Neil's Place

September 13, 2005

10:22 PM How Templates Work XLIV - Conclusion

We have come to end of the guide on how XUL templates work. I hope people reading it have learned a bit more about templates and how to use them. Expect to find the whole series on the Mozilla developer documentation site soon, or even sooner if (hint, hint), someone offers to help format it.

The template guide was originally supposed to be a short series, perhaps eight to ten parts, but quickly grew into a 44 part monster that took three and a half months to complete. One main reason for doing the template guide through my weblog rather than post it to a tutorial section was because I wanted to try a new method of writing documentation where I would write in short pieces rather than large blocks at a time. Clearly, it was still a lot of work. If I write another guide like this, I'll make sure to keep it shorter.

Anyway, I hope you enjoyed the series 'How Templates Work'. Hopefully, the template changes I've been working on for the last half year will make it into a Mozilla build soon. This will make templates much more useful for the future, and, naturally, will require even more documentation.

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September 12, 2005

5:29 PM How Templates Work XLIII - Additional Template Attributes

All the templates used so far have had the <template> placed inside the root element with the datasources attribute. However, you may use the template attribute to refer to a template located elsewhere within the document. This allows you to share the same template among two different parts of the user interface. Firefox uses this for bookmarks to display the bookmarks menu and the bookmarks toolbar, since the same template may be used to display the menu items in both cases. This is accomplished using a shared template and the clever use of multiple rules. To use this technique, place a template attribute on the root element set to the id of a template.

<listbox datasources="template-guide-photos5.rdf"
            ref="http://www.xulplanet.com/rdf/myphotos"
            template="photoTemplate"/>
...
<template id="photoTemplate">
  ...
</template>

This template will be shared with any other element that references the id "photoTemplate". The listbox here does not have any children, although it may do. If you did add children, they act just like the static content as if the template was present. However, it is possible to use different static content for each usage, even though the template is shared. The datasources and ref attributes also differ for each usage, so it is possible to use a shared template to display the same structure multiple times but with different starting nodes in each case. The generated content is always inserted into the root node, in this example the <listbox>, not inside the template.

Normally, the container and member variables are determined by the template builder automatically. The container or starting node variable is specified in the <content> tag inside a rule's conditions, while the member variable is determined by the value of the uri attribute inside the action body. It is also possible to be explicit about the two variables by using two attributes on the <template> element.

<template container="?first" member="?item">

The container and member attributes can be used to specify the container and member variables. This isn't particularly useful although there is a very slight optimization since the builder does not need to scan the action body looking for the member variable when compiling the rules. You might also use these attributes just to make the code clearer when using very complex rules.

One possible advantage is when using the simple syntax where you don't specify variables; instead you use the special 'rdf:*' syntax for the member variable and the container is implied. By using the container and member attributes, you can use specific variables instead.

<hbox datasources="template-guide-photos5.rdf"
      ref="http://www.xulplanet.com/rdf/myphotos">
  <template container="?start" member="?photo">
    <rule>
      <image uri="?photo" src="?photo"/>
    </rule>
  </template>
</hbox>

In this example, the ?photo variable can be used instead of 'rdf:*' (although you can use either even if you specify the member variable). This may make the code clearer as to its function. We could use the container variable ?start in the action body also. If you are using the simple syntax and want to use the container variable in the content, you must use the container attribute since there is no other way to refer to it. Note also that a <rule> element is needed here, otherwise the builder will think the container and member attributes are conditions to check.

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September 10, 2005

6:00 PM How Templates Work XLII - Content Sorting

For content builders (templates that do not use flags="dont-build-content"), sorting is done using a different means. Instead of sorting on a variable generated during the template generation, sorting for content builders may only be performed using a triple pointing out of the member variable. There is no connection between the generated variables and the resource used for sorting, so it doesn't have to be one used in the template.

You specify the resource used for sorting with the sortResource attribute on the root element, as follows:

<hbox datasources="template-guide-photos5.rdf"
      sortResource="http://purl.org/dc/elements/1.1/title"
      sortDirection="ascending"
      ref="http://www.xulplanet.com/rdf/myphotos">
  <template>
    <vbox class="box-padded" uri="rdf:*">
      <image src="rdf:*"/>
      <label value="rdf:http://purl.org/dc/elements/1.1/title"/>
    </vbox>
  </template>
</hbox>

In this example, the generated items will be sorted by title. The sortDirection attribute specifies the sort direction and may be set to "descending" for a reverse sort. The sortDirection attribute functions similarly to how the tree builder uses it. With a tree builder, each column has its own sort specified using the sort attribute, and the sortActive attribute is used to indicate the column that is currently sorted. For other content, there are no columns and only one sort is applicable, so the sortActive attribute is not necessary.

