Android- 举一反三：12 个 View 绘制流程高频面试题，带你全面理解 View 的绘制流程
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
7. LayoutInflate 的流程
既然上一步用到了 LayoutInflate.inflate，那使用 LayoutInflate.inflate 加载一个布局的流程是什么样的呢？
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
// 通过 resourceId 获取 xml 布局内容
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
// ...
View result = root;
try {
// Look for the root node.
int type;
// 找到 xml start 或者 xml end
while ((type = parser.next()) != XmlPullParser.START_TAG &&
type != XmlPullParser.END_DOCUMENT) {
// Empty
}
if (type != XmlPullParser.START_TAG) {
throw new InflateException(parser.getPositionDescription()
": No start tag found!");
}
final String name = parser.getName();
// 处理 merge 标签
if (TAG_MERGE.equals(name)) {
if (root == null || !attachToRoot) {
throw new InflateException("<merge /> can be used only with a valid "
"ViewGroup root and attachToRoot=true");
}
// merge 标签传入的 parent 是 rootView
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// 通过 tag 创建 View
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
// 使用 rootView 默认的 LayoutParams
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
temp.setLayoutParams(params);
}
}
// 创建子 View
rInflateChildren(parser, temp, attrs, true);
if (root != null && attachToRoot) {
// 添加到 rootView
root.addView(temp, params);
}
// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
result = temp;
}
}
} catch (XmlPullParserException e) {
} finally {
}
return result;
}
}
final void rInflateChildren(XmlPullParser parser, View parent, AttributeSet attrs,
boolean finishInflate) throws XmlPullParserException, IOException {
rInflate(parser, parent, parent.getContext(), attrs, finishInflate);
}
void rInflate(XmlPullParser parser, View parent, Context context,
AttributeSet attrs, boolean finishInflate) throws XmlPullParserException, IOException {
while (((type = parser.next()) != XmlPullParser.END_TAG ||
parser.getDepth() > depth) && type != XmlPullParser.END_DOCUMENT) {
if (type != XmlPullParser.START_TAG) {
continue;
}
final String name = parser.getName();
if (TAG_REQUEST_FOCUS.equals(name)) {
pendingRequestFocus = true;
consumeChildElements(parser);
} else if (TAG_TAG.equals(name)) {
parseViewTag(parser, parent, attrs);
} else if (TAG_INCLUDE.equals(name)) {
// 处理 include 标签
if (parser.getDepth() == 0) {
throw new InflateException("<include /> cannot be the root element");
}
parseInclude(parser, context, parent, attrs);
} else if (TAG_MERGE.equals(name)) {
throw new InflateException("<merge /> must be the root element");
} else {
// 通过 xml 标签生成 View
final View view = createViewFromTag(parent, name, context, attrs);
final ViewGroup viewGroup = (ViewGroup) parent;
final ViewGroup.LayoutParams params = viewGroup.generateLayoutParams(attrs);
rInflateChildren(parser, view, attrs, true);
viewGroup.addView(view, params);
}
}
}
上面的流程可以看到，LayoutInflate.inflate 最终是调用 createViewFromTag 从 xml 生成 View 的，其实这里才是关键。
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
/** 如果是 view 标签的话，就取其 class 属性作为 name
比如
<view class="LinearLayout"/>
最终生成的会是一个 LinearLayout
是不是又学会了一种 view 的写法 ^_^
*/
if (name.equals("view")) {
name = attrs.getAttributeValue(null, "class");
}
// 处理 blink 标签
if (name.equals(TAG_1995)) {
return new BlinkLayout(context, attrs);
}
try {
// 通过 mFactory2、mFactory、mPrivateFactory 创建 View
View view;
if (mFactory2 != null) {
view = mFactory2.onCreateView(parent, name, context, attrs);
} else if (mFactory != null) {
view = mFactory.onCreateView(name, context, attrs);
} else {
view = null;
}
if (view == null && mPrivateFactory != null) {
view = mPrivateFactory.onCreateView(parent, name, context, attrs);
}
// 没有设置 Factory，走默认的创建 View 的流程
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
