由于项目没有设计返回键，一旦进入别的应用，就无法回到桌面。只能通过串口输入input keyevent 4（返回键）来返回桌面，为了方便调试，在Framework 中监听手势，实现返回功能。

       在frameworks\base\services\core\java\com\android\server\policy\PhoneWindowManager中有这么一段代码

mSystemGestures = new SystemGesturesPointerEventListener(context,
                new SystemGesturesPointerEventListener.Callbacks() {
                    @Override
                    public void onSwipeFromTop() {
						Log.i("gyx","onSwipeFromTop");
                        if (mStatusBar != null) {
                            requestTransientBars(mStatusBar);
                        }
                    }
                    @Override
                    public void onSwipeFromBottom() {
						Log.i("gyx","onSwipeFromBottom");
                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_BOTTOM) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onSwipeFromRight() {
						Log.i("gyx","onSwipeFromRight");
						sendKeyCode(4);
						Log.i("gyx","sendKeyCode 4");

                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_RIGHT) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onSwipeFromLeft() {
						Log.i("gyx","onSwipeFromLeft");
                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_LEFT) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onFling(int duration) {
                        if (mPowerManagerInternal != null) {
                            mPowerManagerInternal.powerHint(
                                    PowerHint.INTERACTION, duration);
                        }
                    }
                    @Override
                    public void onDebug() {
                        // no-op
                    }
                    @Override
                    public void onDown() {
                        mOrientationListener.onTouchStart();
                    }
                    @Override
                    public void onUpOrCancel() {
                        mOrientationListener.onTouchEnd();
                    }
                    @Override
                    public void onm ouseHoverAtTop() {
                        mHandler.removeMessages(MSG_REQUEST_TRANSIENT_BARS);
                        Message msg = mHandler.obtainMessage(MSG_REQUEST_TRANSIENT_BARS);
                        msg.arg1 = MSG_REQUEST_TRANSIENT_BARS_ARG_STATUS;
                        mHandler.sendMessageDelayed(msg, 500);
                    }
                    @Override
                    public void onm ouseHoverAtBottom() {
                        mHandler.removeMessages(MSG_REQUEST_TRANSIENT_BARS);
                        Message msg = mHandler.obtainMessage(MSG_REQUEST_TRANSIENT_BARS);
                        msg.arg1 = MSG_REQUEST_TRANSIENT_BARS_ARG_NAVIGATION;
                        mHandler.sendMessageDelayed(msg, 500);
                    }
                    @Override
                    public void onm ouseLeaveFromEdge() {
                        mHandler.removeMessages(MSG_REQUEST_TRANSIENT_BARS);
                    }
                });

代码中监听了手势操作，其中onSwipeFromRight()就是当从屏幕的右侧向左滑动时会触发，只要在这里加入返回的功能即可；
实现返回按键的功能代码如下

private void sendKeyCode(final int keyCode) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    // 创建一个Instrumentation对象
                    Instrumentation inst = new Instrumentation();
                    // 调用inst对象的按键模拟方法
                    inst.sendKeyDownUpSync(keyCode);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }