在上一篇文章【从源码角度分析Activity、Window、View的关系】中讲到了View的加载流程，最终会调用ViewRootImpl的invalidate()方法。如果对View的加载流程不熟悉的，可以先去了解下，然后再和这篇文章结合，这样理解的会比较体系化。下面开始今天的View的绘制流程之源码之旅。
1、ViewRootImpl的invalidate()
void invalidate() {
mDirty.set(0, 0, mWidth, mHeight);
if (!mWillDrawSoon) {
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
//1
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
在上面的代码中， invalidate() 中有一个mWillDrawSoon标识位，在进行绘制流程的时候mWillDrawSoon=true，它的作用：在我们调用invalidate()，其未执行完成时，再次调用invalidate()是无效的。也就是说在同一时间我们多次调用invalidate()，只会执行一次。在上面的代码可以发现任务转移到了注释1的位置，其中mTraversalRunnable是一个Runnable对象，mChoreographer是一个Choreographer对象，先进入Choreographer的postCallback()方法看下做了什么事？
public void postCallback(int callbackType, Runnable action, Object token) {
postCallbackDelayed(callbackType, action, token, 0);
}
public void postCallbackDelayed(int callbackType,
Runnable action, Object token, long delayMillis) {
if (action == null) {
throw new IllegalArgumentException("action must not be null");
}
if (callbackType < 0 || callbackType > CALLBACK_LAST) {
throw new IllegalArgumentException("callbackType is invalid");
}
postCallbackDelayedInternal(callbackType, action, token, delayMillis);
}
private void postCallbackDelayedInternal(int callbackType,
Object action, Object token, long delayMillis) {
if (DEBUG_FRAMES) {
Log.d(TAG, "PostCallback: type=" + callbackType
+ ", action=" + action + ", token=" + token
+ ", delayMillis=" + delayMillis);
}
synchronized (mLock) {
final long now = SystemClock.uptimeMillis();
final long dueTime = now + delayMillis;
mCallbackQueues[callbackType].addCallbackLocked(dueTime, action, token);
if (dueTime <= now) {
scheduleFrameLocked(now);
} else {
Message msg = mHandler.obtainMessage(MSG_DO_SCHEDULE_CALLBACK, action);
msg.arg1 = callbackType;
msg.setAsynchronous(true);
mHandler.sendMessageAtTime(msg, dueTime);
}
}
}
在上面代码中，通过一系列的跳转，我们可以发现，最后会在postCallbackDelayedInternal()方法内部通过mHandler发送消息处理，这里调用的是mTraversalRunnable的run方法去执行，并未启动一个新的工作线程，从这里可以知道，ViewRootImpl的invalidate()是同步执行的。现在已经知道怎么启动的了，那么我们回到mTraversalRunnable的run方法。
final class TraversalRunnable implements Runnable {
@Override
public void run() {
//绘制流程开始执行
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
//执行遍历
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
2、ViewRootImpl的performTraversals()方法
由于该方法的代码量实在是太大了，这里就截取关键性的代码，一些细节可能会忽略掉，但是不会影响对整体绘制流程的理解。
private void performTraversals() {
//省略代码....
//在这里获取的是根布局的WidthMeasureSpec和HeightMeasureSpec，
//根布局的参数是从window中得到的，看了文章开头提的那篇文章就可以发现
int childWidthMeasureSpec = getRootMeasureSpec(mWidth, lp.width);
int childHeightMeasureSpec = getRootMeasureSpec(mHeight, lp.height);
//省略代码....
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
//省略代码....
performLayout(lp, mWidth, mHeight);
//省略代码....
performDraw();
//省略代码....
}
在performTraversals() 对于绘制流程主要的就是上面的三个方法了。接下来对这个三个方法进行逐个分析。
3、 performMeasure()
对View进行测量，内部主要调用了View的measure()方法，源码如下：
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
if (mView == null) {
return;
}
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
//对View进行测量
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
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;
//......1
widthMeasureSpec = MeasureSpec.adjust(widthMeasureSpec, optical ? -oWidth : oWidth);
heightMeasureSpec = MeasureSpec.adjust(heightMeasureSpec, optical ? -oHeight : oHeight);
}
// Suppress sign extension for the low bytes
long key = (long) widthMeasureSpec << 32 | (long) heightMeasureSpec & 0xffffffffL;
if (mMeasureCache == null) mMeasureCache = new LongSparseLongArray(2);
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// Optimize layout by avoiding an extra EXACTLY pass when the view is
// already measured as the correct size. In API 23 and below, this
// extra pass is required to make LinearLayout re-distribute weight.