view = onCreateView(parent, name, attrs);
} else {
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
return view;
} catch (InflateException e) {
}
}
这里我们需要了解一下，mFactory、mFactory2、mPrivateFactory 都是什么？
private Factory mFactory;
private Factory2 mFactory2;
private Factory2 mPrivateFactory;
public interface Factory {
public View onCreateView(String name, Context context, AttributeSet attrs);
}
public interface Factory2 extends Factory {
public View onCreateView(View parent, String name, Context context, AttributeSet attrs);
}
mFactory、mFactory2、mPrivateFactory 分别对应 Factory 和 Factory2 方法，对应的是两个 onCreateView 方法，Factory.onCreateView 没有传入 parent 参数，Factory2.onCreateView 传入了 parent 参数。
而 mFactory 和 mFactory2 我们是可以设置的，当然不能重复设置，重复设置会抛出异常。
如果已经有 mFactory 的值，则生成一个 FactoryMerger，这个也是继承了 Factory2，用来控制一下调用顺序。
具体代码如下
public void setFactory(Factory factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = factory;
} else {
mFactory = new FactoryMerger(factory, null, mFactory, mFactory2);
}
}
public void setFactory2(Factory2 factory) {
if (mFactorySet) {
throw new IllegalStateException("A factory has already been set on this LayoutInflater");
}
if (factory == null) {
throw new NullPointerException("Given factory can not be null");
}
mFactorySet = true;
if (mFactory == null) {
mFactory = mFactory2 = factory;
} else {
mFactory = mFactory2 = new FactoryMerger(factory, factory, mFactory, mFactory2);
}
}
private static class FactoryMerger implements Factory2 {
private final Factory mF1, mF2;
private final Factory2 mF12, mF22;
FactoryMerger(Factory f1, Factory2 f12, Factory f2, Factory2 f22) {
mF1 = f1;
mF2 = f2;
mF12 = f12;
mF22 = f22;
}
public View onCreateView(String name, Context context, AttributeSet attrs) {
View v = mF1.onCreateView(name, context, attrs);
if (v != null) return v;
return mF2.onCreateView(name, context, attrs);
}
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
View v = mF12 != null ? mF12.onCreateView(parent, name, context, attrs)
: mF1.onCreateView(name, context, attrs);
if (v != null) return v;
return mF22 != null ? mF22.onCreateView(parent, name, context, attrs)
: mF2.onCreateView(name, context, attrs);
}
}
然后我们再看 mPrivateFactory，看名称就知道是系统的隐藏方法。
调用时机是在 Activity.attach 中，Activity 其实是实现了 Factory2 的 onCreateView 方法，其中对 fragment 做了处理，如果是 fragment 标签，就调用 fragment 的 onCreateView，这里就不详细往下面看了，如果是非 fragment 的标签，就返回 null，走默认的创建 View 的方法。
/**
@hide for use by framework
*/
public void setPrivateFactory(Factory2 factory) {
if (mPrivateFactory == null) {
mPrivateFactory = factory;
} else {
mPrivateFactory = new FactoryMerger(factory, factory, mPrivateFactory, mPrivateFactory);
}
}
// Activity
final void attach(...)
mWindow.getLayoutInflater().setPrivateFactory(this);
}
public View onCreateView(String name, Context context, AttributeSet attrs) {
return null;
}
public View onCreateView(View parent, String name, Context context, AttributeSet attrs) {
if (!"fragment".equals(name)) {
return onCreateView(name, context, attrs);
}
return mFragments.onCreateView(parent, name, context, attrs);
}
所以上面的 Factory 和 Factory2，是系统留给我们的 hook View 创建流程的接口。
如果都没有设置，那就走到默认的创建 View 的方法。
默认创建 View 的方法比较简单，就是反射调用 View 的构造函数，然后做一个缓存，然后创建 View。
具体代码如下
// LayoutInflate
View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
boolean ignoreThemeAttr) {
// 前面的 mFactory、mFactory2、mPrivateFactory 都没有去创建 View
if (view == null) {
final Object lastContext = mConstructorArgs[0];
mConstructorArgs[0] = context;
try {
if (-1 == name.indexOf('.')) {
// 如果名称里没有 “.”，也就是系统的 View，需要添加 android.view. 前缀，比如 <LinearLayout />，最终去创建的名称是 android.view.LinearLayout
view = onCreateView(parent, name, attrs);
} else {
// 如果是自定义 View，则直接去创建
view = createView(name, null, attrs);
}
} finally {
mConstructorArgs[0] = lastContext;
}
}
// ...
