public class GraphicOverlay extends View {
    private final Object lock = new Object();
    private int previewWidth;
    private float widthScaleFactor = 1.0f;
    private int previewHeight;
    private float heightScaleFactor = 1.0f;
    private int facing = CameraCharacteristics.LENS_FACING_BACK;
    private Set graphics = new HashSet<>();

    /**
     * Base class for a custom graphics object to be rendered within the graphic overlay. Subclass
     * this and implement the {@link Graphic#draw(Canvas)} method to define the graphics element. Add
     * instances to the overlay using {@link GraphicOverlay#add(Graphic)}.
     */
    public abstract static class Graphic {
        private GraphicOverlay overlay;

        public Graphic(GraphicOverlay overlay) {
            this.overlay = overlay;
        }

        /**
         * Draw the graphic on the supplied canvas. Drawing should use the following methods to convert
         * to view coordinates for the graphics that are drawn:
         * 

         * 

         * 
{@link Graphic#scaleX(float)} and {@link Graphic#scaleY(float)} adjust the size of the
         * supplied value from the preview scale to the view scale.
         * 
{@link Graphic#translateX(float)} and {@link Graphic#translateY(float)} adjust the
         * coordinate from the preview's coordinate system to the view coordinate system.
         * 
         *
         * @param canvas drawing canvas
         */
        public abstract void draw(Canvas canvas);

        /**
         * Adjusts a horizontal value of the supplied value from the preview scale to the view scale.
         */
        public float scaleX(float horizontal) {
            return horizontal * overlay.widthScaleFactor;
        }

        /**
         * Adjusts a vertical value of the supplied value from the preview scale to the view scale.
         */
        public float scaleY(float vertical) {
            return vertical * overlay.heightScaleFactor;
        }

        /**
         * Returns the application context of the app.
         */
        public Context getApplicationContext() {
            return overlay.getContext().getApplicationContext();
        }

        /**
         * Adjusts the x coordinate from the preview's coordinate system to the view coordinate system.
         */
        public float translateX(float x) {
            if (overlay.facing == CameraCharacteristics.LENS_FACING_FRONT) {
                return overlay.getWidth() - scaleX(x);
            } else {
                return scaleX(x);
            }
        }

        /**
         * Adjusts the y coordinate from the preview's coordinate system to the view coordinate system.
         */
        public float translateY(float y) {
            return scaleY(y);
        }

        public void postInvalidate() {
            overlay.postInvalidate();
        }
    }

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

    /**
     * Removes all graphics from the overlay.
     */
    public void clear() {
        synchronized (lock) {
            graphics.clear();
        }
        postInvalidate();
    }

    /**
     * Adds a graphic to the overlay.
     */
    public void add(Graphic graphic) {
        synchronized (lock) {
            graphics.add(graphic);
        }
        postInvalidate();
    }

    /**
     * Removes a graphic from the overlay.
     */
    public void remove(Graphic graphic) {
        synchronized (lock) {
            graphics.remove(graphic);
        }
        postInvalidate();
    }

    /**
     * Sets the camera attributes for size and facing direction, which informs how to transform image
     * coordinates later.
     */
    public void setCameraInfo(int previewWidth, int previewHeight, int facing) {
        synchronized (lock) {
            this.previewWidth = previewWidth;
            this.previewHeight = previewHeight;
            this.facing = facing;
        }
        postInvalidate();
    }

    /**
     * Draws the overlay with its associated graphic objects.
     */
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        synchronized (lock) {
            if ((previewWidth != 0) && (previewHeight != 0)) {
                widthScaleFactor = (float) canvas.getWidth() / (float) previewWidth;
                heightScaleFactor = (float) canvas.getHeight() / (float) previewHeight;
            }

            for (Graphic graphic : graphics) {
                graphic.draw(canvas);
            }
        }
    }
}