ZRenderCheckbox

ZRenderer check_var Example

Suppose you were working with a web designer, who knew very little about Zope and less about DTML. They might provide you with HTML forms full of tags like:

    <input name="is_larch" type="checkbox" class="fancy_checkbox">

Suppose you want to control the state of these checkboxes based on the value of the tree variable, which may or may not be defined. The usual Zope idiom for this would be:

    <dtml-if tree>
      <dtml-if "tree == 'larch'">
        <input name="is_larch" type="checkbox" class="fancy_checkbox" CHECKED>
      <dtml-else>
        <input name="is_larch" type="checkbox" class="fancy_checkbox"> 
      </dtml-if>
    </dtml-if>

This is pretty ugly no matter how you look at it. Now suppose that you need the designer to make further changes to the HTML. They will see two checkboxes, and any changes will have to be made to both copies. If they rearrange the form, the checkboxes could easily become separated from the DTML logic.

With ZRenderer implemented, they (or you) would add three attributes to each of the input tags:

    <input name="is_larch" type="checkbox" class="fancy_checkbox" 
      z-render=check_var var=tree value=larch>

You would write (or have written) a check_var which tests whether tree exists and equals larch, and adds a CHECKED attribute to the tag if so. Something like this:

    <dtml-if expr="_.has_key(tag.var) and _[tag.var] == tag.value">
        <dtml-call expr="tag.set('CHECKED')>
    </dtml-if>
    <dtml-var tag>

Then, unless the designer's editor is brain-dead enough to strip out attributes it doesn't recognize, you can send this HTML back and forth and have it work unmodified in both environments.

Notice that this method is completely generic, relying on its ability to read the tag's attributes in order to get the variable and value to test against each other.