DecorView是activity窗口的根视图，本文介绍DecorView的初始化以及和actionbar，contentview，viewRootImpl的关系
1.DecorView的视图结构
图片1.png
每个activity都对应一个窗口window，这个窗口是PhoneWindow的实例，PhoneWindow对应的布局是DecirView，是一个FrameLayout，DecorView内部又分为两部分，一部分是ActionBar，另一部分是ContentParent，即activity在setContentView对应的布局。
2.DecorView的初始化
2.1Activity的setContentView
从Activity的源码开始
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
其中，getWindow()拿到的是PhoneWindow对象，所以继续看PhoneWindow中的方法
public void setContentView(int layoutResID) {
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;
}
2.2installDecor
private void installDecor() {
mForceDecorInstall = false;
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);
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);
...
}
}
mDecor是PhoneWindow对应的DecorView对象，如果mDecor为空，则调用generateDecor方法完成DecorView的初始化
protected DecorView generateDecor(int featureId) {
Context context;
if (mUseDecorContext) {
Context applicationContext = getContext().getApplicationContext();
if (applicationContext == null) {
context = getContext();
} else {
context = new DecorContext(applicationContext, getContext().getResources());
if (mTheme != -1) {
context.setTheme(mTheme);
}
}
} else {
context = getContext();
}
return new DecorView(context, featureId, this, getAttributes());
}
后面具体的就是DecorView与window和Activity的绑定，不再详细介绍
2.3ContentParent
继续看setContent的方法，如果没有特别指定过渡动画相关的参数，则调用LayoutInflater的inflate方法，把mContentParent作为参数传进去，mContentParent是一个ViewGroup对象，即xml文件所对应的layout，LayoutInflater最终会把xml文件解析并复制给mContentParent
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
final Resources res = getContext().getResources();
if (DEBUG) {
Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
+ Integer.toHexString(resource) + ")");
}
final XmlResourceParser parser = res.getLayout(resource);
try {
return inflate(parser, root, attachToRoot);
} finally {
parser.close();
}
}
2.4Actionbar
回到setContentView，看第二个执行的方法
private void initWindowDecorActionBar() {
Window window = getWindow();
// Initializing the window decor can change window feature flags.
// Make sure that we have the correct set before performing the test below.
window.getDecorView();
if (isChild() || !window.hasFeature(Window.FEATURE_ACTION_BAR) || mActionBar != null) {
return;
}
mActionBar = new WindowDecorActionBar(this);
mActionBar.setDefaultDisplayHomeAsUpEnabled(mEnableDefaultActionBarUp);
mWindow.setDefaultIcon(mActivityInfo.getIconResource());
mWindow.setDefaultLogo(mActivityInfo.getLogoResource());
}
这里的window依然是phonewindow对象，window.getDecorView()的作用是确保decorview不为空，mActionBar代表actionbar，继续看
public WindowDecorActionBar(View layout) {
assert layout.isInEditMode();
init(layout);
}
private void init(View decor) {
mOverlayLayout = (ActionBarOverlayLayout) decor.findViewById(
com.android.internal.R.id.decor_content_parent);
if (mOverlayLayout != null) {
mOverlayLayout.setActionBarVisibilityCallback(this);
}
mDecorToolbar = getDecorToolbar(decor.findViewById(com.android.internal.R.id.action_bar));
mContextView = (ActionBarContextView) decor.findViewById(
com.android.internal.R.id.action_context_bar);
mContainerView = (ActionBarContainer) decor.findViewById(
com.android.internal.R.id.action_bar_container);
mSplitView = (ActionBarContainer) decor.findViewById(
com.android.internal.R.id.split_action_bar);
if (mDecorToolbar == null || mContextView == null || mContainerView == null) {
throw new IllegalStateException(getClass().getSimpleName() + " can only be used " +
"with a compatible window decor layout");
}
mContext = mDecorToolbar.getContext();
mContextDisplayMode = mDecorToolbar.isSplit() ?
CONTEXT_DISPLAY_SPLIT : CONTEXT_DISPLAY_NORMAL;
// This was initially read from the action bar style
final int current = mDecorToolbar.getDisplayOptions();
final boolean homeAsUp = (current & DISPLAY_HOME_AS_UP) != 0;
if (homeAsUp) {
mDisplayHomeAsUpSet = true;
}
ActionBarPolicy abp = ActionBarPolicy.get(mContext);
setHomeButtonEnabled(abp.enableHomeButtonByDefault() || homeAsUp);
setHasEmbeddedTabs(abp.hasEmbeddedTabs());
final TypedArray a = mContext.obtainStyledAttributes(null,
com.android.internal.R.styleable.ActionBar,
com.android.internal.R.attr.actionBarStyle, 0);
if (a.getBoolean(R.styleable.ActionBar_hideOnContentScroll, false)) {
setHideOnContentScrollEnabled(true);
}
final int elevation = a.getDimensionPixelSize(R.styleable.ActionBar_elevation, 0);
if (elevation != 0) {
setElevation(elevation);
}
a.recycle();
}
actionbar对应的布局文件是screen_action_bar.xml，在init方法里就会找到其中的子控件或者布局完成初始化
3.actionBar和contentParent如何添加到decorView
准确来说，actionBar和contentParent并非是添加到decorView上去的，而是本身就存在于decorView，
- 对于有actionBar的activity，decorView的默认布局是screen_action_bar.xml，里面就会包含actionBar和contentParent
- 对于没有actionBar的activity，会根据activity所带的参数选择decorView的默认布局，例如screen_simple.xml
选择decorView的默认布局的相关的判断逻辑是installDecor方法中调用generateLayout完成的，以screen_action_bar.xml为例，可以看一下DecorView的默认布局
其中，id为action_bar_container和content分别对应actionBar和contentParent，setContentView()和initWindowDecorActionBar()会完成两者的初始化
4.decorView建立与viewRootImpl的联系
viewRootImpl是用于管理activity的view，其成员mView对应的就是activity的decorView，viewRootImpl设置decorView的方法是setView
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
...
}
}
}
在源码中搜索setView，可以在WindowManagerGlobal中找到使用的地方
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);
}
}
}
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 {
root.setView(view, wparams, panelParentView);
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
if (index >= 0) {
removeViewLocked(index, true);
}
throw e;
}
}
}
看addView方法的最后的部分，这里会创建一个viewRootImpl对象root，而view则是decorView，之后就会把decorView设置到viewRootImpl中去，方法addView调用流程有些复杂，与activity与window的创建绑定有关，放到后面再讲。