Expression


Object Hierarchy:

Object hierarchy for Expression

Description:

[ CCode ( type_id = "gtk_expression_get_type ()" ) ]
public abstract class Expression

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 evaluate for evaluating an expression.

Various methods for defining expressions exist, from simple constants via ConstantExpression to looking up properties in a Object (even recursively) via PropertyExpression or providing custom functions to transform and combine expressions via ClosureExpression .

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, DropDown 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. StringFilter 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 ExpressionWatch, which will cause a callback to be called whenever the value of the expression may have changed. watch starts watching an expression, and unwatch stops.

Watches can be created for automatically updating the property of an object, similar to GObject's Binding mechanism, by using bind.

GtkExpression in GObject properties

In order to use a Expression as a Object property, you must use the ParamSpecExpression when creating a ParamSpec to install in the Object 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 value_get_expression, to retrieve the stored Expression from the Value container, and value_set_expression, to store the Expression into the Value; 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 Expression object 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.

Example:

  <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.

Example:

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


Namespace: Gtk
Package: gtk4

Content:

Creation methods:

Methods: