When the interface is full screen, the implementation principle of the statusbar will be displayed when the top is pulled down

When the interface is full screen, the implementation principle of statusbar will be displayed when the top is pulled down:
/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);
                    }
                });

Among them requestTransientBars(mStatusBar); is to ask to display statusbar.
Let's look at /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

The PhoneWindowManager will make the SystemGesturesPointerEventListener

mWindowManagerFuncs.registerPointerEventListener(mSystemGestures);

mWindowManagerFuncs is the WindowManagerService implementation
/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);
    }
//Here establishes the input channel of the window manager
mPointerEventDispatcher = new PointerEventDispatcher(mInputManager.monitorInput(TAG_WM));

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

application also has its own input channel, in addWindow of WindowManagerService

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);

How the underlying event is passed to the input channel depends on
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 here corresponds to the input channel of the application, and 
addMonitoringTargetsLocked corresponds to the input channel of the window manager.
Through this section, we know that when an event is generated, it will be dispatched to the application window as well as to the window manager. The window manager mainly deals with the system. events, such as displaying statusbar, navigationbar.
There are still many details about the input channel. You should study the code slowly by yourself. Only a general code flow is recorded here.