Android View视图层次
参考：
1. Android应用setContentView与LayoutInflater加载解析机制源码分析
之前一直不明白为什么android的view层次是这么个布局，现在终于有所了解了。。其实上图还有点问题，其实在DecorWindow中还有一个id为statusBarBackgroud和一个id为navigationBarBackground的View，分别表示手机的顶部的状态栏和手机底部的导航栏。。
Activity中加载布局，都是通过在onCreate中调用setContentView方法开始：
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.main_layout);
ButterKnife.bind(this);
}Activity中有setContentView重载方法，一个是layoutid，一个直接传入view
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
initWindowDecorActionBar();
}
public void setContentView(View view) {
getWindow().setContentView(view);
initWindowDecorActionBar();
}实际上调用的都是PhoneWindow的setContentView方法。initWindowDecorActionBar一看就知道是初始化actionbar的
@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();//如果mContentParent为空调用installDecor，初始化
} else if (!hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
mContentParent.removeAllViews();
}
if (hasFeature(FEATURE_CONTENT_TRANSITIONS)) {
final Scene newScene = Scene.getSceneForLayout(mContentParent, layoutResID,
getContext());
transitionTo(newScene);
} else {
//可以看到mContentParent这个view作为layoutResID的parent，所以layoutResID根width/height参数有效
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}
@Override
public void setContentView(View view) {
setContentView(view, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));//所以始终是MATCH_PARENT，所以该view的width/height无效
}
@Override
public void setContentView(View view, ViewGroup.LayoutParams params) {
// 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)) {
view.setLayoutParams(params);
final Scene newScene = new Scene(mContentParent, view);
transitionTo(newScene);
} else {
mContentParent.addView(view, params);//把这个view加入到mContentParent中了，同时layoutparam为MATCH_PARENT
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
}setContentView(int layoutResID) 因为是通过inflate(id,mParent) 所以设置的layout_width/layout_height有效。。。
inflate方法的解析，可以查看Android LayoutInflater原理分析
既然mContentParent是在installDecor方法中初始化的，那么
private void installDecor() {
if (mDecor == null) {
mDecor = generateDecor();//初始化mDecor这个View
mDecor.setDescendantFocusability(ViewGroup.FOCUS_AFTER_DESCENDANTS);
mDecor.setIsRootNamespace(true);
if (!mInvalidatePanelMenuPosted && mInvalidatePanelMenuFeatures != 0) {
mDecor.postOnAnimation(mInvalidatePanelMenuRunnable);
}
}
if (mContentParent == null) {
mContentParent = generateLayout(mDecor);//通过mDecor对象初始化mContentParent
.....
}
.....
}private final class DecorView extends FrameLayout implements RootViewSurfaceTaker然后在generateLayout方法中初始化了mContentParent对象
protected ViewGroup generateLayout(DecorView decor) {
// Apply data from current theme.
TypedArray a = getWindowStyle();
.........
if (a.getBoolean(R.styleable.Window_windowNoTitle, false)) { //获取theme中的设置，如果是NoTitle
requestFeature(FEATURE_NO_TITLE);//调用该方法，表示无标题
} else if (a.getBoolean(R.styleable.Window_windowActionBar, false)) { //获取theme中的设置，如果是ActionBar
// Don't allow an action bar if there is no title.
requestFeature(FEATURE_ACTION_BAR);//调用该方法，表示actionbar
}
.........
int layoutResource;
int features = getLocalFeatures();//记得上文中的requestFeature，然后就可以通过getLocalFeatures方法获取了。。其实在activity中可以调用requestFeature
// System.out.println("Features: 0x" + Integer.toHexString(features));
if ((features & (1 << FEATURE_SWIPE_TO_DISMISS)) != 0) {
layoutResource = R.layout.screen_swipe_dismiss;
}
..........
else if ((features & (1 << FEATURE_NO_TITLE)) == 0) {
// If no other features and not embedded, only need a title.
// If the window is floating, we need a dialog layout
if (mIsFloating) {
TypedValue res = new TypedValue();
getContext().getTheme().resolveAttribute(
R.attr.dialogTitleDecorLayout, res, true);
layoutResource = res.resourceId;
} else if ((features & (1 << FEATURE_ACTION_BAR)) != 0) {
layoutResource = a.getResourceId(
R.styleable.Window_windowActionBarFullscreenDecorLayout,
R.layout.screen_action_bar);
} else {
layoutResource = R.layout.screen_title;
}
// System.out.println("Title!");
} else if ((features & (1 << FEATURE_ACTION_MODE_OVERLAY)) != 0) {
layoutResource = R.layout.screen_simple_overlay_action_mode;
} else {
// Embedded, so no decoration is needed.
layoutResource = R.layout.screen_simple;
// System.out.println("Simple!");
}
............
