1.簡(jiǎn)介
在開發(fā)中，我們經(jīng)常會(huì)遇到各種各樣的View，這些View有的是系統(tǒng)提供的，有的是我們自定義的View，可見View在開發(fā)中的重要性，那么了解Android View的繪制流程對(duì)于我們更好地理解View的工作原理和自定義View相當(dāng)有益，本文將依據(jù)Android源碼（API=30）探究View的繪制流程，加深大家對(duì)其的理解和認(rèn)知。
2.View繪制流程概覽
應(yīng)用的一個(gè)頁(yè)面是由各種各樣的View組合而成的，它們能夠按照我們的期望呈現(xiàn)在屏幕上，實(shí)現(xiàn)我們的需求，其背后是有一套復(fù)雜的繪制流程的，主要涉及到以下三個(gè)過程：
measure：顧名思義，是測(cè)量的意思，在這個(gè)階段，做的主要工作是測(cè)量出View的尺寸大小并保存。
layout：這是布局階段，在這個(gè)階段主要是根據(jù)上個(gè)測(cè)量階段得到的View尺寸大小以及View本身的參數(shù)設(shè)置來確定View應(yīng)該擺放的位置。
-
draw：這是階段相當(dāng)重要，主要執(zhí)行繪制的任務(wù)，它根據(jù)測(cè)量和布局的結(jié)果，完成View的繪制，這樣我們就能看到豐富多彩的界面了。
這些階段執(zhí)行的操作都比較復(fù)雜，幸運(yùn)的是系統(tǒng)幫我們處理了很多這樣的工作，并且當(dāng)我們需要實(shí)現(xiàn)自定義View的時(shí)候，系統(tǒng)又給我們提供了onMeasure()、onLayout()、onDraw()方法，一般來說，我們重寫這些方法，在其中加入我們自己的業(yè)務(wù)邏輯，就可以實(shí)現(xiàn)我們自定義View的需求了。
3.View繪制的入口
講到View繪制的流程，就要提到ViewRootImpl類中的performTraversals()方法，這個(gè)方法中涉及到performMeasure()、performLayout()、performDraw()三個(gè)方法，其中performMeasure()方法是從ViewTree的根節(jié)點(diǎn)開始遍歷執(zhí)行測(cè)量View的工作，performLayout()方法是從ViewTree的根節(jié)點(diǎn)開始遍歷執(zhí)行View的布局工作，而performDraw()方法是從ViewTree的根節(jié)點(diǎn)開始遍歷執(zhí)行繪制View的工作，ViewTree的根節(jié)點(diǎn)是DecorView。performTraversals()方法內(nèi)容很長(zhǎng)，以下只是部分代碼。
//ViewRootImpl
private void performTraversals() {
final View host = mView;
...
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
...
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
...
performLayout(lp, mWidth, mHeight);
...
performDraw();
}
4.measure階段
measure是繪制流程的第一個(gè)階段，在這個(gè)階段主要是通過測(cè)量來確定View的尺寸大小。
4.1 MeasureSpec介紹
- MeasureSpec封裝了從父View傳遞到子View的布局要求，MeasureSpec由大小和模式組成，它可能有三種模式。
- UNSPECIFIED模式：父View沒有對(duì)子View施加任何約束，子View可以是它想要的任何大小。
- EXACTLY模式：父View已經(jīng)為子View確定了精確的尺寸，不管子View想要多大尺寸，它都要在父View給定的界限內(nèi)。
- AT_MOST模式：在父View指定的大小范圍內(nèi)，子View可以是它想要的大小。
4.2 View測(cè)量的相關(guān)方法
-
ViewRootImpl.performMeasure()方法
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) { if (mView == null) { return; } Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure"); try { mView.measure(childWidthMeasureSpec, childHeightMeasureSpec); } finally { Trace.traceEnd(Trace.TRACE_TAG_VIEW); } }在performMeasure()中，從根布局DecorView開始遍歷執(zhí)行measure()操作。
-
View.measure()方法
public final void measure(int widthMeasureSpec, int heightMeasureSpec) { boolean optical = isLayoutModeOptical(this); if (optical != isLayoutModeOptical(mParent)) { Insets insets = getOpticalInsets(); int oWidth = insets.left + insets.right; int oHeight = insets.top + insets.bottom; widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth); heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight); } ... if (forceLayout || needsLayout) { // first clears the measured dimension flag mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET; resolveRtlPropertiesIfNeeded(); int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key); if (cacheIndex < 0 || sIgnoreMeasureCache) { // measure ourselves, this should set the measured dimension flag back onMeasure(widthMeasureSpec, heightMeasureSpec); mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } else { long value = mMeasureCache.valueAt(cacheIndex); // Casting a long to int drops the high 32 bits, no mask needed setMeasuredDimensionRaw((int) (value >> 32), (int) value); mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT; } ... } ... }調(diào)用這個(gè)方法是為了找出視圖應(yīng)該有多大，父View在寬度和高度參數(shù)中提供約束信息，其中widthMeasureSpec參數(shù)是父View強(qiáng)加的水平空間要求，heightMeasureSpec參數(shù)是父View強(qiáng)加的垂直空間要求，這是一個(gè)final方法，實(shí)際的測(cè)量工作是通過調(diào)用onMeasure()方法執(zhí)行的，因此只有onMeasure()方法可以被子類重寫。
-
View.onMeasure()方法