final boolean specChanged = widthMeasureSpec != mOldWidthMeasureSpec
|| heightMeasureSpec != mOldHeightMeasureSpec;
final boolean isSpecExactly = MeasureSpec.getMode(widthMeasureSpec) == MeasureSpec.EXACTLY
&& MeasureSpec.getMode(heightMeasureSpec) == MeasureSpec.EXACTLY;
final boolean matchesSpecSize = getMeasuredWidth() == MeasureSpec.getSize(widthMeasureSpec)
&& getMeasuredHeight() == MeasureSpec.getSize(heightMeasureSpec);
final boolean needsLayout = specChanged
&& (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
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) {
//......2
// 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;
}
// flag not set, setMeasuredDimension() was not invoked, we raise
// an exception to warn the developer
if ((mPrivateFlags & PFLAG_MEASURED_DIMENSION_SET) != PFLAG_MEASURED_DIMENSION_SET) {
throw new IllegalStateException("View with id " + getId() + ": "
+ getClass().getName() + "#onMeasure() did not set the"
+ " measured dimension by calling"
+ " setMeasuredDimension()");
}
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
mOldWidthMeasureSpec = widthMeasureSpec;
mOldHeightMeasureSpec = heightMeasureSpec;
mMeasureCache.put(key, ((long) mMeasuredWidth) << 32 |
(long) mMeasuredHeight & 0xffffffffL); // suppress sign extension
}
从上面代码可发现，View的measure方法是不可重写的。在注释1，通过MeasureSpec调整widthMeasureSpec和heightMeasureSpec；而这个MeasureSpec是什么呢？
A MeasureSpec encapsulates the layout requirements passed from parent to child. Each MeasureSpec represents a requirement for either the width or the height. A MeasureSpec is comprised of a size and a mode.
上面是Google官方文档对MeasureSpec的定义。大概意思：MeasureSpec内部封装了父View对子View的布局规格要求，每一个MeasureSpec都代表一个宽度和高度的规格，由一个尺寸和一个模式组成。
MeasureSpec的三种模式：
- UNSPECIFIED： 父View对子View不施加任何要求，子View想要多大就多大。
- EXACTLY ：父View对子View设定了确切的尺寸，不管子View想要多大，都会是父View给予的大小
- AT_MOST：子View可以达到它自己想要的大小，但是不会超过父View所剩余的空间。
我们回到绘制流程中，继续跟踪代码，在注释2，调用了onMeasure()并且把widthMeasureSpec和heightMeasureSpec作为参数传入。
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}
可以发现onMeasure()方法很简单，只是调用了setMeasuredDimension()方法；首先，看下setMeasuredDimension()方法中的getDefaultSize(getSuggestedMinimumWidth())；宽度和高度的设置都是同一个原理，所以这里就选一个宽度来分析。
//measureSpec也就是从父View传递过来的widthMeasureSpec
//size是getSuggestedMinimumWidth()返回的值
public static int getDefaultSize(int size, int measureSpec) {
int result = size;
//获取父View的测量模式
int specMode = MeasureSpec.getMode(measureSpec);
//获取父View的宽的尺寸大小
int specSize = MeasureSpec.getSize(measureSpec);
//根据父View的测量模式返回一个默认的尺寸
switch (specMode) {
case MeasureSpec.UNSPECIFIED:
result = size;
break;
case MeasureSpec.AT_MOST:
case MeasureSpec.EXACTLY:
result = specSize;
break;
}
return result;
}
//这里先判断是否设置了背景，如果没有就返回mMinWidth （即：android:minWidth）
//如果有，则返回背景的最小宽度
protected int getSuggestedMinimumWidth() {
return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());
}
//保存measuredWidth和measuredHeight
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);
}
到这里，View的基本的measure就已经完成了，但是，根视图是DecorView，是一个ViewGroup，那么对于ViewGroup而言，其测量又是怎样的呢？ViewGroup也是继承自View，不过由于它的特殊性，它存放的是一个个子View，所以并不是直接对ViewGroup进行measure，而是通过遍历的方式对子View进行measure，等遍历完所有子View后确认自身大小。在ViewGroup中measure的方法有measureChildWithMargins()、measureChildren()、measureChild()；measureChildren()这个方法遍历时直接调用measureChild()对子View进行measure。measureChild()和measureChildWithMargins()的区别是在于有没有把margin和padding作为子视图的大小。对measureChildren()这个方法，里面的实现比较简单，通过传入父View的WidthMeasureSpec、HeightMeasureSpec和自身的LayoutParams结合，生成自己的childWidthMeasureSpec；
下面我们看下measureChildWithMargins()这个方法：
protected void measureChildWithMargins(View child,
int parentWidthMeasureSpec, int widthUsed,
int parentHeightMeasureSpec, int heightUsed) {
//子View的LayoutParams
final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
//根据自身的LayoutParams和父view的MeasureSpec生成自生的childMeasureSpec
//同时计算了Padding 和 Margin值，已使用的尺寸
