研究源码的目的，我个人理解是这样的：懂得底层工作原理，可以更好更快的定位BUG，写项目更得心应手，比如，知道了ViewRootImpl什么时候创建，所以为什么在onCreate中View.getMeasure会是0，知道了ViewRootImpl检查线程的时机，知道了为什么子线程中更新UI为什么不一定会报错等等，最重要的是，我们改BUG，定位BUG更有自信，从而可以去分析解剖别人的框架，一步一步，你也可以写出好框架，当然，不要忘了还要有设计模式的思想起看别人的代码。所以，参考了前辈的分析，自己过一遍，记录下自己理解分析的过程，不过有个前提是，会先懂Binder机制，这一点对看Android源码很重要。
分析之前，先有个概念：
就是每个Window都对应着一个View和一个ViewRootImpl，Window和View是通过ViewRootImpl来建立联系，所以一个Activity会有一个Window，而最终Window的请求添加View的操作会交给WindowManagerService去处理，WindowManagerService存在于系统进程，不是我们应该的进程，所以这是一个跨进程的过程，通过Binder通信。
我们通常通过getSystemService拿到WindowManager，然后通过WindowManager的addView方法就可以将一个View添加到屏幕上，所以就先从addView入手：
点进去你发现不是WindowManager，而是ViewManager：
public interface ViewManager {
public void addView(View view, ViewGroup.LayoutParams params);
public void updateViewLayout(View view, ViewGroup.LayoutParams params);
public void removeView(View view);
}
可以看到ViewManager 定义了Window的添加、更新和操作，我相信你就会去想WindowManager实现了ViewManager ，找WindowManager，没错，确实WindowManager实现了ViewManager，但是你会发现WindowManager也是一个接口，没做什么：
public interface WindowManager extends ViewManager
这时候我们是不是想这找WindowManager的实现类，没错确实是这样，按照套路很多时候都是后面加个Impl，也就是WindowManagerImpl，那么全局搜索下WindowManagerImpl，果然就是它了：
@Override
public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.addView(view, params, mContext.getDisplay(), mParentWindow);
}
@Override
public void updateViewLayout(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
applyDefaultToken(params);
mGlobal.updateViewLayout(view, params);
}
@Override
public void removeView(View view) {
mGlobal.removeView(view, false);
}
可以看到，交给了一个成员mGlobal去处理，先看下这个mGlobal：
private final WindowManagerGlobal mGlobal = WindowManagerGlobal.getInstance();
可以看到交给了WindowManagerGlobal 去处理了，那么点进去看下WindowManagerGlobal 的addView，跟着源码和注释看下去：
private final ArrayList<View> mViews = new ArrayList<View>(); //存放View
private final ArrayList<ViewRootImpl> mRoots = new ArrayList<ViewRootImpl>(); //存放ViewRootImpl
private final ArrayList<WindowManager.LayoutParams> mParams =
new ArrayList<WindowManager.LayoutParams>(); //存放WindowManager.LayoutParams
private final ArraySet<View> mDyingViews = new ArraySet<View>(); //存放将要被删除的View
-----------------------------------上面是WindowManagerGlobal 的成员-----------------------------
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
if (view == null) { //判断
throw new IllegalArgumentException("view must not be null");
}
if (display == null) { //判断
throw new IllegalArgumentException("display must not be null");
}
if (!(params instanceof WindowManager.LayoutParams)) { //判断，必须是WindowManager.LayoutParams
throw new IllegalArgumentException("Params must be WindowManager.LayoutParams");
}
final WindowManager.LayoutParams wparams = (WindowManager.LayoutParams) params;
if (parentWindow != null) { //parentWindow 一般就是PhoneWindow
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 //硬件加速相关
& ApplicationInfo.FLAG_HARDWARE_ACCELERATED) != 0) {
wparams.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;
}
}
ViewRootImpl root; //可以看到，会在下面在创建ViewRootImpl
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); //查询View有没已经添加了，就是通过上面存放View的集合查找
if (index >= 0) { 没有添加过会返回-1，返回是集合的索引值
if (mDyingViews.contains(view)) {
// Don't wait for MSG_DIE to make it's way through root's queue.
mRoots.get(index).doDie(); //在等待删除集合了就调用去删除
} else {
throw new IllegalStateException("View " + view //这个异常可能很多人遇到过
+ " 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); //创建ViewRootImpl
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); //将View和ViewRootImpl关联
} catch (RuntimeException e) {
// BadTokenException or InvalidDisplayException, clean up.