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec), getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec)); }這個(gè)方法的作用是測(cè)量視圖及其內(nèi)容，以確定測(cè)量的寬度和高度，這個(gè)方法被measure()方法調(diào)用，并且應(yīng)該被子類重寫去對(duì)它們的內(nèi)容進(jìn)行準(zhǔn)確和有效的測(cè)量，當(dāng)重寫此方法時(shí)，必須調(diào)用setMeasuredDimension()方法去存儲(chǔ)這個(gè)View被測(cè)量出的寬度和高度。
-
View.setMeasuredDimension()方法
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); }setMeasuredDimension()方法必須被onMeasure()方法調(diào)用去存儲(chǔ)被測(cè)量出的寬度和高度，在測(cè)量的時(shí)候如果setMeasuredDimension()方法執(zhí)行失敗將會(huì)拋出異常。
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) { mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET; }setMeasuredDimensionRaw()方法被setMeasuredDimension()方法調(diào)用來設(shè)置出被測(cè)量出的寬度和高度給View的變量mMeasuredWidth和mMeasuredHeight。
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; }參數(shù)size是這個(gè)View的默認(rèn)大小，參數(shù)measureSpec是父View對(duì)子View施加的約束，通過計(jì)算的得出這個(gè)View 應(yīng)該的大小，如果MeasureSpec沒有施加約束則使用提供的大小，如果是MeasureSpec.AT_MOST或MeasureSpec.EXACTLY模式則會(huì)使用specSize。
protected int getSuggestedMinimumWidth() { return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth()); }getSuggestedMinimumWidth()方法返回View應(yīng)該使用的最小寬度，這個(gè)返回值是View的最小寬度和背景的最小寬度二者之中較大的那一個(gè)值。當(dāng)在onMeasure()方法內(nèi)被使用的時(shí)候，調(diào)用者依然應(yīng)該確保返回的寬度符合父View的要求。
4.3 ViewGroup測(cè)量的相關(guān)方法
ViewGroup繼承View，是一個(gè)可以包含其他子View的一個(gè)特殊的View，在執(zhí)行測(cè)量工作的時(shí)候，它有幾個(gè)比較重要的方法，measureChildren()、measureChild()和getChildMeasureSpec()。
protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
final int size = mChildrenCount;
final View[] children = mChildren;
for (int i = 0; i < size; ++i) {
final View child = children[i];
if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
measureChild(child, widthMeasureSpec, heightMeasureSpec);
}
}
}
measureChildren()方法要求這個(gè)View的子View們?nèi)y(cè)量它們自己，處于GONE狀態(tài)的子View不會(huì)執(zhí)行measureChild()方法。
protected void measureChild(View child, int parentWidthMeasureSpec,
int parentHeightMeasureSpec) {
final LayoutParams lp = child.getLayoutParams();
final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
mPaddingLeft + mPaddingRight, lp.width);
final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
mPaddingTop + mPaddingBottom, lp.height);
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
measureChild()方法要求子View去測(cè)量它自身，測(cè)量的同時(shí)需要考慮到父布局的MeasureSpec要求和它自身的padding。
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
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 = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
} else if (childDimension == LayoutParams.WRAP_CONTENT) {
// Child wants to determine its own size.... find out how
// big it should be
resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
resultMode = MeasureSpec.UNSPECIFIED;
}
break;
}
//noinspection ResourceType
return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
這個(gè)方法做了測(cè)量子View過程中復(fù)雜的工作，計(jì)算出MeasureSpec傳遞給特定的子節(jié)點(diǎn)，目標(biāo)是根據(jù)來自MeasureSpec的信息以及子View的LayoutParams信息去得到一個(gè)最可能的結(jié)果。
4.4 DecorView的測(cè)量
DecorView繼承了FrameLayout，F(xiàn)rameLayout又繼承了ViewGroup，它重寫了onMeasure()方法，并且調(diào)用了父類的onMeasure()方法,在遍歷循環(huán)去測(cè)量它的子View，之后又調(diào)用了setMeasuredDimension()。
//DecorView
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
final DisplayMetrics metrics = getContext().getResources().getDisplayMetrics();
final boolean isPortrait = getResources().getConfiguration().orientation == ORIENTATION_PORTRAIT;
final int widthMode = getMode(widthMeasureSpec);
final int heightMode = getMode(heightMeasureSpec);
...
