XForms, Calculator

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One of the classic examples of a web application program is a calculator.

Program

<title>Calculator Sample</title>
<xf:model>
<xf:instance>
<equation xmlns="">
<display>0</display>
<displaybuffer>0</displaybuffer>
<first>0</first>
<second>0</second>
<memory>0</memory>
<result />
</equation>
</xf:instance>
</xf:model>
<style type="text/css">
table {
border: thin outset;
}
td {
border: thin inset;
}
.display {
text-align: right;
}
</style>
</head>
<body>
<p>A simple calculator</p>
<table>
<tr>
<td colspan="6" class="display">
<xf:output ref="/equation/display" />
</td>
</tr>
<tr>
<td>
<xf:output ref="/equation/memory">
<xf:label>M:</xf:label>
</xf:output>
</td>
<td />
<td />
<td />
<td colspan="2">
<xf:trigger>
<xf:label>Clear</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/first" value="0" />
<xf:setvalue ref="/equation/second" value="0" />
<xf:setvalue ref="/equation/result" value="0" />
<xf:setvalue ref="/equation/display" value="0" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
<xf:toggle case="add" />
</xf:action>
</xf:trigger>
</td>
</tr>
<tr>
<td>
<xf:trigger>
<xf:label>MC</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/memory" value="0" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>7</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 7" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>8</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 8" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>9</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 9" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>/</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/first" value="/equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
<xf:toggle case="divide" />
</xf:action>
</xf:trigger>
</td>
<td />
</tr>
<tr>
<td>
<xf:trigger>
<xf:label>MR</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/display" value="/equation/memory" />
<xf:setvalue ref="/equation/displaybuffer" value="/equation/memory" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>4</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 4" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>5</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 5" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>6</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 6" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>*</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/first" value="/equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
<xf:toggle case="multiply" />
</xf:action>
</xf:trigger>
</td>
</tr>
<tr>
<td>
<xf:trigger>
<xf:label>MS</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/memory" value="/equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>1</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 1" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>2</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 2" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>3</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10 + 3" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>-</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/first" value="/equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
<xf:toggle case="subtract" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>1/x</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/display" value="1 div /equation/display" />
</xf:action>
</xf:trigger>
</td>
</tr>
<tr>
<td>
<xf:trigger>
<xf:label>M+</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/memory" value="/equation/memory + /equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>0</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/displaybuffer" value="/equation/displaybuffer * 10" />
<xf:setvalue ref="/equation/display" value="/equation/displaybuffer" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:trigger>
<xf:label>+/-</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/display" value="/equation/display * -1" />
</xf:action>
</xf:trigger>
</td>
<td />
<td>
<xf:trigger>
<xf:label>+</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/first" value="/equation/display" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
<xf:toggle case="add" />
</xf:action>
</xf:trigger>
</td>
<td>
<xf:switch>
<xf:case id="add" selected="true">
<xf:trigger>
<xf:label>=</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/second" value="/equation/displaybuffer" />
<xf:setvalue ref="/equation/result" value="/equation/first + /equation/second" />
<xf:setvalue ref="/equation/display" value="/equation/result" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</xf:case>
<xf:case id="subtract">
<xf:trigger>
<xf:label>=</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/second" value="/equation/displaybuffer" />
<xf:setvalue ref="/equation/result" value="/equation/first - /equation/second" />
<xf:setvalue ref="/equation/display" value="/equation/result" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</xf:case>
<xf:case id="multiply">
<xf:trigger>
<xf:label>=</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/second" value="/equation/displaybuffer" />
<xf:setvalue ref="/equation/result" value="/equation/first * /equation/second" />
<xf:setvalue ref="/equation/display" value="/equation/result" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</xf:case>
<xf:case id="divide">
<xf:trigger>
<xf:label>=</xf:label>
<xf:action ev:event="DOMActivate">
<xf:setvalue ref="/equation/second" value="/equation/displaybuffer" />
<xf:setvalue ref="/equation/result" value="/equation/first div /equation/second" />
<xf:setvalue ref="/equation/display" value="/equation/result" />
<xf:setvalue ref="/equation/displaybuffer" value="0" />
</xf:action>
</xf:trigger>
</xf:case>
</xf:switch>
</td>
</tr>
</table>
</body>
</html>

Discussion

The calculator program is about 300 lines long. But much of the code is just telling what actions should happen when a button gets pressed.

The model of the calculator is very simple. Just a five variables:

display - the visible display
displaybuffer - a secondary display that is not visible
first - the first operator
second - the second operator
memory - used by the memory function

Here is the trigger for the "5" button:

The actions for the trigger is to do just two things:

are to multiply the value of the display buffer by 10 (shifting the digits over by one) and then to add a five.
copy the value of the display buffer to the display.

There are also scientific function calculators but they have a smiliar structure.

It is interesting to compare a JavaScript version to the XForms version. The JavaScript version is shorter but the JavaScript can take advantage of some of the tools that are available to JavaScript.

References

Hixie's Natural Log

Where this page came from

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