Android 應用介面繪製流程
上一篇文章《Android 應用介面顯示流程》講到,從Activity.setContentView(int)方法,把佈局檔案某xml交給了PhoneWindow,PhoneWindow把佈局inflate出來附在了DecorView裡,然後交給了WMS管理,WMS和AMS排程後最終把畫面顯示出來。
這篇文章講ViewRootImpl.performTraversals(),是如何把DecorView繪製出來的。
我們知道自定義View中,需要搞懂三個重要方法onMeasure、onLayout和onDraw。performTraversals()也是分別從measure、layout和draw這3個流程去把View繪製出來的。
方法流程大概是這樣的,原始碼基於Android5.1.1
private void performTraversals() { ... performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ... performLayout(lp, desiredWindowWidth, desiredWindowHeight); ... performDraw(); ... } private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { ... mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); ... } private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth, int desiredWindowHeight) { ... final View host = mView; ... host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight()); ... } private void performDraw() { ... draw(fullRedrawNeeded); ... } private void draw(boolean fullRedrawNeeded) { ... if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) { return; } ... } private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff, boolean scalingRequired, Rect dirty) ... mView.draw(canvas); ... }
1.performMeasure
我們知道View裡面有個MeasureSpec的概念,型別是int。這個int型的32位的二進位制數裡由兩部分組成分別是mode和size,頭兩位表示型別mode,00表示未指定,01表示精確值,10表示最大值。後面30位表示具體的數值size。舉兩個例子就是可以有『精確值|100』『最大值|1080』這種長度規格。
private void performTraversals() { ... int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width); int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height); performMeasure(childWidthMeasureSpec, childHeightMeasureSpec); ... } private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { ... mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); ... } private static int getRootMeasureSpec(int windowSize, int rootDimension) { int measureSpec; switch (rootDimension) { case ViewGroup.LayoutParams.MATCH_PARENT: // Window can't resize. Force root view to be windowSize. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY); break; case ViewGroup.LayoutParams.WRAP_CONTENT: // Window can resize. Set max size for root view. measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST); break; default: // Window wants to be an exact size. Force root view to be that size. measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY); break; } return measureSpec; }
執行performMeasure需要MeasureSpec,所以首先需要構建一個MeasureSpec,通過getRootMeasureSpec傳入了視窗的實際大小和LayoutParams.MATCH_PARENT(來自WindowManager.LayoutParam的預設值)。MeasureSpec.makeMeasureSpec用於把這兩個mode和size合併為一個int型整數。然後performMeasure裡,執行了mView.measure,mView就是DecorView了。
View.java
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
==================================================================================
FrameLayout.java
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
...
//遍歷測量子View
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
//需要判斷子View不是GONE的狀態
if (mMeasureAllChildren || child.getVisibility() != GONE) {
//呼叫ViewGroup的measureChildWithMargins測量子View
measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
maxWidth = Math.max(maxWidth,
child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
maxHeight = Math.max(maxHeight,
child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
childState = combineMeasuredStates(childState, child.getMeasuredState());
if (measureMatchParentChildren) {
if (lp.width == LayoutParams.MATCH_PARENT ||
lp.height == LayoutParams.MATCH_PARENT) {
mMatchParentChildren.add(child);
}
}
}
}
...
}
==================================================================================
ViewGroup.java
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
//獲取子類的LayoutParams
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//getChildMeasureSpec的第二個引數,是已經用掉了多少空間。
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
+ widthUsed, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
+ heightUsed, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
//根據父View的spec,padding+margin已消耗的空間,子View的LayoutParam最終確定子View的spec
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//父View mode
int specMode = MeasureSpec.getMode(spec);
//父View size
int specSize = MeasureSpec.getSize(spec);
//如果父View的大小-已消耗空間 = 負數了,子View的大小就設為0
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
//判斷父View的mode
switch (specMode) {
// Parent has imposed an exact size on us
case MeasureSpec.EXACTLY:
if (childDimension >= 0) {
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size. So be it.
resultSize = size;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent has imposed a maximum size on us
case MeasureSpec.AT_MOST:
if (childDimension >= 0) {
// Child wants a specific size... so be it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size, but our size is not fixed.
