a. display & window之显示

window对应用层来说来，是view跟display解耦，对framework来说，是用来display跟layer（即surfaceflinger）解耦，简单的理解：display ->layer/view<-window。

从显示的角度来看，并没有windows这个概念，只有layer图层，display显示屏的概念，window提供了一个桥梁来连接display和layer，因为安卓系统只允许应用处理自己的view关系，而且display并不属于应用可以管理的范围，于是就在activity的生命周期加了入window来桥接displayt和view，window决定了view的显示，具体window和display创建的流程如下：

一、window创建流程：

如上图，应用进程activity的生命周期函数里面按步骤创建window，view，关联view和window，会binder调到WMS的addwindow。

注：此处的WindowSession.addToDisplay 应为**com.android.server.wm**.Session
WMS, WindowToken 里的displaycontent即是Display。

addToDisplay 里会执行 WindowManagerService的addwindow。

@Override
public int addToDisplay(IWindow window, WindowManager.LayoutParams attrs,
        int viewVisibility, int displayId, InsetsVisibilities requestedVisibilities,
        InputChannel outInputChannel, InsetsState outInsetsState,
        InsetsSourceControl[] outActiveControls) {
    return mService.addWindow(this, window, attrs, viewVisibility, displayId,
            UserHandle.getUserId(mUid), requestedVisibilities, outInputChannel, outInsetsState,
            outActiveControls);
}
 
@Override
public int addToDisplayAsUser(IWindow window, WindowManager.LayoutParams attrs,
        int viewVisibility, int displayId, int userId, InsetsVisibilities requestedVisibilities,
        InputChannel outInputChannel, InsetsState outInsetsState,
        InsetsSourceControl[] outActiveControls) {
    return mService.addWindow(this, window, attrs, viewVisibility, displayId, userId,
            requestedVisibilities, outInputChannel, outInsetsState, outActiveControls);
}
 
@Override
public int addToDisplayWithoutInputChannel(IWindow window, WindowManager.LayoutParams attrs,
        int viewVisibility, int displayId, InsetsState outInsetsState) {
    return mService.addWindow(this, window, attrs, viewVisibility, displayId,
            UserHandle.getUserId(mUid), mDummyRequestedVisibilities, null /* outInputChannel */,
            outInsetsState, mDummyControls);
}

在addwindow中对应不同场景。

final boolean openInputChannels = (outInputChannel != null
        && (attrs.inputFeatures & INPUT_FEATURE_NO_INPUT_CHANNEL) == 0);
if  (openInputChannels) {
    win.openInputChannel(outInputChannel);
}

int userId = UserHandle.getUserId(session.mUid);
if (requestUserId != userId) {
    try {
        mAmInternal.handleIncomingUser(callingPid, callingUid, requestUserId,
                false /*allowAll*/, ALLOW_NON_FULL, null, null);
    } catch (Exception exp) {
        ProtoLog.w(WM_ERROR, "Trying to add window with invalid user=%d",
                requestUserId);
        return WindowManagerGlobal.ADD_INVALID_USER;
    }
    // It's fine to use this userId
    userId = requestUserId;
}

ViewRootImpl中requestLayout函数调用mWindowSession.relayout，最终调用WindowManagerservice 的relayoutWindow，这个函数会配置Display信息; WindowStateAnimator创建Surfacecontrol，通过SurfaceControl，设置Layerstack。即在触发view更新绘制的时候才会更新图层数据。

com.android.server.wm.WindowManagerService#relayoutWindow

if (shouldRelayout) {
    try {
        result = createSurfaceControl(outSurfaceControl, result, win, winAnimator);
    } catch (Exception e) {
        displayContent.getInputMonitor().updateInputWindowsLw(true /*force*/);
 
        ProtoLog.w(WM_ERROR,
                "Exception thrown when creating surface for client %s (%s). %s",
                client, win.mAttrs.getTitle(), e);
        Binder.restoreCallingIdentity(origId);
        return 0;
    }
}

至此，一个Windows对应一个ViewRootImpl，用来管理应用进程的view的绘制生命周期，事件机制等等，后续的流程是layer会在surfaceflinger里面处理后，经由GPU或者硬件合成渲染。