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HTML: Translating touch events to relative

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    HTML: Translating touch events to relative

    This is one of those "I can't believe it's this hard" problems.

    I need to translate a mouse press or touch on a web page to a div's local coordinates. Any sensible system would do this for you, but that's too simple for the folks behind the HTML standards. After a lot of faffing, I've come up with this (ignoring x for brevity):

    Code:
    function onTouchStart( ev )
    {
    e = new Object;
    e.y = ev.touches[0].pageY;
    
    // translate
    r = rootDiv.getBoundingClientRect();
    e.y -= r.top;
    e.y -= window.scrollY;
    }
    If I have a DIV that starts 300 pixels down the page and I touch its top left, e.y = 300 and r.top is 300, and so it works. If the window is scrolled down so my div is at the top of the window then r.top is now 0 because it's offset by the scroll position (bizarrely - I don't know why anybody would think that's how it should be) which is why you also need to subtract the scroll position.

    This is fine. Until I zoom in on Android (Chrome).

    Once zoomed if I scroll so my div is at the top r now doesn't take into account the scroll position, so that r.top is 300 and scrollY is 300 and I therefore end up 300 pixels out.

    So how the ***** am I meant to make that work? Is there some alternate strategy?

    Any insights?
    Will work inside IR35. Or for food.

    #2
    I suspect you are trying to reinvent the wheel.

    Try looking at a library:

    Hammer.JS - Hammer.js
    QuoJS
    Daniel Glyde: TouchIt: jQuery Plugin for Touch events

    Comment


      #3
      Not really. I'm just trying to use the wheel; it's other people that have implemented it wrong.

      Found a bodge. Basically create a DIV at 0,0 and then call its getClientBoundingRect() and measure the distance. Then the error cancels itself out. Phew.
      Will work inside IR35. Or for food.

      Comment

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