PhoneWindow DecorView 和 View之间的关系如图所示
PhoneWindow_DecorView_View关系图.png
在activity中，通常我们通过 setContentView(R.layout.activity_main) 加载一个布局就能显示出一个界面，那么我们看看源码，瞧瞧这行代码背后做了些什么，可以让界面显示出来。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
setContentView源码分析
AppCompatActivity
@Override
public void setContentView(@LayoutRes int layoutResID) {
//点击setContentView发现是一个抽象方法，那么找它的实现类
getDelegate().setContentView(layoutResID);
}
@NonNull
public AppCompatDelegate getDelegate() {
if (mDelegate == null) {
mDelegate = AppCompatDelegate.create(this, this);
}
return mDelegate;
}
public static AppCompatDelegate create(Activity activity, AppCompatCallback callback) {
return create(activity, activity.getWindow(), callback);
}
private static AppCompatDelegate create(Context context, Window window,
AppCompatCallback callback) {
final int sdk = Build.VERSION.SDK_INT;
if (BuildCompat.isAtLeastN()) {
return new AppCompatDelegateImplN(context, window, callback);
} else if (sdk >= 23) {
return new AppCompatDelegateImplV23(context, window, callback);
} else if (sdk >= 14) {
return new AppCompatDelegateImplV14(context, window, callback);
} else if (sdk >= 11) {
return new AppCompatDelegateImplV11(context, window, callback);
} else {
return new AppCompatDelegateImplV9(context, window, callback);
}
}
//随便找一个类进入查看实现方法 这里我点击进入 AppCompatDelegateImplV9
//在AppCompatDelegateImplV9里面找到setContentView方法
@Override
public void setContentView(View v) {//这个方法由几个构造，有 //setContentView(int resId) 通过资源id的形式加载我们的布局
//创建DecorView加载系统布局
ensureSubDecor();
//在DecorView添加的系统布局Fraglayout(ViewGroup)里
//findViewById获取系统跟布局的id android.R.id.content获取跟布局容器
ViewGroup contentParent = (ViewGroup) mSubDecor.findViewById(android.R.id.content);
//移除跟布局容器里面的所有子View
contentParent.removeAllViews();
//添加我们通过setContentView设置过来的布局
contentParent.addView(v);
mOriginalWindowCallback.onContentChanged();
}
private void ensureSubDecor() {
if (!mSubDecorInstalled) {
mSubDecor = createSubDecor();
...代码省略 我们只关心我们关注的重点
onSubDecorInstalled(mSubDecor);
mSubDecorInstalled = true;
...代码省略
}
}
private ViewGroup createSubDecor() {
..代码省略
// Now let's make sure that the Window has installed its decor by retrieving it
mWindow.getDecorView();//获取DecorView
final LayoutInflater inflater = LayoutInflater.from(mContext);
ViewGroup subDecor = null;
...代码省略
final ContentFrameLayout contentView = (ContentFrameLayout) subDecor.findViewById(
R.id.action_bar_activity_content);
final ViewGroup windowContentView = (ViewGroup) mWindow.findViewById(android.R.id.content)；//系统根布局
if (windowContentView != null) {
// There might be Views already added to the Window's content view so we need to
// migrate them to our content view
while (windowContentView.getChildCount() > 0) {
final View child = windowContentView.getChildAt(0);
windowContentView.removeViewAt(0);
contentView.addView(child);
}
// Change our content FrameLayout to use the android.R.id.content id.
// Useful for fragments.
windowContentView.setId(View.NO_ID);
contentView.setId(android.R.id.content);
// The decorContent may have a foreground drawable set (windowContentOverlay).
// Remove this as we handle it ourselves
if (windowContentView instanceof FrameLayout) {
((FrameLayout) windowContentView).setForeground(null);
}
}
// Now set the Window's content view with the decor
mWindow.setContentView(subDecor);//调用windwow的setContentView
return subDecor;
}
PhoneWindow(Window的实现类)的setContentView
@Override
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.
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 {
//调用此方法会走 ViewRootImpl的requestLayout方法
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
查看ViewRootImpl源码 对View的绘制流程分析
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
//检查线程 子线程不能更新UI的原因也是这个，更新UI会走ViewRootImpl，然后会进行线程检查
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void scheduleTraversals() {
if (!mTraversalScheduled) {
mTraversalScheduled = true;
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
//重点关注mTraversalRunnable
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
if (!mUnbufferedInputDispatch) {
scheduleConsumeBatchedInput();
}
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}
final TraversalRunnable mTraversalRunnable = new TraversalRunnable();
final class TraversalRunnable implements Runnable {
@Override
public void run() {
doTraversal();
}
}
void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
if (mProfile) {
Debug.startMethodTracing("ViewAncestor");
}
//View的绘制流程从这里开始
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
View的绘制流程从这里开始
private void performTraversals() {
···
//View的绘制流程第一步 测量
performMeasure(childWidthMeasureSpec, childHeightMeasureSpec);
···
//View的绘制流程第二部 摆放
performLayout(lp, mWidth, mHeight);
···
//View的绘制流程第三部 绘制
performDraw();
}
private void performMeasure(int childWidthMeasureSpec, int childHeightMeasureSpec) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "measure");
try {
//调用View的measure方法
mView.measure(childWidthMeasureSpec, childHeightMeasureSpec);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
}
private void performLayout(WindowManager.LayoutParams lp, int desiredWindowWidth,
int desiredWindowHeight) {
mLayoutRequested = false;
mScrollMayChange = true;
mInLayout = true;
final View host = mView;
···
调用View的layout
host.layout(0, 0, host.getMeasuredWidth(), host.getMeasuredHeight());
···
mInLayout = false;
}
private void performDraw() {
if (mAttachInfo.mDisplayState == Display.STATE_OFF && !mReportNextDraw) {
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);
}
····
}
private void draw(boolean fullRedrawNeeded) {
...
if (!drawSoftware(surface, mAttachInfo, xOffset, yOffset, scalingRequired, dirty)) {
return;
}
}
}
if (animating) {
mFullRedrawNeeded = true;
scheduleTraversals();
}
}
private boolean drawSoftware(Surface surface, AttachInfo attachInfo, int xoff, int yoff,
boolean scalingRequired, Rect dirty) {
···
//调用View的Draw方法
mView.draw(canvas);
··
}
