfalls