View依附于Window，而Activity负责管理Window。为什么会产生这样的关系呢？文章围绕这个问题。将会从Activity加载View的整个流程去分析Activity、Window、View三者之间的关系。
1、在启动Activity时，会在onCreate()方法中调用setContentView()去加载布局，在这一阶段，只是把布局添加到了DecorView（根视图）中，并没有真正的依附于Window，那么是什么时候添加到Window的呢？这个问题先记着，接下来通过分析源码揭开其庐山真面目。
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//......
}
public void setContentView(@LayoutRes int layoutResID) {
getWindow().setContentView(layoutResID);
//初始化ActionBar
initWindowDecorActionBar();
}
上述代码中，可以发现，直接把任务转移到Activity的setContentView()方法中，调用了getWindow().setContentView()加载我们的布局资源文件，然后initWindowDecorActionBar()初始化ActionBar，插个画：
getWindow()会返回一个Window对象mWindow，实际指向了Window的唯一实现类PhoneWindow，调用setContentView()是把资源文件加载到图中的Content部分。initWindowDecorActionBar()初始化图中的DecorActionBar。mWindow的初始化会在下一篇文章分析，在这里先提下，其实就是在启动Activity的时候，调用了Activity的attach()方法。
2、通过1的分析，这时把视线转移到PhoneWindow的setContentView()中，看下面源码：
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 {
//对布局文件进行解析
mLayoutInflater.inflate(layoutResID, mContentParent);
}
mContentParent.requestApplyInsets();
final Callback cb = getCallback();
if (cb != null && !isDestroyed()) {
cb.onContentChanged();
}
mContentParentExplicitlySet = true;
}
private void installDecor() {
mForceDecorInstall = false;
if (mDecor == null) {
mDecor = generateDecor(-1);//加载一个DecorView
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);//获取content部分
//省略代码....
}
//省略代码....
protected DecorView generateDecor(int featureId) {
// System process doesn't have application context and in that case we need to directly use
// the context we have. Otherwise we want the application context, so we don't cling to the
// activity.
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());
}
由于代码量太大，所以选择了一些关键性的代码来进行分析。接着分析，在上面的代码中，首先第一次启动mContentParent 肯定为null，调用installDecor()，在installDecor()方法里面通过generateDecor()生成一个DecorView对象mDecor，然后通过generateLayout(mDecor)获取一个mContentParent，mContentParent也就是上图的Content部分。
3、在2中分析了DecorView和mContentParent的初始化，下面将任务转移到布局文件的解析mLayoutInflater.inflate(layoutResID, mContentParent)中，mLayoutInflater是一个LayoutInflater的对象，进入LayoutInflater的inflate()源码看下：
public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
return inflate(resource, root, root != null);
}
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();
}
}
在上面代码中，利用Resources的getLayout()获取一个XmlResourceParser对象parser ，再调用inflate(),
public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
synchronized (mConstructorArgs) {
Trace.traceBegin(Trace.TRACE_TAG_VIEW, "inflate");
final Context inflaterContext = mContext;
final AttributeSet attrs = Xml.asAttributeSet(parser);
Context lastContext = (Context) mConstructorArgs[0];
mConstructorArgs[0] = inflaterContext;
View result = root;
try {
// Look for the root node.
int type;
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();
if (DEBUG) {
System.out.println("**************************");
System.out.println("Creating root view: "
+ name);
System.out.println("**************************");
}
//这里判断是否是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");
}
rInflate(parser, root, inflaterContext, attrs, false);
} else {
// Temp is the root view that was found in the xml
final View temp = createViewFromTag(root, name, inflaterContext, attrs);
ViewGroup.LayoutParams params = null;
if (root != null) {
if (DEBUG) {
System.out.println("Creating params from root: " +
root);
}
// Create layout params that match root, if supplied
params = root.generateLayoutParams(attrs);
if (!attachToRoot) {
// Set the layout params for temp if we are not
// attaching. (If we are, we use addView, below)
temp.setLayoutParams(params);
}
}
if (DEBUG) {
System.out.println("-----> start inflating children");
}
// Inflate all children under temp against its context.
