class Gtk::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 Gtk::Expression#evaluate for evaluating an expression.

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

Here is an example of a complex expression:

WARNING ⚠️ The following code is in c ⚠️

  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


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 Gtk#ExpressionWatch, 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::ExpressionWatch#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:

WARNING ⚠️ The following code is in c ⚠️

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

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:

WARNING ⚠️ The following code is in c ⚠️

  // in set_property()...
    foo_widget_set_expression (foo, gtk_value_get_expression (value));

  // in get_property()...
    gtk_value_set_expression (value, foo->expression);

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 specfiying the expression to use the object, or a string that specifies the name of the object to use.


WARNING ⚠️ The following code is in xml ⚠️

  <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:

WARNING ⚠️ The following code is in xml ⚠️

  <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:

WARNING ⚠️ The following code is in xml ⚠️

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

Direct Known Subclasses

Defined in:


Class Method Summary

Instance Method Summary

Class Method Detail

def self.g_type : UInt64 #

Returns the type id (GType) registered in GLib type system.

Instance Method Detail

def bind(target : GObject::Object, property : String, this_ : GObject::Object?) : Gtk::ExpressionWatch #

Bind target's property named property to self.

The value that self evaluates to is set via g_object_set() on target. This is repeated whenever self changes to ensure that the object's property stays synchronized with self.

If self's evaluation fails, target's property is not updated. You can ensure that this doesn't happen by using a fallback expression.

Note that this function takes ownership of self. If you want to keep it around, you should Gtk::Expression#ref it beforehand.

def evaluate(this_ : GObject::Object?, value : _) : Bool #

Evaluates the given expression and on success stores the result in @value.

The GType of value will be the type given by Gtk::Expression#value_type.

It is possible that expressions cannot be evaluated - for example when the expression references objects that have been destroyed or set to NULL. In that case value will remain empty and FALSE will be returned.

def finalize #

Called by the garbage collector. Decreases the reference count of object. (i.e. its memory is freed).

def is_static : Bool #

Checks if the expression is static.

A static expression will never change its result when Gtk::Expression#evaluate is called on it with the same arguments.

That means a call to Gtk::Expression#watch is not necessary because it will never trigger a notify.

def to_unsafe : Pointer(Void) #

Returns a pointer to the C object.

def value_type : UInt64 #

Gets the GType that this expression evaluates to.

This type is constant and will not change over the lifetime of this expression.

Watch the given expression for changes.

The @notify function will be called whenever the evaluation of self may have changed.

GTK cannot guarantee that the evaluation did indeed change when the @notify gets invoked, but it guarantees the opposite: When it did in fact change, the @notify will be invoked.