Touch move not triggering filter

[ChrisGatzo]

Hello,

I'm trying to implement a filter that would circumvent execution of touchmove.zoom event and I noticed in the source code that the touchmoved method does not do

if (!filter.apply(this, arguments)) return;

The obvious question is why? Will something break if make this change? So far it works fine.

The reason I want to do this is because I want zoom to only handle multi touch events and let the single touch bubble up to the browser, so that the user can still scroll the page with one finger and zoom/pan with two fingers. So then the filter can be something like

      d3
      .zoom()
      .filter(() => {
        return !(d3.event.type === 'touchmove' && d3.event.touches.length <= 1);
      })

Thanks

[mbostock]

It’s because zoom.filter only applies to events that can start zoom gestures, not events that occur on a gesture that’s already started. For the same reason, zoom.filter doesn’t apply to mousemove and mouseup events.

If you want to only allow multitouch zoom gestures, then ideally you’d use the existing zoom.filter application on the touchstart event rather than the touchmove event. However, the code currently cancels touchmove (preventing scrolling) even if the touchstart event didn’t initiate a zoom gesture. My guess is we want a fix like this?

function touchmoved() {
  if (!gestures.length) return; // No active gesture!
  var g = gesture(this, arguments),
      touches = event.changedTouches,
      n = touches.length, i, t, p, l;
  
  ... etc. remainder of existing touchmoved function …
}

[hohlb]

Was there any progress on this issue?

I have a similar situation: I would like to enable drag AND zoom behaviour for touch at the same time. The logic would be:

• if one finger is down: drag
• if more fingers are down: zoom

I tried to make it work by cancelling the drag event as soon as there is more than one finger on the screen, but it did not work smooth enough. Related to d3/d3-drag#50

[testower]

I've added a PR with @mbostock`s suggested fix, which solves our problem.

[robinhouston]

If you want to only allow multitouch zoom gestures, then ideally you’d use the existing zoom.filter application on the touchstart event rather than the touchmove event.

We’ve found that to make this work correctly on Android Chrome we need another change as well, which is proposed as PR #176.

[mbostock]

Closing as this is covered by #169 #176.