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
...
}
//FrameLayout
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int count = getChildCount();
...
for (int i = 0; i < count; i++) {
final View child = getChildAt(i);
if (mMeasureAllChildren || child.getVisibility() != GONE) {
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);
}
}
}
}
...
setMeasuredDimension(resolveSizeAndState(maxWidth, widthMeasureSpec, childState),
resolveSizeAndState(maxHeight, heightMeasureSpec,
childState << MEASURED_HEIGHT_STATE_SHIFT));
...
}
5.layout階段
當(dāng)measure階段完成后，就會(huì)進(jìn)入到layout布局階段，根據(jù)View測(cè)量的結(jié)果和其他參數(shù)來確定View應(yīng)該擺放的位置。
5.1 performLayout()方法
測(cè)量完成后，在performTraverserals()方法中，會(huì)執(zhí)行performLayout()方法，開始布局過程。
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
if (host == null) {
return;
}
...
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
...
}
5.2 layout()方法
//ViewGroup
@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;
}
}
這個(gè)是ViewGroup的layout()方法，它是一個(gè)final類型的方法，在其內(nèi)部又調(diào)用了父類View的layout()方法。
//View
@SuppressWarnings({"unchecked"})
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);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
...
}
...
}
View的layout()方法作用是為它本身及其后代View分配大小和位置，派生類不應(yīng)重寫此方法，帶有子View的派生類應(yīng)該重寫onLayout()方法，參數(shù)l、t、r、b指的是相對(duì)于父View的位置。
5.3 setFrame()方法
//View
protected boolean setFrame(int left, int top, int right, int bottom) {
boolean changed = false;
...
if (mLeft != left || mRight != right || mTop != top || mBottom != bottom) {
changed = true;
// Remember our drawn bit
int drawn = mPrivateFlags & PFLAG_DRAWN;
int oldWidth = mRight - mLeft;
int oldHeight = mBottom - mTop;
int newWidth = right - left;
int newHeight = bottom - top;
boolean sizeChanged = (newWidth != oldWidth) || (newHeight != oldHeight);
// Invalidate our old position
invalidate(sizeChanged);
mLeft = left;
mTop = top;
mRight = right;
mBottom = bottom;
mRenderNode.setLeftTopRightBottom(mLeft, mTop, mRight, mBottom);
mPrivateFlags |= PFLAG_HAS_BOUNDS;
if (sizeChanged) {
sizeChange(newWidth, newHeight, oldWidth, oldHeight);
}
...
}
return changed;
}
在View的layout()方法內(nèi)會(huì)調(diào)用setFrame()方法，其作用是給這個(gè)視圖分配一個(gè)大小和位置，如果新的大小和位置與原來的不同，那么返回值為true。
5.4 onLayout()方法
//View
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
View的onLayout()方法是一個(gè)空方法，內(nèi)部沒有代碼實(shí)現(xiàn)，帶有子節(jié)點(diǎn)的派生類應(yīng)該重寫此方法，并在其每個(gè)子節(jié)點(diǎn)上調(diào)用layout。
//ViewGroup
@Override
protected abstract void onLayout(boolean changed,
int l, int t, int r, int b);
ViewGroup的onLayout()方法是一個(gè)抽象方法，因此直接繼承ViewGroup的類需要重寫此方法。
//DecorView
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
...
}
5.5 DecorView的布局
//DecorView
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
if (mApplyFloatingVerticalInsets) {
offsetTopAndBottom(mFloatingInsets.top);
}
if (mApplyFloatingHorizontalInsets) {
offsetLeftAndRight(mFloatingInsets.left);
}
// If the application changed its SystemUI metrics, we might also have to adapt
// our shadow elevation.
updateElevation();
mAllowUpdateElevation = true;
if (changed
&& (mResizeMode == RESIZE_MODE_DOCKED_DIVIDER
|| mDrawLegacyNavigationBarBackground)) {
getViewRootImpl().requestInvalidateRootRenderNode();
}
}
DecorView重寫了onLayout()方法，并且調(diào)用了其父類FrameLayout的onLayout()方法。
//FrameLayout
@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();
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();
...
child.layout(childLeft, childTop, childLeft + width, childTop + height);
}
}
}
在FrameLayout的onLayout()方法中，調(diào)用了layoutChildren()方法，在此方法內(nèi)開啟循環(huán)，讓子View調(diào)用layout()去完成布局。
6.draw階段
當(dāng)測(cè)量和布局階段完成后，就進(jìn)入了繪制階段，在這個(gè)階段，將View繪制到畫布上。
6.1 performDraw()方法
在performTraverserals()方法中，會(huì)執(zhí)行performDraw()方法，開始繪制過程。
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
} else if (mView == null) {
return;
}
...
boolean canUseAsync = draw(fullRedrawNeeded);
...
