How to make HTML5 draggable objects over canvas?

Question

I just started learning html5 and I am trying to create a battleship interface with draggable ships. I need help making my dragging methods work. I am purposely not using a libr

Answer 1

This is the procedure for making an html shape draggable

Note that this has been answered before on SO (many times!)

But this answer illustrates the new context.isPointInPath method to hit-test whether a point is inside an html canvas path.

Hopefully, this new hit-testing method will be new & useful to the OP and others :)

Here's the general procedure for dragging shapes in html canvas:

On mouseDown:

• save this mouseX position in a variable (lastX)
• save this mouseY position in a variable (lastY)
• set the mouseIsDown flag to true

On mouseUp

• set the mouseIsDown flag to false

On mouseMove

• Hit-test each ship to see if it should be dragged.
• If the lastX/lastY was inside a ship, that ship is being dragged
• Move dragging ships by the distance the mouse has just moved

MouseDown handler code:

function handleMouseDown(e){

  // get the current mouse position relative to the canvas

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  // save this last mouseX/mouseY

  lastX=mouseX;
  lastY=mouseY;

  // set the mouseIsDown flag

  mouseIsDown=true;
}

MouseUp handler code:

function handleMouseUp(e){

  // clear the mouseIsDown flag

  mouseIsDown=false;
}

MouseMove handler code:

This code illustrates using context.isPointInPath to hit-test an html canvas path

The procedure to do that is:

• define a path (but not draw it -- no fill, no stroke)
• use context.isPointInPath(x,y) to test if x,y are inside the path defined above.

Here's the mouseMove handler using context.isPointInPath

function handleMouseMove(e){

  // if the mouseIsDown flag is’nt set, no work to do

  if(!mouseIsDown){ return; }
  // get mouseX/mouseY

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  // for each ship in the ships array
  // use context.isPointInPath to test if it’s being dragged

  for(var i=0;i

Note about enhancing performance:

• In production, you'll want to have the mouseMove just save the mouse positions.
• Then have another procedure retrieve those saved positions and do hit-testing/redrawing.
• That other procedure will probably be inside a timed loop like requestAnimationFrame.

Here’s code and a Fiddle: http://jsfiddle.net/m1erickson/sEBAC/