当界面全屏时，在顶部下拉时会显示statusbar的实现原理

当界面全屏时，在顶部下拉时会显示statusbar的实现原理：
/frameworks/base/services/core/java/com/android/server/policy/PhoneWindowManager.java

        // monitor for system gestures
        mSystemGestures = new SystemGesturesPointerEventListener(context,
                new SystemGesturesPointerEventListener.Callbacks() {
                    @Override
                    public void onSwipeFromTop() {
                        if (mStatusBar != null) {
                            requestTransientBars(mStatusBar);
                        }
                    }
                    @Override
                    public void onSwipeFromBottom() {
                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_BOTTOM) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onSwipeFromRight() {
                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_RIGHT) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onSwipeFromLeft() {
                        if (mNavigationBar != null && mNavigationBarPosition == NAV_BAR_LEFT) {
                            requestTransientBars(mNavigationBar);
                        }
                    }
                    @Override
                    public void onFling(int duration) {
                        if (mPowerManagerInternal != null) {
                            mPowerManagerInternal.powerHint(
                                    PowerManagerInternal.POWER_HINT_INTERACTION, duration);
                        }
                    }
                    @Override
                    public void onDebug() {
                        // no-op
                    }
                    @Override
                    public void onDown() {
                        mOrientationListener.onTouchStart();
                    }
                    @Override
                    public void onUpOrCancel() {
                        mOrientationListener.onTouchEnd();
                    }
                    @Override
                    public void onMouseHoverAtTop() {
                        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 onMouseHoverAtBottom() {
                        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 onMouseLeaveFromEdge() {
                        mHandler.removeMessages(MSG_REQUEST_TRANSIENT_BARS);
                    }
                });

其中requestTransientBars(mStatusBar);就是要求显示statusbar.
我们看看/frameworks/base/services/core/java/com/android/server/policy/SystemGesturesPointerEventListener.java

/*
 * Listens for system-wide input gestures, firing callbacks when detected.
 * @hide
 */
public class SystemGesturesPointerEventListener implements PointerEventListener

在PhoneWindowManager会将SystemGesturesPointerEventListener进行

mWindowManagerFuncs.registerPointerEventListener(mSystemGestures);

mWindowManagerFuncs是WindowManagerService实现
/frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java

public class WindowManagerService extends IWindowManager.Stub
        implements Watchdog.Monitor, WindowManagerPolicy.WindowManagerFuncs

  @Override
    public void registerPointerEventListener(PointerEventListener listener) {
        mPointerEventDispatcher.registerInputEventListener(listener);
    }
//这里建立了window manager的input channel
mPointerEventDispatcher = new PointerEventDispatcher(mInputManager.monitorInput(TAG_WM));

mPointerEventDispatcher为
/frameworks/base/services/core/java/com/android/server/wm/PointerEventDispatcher.java

应用也有自己的input channel,在WindowManagerService的addWindow

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

/frameworks/base/services/core/java/com/android/server/wm/WindowState.java
   

void openInputChannel(InputChannel outInputChannel) {
        if (mInputChannel != null) {
            throw new IllegalStateException("Window already has an input channel.");
        }
        String name = makeInputChannelName();
        InputChannel[] inputChannels = InputChannel.openInputChannelPair(name);
        mInputChannel = inputChannels[0];
        mClientChannel = inputChannels[1];
        mInputWindowHandle.inputChannel = inputChannels[0];
        if (outInputChannel != null) {
            mClientChannel.transferTo(outInputChannel);
            mClientChannel.dispose();
            mClientChannel = null;
        } else {
            // If the window died visible, we setup a dummy input channel, so that taps
            // can still detected by input monitor channel, and we can relaunch the app.
            // Create dummy event receiver that simply reports all events as handled.
            mDeadWindowEventReceiver = new DeadWindowEventReceiver(mClientChannel);
        }
        mService.mInputManager.registerInputChannel(mInputChannel, mInputWindowHandle);

那底层事件是怎么传递到input channel,那就得看
frameworks/native/services/inputflinger/InputDispatcher.cpp

dispatchMotionLocked{
....
       if (isPointerEvent) {
        // Pointer event.  (eg. touchscreen)
        injectionResult = findTouchedWindowTargetsLocked(currentTime,
                entry, inputTargets, nextWakeupTime, &conflictingPointerActions);
    } else {
        // Non touch event.  (eg. trackball)
        injectionResult = findFocusedWindowTargetsLocked(currentTime,
                entry, inputTargets, nextWakeupTime);
    }
....
if (isMainDisplay(entry->displayId)) {
        addMonitoringTargetsLocked(inputTargets);
    }

findTouchedWindowTargetsLocked 这里对应的就是应用的input channel, 
addMonitoringTargetsLocked这个对应的就是window manager的input channel.
通过这段我们就知道产生事件的时候既会派发给应用window也会派发给window manager,window manager主要是处理system 的事件，比如显示statusbar,navigationbar.
关于input channel还有很多细节问题，要自己慢慢研读代码，这里只记录一个大概的代码流程