}
protected View onCreateView
(String name, AttributeSet attrs)
throws ClassNotFoundException {
return createView(name, "android.view.", attrs);
}
public final View createView(String name, String prefix, AttributeSet attrs)
throws ClassNotFoundException, InflateException {
Constructor<? extends View> constructor = sConstructorMap.get(name);
if (constructor != null && !verifyClassLoader(constructor)) {
constructor = null;
sConstructorMap.remove(name);
}
Class<? extends View> clazz = null;
try {
if (constructor == null) {
// 加载对应的类
clazz = mContext.getClassLoader().loadClass(
prefix != null ? (prefix + name) : name).asSubclass(View.class);
// 反射获取构造函数
constructor = clazz.getConstructor(mConstructorSignature);
constructor.setAccessible(true);
// 做个缓存，下次直接使用，提高效率
sConstructorMap.put(name, constructor);
} else {
}
Object lastContext = mConstructorArgs[0];
if (mConstructorArgs[0] == null) {
// Fill in the context if not already within inflation.
mConstructorArgs[0] = mContext;
}
Object[] args = mConstructorArgs;
args[1] = attrs;
// 调用构造函数创建 View
final View view = constructor.newInstance(args);
if (view instanceof ViewStub) {
// 处理 ViewStub
final ViewStub viewStub = (ViewStub) view;
viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
}
mConstructorArgs[0] = lastContext;
return view;
} catch (NoSuchMethodException e) {
}
}
所以上面就是 LayoutInflate.inflate 的整个流程。
8. Activity、PhoneWindow、DecorView、ViewRootImpl 的关系？
其实上面的问题中，我们经常会说到 PhoneWindow 这个角色，PhoneWindow 其实是 Window 的唯一子类，是 Activity 和 View 交互系统的中间层，而 DecorView 是整个 View 层级的最顶层，ViewRootImpl 是 DecorView 的 parent，但是他并不是一个真正的 View，只是继承了 ViewParent 接口，用来掌管 View 的各种事件，包括 requestLayout、invalidate、dispatchInputEvent 等等。
9. PhoneWindow 的创建时机？
既然上面又提到了 PhoneWindow，那么 PhoneWindow 是什么时候创建的呢？是在 Activity.attach 里创建的，而 Activity.attach 又是在 ActivityThread.performLaunchActivity 里创建的。
这里就又能引申出 Activity 的启动流程，这里就先不讲了。
10. 如何触发重新绘制？
既然上面说到 View 的绘制流程，那我们怎么触发 View 的重新绘制呢？
就是调用 requestLayout 和 invalidate。
11. requestLayout 和 invalidate 的流程
requestLayout 流程
// View
public void requestLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null && viewRoot.isInLayout()) {
if (!viewRoot.requestLayoutDuringLayout(this)) {
// 如果当前在 layout 流程中，并且是在处理 requestLayout，那么就直接返回，这个时候需要注意，mPrivateFlags 并没有设置 FORCE_LAYOUT
// 这个时候 reqeustLayout 会在下一个 frame 里执行
return;
}
}
mAttachInfo.mViewRequestingLayout = this;
}
// 如果当前在 layout 流程中，但是没有处理 requestLayout，那么就继续后面的流程，这个时候 mPrivateFlags 是设置为 FORCE_LAYOUT
// 这个时候 requestLayout 会在下一次 layout 过程中进行执行
// 设置 FORCE_LAYOUT 和 INVALIDETED flag
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
if (mParent != null && !mParent.isLayoutRequested()) {
// 层层调用 parent 的 requestLayout
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}
从上面代码可以看到，会一层层调用 parent 的 requestLayout，而上面的问题中我们也分析到了，DecorView 是整个 View 层级的最顶层，ViewRootImpl 又是 DecorView 的 parent，所以最终调用到 ViewRootImpl 的 requestLayout。
// ViewRootImpl
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
ViewRootImpl.requestLayout 调用 scheduleTraversals -> doTraversal -> performTraversals 开启绘制流程。
其实这里又涉及到了 Choreographer 的一些流程，这里也暂时不展开讲了。
在 performTraversals 里，就是熟悉的 performMeasure -> performLayout -> performDraw 三个流程了。
先看 performMeasure，最终调用的是 View.measure
// View
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
// 这里就是 requestLayout 时设置的 flag，如果执行了 requestLayout，这里 forceLayout 一定是 true
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// needsLayout 是 measureSpec 和 oldMeasureSpec 不相符的时候会为 true
if (forceLayout || needsLayout) {
onMeasure(widthMeasureSpec, heightMeasureSpec);
}
// 设置 LAYOUT_REQUIRED flag，在 layout 中会用到
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
再看 performLayout
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
// 表明在 layout 流程中
mInLayout = true;
final View host = mView;
try {
// 先执行 layout
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mInLayout = false;
// 这里处理在上一次 layout 过程中，调用了 requestLayout 的 View
int numViewsRequestingLayout = mLayoutRequesters.size();
if (numViewsRequestingLayout > 0) {
// requestLayout() was called during layout.
