GtkExpression

GtkExpression provides a way to describe references to values.

An important aspect of expressions is that the value can be obtained from a source that is several steps away. For example, an expression may describe ‘the value of property A of object1, which is itself the value of a property of object2’. And object1 may not even exist yet at the time that the expression is created. This is contrast to GObject property bindings, which can only create direct connections between the properties of two objects that must both exist for the duration of the binding.

An expression needs to be “evaluated” to obtain the value that it currently refers to. An evaluation always happens in the context of a current object called this (it mirrors the behavior of object-oriented languages), which may or may not influence the result of the evaluation. Use gtk_expression_evaluate() for evaluating an expression.

Various methods for defining expressions exist, from simple constants via gtk_constant_expression_new() to looking up properties in a GObject (even recursively) via gtk_property_expression_new() or providing custom functions to transform and combine expressions via gtk_closure_expression_new().

Here is an example of a complex expression:

  color_expr = gtk_property_expression_new (GTK_TYPE_LIST_ITEM,
                                            NULL, "item");
  expression = gtk_property_expression_new (GTK_TYPE_COLOR,
                                            color_expr, "name");

when evaluated with this being a GtkListItem, it will obtain the “item” property from the GtkListItem, and then obtain the “name” property from the resulting object (which is assumed to be of type GTK_TYPE_COLOR).

A more concise way to describe this would be

  this->item->name

The most likely place where you will encounter expressions is in the context of list models and list widgets using them. For example, GtkDropDown is evaluating a GtkExpression to obtain strings from the items in its model that it can then use to match against the contents of its search entry. GtkStringFilter is using a GtkExpression for similar reasons.

By default, expressions are not paying attention to changes and evaluation is just a snapshot of the current state at a given time. To get informed about changes, an expression needs to be “watched” via a GtkExpressionWatch, which will cause a callback to be called whenever the value of the expression may have changed; gtk_expression_watch() starts watching an expression, and gtk_expression_watch_unwatch() stops.

Watches can be created for automatically updating the property of an object, similar to GObject’s GBinding mechanism, by using gtk_expression_bind().

GtkExpression in GObject properties

In order to use a GtkExpression as a GObject property, you must use the gtk_param_spec_expression() when creating a GParamSpec to install in the GObject class being defined; for instance:

obj_props[PROP_EXPRESSION] =
  gtk_param_spec_expression ("expression",
                             "Expression",
                             "The expression used by the widget",
                             G_PARAM_READWRITE |
                             G_PARAM_STATIC_STRINGS |
                             G_PARAM_EXPLICIT_NOTIFY);

When implementing the GObjectClass.set_property and GObjectClass.get_property virtual functions, you must use gtk_value_get_expression(), to retrieve the stored GtkExpression from the GValue container, and gtk_value_set_expression(), to store the GtkExpression into the GValue; for instance:

  // in set_property()...
  case PROP_EXPRESSION:
    foo_widget_set_expression (foo, gtk_value_get_expression (value));
    break;

  // in get_property()...
  case PROP_EXPRESSION:
    gtk_value_set_expression (value, foo->expression);
    break;

GtkExpression in .ui files

GtkBuilder has support for creating expressions. The syntax here can be used where a GtkExpression object is needed like in a <property> tag for an expression property, or in a <binding name="property"> tag to bind a property to an expression.

To create a property expression, use the <lookup> element. It can have a type attribute to specify the object type, and a name attribute to specify the property to look up. The content of <lookup> can either be an element specifying the expression to use the object, or a string that specifies the name of the object to use.

Example:

  <lookup name='search'>string_filter</lookup>

Since the <lookup> element creates an expression and its element content can itself be an expression, this means that <lookup> tags can also be nested. This is a common idiom when dealing with GtkListItems. See GtkBuilderListItemFactory for an example of this technique.

To create a constant expression, use the <constant> element. If the type attribute is specified, the element content is interpreted as a value of that type. Otherwise, it is assumed to be an object. For instance:

  <constant>string_filter</constant>
  <constant type='gchararray'>Hello, world</constant>

To create a closure expression, use the <closure> element. The function attribute specifies what function to use for the closure, and the type attribute specifies its return type. The content of the element contains the expressions for the parameters. For instance:

  <closure type='gchararray' function='combine_args_somehow'>
    <constant type='gchararray'>File size:</constant>
    <lookup type='GFile' name='size'>myfile</lookup>
  </closure>

To create a property binding, use the <binding> element in place of where a <property> tag would ordinarily be used. The name and object attributes are supported. The name attribute is required, and pertains to the applicable property name. The object attribute is optional. If provided, it will use the specified object as the this object when the expression is evaluated. Here is an example in which the label property of a GtkLabel is bound to the string property of another arbitrary object:

  <object class='GtkLabel'>
    <binding name='label'>
      <lookup name='string'>some_other_object</lookup>
    </binding>
  </object>