View的绘制流程
今天来讲讲View的绘制流程吧，这一块也是我比较薄弱的地方，而且之前没有什么耐心去看这一块。今天就来学习这一块的知识点。主要分为3部分：
-
View树的创建
-
ViewRootImpl的创建 -
真正的绘制流程开始performTraversals
View树的创建
首先思考一个问题View的绘制从那里开始？关于这个问题我一开始也谷歌百度了很多东西，但是还是不能解决我的疑惑。所以今天我打算从根本上解决这个问题——从Activity的setContentView开始看。下面开始我的源码阅读过程。
Activity#setContentView
public void setContentView(@LayoutRes int layoutResID) {
//注释1 getWindow()返回的是一个PhoneWindow对象，所以下一步，就是要去PhoneWindow中去看看这里面做了什么。PhoneWindow也是Window类的唯一实现类
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
PhoneWindow#setContentView
public void setContentView(int layoutResID) {
// Note: FEATURE_CONTENT_TRANSITIONS may be set in the process of installing the window
// decor, when theme attributes and the like are crystalized. Do not check the feature
// before this happens.
//注释2 mContentParent一开始的时候为空，所以会执行installDecor函数，mContextParent是一个ViewGroup对象
if (mContentParent == null) {
installDecor();
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
//installDecor的注释很长，我们一步一步看
private void installDecor() {
mForceDecorInstall = false;
//注释3 mDecor是一个DecorView，这是窗口的顶层视图，包含窗口装饰。首次进来的时候mDecor是空的，所以获取执行generateDecor，在generateDecor里面会去创建一个DecorView，注意这个函数是在PhoneWindow里面的，也就是说PhoneWindow里面的mDecor有了初始值，那么就是DecorView与PhoneWindow建立了关联。
if (mDecor == null) {
mDecor = generateDecor(-1);
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
} else {
mDecor.setWindow(this);
}
//注释4 注意这里mContentParent还是没有进行任何的初始化，所以还是为null，所以会执行generateLayout方法，而generateLayout的方法参数是上面干初始化的mDecor，而这个函数的返回值是给mContentParent赋值。
if (mContentParent == null) {
//这里面得到的是一个FrameLayout
mContentParent = generateLayout(mDecor);
//....省略未分析的代码
}
}
PhoneWindow#generateLayout
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
//...省略很多代码，主要是获取Activity主题的一些属性，然后根据属性配置
//注释5 这个默认情况下会为R.layout.screen_simple,screen_simple.xml的布局内容是一个LinearLayout里面包含两个子view，其中一个子view的id是content，并且是FrameLayout。
int layoutResource;
int features = getLocalFeatures();//获取Activity.java 代码的里面的配置，注意这里很上面的不一样，上面是根据Activity主题的配置，这里获取的是java代码里面的动态配置
//...省略很多代码，
mDecor.startChanging();
//注释6 参数为一个LayoutInflate，和刚刚的布局文件id，这里面会调用mLayoutInflater的inflater创建view，然后调用DecorView的addView方法，也就是说刚才那个布局里面的控件都加入了DecorView中，也是就DecorView包含了他们
mDecor.onResourcesLoaded(mLayoutInflater, layoutResource);
//注释7 调用findViewById，其实调用的是DecorView的findViewById方法，然后返回一个ViewGroup。而这个id正好是注释5里面提到的content，所以这里面相当于返回了一个FrameLayout
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);
//省略一些配置代码
//注释8 这里面将FrameLayout返回
return contentParent;
}
好分析到这里，很多人已经蒙了，什么跟什么啊，一点绘制的流程都没讲。确实这里面还是没有讲解到view真正去绘制的代码，但是这里面的知识点确实很重要。如果直接讲解view的绘制流程的代码，很多人会不知道为什么执行到了具体哪一个步骤。所以上面的流程步骤先耐心看完吧。我们现在有了什么？总结一下，我们有了DecorView，有了一个FrameLayout，还有一些可能配置的信息，比如TitleBar，他们的关系是怎么样的呢？如下图。
想一个问题，这里面是系统的层面的布局给改好了，我们传入的布局呢？这时候就需要回到PhoneWindow#setContentView里面了。
public void setContentView(int layoutResID) {
//省略已经分析过的代码
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
//最终会执行到这里,mContentParent也就是上面得到的id为content的frameLayout
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
如图：
那么我们总结一下上面的流程和梳理一些对应的关系。（注意箭头的指向）
分析到这里整一个View树的创建过程就分析完了，看起来好像是DecorView是我们的最顶层的父类了，但是DecorView还有没有父类呢？答案是有的，它的顶层就是就是ViewRootImpl。
ViewRootImpl的创建
所以接下来我们就需要看一下ViewRootImpl是怎么和DecorView建立关系的
看一下ViewRootImpl类的描述：视图层次结构的顶部，实现 View 和 WindowManager 之间所需的协议。这在很大程度上是 WindowManagerGlobal 的内部实现细节。
ViewRootImpl的创建是在WindowManagerGlobal中的addView中创建的。而WindowManagerGlobal的addView是在ActivityThread中的handleResumeActivity中去调用的。Activity的生命周期都是由这个类管理的。这里我们直接看handleResumeActivity里面的方法
ActivityThread#handleResumeActivity
public void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,
String reason) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// TODO Push resumeArgs into the activity for consideration
//注释9 执行了这个方法之后，也就是Activity的onResume方法没执行了
final ActivityClientRecord r = performResumeActivity(token, finalStateRequest, reason);
if (r == null) {
// We didn't actually resume the activity, so skipping any follow-up actions.