// If no layout-request flags are set on the requesting views, there is no problem.
// If some requests are still pending, then we need to clear those flags and do
// a full request/measure/layout pass to handle this situation.
// 获取有效的需要 layout 的 View，此时获取的是 mPrivateFlags == PFLAG_FORCE_LAYOUT 的 View，也就是在 View.requestLayout 里设置了 PFLAG_FORCE_LAYOUT 的 View
ArrayList<View> validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters,
false);
if (validLayoutRequesters != null) {
// Set this flag to indicate that any further requests are happening during
// the second pass, which may result in posting those requests to the next
// frame instead
// 表明当前在处理 requestLayout
mHandlingLayoutInLayoutRequest = true;
// Process fresh layout requests, then measure and layout
int numValidRequests = validLayoutRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = validLayoutRequesters.get(i);
view.requestLayout();
}
// 执行 measure
measureHierarchy(host, lp, mView.getContext().getResources(),
desiredWindowWidth, desiredWindowHeight);
mInLayout = true;
// 执行 Layout
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
mHandlingLayoutInLayoutRequest = false;
// Check the valid requests again, this time without checking/clearing the
// layout flags, since requests happening during the second pass get noop'd
// 获取 mPrivateFlags != PFLAG_FORCE_LAYOUT 的 View，也就是在 View.requestLayout 里没有设置 PFLAG_FORCE_LAYOUT 的 View
validLayoutRequesters = getValidLayoutRequesters(mLayoutRequesters, true);
if (validLayoutRequesters != null) {
final ArrayList<View> finalRequesters = validLayoutRequesters;
// 在下一次 frame 里再执行一次 requestLayout
// 下一次 performTraversals 里会执行 getRunQueue().executeActions(mAttachInfo.mHandler);
getRunQueue().post(new Runnable() {
@Override
public void run() {
int numValidRequests = finalRequesters.size();
for (int i = 0; i < numValidRequests; ++i) {
final View view = finalRequesters.get(i);
view.requestLayout();
}
}
});
}
}
}
} finally {
}
mInLayout = false;
}
上面 performLayout 里一共执行了三件事：
执行 View.layout
执行调用过 requestLayout 的 View 的 measure 和 layout
将还没有执行的 requestLayout 加到队列中，下一次 frame 中进行执行
然后看 View.layout 的流程：
public void layout(int l, int t, int r, int b) {
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;
// 判断是否位置有变化
boolean changed = isLayoutModeOptical(mParent) ?
setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 如果位置有变化，或者设置了 PFLAG_LAYOUT_REQUIRED，PFLAG_LAYOUT_REQUIRED 是在 View.measure 结束以后设置的
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
// ...
// 取消 flag
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
// ...
}
// 取消 flag
mPrivateFlags &= ~PFLAG_FORCE_LAYOUT;
mPrivateFlags3 |= PFLAG3_IS_LAID_OUT;
// ...