// Constrain child to not be bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size. It can't be
// bigger than us.
resultSize = size;
resultMode = MeasureSpec.AT_MOST;
}
break;
// Parent asked to see how big we want to be
case MeasureSpec.UNSPECIFIED:
if (childDimension >= 0) {
// Child wants a specific size... let him have it
resultSize = childDimension;
resultMode = MeasureSpec.EXACTLY;
} else if (childDimension == LayoutParams.MATCH_PARENT) {
// Child wants to be our size... find out how big it should
// be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = 0;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
View.measure(int, int)方法是final型別的,意味著子類均不可重寫。所以對於不同的View測量自身,需要重寫onMeasure方法。不同的ViewGroup有不同佈局策略(比如對與一個wrap_contentde的ViewGroup佈局,LinearLayout是對子View順序擺放,不會重疊,所以需要的空間是累加的,而FrameLayout只需要求子View空間的最大值就行),所以ViewGroup有不同的onMeasure方法。
這裡還有一個重要的前提,就是對於ViewGroup,是有義務測量自己的子View的。Android正式通過這種遞迴方式,才使得代理裡只需要對DecorView操作,自然會遞迴到最上層的子View。
子View的MeasureSpec是根據父View的MeasureSpec和自己的LayoutParams共同決定的。LayoutParams也就是我們平時在xml中寫的layout_widht, layout_height, layout_gravity等這些帶layout_字首的屬性。根據這兩種排列組合,可以有這幾種最終結果。
豎排父View\橫排子View | childDimension >= 0 | childDimension == LayoutParams.MATCH_PARENT | childDimension == LayoutParams.WRAP_CONTENT |
MeasureSpec.EXACTLY | 準確值|子View大小 | 準確值|父View大小-padding-margin | 最大值|父View大小-padding-margin |
MeasureSpec.AT_MOST | 準確值|子View大小 | 最大值|父View大小-padding-margin | 最大值|父View大小-padding-margin |
MeasureSpec.UNSPECIFIED | 準確值|子View大小 | 未指定|0 | 未指定|0 |
這幾種結果最終也是以MeasureSpec呈現,然後會通過呼叫到子View的measure方法,把childWidthMeasureSpec和childHeightMeasureSpec傳到了子View裡。到此完成了一次迴圈。之後就是不斷重複此迴圈,直到某個View再也沒有子View為止。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) {
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
int specMode = MeasureSpec.getMode(measureSpec);
int specSize = MeasureSpec.getSize(measureSpec);
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
上面是View的onMeasure方法實現,根據文件說明,在此方面必須呼叫了setMeasureDimension,主要是為了給屬性mMeasuredWidth和mMeasuredHeight賦值。後面的佈局會使用到這兩個屬性。
不同的View有不同的onMeasure方法,它應該根據傳進來的MeasureSpec來補充自己的測量邏輯。一個正常的View如果拿到MeasureSpec的mode為準確值(MeasureSpec.EXACTLY),它應該直接設定大小為size大小。如果拿到的MeasureSpec的mode為最大值(MeasureSpec.AT_MOST),它應該自己根據自身需要測量自身內容,比如TextView就是測量文字需要佔據多少空間,最終結果取自身測量結果和size的最小值,也就是所謂的不要超過MeasureSpec要求的最大值。
對於ViewGroup,需要把子View都測量完才會最終確定確定自己的尺寸。如上面的例子說的,如果是個wrap_contentde的LinearLayout需要測量完所有子View佔據多少空間才能知道自己應該多大。
2.performLayout
onMeasure方法中會給賦值,在layout這步驟就會使用到
ViewRootImpl.java
private void performTraversals() {
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
...
performDraw();
...
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
...
final View host = mView;
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
==================================================================================
ViewGroup.java
@Override
public final void layout(int l, int t, int r, int b) {
if (!mSuppressLayout && (mTransition == null || !mTransition.isChangingLayout())) {
if (mTransition != null) {
mTransition.layoutChange(this);
}
super.layout(l, t, r, b);
} else {
// record the fact that we noop'd it; request layout when transition finishes
mLayoutCalledWhileSuppressed = true;
}
}
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);
==================================================================================
View.java
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);
...
onLayout(changed, l, t, r, b);
...
}
protected boolean setFrame(int left, int top, int right, int bottom) {
...
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
...
