Thursday, December 22, 2011

jQuery Mobile and Duplicated Page IDs

In the previous post, I touched upon how browsers safely handle duplicate IDs in a html file. Things seem to work fine for the three main uses of element IDs, i.e. linking to fragments, styling and referencing the element in javascript. The first element with matching ID is picked up and styling is done for all matches.


Now let us extend this test to the jQuery Mobile framework. You can launch the below multi-page code here.


 Launch 


<!DOCTYPE html>
<html>
  <head>
    <title>jQuery Mobile Duplicate IDs</title>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="http://code.jquery.com/mobile/1.0.1/jquery.mobile-1.0.1.min.css" /> 
    <script src="http://code.jquery.com/jquery-1.6.4.min.js"></script>
    <script src="http://code.jquery.com/mobile/1.0.1/jquery.mobile-1.0.1.min.js"></script>
  </head>
  <body>
    <div data-role="page" id="p1">
      <div data-role="content">
        <p><a href="#p2" data-role="button">duplicate page id</a></p>
      </div>
    </div>
    <div data-role="page" id="p2">
      <div data-role="content">
        <a data-role="button" data-rel="back" data-direction="reverse">1 1</a>
        <a href="#p1" data-role="button" data-rel="dialog">Open Dialog</a>
      </div>
    </div>
    <div data-role="page" id="p2">
      <div data-role="content">
        <a data-role="button" data-rel="back" data-direction="reverse"> 0 </a>
      </div>
    </div>
  </body>
</html>


On load, when you click on the button, ajax is used to navigate to the page with ID p2, but here there are two IDs that match. The framework is not expecting this, and correctly so. Things go haywire. You have the resulting page rendered with seemingly garbled text, Actually both pages are rendered, but in a weird way. You will see the first button from both the pages, at the same location, merging the texts "1  1" and "  0  " to show "101". Then there is the second button which gets hidden, but its text "Dialog" is shown.


It gets interesting when you click on the button "101" now shown, as you can see from the code, it should take you back to the page ID p1. But here, two pages were rendered, and the first occurring page div slides out and for a brief instant you can see the second page container, which also slides out.


If you use the browser code inspector, you will see the data-url attribute being automatically added to all page divs. Since no data-url was specified here, the page ID is copied as is. In this case, both data-urls are the same. You can try the above example with different page IDs but having the same data-url and the resultant output is the same. 


<div data-role="page" id="p2" data-url="durl1">
......
<div data-role="page" id="p3"  data-url="durl1" >

One final scenario, suppose if the page IDs are duplicated but each page has a different data-url specified. 


<div data-role="page" id="p2" data-url="durl1">
......
<div data-role="page"  id="p2"  data-url="durl2">

And now try to navigate to any page using the data-url. 

<a href="#durl1" data-role="button">Open Page 1</a>
<a href="#durl2" data-role="button">Open Page 2</a>

Bingo! Both links work just works fine.


So jQuery Mobile primarily uses data-url for page navigation and as long as they are unique, navigation works just fine. And it was a purely accidental discovery that having duplicate data-urls messes up the rendered content.


Now is this a valid bug? I feel the framework should gracefully handle this and render only the first instance of matching data-url. The behavior would then be consistent with how various browser engines handle duplicate IDs (from my previous post). But again is this an overkill? Should the framework focus on adding more features that work correctly on multiple devices/platforms/browsers or fix such scenarios. That is a call that the jQuery Mobile developer community has to take.

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