Defining and Using XData Blocks

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Defining and Using Classes

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An XData block is a class member that consists of a name and a unit of data that you include in a class definition for use by the class after compilation. This chapter discusses XData blocks and covers the following topics:

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Basics

An XData block is a named unit of data that you include in a class definition, typically for use by a method in the class. Most frequently, it is a well-formed XML document, but it could consist of other forms of data, such as JSON.

You can create an XData block by typing it directly in Atelier.

An XData block is a named class member (like properties, methods, and so on). The available XData block keywords include:

SchemaSpec — Optionally specifies an XML schema against which the XData can be validated.
XMLNamespace — Optionally specifies the XML namespace to which the XData block belongs. You can also, of course, include namespace declarations within the XData block itself.
MimeType — The MIME type (more formally, the Internet media type) of the contents of the XData block. The default is text/xml.

If used to store XML, the XData block must consist of one root XML element, with any valid contents.

Using XData (XML Example)

To access an XML document in an arbitrary XData block programmatically, you use %Dictionary.CompiledXData and other classes in the %Dictionary package.

An XData block is useful if you want to define a small amount of system data. For example, suppose that the EPI.AllergySeverity class includes the properties Code (for internal use) and Description (for display to the users). This class could include an XData block like the following:

XData LoadData
{
<table>
 <row>1^Minor</row>
 <row>2^Moderate</row>
 <row>3^Life-threatening</row>
 <row>9^Inactive</row>
 <row>99^Unable to determine</row>
</table>
}

The same class could also include a class method that reads this XData block and populates the table, as follows:

/// called by EPI.Utils.GenerateData
ClassMethod Setup() As %Status
{
   //first kill extent
   do ..%KillExtent()
   
   // Get a stream of XML from the XData block contained in this class
   Set xdataID="EPI.AllergySeverity||LoadData"
   Set compiledXdata=##class(%Dictionary.CompiledXData).%OpenId(xdataID)
   Set tStream=compiledXdata.Data
   If '$IsObject(tStream) Set tSC=%objlasterror Quit
   
   set status=##class(%XML.TextReader).ParseStream(tStream,.textreader)
   //check status
   if $$$ISERR(status) do $System.Status.DisplayError(status) quit
   
   //iterate through document, node by node
   while textreader.Read()
   {
       if (textreader.NodeType="chars")
       {
           set value=textreader.Value
           set obj=..%New()
           set obj.Code=$Piece(value,"^",1)
           set obj.Description=$Piece(value,"^",2)
           do obj.%Save()
       }
   }
}

Notice the following:

The XML within the XData is minimal. That it, instead of presenting the allergy severities as XML element with their own elements or attributes, the XData block simply presents rows of data as delimited strings. This approach allows you to write the setup data in a visually compact form.
The EPI.AllergySeverity class is not XML-enabled and does not need to be XML-enabled.

Using XData (JSON Example)

A class could also include an XData block containing JSON content, like the following:

XData LoadJSONData [MimeType = "application/json"]
{
   {
      "person":"John", 
      "age":30, 
      "car":"Ford"
   }
}

The same class could also include a class method that reads this XData block and populates a dynamic object, as follows:

/// Reads a JSON XData block
ClassMethod SetupJSON() As %Status
{
   
   // Get a stream of JSON from the XData block contained in this class
   Set xdataID="Demo.XData||LoadJSONData"
   Set compiledXdata=##class(%Dictionary.CompiledXData).%OpenId(xdataID)
   Set tStream=compiledXdata.Data
   If '$IsObject(tStream) Set tSC=%objlasterror Quit

   // Create a dynamic object from the JSON content and write it as a string
   Set dynObject = {}.%FromJSON(tStream)
   Write dynObject.%ToJSON()

}