View in = mLayoutInflater.inflate(layoutResource, null);
decor.addView(in, new ViewGroup.LayoutParams(MATCH_PARENT, MATCH_PARENT));//把上述得到的layoutResource添加到decor中
mContentRoot = (ViewGroup) in;
ViewGroup contentParent = (ViewGroup)findViewById(ID_ANDROID_CONTENT);//获取id=content的view作为contenparent
............
}另一方面如果要设置窗口风格，必须放在setContentView的前面
requestWindowFeature(Window.FEATURE_NO_TITLE);//getLocalFeatures方法中被获取
setContentView(R.layout.test_layout);当窗口风格是NoTitle，同时没有actionbar的时候，那么窗口根布局就是screen_title.xml.xml
为了验证这个猜测，打开Android\android-sdk\tools\hierarchyviewer工具验证
在我的代码中，既没有设置title也没有设置actionbar
main_activity.xml
DecorView果然作为根View，其下有三个子view，id为statusBarBackgroud和id为navigationBarBackground的View，分别表示手机的顶部的状态栏和手机底部的导航栏。然后是一个LinearLayout，很明显这个就是上面提到的screen_title.xml.xml中的根LinearLayout。然后包括一个@+id/action_mode_bar_stub的ViewStub，一个@android:id/content的FrameLayout，，，然后因为在setContentView中，下面两种方式都把这个FrameLayout作为了main_activity.xml的根视图。。
mLayoutInflater.inflate(layoutResID, mContentParent);
mContentParent.addView(view, params);
最后inflate内部其实还是调用addview，然后一直如果该view是viewgroup，那么viewgroup又会把其中所有的子view都add进去，所以最后view就形成了一个视图层次。
inflate原理可以参考 Android LayoutInflater原理分析
那最后知道了所有的view的根viewgroup就是上面的decorview，那么这个decorview又是在哪里被添加的呢？
handler机制我们知道了启动Activity其实都会启动activityThread的main方法，这个方法里面会创建主线程的looper。
启动Activity调用完ActivityThread的main方法之后，接着调用ActivityThread类performLaunchActivity来创建要启动的Activity组件，
在创建Activity组件的过程中，还会为该Activity组件创建窗口对象和视图对象；接着Activity组件创建完成之后，通过调用ActivityThread类的handleResumeActivity将它激活。
(在onCreate中调用了setContentView，所以把除了decorView之外的所有的view都已经添加进去了。)handleResumeActivity中把decorView添加进去了
handleResumeActivity方法中调用了r.activity.makeVisible();
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
......
// TODO Push resumeArgs into the activity for consideration
ActivityClientRecord r = performResumeActivity(token, clearHide);
if (r != null) {
......
// 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.
......
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
......
// 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.activity.mVisibleFromClient) {
r.activity.makeVisible();
}
}
......
} else {
// If an exception was thrown when trying to resume, then
// just end this activity.
......
}
}Activity中makeVisible方法，把这个DecorView添加到wm中。。
void makeVisible() {
if (!mWindowAdded) {
ViewManager wm = getWindowManager();
wm.addView(mDecor, getWindow().getAttributes());
mWindowAdded = true;
}
mDecor.setVisibility(View.VISIBLE);
}另一方面， wm.addView实际上调用的是WindowManagerGlobal中addView方法，此时创建了ViewRootImpl对象。。ViewRootImpl有木有很熟悉，在绘制View过程中，就是从ViewRootImpl的performTraversals方法开始的，然后依次经过测量，布局，绘制过程。。invalidate其实最后也是调用了ViewRootImpl的performTraversals方法
public void addView(View view, ViewGroup.LayoutParams params, Display display, Window parentWindow) {
............
ViewRootImpl root;
View panelParentView = null;
............
root = new ViewRootImpl(view.getContext(), display);
view.setLayoutParams(wparams);
mViews.add(view);
mRoots.add(root);
mParams.add(wparams);
} 总结一下：
1.创建一个DecorView的对象mDecor，该mDecor对象将作为整个应用窗口的根视图。
2.依据Feature等style theme创建不同的窗口修饰布局文件，并且通过findViewById获取Activity布局文件该存放的地方（窗口修饰布局文件中id为content的FrameLayout）。
3.将Activity的布局文件添加至id为content的FrameLayout内。