}
private boolean draw(boolean fullRedrawNeeded) {
Surface surface = mSurface;
if (!surface.isValid()) {
return false;
}
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
...
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty, Rect surfaceInsets) {
// Draw with software renderer.
final Canvas canvas;
...
mView.draw(canvas);
...
}
6.2 draw()方法
//View
@CallSuper
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
// Step 1, draw the background, if needed
int saveCount;
drawBackground(canvas);
...
// Step 3, draw the content
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;
...
canvas.restoreToCount(saveCount);
drawAutofilledHighlight(canvas);
// Overlay is part of the content and draws beneath Foreground
if (mOverlay != null && !mOverlay.isEmpty()) {
mOverlay.getOverlayView().dispatchDraw(canvas);
}
// Step 6, draw decorations (foreground, scrollbars)
onDrawForeground(canvas);
// Step 7, draw the default focus highlight
drawDefaultFocusHighlight(canvas);
if (isShowingLayoutBounds()) {
debugDrawFocus(canvas);
}
}
View的draw()方法將View和它的子View渲染到給定的畫布，此方法在被調(diào)用之前需要先完成布局工作。
6.3 onDraw()方法
//View
protected void onDraw(Canvas canvas) {
}
View的onDraw()方法是一個(gè)空方法，內(nèi)部沒有代碼實(shí)現(xiàn)，通過重寫這個(gè)方法，可以實(shí)現(xiàn)我們自己的繪制。
6.4 dispatchDraw()方法
//View
protected void dispatchDraw(Canvas canvas) {
}
View的dispatchDraw()方法是一個(gè)空方法，它被draw()調(diào)用去繪制子View，在派生類的子節(jié)點(diǎn)View被繪制之前，它可以被派生類重寫，這樣派生類就獲得了控制權(quán)。
//ViewGroup
@Override
protected void dispatchDraw(Canvas canvas) {
boolean usingRenderNodeProperties = canvas.isRecordingFor(mRenderNode);
final int childrenCount = mChildrenCount;
final View[] children = mChildren;
...
for (int i = 0; i < childrenCount; i++) {
while (transientIndex >= 0 && mTransientIndices.get(transientIndex) == i) {
final View transientChild = mTransientViews.get(transientIndex);
if ((transientChild.mViewFlags & VISIBILITY_MASK) == VISIBLE ||
transientChild.getAnimation() != null) {
more |= drawChild(canvas, transientChild, drawingTime);
}
transientIndex++;
if (transientIndex >= transientCount) {
transientIndex = -1;
}
}
final int childIndex = getAndVerifyPreorderedIndex(childrenCount, i, customOrder);
final View child = getAndVerifyPreorderedView(preorderedList, children, childIndex);
if ((child.mViewFlags & VISIBILITY_MASK) == VISIBLE || child.getAnimation() != null) {
more |= drawChild(canvas, child, drawingTime);
}
}
...
}
ViewGroup的dispatchDraw()方法重寫了View的dispatchDraw()方法，并在循環(huán)體內(nèi)調(diào)用drawChild()方法繪制其子View。
protected boolean drawChild(Canvas canvas, View child, long drawingTime) {
return child.draw(canvas, this, drawingTime);
}
drawChild()方法的作用是繪制ViewGroup的子View。
6.5 DecorView的繪制
//DecorView
@Override
public void draw(Canvas canvas) {
super.draw(canvas);
if (mMenuBackground != null) {
mMenuBackground.draw(canvas);
}
}
DecorView重寫了draw()方法，在其內(nèi)部調(diào)用了super.draw(canvas)，因?yàn)镕rameLayout和ViewGroup都沒有重寫draw()方法，所以super.draw(canvas)調(diào)用的其實(shí)是View的draw()方法，在執(zhí)行onDraw()后，會(huì)調(diào)用dispatchDraw()方法去遍歷繪制子view。
7.總結(jié)
Android View的繪制流程經(jīng)歷measure、layout、draw三個(gè)階段，ViewRootImpl類中的performTraversals()方法是繪制流程開始的地方，performTraversals()方法中包含有performMeasure()、performLayout()、performDraw()三個(gè)與View繪制相關(guān)的方法，測(cè)量、布局和繪制都是從ViewTree的根節(jié)點(diǎn)DecorView遍歷執(zhí)行的，一般我們自定義View的時(shí)候，重寫onMeasure()、onLayout()、onDraw()方法，在其中加入我們自己的業(yè)務(wù)邏輯去實(shí)現(xiàn)需求。