rInflateChildren(parser, temp, attrs, true);
if (DEBUG) {
System.out.println("-----> done inflating children");
}
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
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) {
final InflateException ie = new InflateException(e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} catch (Exception e) {
final InflateException ie = new InflateException(parser.getPositionDescription()
+ ": " + e.getMessage(), e);
ie.setStackTrace(EMPTY_STACK_TRACE);
throw ie;
} finally {
// Don't retain static reference on context.
mConstructorArgs[0] = lastContext;
mConstructorArgs[1] = null;
Trace.traceEnd(Trace.TRACE_TAG_VIEW);
}
return result;
}
}
先遍历属性，然后判断是否是merge标签，如果是，则利用rInflate解析布局。否则，createViewFromTag()创建一个临时的temp；然后通过rInflateChildren()解析布局文件，内容添加到temp中，再利用root.addView()把temp添加进来，root是最初调用LayoutInflater的inflate(layoutResID, mContentParent)传进来的mContentParent，这个时候就完成了添加。
4、在第3步中，我们已经把布局文件添加进了content中，那么视图是怎么显示出来的呢？下面接着分析，在源码中可以发现mContentParent是一个ViewGroup对象，即调用了ViewGroup的addView()方法：
public void addView(View child, int index, LayoutParams params) {
if (DBG) {
System.out.println(this + " addView");
}
if (child == null) {
throw new IllegalArgumentException("Cannot add a null child view to a ViewGroup");
}
// addViewInner() will call child.requestLayout() when setting the new LayoutParams
// therefore, we call requestLayout() on ourselves before, so that the child's request
// will be blocked at our level
requestLayout();
invalidate(true);
addViewInner(child, index, params, false);
}
在上面代码中，requestLayout()重新请求布局，会从父View开始，invalidate()会让View重绘，调用onDraw()方法，addViewInner()是把之前加载的View添加进ViewGroup中，并且当前ViewGroup会被当成一个parent，在View进行绘制的时候，调用parent遍历其中的childView。不管是requestLayout()还是invalidate()最终都会调用ViewParent接口，而ViewRootImpl是ViewParent的实现类，反过来说，也就是调用了ViewRootImpl的requestLayout()和invalidate()。
5、在第4步中已经找到了View的绘制入口了，由于这部分内容也比较多，所以另写了一篇文章【从源码角度分析View的绘制流程】。在前面我们分析完了View的加载过程，并没有涉及到Window这个东西，只是把View添加进了DecorView的content部分。下面我们看一段Acitivity的启动流程中的源码【从源码探索Activity的启动过程】：
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
//省略代码.........
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager(); //1
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;
wm.addView(decor, l); //2
} 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);
}
}
}
上面是Activity在onResume时执行的代码，注意下注释1的部分，ViewManager wm = a.getWindowManager();获取一个wm对象，a.getWindowManager()实际返回的是WindowManagerImpl对象（WindowManager（继承了ViewManager ）的实现类），，在注释2中，那么我们就能知道实际是调用了WindowManagerImpl的addView()方法。
6、经过第5步分析，接下来看下WindowManagerImpl的addView()方法。
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
//WindowManagerGlobal类中的addView方法
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) { //通过window调整参数
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;
}
}
}
根据上面代码可以发现，任务转移到了WindowManagerGlobal的addView方法中，内部创建了一个ViewRootImpl对象root ，然后调用root.setView()。同时也把WindowManager.LayoutParams传到了ViewRootImpl中。
总结： Activity中利用了WindowManager管理Window，而Window和View中间是通过ViewRootImpl建立关联。 流程分析到这里就结束了，文章中并没有深入，只是表层去探究了Activity、Window、View的关系。深层次还涉及到了AMS、WMS等IPC机制。后面再另起一篇文章来分析。最后看一张图，对上面流程的简述。