return;
}
final Activity a = r.activity;
if (localLOGV) {
Slog.v(TAG, "Resume " + r + " started activity: " + a.mStartedActivity
+ ", hideForNow: " + r.hideForNow + ", finished: " + a.mFinished);
}
final int forwardBit = isForward
? WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
// If the window hasn't yet been added to the window manager,
// and this guy didn't finish itself or start another activity,
// then go ahead and add the window.
boolean willBeVisible = !a.mStartedActivity;
if (!willBeVisible) {
try {
willBeVisible = ActivityManager.getService().willActivityBeVisible(
a.getActivityToken());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
//注释10 这里面的ViewManager实例时ViewManager
ViewManager wm = a.getWindowManager();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
//注释11 将decor传入ViewManagerImpl的addView,addView，在ViewManagerImpl中的addView会去调用WindowManagerGlobal的addView方法，所以这里就会创建了ViewRootImpl
wm.addView(decor, l);
} else {
// The activity will get a callback for this {@link LayoutParams} change
// earlier. However, at that time the decor will not be set (this is set
// in this method), so no action will be taken. This call ensures the
// callback occurs with the decor set.
a.onWindowAttributesChanged(l);
}
}
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// Get rid of anything left hanging around.
cleanUpPendingRemoveWindows(r, false /* force */);
// The window is now visible if it has been added, we are not
// simply finishing, and we are not starting another activity.
if (!r.activity.mFinished && willBeVisible && r.activity.mDecor != null && !r.hideForNow) {
if (r.newConfig != null) {
performConfigurationChangedForActivity(r, r.newConfig);
if (DEBUG_CONFIGURATION) {
Slog.v(TAG, "Resuming activity " + r.activityInfo.name + " with newConfig "
+ r.activity.mCurrentConfig);
}
r.newConfig = null;
}
if (localLOGV) Slog.v(TAG, "Resuming " + r + " with isForward=" + isForward);
WindowManager.LayoutParams l = r.window.getAttributes();
if ((l.softInputMode
& WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION)
!= forwardBit) {
l.softInputMode = (l.softInputMode
& (~WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION))
| forwardBit;
if (r.activity.mVisibleFromClient) {
ViewManager wm = a.getWindowManager();
View decor = r.window.getDecorView();
wm.updateViewLayout(decor, l);
}
}
r.activity.mVisibleFromServer = true;
mNumVisibleActivities++;
if (r.activity.mVisibleFromClient) {
r.activity.makeVisible();
}
}
r.nextIdle = mNewActivities;
mNewActivities = r;
if (localLOGV) Slog.v(TAG, "Scheduling idle handler for " + r);
Looper.myQueue().addIdleHandler(new Idler());
}
WindowManagerGlobal#addView
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) {
throw new IllegalArgumentException("view must not be null");
}
if (display == null) {
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) {
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
if (parentWindow != null) {
parentWindow.adjustLayoutParamsForSubWindow(wparams);
} else {
// If there's no parent, then hardware acceleration for this view is
// set from the application's hardware acceleration setting.
final Context context = view.getContext();
if (context != null
&& (context.getApplicationInfo().flags
& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) {
// Start watching for system property changes.
if (mSystemPropertyUpdater == null) {
mSystemPropertyUpdater = new Runnable() {
@Override public void run() {
synchronized (mLock) {
for (int i = mRoots.size() - 1; i >= 0; --i) {
mRoots.get(i).loadSystemProperties();
}
}
}
};
SystemProperties.addChangeCallback(mSystemPropertyUpdater);
}
int index = findViewLocked(view, false);
if (index >= 0) {
if (mDyingViews.contains(view)) {
// Don't wait for MSG_DIE to make it's way through root's queue.
mRoots.get(index).doDie();
} else {
throw new IllegalStateException("View " + view
+ " has already been added to the window manager.");
}
// The previous removeView() had not completed executing. Now it has.
