common-lisp:standard-object, common-lisp:t

None

The gtk:expression type 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 the gtk:expression-evaluate function for evaluating an expression.

Various methods for defining expressions exist, from simple constants via the gtk:constant-expression-new function to looking up properties in a GObject (even recursively) via the gtk:property-expression-new function or providing custom functions to transform and combine expressions via the gtk:closure-expression-new function.

Here is an example of a complex expression:

(setf color (gtk:property-expression-new "GtkListItem" nil "item"))
(setf expression (gtk:property-expression-new "GtkColor" color "name"))  

when evaluated with this being a gtk:list-item object, it will obtain the item property from the gtk:list-item object, and then obtain the name property from the resulting object, which is assumed to be of type "GtkColor". 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, the gtk:drop-down widget is evaluating a gtk:expression instance to obtain strings from the items in its model that it can then use to match against the contents of its search entry. The gtk:string-filter object is using a gtk:expression instance 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 gtk:expression-watch instance, which will cause a callback to be called whenever the value of the expression may have changed. The gtk:expression-watch function starts watching an expression, and the gtk:expression-watch-unwatch function stops.

Watches can be created for automatically updating the property of an object, similar to GObject's GBinding mechanism, by using the gtk:expression-bind function.

GtkExpression in GObject properties

In order to use a gtk:expression instance as a GObject property, you must use the the gtk:param-spec-expression function 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 the gtk:value-get-expression function, to retrieve the stored gtk:expression instance from the GValue container, and the gtk:value-set-expression, to store the gtk:expression instance 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

The gtk:builder class has support for creating expressions. The syntax here can be used where a gtk:expression instance is needed like in a <property> tag for an expression property, or in a <binding> tag to bind a property to an expression.

To create an 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 specfiying 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>  

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 type and function attributes specify what function to use for the closure, 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>