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A Juice Metadef is a named description, associated with a particular User Interface type, of where information to be used by Juice extensions can be identified on a html page. The description is identified by a name that can then be used within an extension to access the data displayed on the page.

The Metadef file

Following the theme of all Juice components, the Metadef for a particular interface type is contained in a separate JavaScript file. The convention being that the file will be of the format xxx_metadef.js.

The first step in creating a Metadef is to first check if one has already been created. As the layout of interfaces for a particular product all tend to share the same format, it is possible that someone else has already created one that you can use.

The anatomy of a Metadef

Lets take a look at an example Metadef - in this case the one for the Talis Aspire Resource List system.

function talis_aspire_metadef(){
	juice.findMeta("isbns","#isbn10 > .fieldValue, #isbn13 > .fieldValue ",juice.stringToAlphnumAray);
	juice.findMeta("author","#authors > .fieldValue "); 
	juice.findMeta("title","#itemDetailsContainer > h1 "); 
}

The Metadef is contained in a single, appropriately named, function talis_aspire_metadef().

In this example we create three definitions - 'isbns', 'author', and 'title'. For each one we create an instance of the JuiceMeta class. The constructor of which has three arguments:

• id - the id to reference the value by.
• selector - jQuery selector string for element within page
• filterFunc - optional function used to process retrieved data before storage

For information on jQuery selectors check out the jQuery documentation on selectors, but for example the title definition in our example is selecting the contents of the <h1> element contained within an element with an id attribute of itemDetailsContainer.

Remember that a jQuery selector can return multiple values, a feature used in the isbns meta in our example. This also highlights the optional filterFunc - a JavaScript function used to filter or change the values of the returned before storing them. juice.stringToAlphnumAray is a utility which removes words which contain non alphanumeric characters from a string and then returns the remaining words as an array which is stored in the instance of JuiceMeta.

What about attributes?

JuiceMeta will retrieve and store the contents of a document element from the html page. If you need to retrieve a value held in an element attribute you should use the associated class - JuiceMetaAttr. It's constructor takes one extra argument defining the name of the attribute:

JuiceMetaAttr(id, selector, attName, filterFunc)

Making the value available

In our example the constructed instances of JuiceMeta are passed to the Juice function juice.addMeta. This makes them available to other components of the Juice framework.

Debugging Metadefs

If you enabled debugging, a list of all defined Metadefs and their selected content can be showed:

juice.setDebug(true);
juice.debugMeta();