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Layout Traversals (Droidcon Turin 2015)
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Lucas Rocha
April 10, 2015
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Layout Traversals (Droidcon Turin 2015)
Presented at Droidcon Turin 2015
Lucas Rocha
April 10, 2015
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Transcript
LAYOUT TRAVERSALS Droidcon Turin, 2015
LUCAS ROCHA +LucasRocha | @lucasratmundo
None
BE DELIBERATE Less handwavy UI code
THE BASICS From inside out.
UI TOOLKITS Layout + Rendering + Input Events
OLD SCHOOL Nested boxes, rudimentary motion API.
PRE-JELLY BEAN public final class ViewRoot extends Handler ... {
... public void scheduleTraversals() { if (!mTraversalScheduled) { mTraversalScheduled = true; sendEmptyMessage(DO_TRAVERSAL); } } ... public void handleMessage(Message msg) { ... case DO_TRAVERSAL: performTraversals(); ... } } ViewRoot.java
MODERNIZE Predictable notion of time that drives input, layout, and
motion.
TICK TOCK VSYNC sets the pace. No tearing, no extra
work.
CHOREOGRAPHER f1 f2 f3 f4 f5 . . . .
. . VSYNC (60fps) Resize view Redraw view Input Events
CHOREOGRAPHER public void onVsync(long timestampNanos, int builtInDisplayId, int frame) {
... scheduleVsync(); ... } ... void doFrame(long frameTimeNanos, int frame) { ... if (!mFrameScheduled) { return; } ... doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos); ... } Choreographer.java
ViewRootImpl Connects WindowManager with the View framework
THE ROOT * * SurfaceFlinger ViewRootImpl
VIEWS Measure + Layout + Draw
TRAVERSAL * performTraversals() f2 performTraversals()
MEASURE * measure(int, int) f2 M M M M M
M → onMeasure(int, int) measureHierarchy(...)
LAZY MEASURE Multi-MeasureSpec cache, invalidated in requestLayout()
LAZY MEASURE public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
... int cacheIndex = mMeasureCache.indexOfKey(key); if (cacheIndex < 0 || sIgnoreMeasureCache) { onMeasure(widthMeasureSpec, heightMeasureSpec); mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } else { long value = mMeasureCache.valueAt(cacheIndex); setMeasuredDimensionRaw((int) (value >> 32), (int) value); mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } ... } View.java
LAZY MEASURE public void layout(int l, int t, int r,
int b) { if ((flags & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) { onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec); mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } ... } View.java
LAYOUT * f2 M L M L M L M
L M L M L layout(int, int, int, int) → onLayout(boolean, int, int, int, int) performLayout()
FRAME IT public void layout(int l, int t, int r,
int b) { ... boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b); ... } View.java
DRAW * f2 M L D M L D M
L D M L D M L D M L D draw(Canvas) → onDraw(Canvas) performDraw()
DISPLAY LISTS private DisplayList getDisplayList(...) { ... final HardwareCanvas canvas
= displayList.start(width, height); ... draw(canvas); ... displayList.end(); ... } View.java
INVARIANTS Layout means measurement is done. Drawing means layout is
done.
SMELLS 1. getMeasured*() calls outside onLayout() 2. New allocations during
traversal · onLayout: ok-ish · onMeasure: avoid · onDraw: nope 3. post(Runnable) to mean “after layout”
HAPPENS TO WORK public final class MyActivity extends Activity {
... @Override public void onCreate() { final View myView = findViewById(R.id.some_id); myView.post(new Runnable() { @Override public void run() { Log.d(“LOGTAG”, myView.getWidth()); } }); } ... }
LOOPER BARRIERS void scheduleTraversals() { ... mTraversalBarrier = mHandler.getLooper().postSyncBarrier(); ...
} ViewRootImpl.java
TREE OBSERVER Use OnPreDrawListener!
TRANSITIONS http://lucasr.org/?p=3902
OnPreDrawListener // 1. Save layout state and wait for next
frame. getViewTreeObserver().addOnPreDrawListener(new OnPreDrawListener() { @Override public boolean onPreDraw() { getViewTreeObserver().removeOnPreDrawListener(this); // 2. Restore original layout state. // 3. Trigger animators towards new layout state. } }
CHANGES Resize, redraw & animate.
* requestLayout() f1 f2 f3 f4 f5 . . .
. . .
requestLayout() public void requestLayout() { ... if (mParent != null
&& !mParent.isLayoutRequested()) { mParent.requestLayout(); } ... } void scheduleTraversals() { ... mChoreographer.postCallback( Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null); ... } View.java ViewRootImpl.java
* Invalidate(...) f1 f2 f3 f4 f5 . . .
. . .
invalidate(...) public void invalidateInternal(...) { ... mPrivateFlags |= PFLAG_INVALIDATED; ...
if (mParent != null && mAttachInfo != null && l < r && t < b) { final Rect damage = mAttachInfo.mTmpInvalRect; damage.set(l, t, r, b); mParent.invalidateChild(this, damage); } ... } boolean draw(...) { ... mRecreateDisplayList = (mPrivateFlags & PFLAG_INVALIDATED) == PFLAG_INVALIDATED; ... } View.java
* postOnAnimation() f1 f2 f3 f4 f5 . . .
. . . ValueAnimator ...
postOnAnimation() public void postOnAnimation(Runnable action) { ... attachInfo.mViewRootImpl.mChoreographer.postCallback( Choreographer.CALLBACK_ANIMATION, action,
null); ... } void doFrame(long frameTimeNanos, int frame) { ... doCallbacks(Choreographer.CALLBACK_INPUT, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_ANIMATION, frameTimeNanos); doCallbacks(Choreographer.CALLBACK_TRAVERSAL, frameTimeNanos); ... } View.java Choreographer.java
BASIC TIPS 1. No layout requests during layout 2. No
layout requests during animations 3. Invalidate regions when possible
PERFORMANCE 1. Simplify your view hierarchy 2. Avoid multi-pass measurement
GO CUSTOM Simplify view hierarchy, performance, missing features. http://lucasr.org/?p=3920
PROBE IT! Intercept view methods. Dissect layout traversals. https://github.com/lucasr/probe
WRAP VIEW METHODS public class LogRequestLayout extends Interceptor { @Override
public void requestLayout(View view) { super.requestLayout(view); Log.d(LOGTAG, “requestLayout() on ” + view); } }
DEPLOY public final class MainActivity extends Activity { @Override protected
void onCreate(Bundle savedInstanceState) { Probe.deploy(this, new DrawGreen(), new Filter.ViewId(R.id.view2)); super.onCreate(savedInstanceState); setContentView(R.id.main_activity); } }
FANCY HACKING ON DEEP LAYOUT STUFF? We're hiring :-)
QUESTIONS? +LucasRocha | @lucasratmundo