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);
//这里会回调到我们之前分析的View的onMeasure()方法
child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}
public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
//获取父View的Size和Mode
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding);
int resultSize = 0;
int resultMode = 0;
//根据父View不同的模式，对子View进行不同的调整
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);
}
对于绘制流程的第一步到这里就结束了，这里对前面的信息再回顾下，通过performMeasure()进行测量时，然后调用View的measure()，然后又切换onMeasure()；对于ViewGroup，是通过遍历测量子View来确定自身的最终大小的。在测量过程中，View的大小是有父View的测量规格和View自身的LayoutParams决定的。在实际开发中，如果我们想要获取View的宽度，就必须在onMeasure()后获取才有效。
写的有点飘了~~~补充下查克拉，继续
4、performLayout()
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
//............省略代码
final View host = mView;
if (host == null) {
return;
}
//............省略代码
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
//............省略代码
}
public void layout(int l, int t, int r, int b) {
//已进行Measure的会跳过这里
if ((mPrivateFlags3 & PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT) != 0) {
onMeasure(mOldWidthMeasureSpec, mOldHeightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
int oldL = mLeft;
int oldT = mTop;
int oldB = mBottom;
int oldR = mRight;
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
//........1
onLayout(changed, l, t, r, b);
if (shouldDrawRoundScrollbar()) {
if(mRoundScrollbarRenderer == null) {
mRoundScrollbarRenderer = new RoundScrollbarRenderer(this);
}
} else {
mRoundScrollbarRenderer = null;
}
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
ListenerInfo li = mListenerInfo;
if (li != null && li.mOnLayoutChangeListeners != null) {
ArrayList listenersCopy =
(ArrayList)li.mOnLayoutChangeListeners.clone();
int numListeners = listenersCopy.size();
for (int i = 0; i < numListeners; ++i) {
listenersCopy.get(i).onLayoutChange(this, l, t, r, b, oldL, oldT, oldR, oldB);
}
}
}
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
if ((mPrivateFlags3 & PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT) != 0) {
mPrivateFlags3 &= ~PFLAG3_NOTIFY_AUTOFILL_ENTER_ON_LAYOUT;
notifyEnterOrExitForAutoFillIfNeeded(true);
}
}
在上面的代码中可以发现，任务转移到到了注释1的 onLayout(changed, l, t, r, b)；l和t是左上角坐标，r和b是右下角坐标。接下我们看下onLayout源码：
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
}
这里竟然是空实现，那么我们就找一个具体的来分析，下面选了RelativeLayout中的onLayout。
@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
// The layout has actually already been performed and the positions
// cached. Apply the cached values to the children.
final int count = getChildCount();
for (int i = 0; i < count; i++) {
View child = getChildAt(i);
if (child.getVisibility() != GONE) {
RelativeLayout.LayoutParams st =
(RelativeLayout.LayoutParams) child.getLayoutParams();
child.layout(st.mLeft, st.mTop, st.mRight, st.mBottom);
}
}
}
RelativeLayout中的onLayout的实现，首先获取子View的总数，然后for循环遍历，得到每个子View的LayoutParams ，再获取子View的左上角坐标，和右下角坐标，开始layout。
Layout过程总结：layout在实现方面也比较灵活，因为对于每一个ViewGroup而言，绝大多数时候，其内容都是不同的，通过遍历每一个子View去layout，而相应参数在measure过程已经保存了，会传递到layout这个过程。
5、 performDraw()
接下来看绘制流程的最后一个，绘制（Draw）。首先进入performDraw()方法。
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
return;
} else if (mView == null) {
return;
}
final boolean fullRedrawNeeded = mFullRedrawNeeded;
mFullRedrawNeeded = false;
mIsDrawing = true;
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "draw");
try {
draw(fullRedrawNeeded);
} finally {
mIsDrawing = false;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
//..........代码省略
}
从上面的代码中，找到关键点draw()，接着往下看
private void draw(boolean fullRedrawNeeded) {
Surface surface = mSurface; //原始图片的处理缓冲区
if (!surface.isValid()) {
return;
}
//.........省略代码
//开始绘制
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}
在draw()方法里只选择了关键性的代码作为参考，把任务切换到了drawSoftware()方法。
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
// Draw with software renderer.