}
performLayout裡面會有其他功能的程式碼,比如一定情況下,使用desiredWindowWidth和desiredWindowHeight重新執行測量流程。但是重點是在執行host這個DecorView的Layout方法。Layout方法裡會呼叫到setFrame方法,裡面會對四個屬性賦值,mLeft、mTop、mRight和mBottom。這4個屬性是最終確定該View邊界的欄位。需要注意的是四個值是相對父View的位置,而不是整個螢幕的。
然後是流程是ViewGroup.layout -> View.layout -> onLayout方法。如果是繼承ViewGroup,就需要實現onLayout方法。因為ViewGroup.onLayout是抽象型別的。如果是繼承View,則無需實現onLayout。
對於DecorView,會執行到FrameLayout.onLayout方法。FrameLayout是ViewGroup,所以需要確定所有子View的位置。在layouChildren裡,會根據佈局的Gravity屬性進行子View的擺放。同樣也是遞迴,直到View再也沒有子View停止。
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
layoutChildren(left, top, right, bottom, false /* no force left gravity */);
}
void layoutChildren(int left, int top, int right, int bottom,
boolean forceLeftGravity) {
final int count = getChildCount();
final int parentLeft = getPaddingLeftWithForeground();
final int parentRight = right - left - getPaddingRightWithForeground();
final int parentTop = getPaddingTopWithForeground();
final int parentBottom = bottom - top - getPaddingBottomWithForeground();
mForegroundBoundsChanged = true;
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (child.getVisibility() != GONE) {
final LayoutParams lp = (LayoutParams) child.getLayoutParams();
final int width = child.getMeasuredWidth();
final int height = child.getMeasuredHeight();
int childLeft;
int childTop;
int gravity = lp.gravity;
if (gravity == -1) {
gravity = DEFAULT_CHILD_GRAVITY;
}
final int layoutDirection = getLayoutDirection();
final int absoluteGravity = Gravity.getAbsoluteGravity(gravity, layoutDirection);
final int verticalGravity = gravity & Gravity.VERTICAL_GRAVITY_MASK;
switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
case Gravity.CENTER_HORIZONTAL:
childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
lp.leftMargin - lp.rightMargin;
break;
case Gravity.RIGHT:
if (!forceLeftGravity) {
childLeft = parentRight - width - lp.rightMargin;
break;
}
case Gravity.LEFT:
default:
childLeft = parentLeft + lp.leftMargin;
}
switch (verticalGravity) {
case Gravity.TOP:
childTop = parentTop + lp.topMargin;
break;
case Gravity.CENTER_VERTICAL:
childTop = parentTop + (parentBottom - parentTop - height) / 2 +
lp.topMargin - lp.bottomMargin;
break;
case Gravity.BOTTOM:
childTop = parentBottom - height - lp.bottomMargin;
break;
default:
childTop = parentTop + lp.topMargin;
}
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
3.performDraw
ViewRootImpl.java
private void performTraversals() {
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, desiredWindowWidth, desiredWindowHeight);
...
performDraw();
...
}
private void performDraw() {
...
draw(fullRedrawNeeded);
...
}
private void draw(boolean fullRedrawNeeded) {
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
...
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty)
...
mView.draw(canvas);
...
}
==================================================================================
View.java
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
/*
* Draw traversal performs several drawing steps which must be executed
* in the appropriate order:
*
* 1. Draw the background
* 2. If necessary, save the canvas' layers to prepare for fading
* 3. Draw view's content
* 4. Draw children
* 5. If necessary, draw the fading edges and restore layers
* 6. Draw decorations (scrollbars for instance)
*/
// Step 1, draw the background, if needed
int saveCount;
if (!dirtyOpaque) {
drawBackground(canvas);
}
// skip step 2 & 5 if possible (common case)
final int viewFlags = mViewFlags;
boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
if (!verticalEdges && !horizontalEdges) {
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// we're done...
return;
}
/*
* Here we do the full fledged routine...
