MonthJune 2010

Gelernter nonsense

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Annotating DOM nodes with JSON, Part 2

It’s been a while since I wrote Annotating DOM nodes with JSON and in retrospective I can say that I never really used the method described in a real life project. Now I’d like to present another method of decorating DOM nodes with JSON based on classes. This one I actually implemented in OpenSAGA to have arbitrary metadata from some of the OpenSAGA Widgets.

I didn’t really like the idea of misusing onclick for the purpose of meta-data and thought about a better way of doing it. Browsing the w3 HTML specs I came upon the fact that classes can be any character separated by spaces. So for use-cases where I only needed one meta-data value I used classes like

<div class="refId:id-1234">
    DIV content
</div>

A use-case specific prefix is used to mark a class as meta-data container containing the string after the prefix. The code to evaluate this in javascript is very easy

/**
 * Returns the class value with the given prefix using the giving separator
 * @param {DOMElement} elem DOM element to fetch metadata from
 * @param {String} name of the classval value
 * @param {String} separator to use between name and value. Default is ":"
 */
function classval(elem, name, separator)
{
    var match = new RegExp("\\b" + name + (separator || ":") + "([^ ]*)($| )")
                          .exec(elem.className);
    if (match)
    {
        return match[1];
    }
    return null;
}
…
// assume divElement to be DOM element of the div
var refId = classval(divElement, "refId");

I thought about going for a more elaborate prefix scheme to support nested metadata but in the end decided against it because I already have a nicely supported format for exchanging data between server and client: JSON. So I tried to come up with a scheme of using arbitrary JSON for the metadata decoration.

Only problem: Spaces are not valid inside classes, so I needed a method to encode and decode JSON into valid classes. The method should not totally mangle the JSON to keep readability and maybe write the encoded variant by hand for simple cases.

Solution:

  • HTML encode the JSON-String
  • Replace spaces with underlines and underlines with \u005f

The replacement of underlines is valid because underlines can only occur inside quoted JSON strings so they can just be replaced by their escaped unicode value \u005f.

Here is the java code to do the escaping. Since it’s basically a combination of string replacement and HTML encoding this should be easily doable in any server-side language:

    public String escapeDecoration(String s)
    {
        String escaped = StringEscapeUtils.escapeHtml(s);

        StringBuilder sb = new StringBuilder(escaped.length());
        sb.append("deco:");
        for (int i = 0; i < escaped.length() ; i++)
        {
            char c = escaped.charAt(i);
            switch(c)
            {
                case '_':
                    sb.append("\\u005F");
                    break;
                case ' ':
                    sb.append('_');
                    break;
                default:
                    sb.append(c);
                    break;
            }
        }

        return sb.toString();
    }

The escape method uses the escapeHTML method from Apache commons-lang's StringEscapeUtil. Going the other way in javascript is not that complicated either:

/**
 * Decodes the given string containing HTML entities.
 */
function htmlDecode(s)
{
    var helper = document.createElement("SPAN");
    helper.innerHTML = s;
    return helper.innerHTML;
}

/**
 * Returns the JSON decoration of the given element.
 * @param {DOMElement} DOM element
 * @param {String} decorator classval name, default is "deco".
 */
function decoration(elem, name)
{
    var value, data, result;

    value = classval(elem, name || "deco");
    if (value)
    {
       // get raw data from DOM element
       data = value.replace(/_/g, " ");
       // replace HTML entities with the original characters
       data = htmlDecode(data);
       // evaluate JSON
       result = eval("("+data+")");
    }
    return result || {};
}

In order to achieve a better readability of escaped JSON, I also used svenson's ability to deviate from the JSON standard by using single quotes instead of double quotes. Just comparing

<div id="tst2" class="deco:{'foo':'xxx\u005f_yyy','baz':[1,3,5,7,9]}">
JSON annotation
</div>

to

<div id="tst2" class="deco:{&quot;foo&quot;:&quot;xxx\u005f_yyy&quot;,&quot;baz&quot;:[1,3,5,7,9]}">
JSON annotation
</div>

should demonstrate that single quotes are not only much better readable, but also shorter. If you use eval() evaluate the JSON string, the single quotes are no problem at all. If you want json2.js / native JSON-parsing, you might have to replace the quote chars before parsing.

Links:

HTML test page with both metadata strategies

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