Helper addition: M (mouse coordinate translation)

[FireyFly]

I wanted to see if I could add an interactive element to a dweet, so I decided to attach a mousemove listener to take the mouse coordinate into account. Now, paying for the cost of adding an event listener and whatnot is fair game, but I didn't anticipate how expensive it'd be just to get something canvas-usable out of the mouse event. In the end I dweeted this little paint demo.

The annoying boilerplate-y part is the computation of the mouse (x,y) coordinate in relation to canvas coordinates, especially when taking different resolutions (non-fullscreen, fullscreen at different resolutions) into account. It seems reasonable to suggest a helper function to help deal with different resolutions, since that part seems to be meant to be transparent to the dweet (i.e., the canvas is always 1920×1080).

So, I suggest a helper function M:

M = (e) => [ e.offsetX/c.offsetWidth *1920,
             e.offsetY/c.offsetHeight*1080 ]

The dweeter could then get the mouse coordinates in canvas coordinates via a simple [X,Y]=M(e).

Thoughts? I do realise new helpers should have quite a lot of friction to being added, or we'll just end up abbreviating everything…

[sigvef]

Can we make it totally transparent for the dweet code? So we get the correct coordinates without having to add new helpers?

[sigvef]

Fwiw the first iteration was always in fullscreen, so if we have old demos relying on the coordinates being correct, transparently patching the coordinates would restore them!

[FireyFly]

Oh hm… that would probably be better, yes. I'm not sure if there is a nice way to patch offsetX/offsetY up, I guess one could write a setter for c.onmousemove (and -up, -down, -click) and then call the real event hander with an object that inherits from the real MouseEvent object, but overrides those properties.

Something like this maybe? It's a bit icky though.

void function () {
  var handler
  Object.defineProperty(c, 'onmousemove', {
    get: function () {
      return handler
    },
    set: function (f) {
      if (handler != null) c.removeEventListener('mousemove', handler)
      handler = function (ev) {
        var ev_ = Object.create(ev)
        Object.defineProperty(ev_, 'offsetX', { get: () => ev.offsetX / c.offsetWidth  * c.width  })
        Object.defineProperty(ev_, 'offsetY', { get: () => ev.offsetY / c.offsetHeight * c.height })
        return f(ev_)
      }
      c.addEventListener('mousemove', handler, false)
    }
  })
}()

Edit: although this would mess up https://www.dwitter.net/d/809 (which only works in not-fullscreen currently)… not sure if messing with offsetX/offsetY would do more good or harm.