synchronized (mLock) {
final int index = findViewLocked(view, false);
if (index >= 0) {
removeViewLocked(index, true);
}
}
throw e;
}
}
可以看到，最终由ViewRootImpl的setView处理，那么看下setView：
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) {
mView = view;
---------------------省略部分代码-------------------
// Schedule the first layout -before- adding to the window
// manager, to make sure we do the relayout before receiving
// any other events from the system.
requestLayout(); //这一步最终会检查线程，走View的流程
if ((mWindowAttributes.inputFeatures
& WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL) == 0) {
mInputChannel = new InputChannel();
}
mForceDecorViewVisibility = (mWindowAttributes.privateFlags
& PRIVATE_FLAG_FORCE_DECOR_VIEW_VISIBILITY) != 0;
try {
mOrigWindowType = mWindowAttributes.type;
mAttachInfo.mRecomputeGlobalAttributes = true;
collectViewAttributes();
res = mWindowSession.addToDisplay(mWindow, mSeq, mWindowAttributes,
getHostVisibility(), mDisplay.getDisplayId(), //这里是Binder过程，下面分析
mAttachInfo.mContentInsets, mAttachInfo.mStableInsets,
mAttachInfo.mOutsets, mInputChannel);
} catch (RemoteException e) {
mAdded = false;
mView = null;
mAttachInfo.mRootView = null;
mInputChannel = null;
mFallbackEventHandler.setView(null);
unscheduleTraversals();
setAccessibilityFocus(null, null);
throw new RuntimeException("Adding window failed", e);
} finally {
if (restore) {
attrs.restore();
}
}
---------------------省略部分代码-------------------
}
}
requestLayout：
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread(); //检查线程，就是不能在子线程更新UI
mLayoutRequested = true;
scheduleTraversals(); //会调用performTraversals，完成三大流程
}
}
requestLayout执行完， mWindowSession.addToDisplay的完成添加过程，看下mWindowSession的定义，它在ViewRootImpl的构造中被赋值：
final IWindowSession mWindowSession;
mWindowSession = WindowManagerGlobal.getWindowSession();
那么看下 WindowManagerGlobal.getWindowSession()：
public static IWindowSession getWindowSession() {
synchronized (WindowManagerGlobal.class) {
if (sWindowSession == null) {
try {
InputMethodManager imm = InputMethodManager.getInstance();
IWindowManager windowManager = getWindowManagerService();
sWindowSession = windowManager.openSession(
new IWindowSessionCallback.Stub() {
@Override
public void onAnimatorScaleChanged(float scale) {
ValueAnimator.setDurationScale(scale);
}
},
imm.getClient(), imm.getInputContext());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
return sWindowSession;
}
}
上面的 IWindowManager windowManager = getWindowManagerService()，这时候就应该想到系统进程的WindowManagerService，没错，先看下getWindowManagerService：
public static IWindowManager getWindowManagerService() {
synchronized (WindowManagerGlobal.class) {
if (sWindowManagerService == null) {
sWindowManagerService = IWindowManager.Stub.asInterface( //明显是AIDL，返回了WindowManagerService
ServiceManager.getService("window"));
try {
sWindowManagerService = getWindowManagerService();
ValueAnimator.setDurationScale(sWindowManagerService.getCurrentAnimatorScale());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
return sWindowManagerService;
}
}
上面的进一步说下，Android源码中有个IWindowManager.aidl文件，很平时使用AIDL同样的原理，看下WindowManagerService的继承关系就明白了：
public class WindowManagerService extends IWindowManager.Stub
所以说，mWindowSession 是通过AIDL跨进程调用WindowManagerService调用openSession得到，可以看下：
@Override
public IWindowSession openSession(IWindowSessionCallback callback, IInputMethodClient client,
IInputContext inputContext) {
if (client == null) throw new IllegalArgumentException("null client");
if (inputContext == null) throw new IllegalArgumentException("null inputContext");
Session session = new Session(this, callback, client, inputContext); //this，是WindowManagerService
return session;
}
看这就大概明白，mWindowSession 真正的实现是Session，最终所有工作交给WindowManagerService ：
@Override
public int addToDisplay(IWindow window, int seq, WindowManager.LayoutParams attrs,
int viewVisibility, int displayId, Rect outContentInsets, Rect outStableInsets,
Rect outOutsets, InputChannel outInputChannel) { //mService，就是WindowManagerService
return mService.addWindow(this, window, seq, attrs, viewVisibility, displayId,
outContentInsets, outStableInsets, outOutsets, outInputChannel);
}
至于WindowManagerService怎么去添加就不深入研究，知道了这一个原理工程，知道了View出现在WindowManagerService是通过WindowManager的addView，WindowManagerService是系统进程，那么就可以去分析Activity启动过程了。
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