原文转载：https://www.cnblogs.com/tiger-wang-ms/p/6517048.html
三、handleResumeActivity()流程
在文章开头贴出的第一段AcitityThread.handleLauncherActivity()方法的代码中，执行完performLaunchAcitity()创建好Acitivity后，便会执行到handleResumeActivity()方法，该方法代码如下。
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
...// TODO Push resumeArgs into the activity for consideration
// 该方法执行过程中会调用到Acitity的onResume()方法，返回的ActivityClientRecord对象描述的即是创建好的Activity r = performResumeActivity(token, clearHide, reason);
if (r != null) {
final Activity a = r.activity;//返回之前创建的Acitivty
if (localLOGV) Slog.v(
TAG, "Resume " + r + " started activity: " +
a.mStartedActivity + ", hideForNow: " + r.hideForNow
+ ", finished: " + a.mFinished);
final int forwardBit = isForward ?
WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION : 0;
// 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. // 判断该Acitivity是否可见，mStartedAcitity记录的是一个Activity是否还处于启动状态 // 如果还处于启动状态则mStartedAcitity为true，表示该activity还未启动好，则该Activity还不可见 boolean willBeVisible = !a.mStartedActivity; // 如果启动的组建不是全屏的，mStartedActivity也会是true，此时依然需要willBeVisible为true以下的if逻辑就是针对这种情况的校正
if (!willBeVisible) {
try {
willBeVisible = ActivityManagerNative.getDefault().willActivityBeVisible(
a.getActivityToken());
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
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();
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit; //PreserverWindow，一般指主题换了或者configuration变了情况下的Acitity快速重启机制
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 && !a.mWindowAdded) {
a.mWindowAdded = true; //调用了WindowManagerImpl的addView方法
wm.addView(decor, l);
}
...
}重点来看wm.addView()方法，该方法中的decor参数为Acitity对应的Window中的视图DecorView，wm为在创建PhoneWindow是创建的WindowManagerImpl对象，该对象的addView方法实际调用到到是单例对象WindowManagerGlobal的addView方法（前文有提到）。在看addView代码前，我先来看看WindowManagerGlobal对象成员变量。
private static WindowManagerGlobal sDefaultWindowManager;
private static IWindowManager sWindowManagerService;
private static IWindowSession sWindowSession;
private final Object mLock = new Object();
private final ArrayList mViews = new ArrayList();
private final ArrayList mRoots = new ArrayList();
private final ArrayList mParams =
new ArrayList();
private final ArraySet mDyingViews = new ArraySet(); 三个成员变量mViews、mRoots和mParams分别是类型为View、ViewRootImpl和WindowManager.LayoutParams的数组。这里有这样的逻辑关系，每个View都对应着唯一的一个ViewRootImpl和WindowManager.LayoutRarams，即是1:1:1的关系。这三个数组长度始终保持一致，并且在同一个位置上存放的是互相关联的View、ViewRootImpl和WindowManager.LayoutParams对象。此外还有一个成员变量mDyView，保存的则是已经不需要但还未被系统会收到View。
View与LayoutParams比较好理解，那ViewRootImpl对象的作用是什么呢？首先WindowManagerImpl是作为管理类，就像主管一样，根据Acitity和Window的调用请求，找到合适的做事的人；DecorView本身是FrameworkLayout，本事是一个View，所表示的是一种静态的结构；所以这里就需要一个真正做事的人，那就是ViewRootImpl类的工作。总结来讲ViewRootImpl的功能如下
- 完成了绘制过程。在ViewRootImpl类中，实现了perfromMeasure()、performDraw()、performLayout()等绘制相关的方法。
- 与系统服务进行交互，例如与AcitityManagerSerivice，DisplayService、AudioService等进行通信，保证了Acitity相关功能等正常运转。
- 触屏事件等分发逻辑的实现
接下来我们进入WindowManagerGlobal.addView()方法的代码。
public void addView(View view, ViewGroup.LayoutParams params,
Display display, Window parentWindow) {
...
ViewRootImpl root;
View panelParentView = null;
synchronized (mLock) { ... // If this is a panel window, then find the window it is being
// attached to for future reference. // 如果当前添加的是一个子视图，则还需要找他他的父视图 //这里我们分析的是添加DecorView的逻辑，没有父视图，故不会走到这里，panelParentView为null 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);
//保存互相对应的View、ViewRootImpl、WindowManager.LayoutParams到数组中
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;
}
}
} 关注代码中加粗的两个方法，首先会创建一个ViewRootImpl对象，然后调用ViewRootImpl.setView方法，其中panelParentView在addView参数为DecorView是为null。进入ViewRootImpl.setView()代码。
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
synchronized (this) {
if (mView == null) { //初始化成员变量mView、mWindowAttraibutes //mAttachInfo是View类的一个内部类AttachInfo类的对象 //该类的主要作用就是储存一组当View attach给它的父Window的时候Activity各种属性的信息 mView = view;
mAttachInfo.mDisplayState = mDisplay.getState();
mDisplayManager.registerDisplayListener(mDisplayListener, mHandler);
mViewLayoutDirectionInitial = mView.getRawLayoutDirection();
mFallbackEventHandler.setView(view);
mWindowAttributes.copyFrom(attrs);
... //继续初始化一些变量，包含针对panelParentView不为null时的父窗口的一些处理
mAdded = true;
// 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. // 这里调用异步刷新请求，最终会调用performTraversals方法来完成View的绘制 requestLayout();
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(),
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();
}
}
...
}
}
}相关变量初始化完成后，便会将mAdded设置为true，表示ViewRootImpl与setView传入的View参数已经做好了关联。之后便会调用requestLayout()方法来请求一次异步刷新，该方法后来又会调用到performTraversals()方法来完成view到绘制工作。注意到这里虽然完成了绘制的工作，但是我们创建Activity的源头是AMS中发起的，我们从一开始创建Acitivity到相对应的Window、DecorView这一大套对象时，还并未与AMS进程进行反馈。所以之后便会调用mWindowSession.addToDisplay()方法会执行IPC的跨进程通信，最终调用到AMS中的addWindow方法来在系统进程中执行相关加载Window的操作。
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