Android的状态栏和导航栏是在SystemUI中实现的，目前还不支持手动隐藏和显示状态栏和导航栏，接下来我们将对此功能进行讲解。
在系统启动过程中，会加载SystemUI中的状态栏，具体过程在这里不予以分析，状态栏对应的类为：\frameworks\base\packages\SystemUI\src\com\android\systemui\statusbar\phone\PhoneStatusBar.java
加载这个类会首先调用它的start方法。其代码为：

 @Override
    public void start() {
        mDisplay = ((WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE))
                .getDefaultDisplay();
        updateDisplaySize();

        /// M: Support Smartbook Feature.
        if (SIMHelper.isMediatekSmartBookSupport()) {
            /// M: [ALPS01097705] Query the plug-in state as soon as possible.
            mIsDisplayDevice = SIMHelper.isSmartBookPluggedIn(mContext);
            Log.v(TAG, "start, mIsDisplayDevice=" + mIsDisplayDevice);
        }

        super.start(); // calls createAndAddWindows()

        addNavigationBar(); 

        // Lastly, call to the icon policy to install/update all the icons.
        mIconPolicy = new PhoneStatusBarPolicy(mContext);

        mHeadsUpObserver.onChange(true); // set up
        if (ENABLE_HEADS_UP) {
            mContext.getContentResolver().registerContentObserver(
                    Settings.Global.getUriFor(SETTING_HEADS_UP), true,
                    mHeadsUpObserver);
        }
    }

这个方法重写了父类\frameworks\base\packages\SystemUI\src\com\android\systemui\statusbar\BaseStatusBar.java的start方法和调用了一个添加NavigationBar的方法，我们这里比较关心的是父类的这个start方法，来看看他的源码：

public void start() {
        mWindowManager = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        mWindowManagerService = WindowManagerGlobal.getWindowManagerService();
        mDisplay = mWindowManager.getDefaultDisplay();

        mDreamManager = IDreamManager.Stub.asInterface(
                ServiceManager.checkService(DreamService.DREAM_SERVICE));
        mPowerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);

        mProvisioningObserver.onChange(false); // set up
        mContext.getContentResolver().registerContentObserver(
                Settings.Global.getUriFor(Settings.Global.DEVICE_PROVISIONED), true,
                mProvisioningObserver);

        mBarService = IStatusBarService.Stub.asInterface(
                ServiceManager.getService(Context.STATUS_BAR_SERVICE));

        mRecents = getComponent(RecentsComponent.class);

        mLocale = mContext.getResources().getConfiguration().locale;
        mLayoutDirection = TextUtils.getLayoutDirectionFromLocale(mLocale);

        // Connect in to the status bar manager service
        StatusBarIconList iconList = new StatusBarIconList();
        ArrayList<IBinder> notificationKeys = new ArrayList<IBinder>();
        ArrayList<StatusBarNotification> notifications = new ArrayList<StatusBarNotification>();
        mCommandQueue = new CommandQueue(this, iconList);

        int[] switches = new int[7];
        ArrayList<IBinder> binders = new ArrayList<IBinder>();
        try {
            mBarService.registerStatusBar(mCommandQueue, iconList, notificationKeys, notifications,
                    switches, binders);
        } catch (RemoteException ex) {
            // If the system process isn't there we're doomed anyway.
        }

        createAndAddWindows();

        disable(switches[0]);
        setSystemUiVisibility(switches[1], 0xffffffff);
        topAppWindowChanged(switches[2] != 0);
        // StatusBarManagerService has a back up of IME token and it's restored here.
        setImeWindowStatus(binders.get(0), switches[3], switches[4]);
        setHardKeyboardStatus(switches[5] != 0, switches[6] != 0);

        // Set up the initial icon state
        int N = iconList.size();
        int viewIndex = 0;
        for (int i=0; i<N; i++) {
            StatusBarIcon icon = iconList.getIcon(i);
            if (icon != null) {
                addIcon(iconList.getSlot(i), i, viewIndex, icon);
                viewIndex++;
            }
        }

        // Set up the initial notification state
        N = notificationKeys.size();
        if (N == notifications.size()) {
            for (int i=0; i<N; i++) {
                addNotification(notificationKeys.get(i), notifications.get(i));
            }
        } else {
            Log.wtf(TAG, "Notification list length mismatch: keys=" + N
                    + " notifications=" + notifications.size());
        }

        if (DEBUG) {
            Log.d(TAG, String.format(
                    "init: icons=%d disabled=0x%08x lights=0x%08x menu=0x%08x imeButton=0x%08x",
                   iconList.size(),
                   switches[0],
                   switches[1],
                   switches[2],
                   switches[3]
                   ));
        }

        mCurrentUserId = ActivityManager.getCurrentUser();

        IntentFilter filter = new IntentFilter();
        filter.addAction(Intent.ACTION_USER_SWITCHED);
        mContext.registerReceiver(mBroadcastReceiver, filter);
    }

这里实例化了一些服务类和管理类，并且对通知和系统图标做了一些初始化工作，其中调用了一个createAndAddWindows方法，这个方法做了哪些工作呢？这个一个抽象方法，那应该就是在其子类实现，我们回到PhoneStatusBar类中，果然有相应的实现

@Override
    public void createAndAddWindows() {
        addStatusBarWindow();
    }

来看看addStatusBarWindow方法：

 private void addStatusBarWindow() {
        // Put up the view
        final int height = getStatusBarHeight();

        // Now that the status bar window encompasses the sliding panel and its
        // translucent backdrop, the entire thing is made TRANSLUCENT and is
        // hardware-accelerated.
        final WindowManager.LayoutParams lp = new WindowManager.LayoutParams(
                ViewGroup.LayoutParams.MATCH_PARENT,
                height,
                WindowManager.LayoutParams.TYPE_STATUS_BAR,
                WindowManager.LayoutParams.FLAG_NOT_FOCUSABLE
                    | WindowManager.LayoutParams.FLAG_TOUCHABLE_WHEN_WAKING
                    | WindowManager.LayoutParams.FLAG_SPLIT_TOUCH
                    | WindowManager.LayoutParams.FLAG_WATCH_OUTSIDE_TOUCH,
                PixelFormat.TRANSLUCENT);

        lp.flags |= WindowManager.LayoutParams.FLAG_HARDWARE_ACCELERATED;

        lp.gravity = getStatusBarGravity();
        lp.setTitle("StatusBar");
        lp.packageName = mContext.getPackageName();

        makeStatusBarView();

        /// M: [SystemUI] For SystemUI AT.
        if (AutoTestHelper.isNotRunningInTest()) {
            mWindowManager.addView(mStatusBarWindow, lp);
            isStatusBarShow = true;
        }
        int state = Settings.System.getInt(mContext.getContentResolver(), Settings.System.STATUSBAR_HIDE, -