XForms Basics, Part 3 - The Bookworm Turns (
Page 5 of 7 )
In addition to simple comparison tests, you can also use XPath arithmetic and non-arithmetic functions in an XForms model. Consider the following example:
<html xmlns="http://www.w3.org/1999/xhtml"
xmlns:xforms="http://www.w3.org/2002/xforms/cr"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<head>
<!-- define the form model -->
<xforms:model id="bookstore">
<xforms:instance>
<inventory>
<books>
<title>Be Cool</title>
<author>Elmore Leonard</author>
<price>7.97</price>
<quantity>150</quantity>
<title>Mystic River</title>
<author>Dennis Lehane</author>
<price>25.00</price>
<quantity>86</quantity>
<title>Hit List</title>
<author>Lawrence Block</author>
<price>23.76</price>
<quantity>26</quantity>
<title>Silent Joe</title>
<author>T. Jefferson Parker</author>
<price>24.99</price>
<quantity>268</quantity>
<title>The Travel Detective</title>
<author>Peter Greenberg</author>
<price>34.87</price>
<quantity>9</quantity>
</books>
<totalInventory />
<mostExpensive />
<leastExpensive />
<averageCost />
</inventory>
</xforms:instance>
<xforms:bind nodeset="/inventory/books/totalInventory"
calculate="sum(/inventory/books/quantity)" />
<xforms:bind nodeset="/inventory/books/mostExpensive"
calculate="max(/inventory/books/price)" />
<xforms:bind nodeset="/inventory/books/leastExpensive"
calculate="min(/inventory/books/price)" />
<xforms:bind nodeset="/inventory/books/averagePrice"
calculate="avg(/inventory/books/price)" />
</xforms:model>
</head>
<body>
<!-- define the form interface -->
<!-- loop over the instance data -->
<xforms:repeat nodeset="/inventory/books">
<xforms:input id="txttitle" ref="title">
<xforms:label>Title</xforms:label>
</xforms:input>
<xforms:input id="txtauthor" ref="author">
<xforms:label>Author</xforms:label>
</xforms:input>
<xforms:input id="txtprice" ref="price">
<xforms:label>Price</xforms:label>
</xforms:input>
<xforms:input id="txtquantity" ref="quantity">
<xforms:label>Quantity</xforms:label>
</xforms:input>
</xforms:repeat>
<!-- print calculated values -->
Total inventory: <xforms:output ref="/inventory/books/totalInventory" />
<br />
Price of most expensive book: <xforms:output
ref="/inventory/books/mostExpensive" /> <br />
Price of least expensive book: <xforms:output
ref="/inventory/books/leastExpensive" /> <br />
Average price:
<xforms:output ref="/inventory/books/averagePrice" />
</body>
</html>
As usual, all the action is centered around the definition of the XForms model. This time, the instance data has been populated with the inventory of the neighbourhood bookstore, and XPath functions like sum(), min(), max() and avg() have been used to perform calculations on that data, and assign the results of those calculations to specific summary elements in the instance data. This data is then displayed in the form using the
<xforms:output> element.
You'll also notice a new element in the form above - the <xforms:repeat> element. This element gives XForms authors the ability to loop over a collection of nodes without having to resort to complex "while" or "for" loops. In this example, the <xforms:repeat> element causes the XForms processor to iterate over the <books> collection from the instance data, and display the values of the title, author, price and quantity. The "nodeset" attribute specifies the node over which iteration should take place, while the loop counter is handled internally by the processor. Looping will stop once no further match is found for the "nodeset" criteria.
Why stop just at numeric functions, though? This next example uses the days-from-date() function to calculate the number of days between an entered date and January 01 1970.
The XForms model will look something like this:
<!-- form model -->
<xforms:model id="dateCalculator">
<xforms:instance>
<calc>
<dt />
<numDays />
</calc>
</xforms:instance>
<xforms:bind nodeset="/calc/numDays" calculate="days-from-date(/calc/dt)" />
</xforms:model>
Pretty straightforward, isn't it?