* (this is an uncommon case where speed matters less,
* this is why we repeat some of the tests that have been
* done above)
*/
boolean drawTop = false;
boolean drawBottom = false;
boolean drawLeft = false;
boolean drawRight = false;
float topFadeStrength = 0.0f;
float bottomFadeStrength = 0.0f;
float leftFadeStrength = 0.0f;
float rightFadeStrength = 0.0f;
// Step 2, save the canvas' layers
int paddingLeft = mPaddingLeft;
final boolean offsetRequired = isPaddingOffsetRequired();
if (offsetRequired) {
paddingLeft += getLeftPaddingOffset();
}
int left = mScrollX + paddingLeft;
int right = left + mRight - mLeft - mPaddingRight - paddingLeft;
int top = mScrollY + getFadeTop(offsetRequired);
int bottom = top + getFadeHeight(offsetRequired);
if (offsetRequired) {
right += getRightPaddingOffset();
bottom += getBottomPaddingOffset();
}
final ScrollabilityCache scrollabilityCache = mScrollCache;
final float fadeHeight = scrollabilityCache.fadingEdgeLength;
int length = (int) fadeHeight;
// clip the fade length if top and bottom fades overlap
// overlapping fades produce odd-looking artifacts
if (verticalEdges && (top + length > bottom - length)) {
length = (bottom - top) / 2;
}
// also clip horizontal fades if necessary
if (horizontalEdges && (left + length > right - length)) {
length = (right - left) / 2;
}
if (verticalEdges) {
topFadeStrength = Math.max(0.0f, Math.min(1.0f, getTopFadingEdgeStrength()));
drawTop = topFadeStrength * fadeHeight > 1.0f;
bottomFadeStrength = Math.max(0.0f, Math.min(1.0f, getBottomFadingEdgeStrength()));
drawBottom = bottomFadeStrength * fadeHeight > 1.0f;
}
if (horizontalEdges) {
leftFadeStrength = Math.max(0.0f, Math.min(1.0f, getLeftFadingEdgeStrength()));
drawLeft = leftFadeStrength * fadeHeight > 1.0f;
rightFadeStrength = Math.max(0.0f, Math.min(1.0f, getRightFadingEdgeStrength()));
drawRight = rightFadeStrength * fadeHeight > 1.0f;
}
saveCount = canvas.getSaveCount();
int solidColor = getSolidColor();
if (solidColor == 0) {
final int flags = Canvas.HAS_ALPHA_LAYER_SAVE_FLAG;
if (drawTop) {
canvas.saveLayer(left, top, right, top + length, null, flags);
}
if (drawBottom) {
canvas.saveLayer(left, bottom - length, right, bottom, null, flags);
}
if (drawLeft) {
canvas.saveLayer(left, top, left + length, bottom, null, flags);
}
if (drawRight) {
canvas.saveLayer(right - length, top, right, bottom, null, flags);
}
} else {
scrollabilityCache.setFadeColor(solidColor);
}
// Step 3, draw the content
if (!dirtyOpaque) onDraw(canvas);
// Step 4, draw the children
dispatchDraw(canvas);
// Step 5, draw the fade effect and restore layers
final Paint p = scrollabilityCache.paint;
final Matrix matrix = scrollabilityCache.matrix;
final Shader fade = scrollabilityCache.shader;
if (drawTop) {
matrix.setScale(1, fadeHeight * topFadeStrength);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, right, top + length, p);
}
if (drawBottom) {
matrix.setScale(1, fadeHeight * bottomFadeStrength);
matrix.postRotate(180);
matrix.postTranslate(left, bottom);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, bottom - length, right, bottom, p);
}
if (drawLeft) {
matrix.setScale(1, fadeHeight * leftFadeStrength);
matrix.postRotate(-90);
matrix.postTranslate(left, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(left, top, left + length, bottom, p);
}
if (drawRight) {
matrix.setScale(1, fadeHeight * rightFadeStrength);
matrix.postRotate(90);
matrix.postTranslate(right, top);
fade.setLocalMatrix(matrix);
p.setShader(fade);
canvas.drawRect(right - length, top, right, bottom, p);
}
canvas.restoreToCount(saveCount);
// Step 6, draw decorations (scrollbars)
onDrawScrollBars(canvas);
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
}
類似的,也呼叫到的DecorView.draw方法,DecorView和FrameLayout都有draw方法的實現。但最重要的部分還是寫在了View.draw裡面,裡面的程式碼不少,但是通過註釋配合程式碼還是能知道其過程。
1.是繪製背景
2.第二步儲存塗層資訊
3.繪製自身內容
4.繪製子View內容
5.繪製View的邊緣漸變內容,還原圖層
6.繪製裝飾比如滾動條
第三步繪製自身內容是呼叫了onDraw方法,所以對於ViewGroup一般不用重寫這方法。第四步呼叫到dispatchDraw。
@Override
protected void dispatchDraw(Canvas canvas) {
...
// Draw any disappearing views that have animations
if (mDisappearingChildren != null) {
final ArrayList<View> disappearingChildren = mDisappearingChildren;
final int disappearingCount = disappearingChildren.size() - 1;
// Go backwards -- we may delete as animations finish
for (int i = disappearingCount; i >= 0; i--) {
final View child = disappearingChildren.get(i);
more |= drawChild(canvas, child, drawingTime);
}
}
...
}
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
裡面會遍歷所有的子View,呼叫到子View.draw方法,開始下一層的遞迴繪製。