FrameLayout that knows when it first becomes visible on screen

EventView.java

import android.annotation.TargetApi;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Point;
import android.graphics.Rect;
import android.os.Build;
import android.util.AttributeSet;
import android.util.Log;
import android.view.ViewTreeObserver;
import android.widget.FrameLayout;

import java.util.HashSet;
import java.util.Set;

public class EventView extends FrameLayout {

    private static final String TAG = EventView.class.getSimpleName();
    private String eventViewId;

    public EventView(Context context) {
        super(context);
        init();
    }

    public EventView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    public EventView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public EventView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
        init();
    }

    private void init() {
        setWillNotDraw(false);
        getViewTreeObserver().addOnGlobalLayoutListener(new EventViewGlobalLayoutListener(this, onScreenListener));
        getViewTreeObserver().addOnScrollChangedListener(new EventViewScrollChangeListener(this, onScreenListener));
    }

    private final OnScreenListener onScreenListener = new OnScreenListener() {
        private Set<String> seenIds = new HashSet<>();
        @Override
        public void onScreen(EventView eventView) {
            if (!seenIds.contains(eventView.getEventViewId())) {
                firstSeen();
                seenIds.add(eventView.getEventViewId());
            }
        }
    };

    protected void firstSeen() {
        Log.d(TAG, "onScreen() - eventView: " + getEventViewId());
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        if (eventViewId == null || eventViewId.isEmpty()) {
            throw new IllegalStateException("eventViewId must be set");
        }
    }

    public void setEventViewId(String eventViewId) {
        this.eventViewId = eventViewId;
    }

    public String getEventViewId() {
        return eventViewId;
    }

    interface OnScreenListener {
        void onScreen(EventView eventView);
    }

    private static class EventViewSubject {
        private EventView eventView;
        private OnScreenListener onScreenListener;
        private final Rect rect;
        private final Point globalOffset;

        EventViewSubject(EventView view, OnScreenListener onScreenListener) {
            Log.d(TAG, "eventViewSubject() - ev: " + view.hashCode() + ", osl: " + onScreenListener.hashCode());
            this.eventView = view;
            this.onScreenListener = onScreenListener;
            this.rect = new Rect();
            this.globalOffset = new Point();
        }

        public void processEvent() {
            if (eventViewVisible()) {
                onScreenListener.onScreen(eventView);
            }
        }

        private boolean eventViewVisible() {
            if (eventView == null) {
                return false;
            }
            boolean isVisibleOnScreen = eventView.getGlobalVisibleRect(rect, globalOffset);
            Log.d(TAG, "onListener() - ev " + eventView.getEventViewId() + " data - width: " + eventView.getWidth() + ", height: " + eventView.getHeight() + ", x: " + eventView.getX() + ", y: " + eventView.getY() + ", visibleOnScreen: " + isVisibleOnScreen);
            return isVisibleOnScreen;
        }
    }

    private static class EventViewScrollChangeListener implements ViewTreeObserver.OnScrollChangedListener {
        private final EventViewSubject eventViewSubject;

        EventViewScrollChangeListener(EventView view, OnScreenListener onScreenListener) {
            this.eventViewSubject = new EventViewSubject(view, onScreenListener);
        }

        @Override
        public void onScrollChanged() {
            eventViewSubject.processEvent();
        }
    }

    private static class EventViewGlobalLayoutListener implements ViewTreeObserver.OnGlobalLayoutListener {
        private final EventViewSubject eventViewSubject;

        EventViewGlobalLayoutListener(EventView view, OnScreenListener onScreenListener) {
            this.eventViewSubject = new EventViewSubject(view, onScreenListener);
        }

        @Override
        public void onGlobalLayout() {
            eventViewSubject.processEvent();
        }
    }

}