Question

I am currently developing a paint-like application for GWT. I would like to add a mouse handler that runs when the user drags the mouse across the canvas(like making a square,etc;), the problem is that I’m not surewhat handler to use. Looking through the handlers implemented in canvas has lead me to some hints, but the documentation as to what event the apply to is scant.

Does anyone know how I should implement it? Thanks.

Answer 1

There is no “dragging” handler. You imlement “dragging” with MouseDown, MouseMove and MouseUp events.

class YourWidget extends Composite
{
    @UiField
    Canvas yourCanvas;

    private boolean dragging;
    private HandlerRegistration mouseMove;

    @UiHandler("yourCanvas")
    void onMouseDown(MouseDownEvent e) {
      dragging = true;
      // do other stuff related to starting of "dragging"
      mouseMove = yourCanvas.addMouseMoveHandler(new MouseMoveHandler(){
            public void onMouseMove(MouseMoveEvent e) {
                // ...do stuff that you need when "dragging"
            }
      });
    }

    @UiHandler("yourCanvas")
    void onMouseUp(MouseUpEvent e) {
      if (dragging){
          // do other stuff related to stopping of "dragging"
          dragging = false;
          mouseMove.remove(); // in earlier versions of GWT
          //mouseMove.removeHandler(); //in later versions of GWT
      }
    }
}