KTextTemplate::FilterExpression Class

Detailed Description

This class is only relevant if implementing custom tags or filters. Most of the API here is internal. Usually when implementing tags or filters, filter expressions will just be created and resolved.

In template markup, a filter expression is a variable followed by one or more filters separated by pipes:

Filter expressions may appear in variable nodes:

{{ some_var|upper_filter|lower_filter }}

Or as arguments to tags

{% some_tag some_arg1|filter1|filter2 some_arg2|filter3 %}

The **%FilterExpression** class would be used in the getNode implementation of the AbstractNodeFactory implementation for the some_tag tag.

Node* SomeTagFactory::getNode(const QString &tagContent, Parser *p) const {
  auto parts = smartSplit( tagContent );

  parts.removeFirst(); // Remove the "some_tag" part.

  FilterExpression arg1( parts.first(), p );
  FilterExpression arg2( parts.at( 1 ), p );

  return new SomeTagNode( arg1, arg2, p );
}

When implementing the Node::render method, the resolve method may be used to process the filter expression.

For example, if our SomeTagNode was to concatenate the resolved values given as arguments:

void SomeTagNode::render( QTextStream *stream, Context *c ) {
  m_arg1.resolve( stream, c );
  m_arg2.resolve( stream, c );
}

Because Filters are highly generic, they do not all write data to the stream. For example, a Filter might take as input a string, and return a list by splitting the string on commas, or a Filter might compare an input to its argument and return whether they are the same, but not write anything to the stream. For that reason, the resolve method writes data to the given stream, and returns the same data in its returned QVariant.

The suitability of either of the resolve methods will depend on the implementation and requirements of your custom tag. For example if the SomeTagNode ran a comparison of the arguments:

void SomeTagNode::render( QTextStream *stream, Context *c ) {
  QString first = m_arg1.resolve( c ).toString();
  QString second = m_arg2.resolve( c ).toString();

  if ( first == second )
    m_trueList.render( stream, c );
  else
    m_falseList.render( stream, c );
}