Activity Window View

1_Activity加载UI-类图关系和视图结构.png

我们知道Activity的启动过程的最后一步是调用ActivityThread的performLaunchActivity,用类加载器初始化一个activity对象

 Activity activity = null;
        try {
            ClassLoader cl = r.packageInfo.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            if (!mInstrumentation.onException(activity, e)) {
                throw new RuntimeException(
                    "Unable to instantiate activity " + component
                    + ": " + e.toString(), e);
            }
        }

然后调用activity的attach，初始化了mWindow对象，并进行了一系列赋值操作：

        mWindow = new PhoneWindow(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        mUiThread = Thread.currentThread();
.........................省略部分代码..............................................

 mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                 mToken,mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();

mWindo是一个Window对象，系统会创建Activity所属的Window对象并为其设置回调接口，当Window接收到外界的状态改变就会回调到Activity的方法

接下来ActivityThread会调用handleResumeActivity：

    ActivityClientRecord r = performResumeActivity(token, clearHide);
  if (r != null) {
   final Activity a = r.activity;
    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;
                if (a.mVisibleFromClient) {
                    a.mWindowAdded = true;
                    wm.addView(decor, l);
                }

getDecorView最后调用DecorView里面的方法generateDecor:

protected DecorView generateDecor() {
        return new DecorView(getContext(), -1);
    }

wm.addView其实调用了WindowManagerImpl的addView

 public void addView(@NonNull View view, @NonNull ViewGroup.LayoutParams params) {
        applyDefaultToken(params);
        mGlobal.addView(view, params, mDisplay, mParentWindow);
    }

mGlobal.addView

  public void addView(View view, ViewGroup.LayoutParams params,
                        Display display, Window parentWindow) {
                       ViewRootImpl root;
.............................省略部分代码.......................................
  root = new ViewRootImpl(view.getContext(), display);

  // 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.
            synchronized (mLock) {
                final int index = findViewLocked(view, false);
                if (index >= 0) {
                    removeViewLocked(index, true);
                }
            }

上面调用root.setView将ViewRoot和DecorView绑定在一起，ViewRootImpl的setView里面调用 requestLayout();最终调用performTraversals，进行DecorView的绘制

View的绘制流程是从ViewRoot的performTraversals方法开始的，它经过measure，layout和draw三个过程最终将一个View绘制出来，其中measure用来测量View的宽和高，layout用来确定View在父容器中的放置位置，而draw则负责将View绘制在屏幕上。如此反复完成一棵View树的遍历，整个View视图就显示在屏幕上了。

在onCreat()中，我们只有一个setContentView()的操作，在Activity的setContentView的实现可以看出，Activity将具体实现交给了Window处理，而Window的具体实现是PhoneWind在setContentView中创建了DecorView，DecorView是整棵View树的顶级View，然后将View添加到DecorView的mContentParent中，最后回调Activity的onContentChanged方法通知Activity视图已经发生改变。

总结：

Activity是整个模型的控制单元，Window属于承载模型，负责承载视图，View是视图显示模型，ActivityThread创建Activity，调用Activity的attch方法创建Window，并且设置回调，接着ActivityThread调用handleResumeActivity，取得刚才创建的Activity，调用其Window对象生成DecorView，然后将DecorView在ViewManager中和ViewRoot绑定，即ViewRoot对象持有DecorView对象，从performTraversals开始View的绘制，measureHierarchy进行组件测量