}
// If this is a panel window, then find the window it is being
// attached to for future reference.
if (wparams.type >= WindowManager.LayoutParams.FIRST_SUB_WINDOW &&
wparams.type <= WindowManager.LayoutParams.LAST_SUB_WINDOW) {
final int count = mViews.size();
for (int i = 0; i < count; i++) {
if (mRoots.get(i).mWindow.asBinder() == wparams.token) {
panelParentView = mViews.get(i);
}
}
}
//注释12 这里会创建ViewRootImpl。
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
// do this last because it fires off messages to start doing things
try {
//注释13 并且这里会调用ViewRootImpl的setView方法
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
}
ViewRootImpl#setView
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
//..,省略代码
requestLayout();//里面会做两件重要的事，1检查是不是在子线程更新view，2、执行scheduleTraversals。而在scheduleTraversals中会执行一个异步任务，最终去执行performTraversals()，这个方法就是View绘制的开始了
//..,省略代码,将ViewRootImpl设置为decorView的parent，这里执行完成之后，调用decorView.parent就有值了，就是ViewRootImpl
view.assignParent(this);
}
}
}
真正的绘制流程开始performTraversals
ViewRootImpl#performTraversals
这个方法很长，只要是调用了performMeasure，performLayout，performDraw
private void performTraversals() {
//省略代码..
//如果第一次执行为
if (mFirst) {
mFullRedrawNeeded = true;
mLayoutRequested = true;
//省略代码..
}
//这里一般为true
boolean layoutRequested = mLayoutRequested && (!mStopped || mReportNextDraw);
if (layoutRequested) {
//省略代码..
// Ask host how big it wants to be
//这里就是对整一个布局树的测量了
windowSizeMayChange |= measureHierarchy(host, lp, res,
desiredWindowWidth, desiredWindowHeight);
}
if (!mStopped || mReportNextDraw) {
// 执行performMeasure
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
}
final boolean didLayout = layoutRequested && (!mStopped || mReportNextDraw);
boolean triggerGlobalLayoutListener = didLayout
|| mAttachInfo.mRecomputeGlobalAttributes;
//这里为true
if (didLayout) {
//这里就是对整一个布局树的布局
performLayout(lp, mWidth, mHeight);
}
if (triggerGlobalLayoutListener) {
mAttachInfo.mRecomputeGlobalAttributes = false;
//这里的监听可以获取控件的大小了
mAttachInfo.mTreeObserver.dispatchOnGlobalLayout();
}
if (!cancelDraw && !newSurface) {
if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).startChangingAnimations();
}
mPendingTransitions.clear();
}
//执行绘制代码
performDraw();
} else {
if (isViewVisible) {
// Try again
scheduleTraversals();
} else if (mPendingTransitions != null && mPendingTransitions.size() > 0) {
for (int i = 0; i < mPendingTransitions.size(); ++i) {
mPendingTransitions.get(i).endChangingAnimations();
}
mPendingTransitions.clear();
}
}
mIsInTraversal = false;
}
到这里View的绘制流程就已经到了我们最熟悉的三个函数里面了
performTraversals会依次调用performMeasure、performLayout和performDraw三个方法，这三个方法分别完成顶级View的measure、layout和draw这三大流程，其中在performMeasure中会调用measure方法，在measure方法中又会调用onMeasure方法，在onMeasure方法中则会对所有的子元素进行measure过程，这个时候measure流程就从父容器传递到子元素中了，这样就完成了一次measure过程。接着子元素会重复父容器的measure过程，如此反复就完成了整个View树的遍历。同理，performLayout和performDraw的 传递流程和performMeasure是类似的，唯一不同的是，performDraw的传递过程是在draw方法中通过dispatchDraw来实现的，不过这并没有本质区别。
以上就是view的绘制流程了，至于viewGroup的各个方法实现的细节，这里就不展开来讲了。大体流程是这个个流程。后面讲解自定义ViewGroup的时候会讲解具体是怎么测量和怎么布局的。
