android透明视图

 想在Android上做一个透明视图，可在整个屏幕上任意涂鸦（包括状态导航栏），但是不想影响下层窗口的操作，偶然发现developer Option中的Pointer Location接近我的需求。于是顺便研究了一下：

PointerLocationView

PhoneWindowManager.java

     public void updateSettings() {
        ContentResolver resolver = mContext.getContentResolver();
        boolean updateRotation = false;
        View addView = null;
        View removeView = null;
.................................

            if (mSystemReady) {
                int pointerLocation = Settings.System.getInt(resolver,
                        Settings.System.POINTER_LOCATION, 0);
                if (mPointerLocationMode != pointerLocation) {
                    mPointerLocationMode = pointerLocation;
                    if (pointerLocation != 0) {
                        if (mPointerLocationView == null) {
                            mPointerLocationView = new PointerLocationView(mContext);
                            mPointerLocationView.setPrintCoords(false);
                            addView = mPointerLocationView;
                        }
                    } else {
                        removeView = mPointerLocationView;
                        mPointerLocationView = null;
                    }
                }
            }

.................................................

if (addView != null) {
            WindowManager.LayoutParams lp = new WindowManager.LayoutParams(
                    WindowManager.LayoutParams.MATCH_PARENT,
                    WindowManager.LayoutParams.MATCH_PARENT);
            lp.type = WindowManager.LayoutParams.TYPE_SECURE_SYSTEM_OVERLAY;
            lp.flags = WindowManager.LayoutParams.FLAG_FULLSCREEN
                    | WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE
                    | WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    | WindowManager.LayoutParams.FLAG_LAYOUT_IN_SCREEN;
            lp.format = PixelFormat.TRANSLUCENT;
            lp.setTitle("PointerLocation");
            WindowManager wm = (WindowManager)
                    mContext.getSystemService(Context.WINDOW_SERVICE);
            lp.inputFeatures |= WindowManager.LayoutParams.INPUT_FEATURE_NO_INPUT_CHANNEL;
            wm.addView(addView, lp);
            
            if (mPointerLocationInputChannel == null) {
                try {
                    mPointerLocationInputChannel =
                        mWindowManager.monitorInput("PointerLocationView");
                    InputQueue.registerInputChannel(mPointerLocationInputChannel,
                            mPointerLocationInputHandler, mHandler.getLooper().getQueue());
                } catch (RemoteException ex) {
                    Slog.e(TAG, "Could not set up input monitoring channel for PointerLocation.",
                            ex);
                }
            }
        }
        if (removeView != null) {
            if (mPointerLocationInputChannel != null) {
                InputQueue.unregisterInputChannel(mPointerLocationInputChannel);
                mPointerLocationInputChannel.dispose();
                mPointerLocationInputChannel = null;
            }
            
            WindowManager wm = (WindowManager)
                    mContext.getSystemService(Context.WINDOW_SERVICE);
            wm.removeView(removeView);
        }

TYPE_SYSTEM_OVERLAY，TYPE_TOAST，TYPE_SECURE_SYSTEM_OVERLAY三个类型的窗口不接收inputevent, 所以android为它单独注册InputQueue.registerInputChannel以响应触摸事件绘制滑动轨迹。