When the template builder generates a result, the sort service uses the value of the sortResource predicate for the result to determine where in the content the generated output should be inserted. When the RDF datasource changes, and a new result is available, the sort service inserts the new content at the right location.

Unlike with trees, you can sort using a secondary resource with the sortResource2 attribute which is used in the same manner as the sortResource attribute. If the values for the sortResource predicate are the same for two results, the sortResource2 predicate is used to further clarify the order. You can only have one secondary resource, that is, there is no sortResource3 attribute.

To change the sorting for a template's generated contents, you can either change the sort related attributes and rebuild the template, or for listboxes and menus, you can call the sort service's sort method:

var listbox = document.getElementById("aListBox");
var sortService = Components.classes["@mozilla.org/xul/xul-sort-service;1"].
                    getService(Components.interfaces.nsIXULSortService);
sortService.sort(listbox, "http://purl.org/dc/elements/1.1/title", "descending");

This code will sort a listbox by title in a descending order. The arguments to the sort method specify the root node (the listbox), the sort resource and the sort direction.

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September 5, 2005

10:29 PM How Templates Work XLI - Sorting Tree Results

Using a tree builder, you can sort the results in a tree by a column. To do this, place a sort attribute on a <treecol> element referring to the variable to sort by for that column.

<treecol id="name" label="Name" sort="?name" flex="1"/>
<treecol id="date" label="Date" sort="?date" flex="1"/>

In this example, the first column will be sorted by the ?name variable and the second column by the ?date variable. When the sort is ascending, the tree rows will be sorted in alphabetical order. When the sort is descending, the tree rows will be sorted in the reverse order. For natural sorting, the rows will be sorted according to the natural order in the RDF datasource. Only one column applies a sort at a time. If the tree is sorted by name, and the user clicks on the date column header, the sort will change to the date column.

There are two additional attributes used for sorting, which you may set on a column to specify the initial sort. These attributes are modified when the user changes the sort. The sortDirection attribute may be used to specify the initial sort direction for a column. Only one column should have this attribute set, as a tree may only be sorted by one column at a time. The value should be either 'ascending', 'descending' or 'natural'. This last value is the default if the attribute is not specified. The sortActive attribute may be set to true or false and specifies which column the tree is sorted by. Only one column should have the sortActive attribute set to true at a time. The tree will change both attributes as necessary automatically when the column headers are clicked or the tree is sorted by other means.

If you don't want to allow sorting by a certain column, you can leave out the sort attribute. Only specify this attribute on columns that you wish to allow the user to sort by.

Here is a complete example of sorting a tree.

The sort attribute should be set to the variable that holds the values to sort by. Usually, this would be the same variable that is used to generate the label for the cells in that column, however this is not actually necessary. For instance, in the example the second column sorts by date, but if you were to use a different variable such as ?description, assuming a <binding> set it, the tree would sort by the value of the description variable for each row. In almost all situations however, you would normally sort using the same variable used for the label value. However, one situation where this is not desirable is if the displayed values would not generate the correct order as there is a lower representation that is more accurate. For example, the date 'May 15' would appear after 'August 24' when sorted purely alphabetically but before it when sorted chronologically.

Another way to sort by dates is to use the the parseType="Date" construct in the RDF datasource. This marks a literal as being a date value rather than a string. The builder will recognize this and sort chronologically instead. This also has the advantage that the dates will be displayed according to the user's current locale (meaning that the date is formatted so as to be suitable for the user's language). Here is a sample of how to specify this in the RDF/XML datasource:

<rdf:RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
     xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
     xmlns:r="http://www.xulplanet.com/rdf/"
     xmlns:nc="http://home.netscape.com/NC-rdf#">
  <rdf:Description rdf:about="http://www.xulplanet.com/ndeakin/images/t/palace.jpg">
    <r:date nc:parseType="Date">1125966767295<r:date>
  </rdf:Description>
</rdf:RDF>

You can also specify parseType="Integer" for numbers which will allow sorting numerically. By specifing different types for different values, you can sort alphabetically, numerically or by date.

If you are using the simple rule syntax, there are no variables, so you need to specify the full predicate including the rdf: prefix in the sort attribute. For instance:

<treecol id="name" label="Name" sort="rdf:http://purl.org/dc/elements/1.1/title" flex="1"/>

Note that all of this discussion about sorting only applies to tree builders. For other elements or content trees, a different sorting mechanism must be used which will be discussed next.

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