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 = CameraSource.CAMERA_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 == CameraSource.CAMERA_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);
      }
    }
  }
}