Loop Through a Well-Known Input

Mashups in Presto Wires : Customizing Wires : Adding Custom Blocks to Presto Wires Using Macros : Loop Through a Well-Known Input

One very common use case for custom blocks is to take results from a mashup or mashable information source, loop through each item in the results and add or transform data in the results.

When the structure of the input is well known, common steps that a looping macro may need to complete are:

1. Define <output> with a root node that will wrap the items added by the macro during looping. See Creating a Root Node for the Macro Result.

2. Begin looping through the input. See Looping With <foreach> and a Well-Known Input Structure.

3. Generate the appropriate output for each loop. See Using <appendresult> to Transform Input and Build the Output.

Creating a Root Node for the Macro Result

You must add an <output> statement for the macro to allow the custom action in Wires to be wired to other blocks or to the output of the mashup itself. When a macro loops through the input block results, you can also create the root node for the macro output within the <output> statement itself. Simply add an empty element within <output>.

This example defines a root node, <statistics>, for the macro output:

Looping With <foreach> and a Well-Known Input Structure

You use <foreach> to loop through a set of repeating items, in this case the input mashable results.

With <foreach> you define a variable to hold each item and use an XPath expression to define the items to loop through. The input for this example comes from an spreadsheet mashable:

<SessionStatsData>
<item>
<Categories>FormA</Categories>
<Session1>23</Session1><Session2>30</Session2><Session3>35</Session3>
<Session4>11</Session4><Session5>23</Session5><Session6>15</Session6>
</item>
<item>
<Categories>FormC</Categories>
<Session1>15</Session1><Session2>10</Session2><Session3>13</Session3>
<Session4>15</Session4><Session5>5</Session5><Session6>11</Session6>
</item>
<item>
<Categories>FormD</Categories>
<Session1>37</Session1><Session2>78</Session2><Session3>51</Session3>
<Session4>66</Session4><Session5>77</Session5><Session6>25</Session6>
</item>
<item>
<Categories>FormL</Categories>
<Session1>8</Session1><Session2>3</Session2><Session3>6</Session3>
<Session4>4</Session4><Session5>5</Session5><Session6>6</Session6>
</item>
<item>
<Categories>FormR</Categories>
<Session1>19</Session1><Session2>22</Session2><Session3>7</Session3>
<Session4>15</Session4><Session5>10</Session5><Session6>13</Session6>
</item>
</SessionStatsData>

The <foreach> loop would look something like this:

Within the loop, the category variable holds each item during an iteration. The XPath that defines what node to use as an item is specifically defined based on the well known structure of the input.

Using <appendresult> to Transform Input and Build the Output

Within the loop, you add to the output using <appendresult>. You define the structure of the output as literal XML and use dynamic mashup expressions to perform calculations, transform data and fill the output. See Dynamic Mashup Expressions for basic instructions.

This example adds a <category> element for each item in the loop and calculates percentages for the session statistics. The content for <appendresult> is again specifically defined from the known structure of the input.

<macros xmlns:xsi= "http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.openmashup.org/schemas/v1.0/EMML/ ../src/schemas/EMMLSpec.xsd"
xmlns="http://www.openmashup.org/schemas/v1.0/EMML"
xmlns:presto="http://www.jackbe.com/v1.0/EMMLPrestoExtensions"
domain = "myCustomMath">

<macro name="GetSessionPercentages">
<input name="spreadsheet" type="document" />
<output name="result" type="document">
<statistics />
</output>

<foreach variable="$category" items="$spreadsheet//item">

<appendresult outputvariable="$result">
<category>
<name>{$category/Categories/string()}</name>
<session1PC>{$category//Session1/number() div 100}</session1PC>
<session2PC>{$category//Session2/number() div 100}</session2PC>
<session3PC>{$category//Session3/number() div 100}</session3PC>
<session4PC>{$category//Session4/number() div 100}</session4PC>
<session5PC>{$category//Session5/number() div 100}</session5PC>
<session6PC>{$category//Session6/number() div 100}</session6PC>
</category>
</appendresult>
</foreach>
</macro>
...
</macros>>

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