}
最后就是 ViewRootImpl.performDraw -> draw 了。
// ViewRootImpl
private boolean draw(boolean fullRedrawNeeded) {
// 有 dirty 区域会进行重绘
if (!dirty.isEmpty() || mIsAnimating || accessibilityFocusDirty) {
if (fullRedrawNeeded) {
mAttachInfo.mIgnoreDirtyState = true;
// 如果需要全部重绘，把 dirty 区域设置成 DecorView 的区域
dirty.set(0, 0, (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
}
// drawSoftware 调用了 DecorView.draw
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset,
scalingRequired, dirty, surfaceInsets)) {
return false;
}
}
}
// View
public void draw(Canvas canvas) {
final int privateFlags = mPrivateFlags;
// flag 是 PFLAG_DIRTY_OPAQUE 则需要绘制
final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
(mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;
if (!dirtyOpaque) {
drawBackground(canvas);
}
if (!dirtyOpaque) onDraw(canvas);
// 绘制 Child
dispatchDraw(canvas);
// foreground 不管 dirtyOpaque 标志，每次都会绘制
onDrawForeground(canvas);
}
在 View 的绘制过程中，我们可以看到，只有 flag 被设置为 PFLAG_DIRTY_OPAQUE 才会进行绘制（这里划重点）。
这也就是大家经常说的 requestLayout 不会引发 draw。
invalidate 流程
invalidate -> invalidateInternal 的主要流程就是在设置 mPrivateFlags
void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
boolean fullInvalidate) {
// ...
if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
|| (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
|| (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
|| (fullInvalidate && isOpaque() != mLastIsOpaque)) {
if (fullInvalidate) {
mLastIsOpaque = isOpaque();
mPrivateFlags &= ~PFLAG_DRAWN;
}
// 设置 dirty flag
mPrivateFlags |= PFLAG_DIRTY;
if (invalidateCache) {
mPrivateFlags |= PFLAG_INVALIDATED;
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
}
if (p != null && ai != null && l < r && t < b) {
final Rect damage = ai.mTmpInvalRect;
damage.set(l, t, r, b);
p.invalidateChild(this, damage);
}
// ...
}
}
invalidate 会调用 parent.invalidateChild
public final void invalidateChild(View child, final Rect dirty) {
final boolean drawAnimation = (child.mPrivateFlags & PFLAG_DRAW_ANIMATION) != 0;
// child 不透明的条件是没有动画且 child 本身是不透明的
final boolean isOpaque = child.isOpaque() && !drawAnimation &&
child.getAnimation() == null && childMatrix.isIdentity();
// 不透明的话使用的是 PFLAG_DIRTY_OPAQUE flag
int opaqueFlag = isOpaque ? PFLAG_DIRTY_OPAQUE : PFLAG_DIRTY;
do {
View view = null;
if (parent instanceof View) {
view = (View) parent;
}
if ((view.mPrivateFlags & PFLAG_DIRTY_MASK) != PFLAG_DIRTY) {
// 设置 flag 为 PFLAG_DIRTY_OPAQUE
view.mPrivateFlags = (view.mPrivateFlags & ~PFLAG_DIRTY_MASK) | opaqueFlag;
}
// 计算 parent 的 dirty 区域
parent = parent.invalidateChildInParent(location, dirty);
} while (parent != null);
}
上面的 while 循环里，会层层计算 parent 的 dirty 区域，最终会调用到 ViewRootImpl.invalidateChildInParent -> invalidateRectOnScreen
private void invalidateRectOnScreen(Rect dirty) {
final Rect localDirty = mDirty;
if (!localDirty.isEmpty() && !localDirty.contains(dirty)) {
mAttachInfo.mSetIgnoreDirtyState = true;
mAttachInfo.mIgnoreDirtyState = true;
}
// Add the new dirty rect to the current one
localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom);
// Intersect with the bounds of the window to skip
// updates that lie outside of the visible region
final float appScale = mAttachInfo.mApplicationScale;
final boolean intersected = localDirty.intersect(0, 0,
(int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
if (!intersected) {
localDirty.setEmpty();
}
if (!mWillDrawSoon && (intersected || mIsAnimating)) {
// 调用 scheduleTraversals 进行整个绘制流程
scheduleTraversals();
}
}
最终调用 scheduleTraversals 去触发整个绘制流程，然后调用到 View.draw 方法，根据 PFLAG_DIRTY_OPAQUE flag 去决定是否重新绘制。
12. requestLayout 和 invalidate 的区别
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