final Canvas canvas;
try {
final int left = dirty.left;
final int top = dirty.top;
final int right = dirty.right;
final int bottom = dirty.bottom;
//.......1
canvas = mSurface.lockCanvas(dirty);
// The dirty rectangle can be modified by Surface.lockCanvas()
//noinspection ConstantConditions
if (left != dirty.left || top != dirty.top || right != dirty.right
|| bottom != dirty.bottom) {
attachInfo.mIgnoreDirtyState = true;
}
// TODO: Do this in native
canvas.setDensity(mDensity);
} catch (Surface.OutOfResourcesException e) {
handleOutOfResourcesException(e);
return false;
} catch (IllegalArgumentException e) {
Log.e(mTag, "Could not lock surface", e);
// Don't assume this is due to out of memory, it could be
// something else, and if it is something else then we could
// kill stuff (or ourself) for no reason.
mLayoutRequested = true; // ask wm for a new surface next time.
return false;
}
try {
if (DEBUG_ORIENTATION || DEBUG_DRAW) {
Log.v(mTag, "Surface " + surface + " drawing to bitmap w="
+ canvas.getWidth() + ", h=" + canvas.getHeight());
//canvas.drawARGB(255, 255, 0, 0);
}
// If this bitmap's format includes an alpha channel, we
// need to clear it before drawing so that the child will
// properly re-composite its drawing on a transparent
// background. This automatically respects the clip/dirty region
// or
// If we are applying an offset, we need to clear the area
// where the offset doesn't appear to avoid having garbage
// left in the blank areas.
if (!canvas.isOpaque() || yoff != 0 || xoff != 0) {
canvas.drawColor(0, PorterDuff.Mode.CLEAR);
}
dirty.setEmpty();
mIsAnimating = false;
mView.mPrivateFlags |= View.PFLAG_DRAWN;
if (DEBUG_DRAW) {
Context cxt = mView.getContext();
Log.i(mTag, "Drawing: package:" + cxt.getPackageName() +
", metrics=" + cxt.getResources().getDisplayMetrics() +
", compatibilityInfo=" + cxt.getResources().getCompatibilityInfo());
}
try {
canvas.translate(-xoff, -yoff);
if (mTranslator != null) {
mTranslator.translateCanvas(canvas);
}
canvas.setScreenDensity(scalingRequired ? mNoncompatDensity : 0);
attachInfo.mSetIgnoreDirtyState = false;
//.......2
mView.draw(canvas);
drawAccessibilityFocusedDrawableIfNeeded(canvas);
} finally {
if (!attachInfo.mSetIgnoreDirtyState) {
// Only clear the flag if it was not set during the mView.draw() call
attachInfo.mIgnoreDirtyState = false;
}
}
} finally {
try {
//.......3
surface.unlockCanvasAndPost(canvas);
} catch (IllegalArgumentException e) {
Log.e(mTag, "Could not unlock surface", e);
mLayoutRequested = true; // ask wm for a new surface next time.
//noinspection ReturnInsideFinallyBlock
return false;
}
if (LOCAL_LOGV) {
Log.v(mTag, "Surface " + surface + " unlockCanvasAndPost");
}
}
return true;
}
在上面的代码中，注释1处，利用mSurface.lockCanvas()锁定dirty区域，获取Canvas。然后在注释2处，把canvas作为参数传入View的draw()方法进行绘制。注释3，绘制完成后回收surface。接下来切换到View的draw()：
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);
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 (debugDraw()) {
debugDrawFocus(canvas);
}
// we're done...
return;
}
//.........省略代码
}
上面代码中的注释已经很明确了，
step1：进行背景的绘制，
step2 & step5：保存图层，绘制边缘、渐变效果，
step3：绘制view的内容，在这个地方，不同的view，内容也不一样（子View重写），这也就是我们自定义View重写的onDraw()，
step4：同样也是绘制子View，不同的View需要各自重写，
step6：绘制装饰(前台、滚动条)，
step7：绘制默认焦点到画布上。
Android view的绘制流程.png
完篇