pub struct Expression { /* private fields */ }
Expand description

Expression 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 [ExpressionExtManual::evaluate()][crate::prelude::ExpressionExtManual::evaluate()] for evaluating an expression.

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

Here is an example of a complex expression:

⚠️ 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 ListItem, it will obtain the “item” property from the ListItem, 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, DropDown is evaluating a Expression 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 Expression 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; [ExpressionExt::watch()][crate::prelude::ExpressionExt::watch()] starts watching an expression, and ExpressionWatch::unwatch() stops.

Watches can be created for automatically updating the property of an object, similar to GObject’s GBinding mechanism, by using [ExpressionExt::bind()][crate::prelude::ExpressionExt::bind()].

GtkExpression in GObject properties

In order to use a Expression 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:

⚠️ 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 Expression from the GValue container, and gtk_value_set_expression, to store the Expression into the GValue; for instance:

⚠️ 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

Builder 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 name="property"> 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.


  <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 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>

This is an Abstract Base Class, you cannot instantiate it.


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.


the target object to bind to


name of the property on target to bind to


the this argument for the evaluation of self


a ExpressionWatch

Gets the GType that this expression evaluates to.

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


The type returned from [ExpressionExtManual::evaluate()][crate::prelude::ExpressionExtManual::evaluate()]

Checks if the expression is static.

A static expression will never change its result when [ExpressionExtManual::evaluate()][crate::prelude::ExpressionExtManual::evaluate()] is called on it with the same arguments.

That means a call to [ExpressionExt::watch()][crate::prelude::ExpressionExt::watch()] is not necessary because it will never trigger a notify.


TRUE if the expression is static

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.


the this argument to watch


callback to invoke when the expression changes


The newly installed watch. Note that the only reference held to the watch will be released when the watch is unwatched which can happen automatically, and not just via ExpressionWatch::unwatch(). You should call Gtk::ExpressionWatch::ref() if you want to keep the watch around.

Similar to Self::evaluate but panics if the value is of a different type.

Create a PropertyExpression that looks up for property_name with self as parameter. This is useful in long chains of Expressions.

Create a ClosureExpression from a glib::Closure with self as the second parameter and R as the return type. The return type is checked at run-time and must always be specified. This is useful in long chains of Expressions when using the [glib::closure!] macro.

Note that the first parameter will always be the this object bound to the expression. If None is passed as this then the type of the first parameter must be Option<glib::Object> otherwise type checking will panic.

use gtk::prelude::*;
use gtk::glib;
use glib::{closure, Object};

let button = gtk::Button::new();
let label = button
    .chain_closure::<String>(closure!(|_: Option<Object>, label: &str| {
        format!("{} World", label)
    .evaluate_as::<String, _>(gtk::Widget::NONE);
assert_eq!(label.unwrap(), "Hello World");

Create a ClosureExpression with self as the second parameter. This is useful in long chains of Expressions.

Trait Implementations

Performs the conversion.

Performs the conversion.

Performs the conversion.

Performs the conversion.

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

Formats the value using the given formatter. Read more

Formats the value using the given formatter. Read more

Value type checker.

Get the contained value from a Value. Read more

Returns the type identifier of Self.

Convert a value to a Value.

Returns the type identifer of self. Read more

Convert an Option to a Value.

Type to get the Type from. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Performs the conversion.

Performs the conversion.

The resulting type after obtaining ownership.

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into)

Uses borrowed data to replace owned data, usually by cloning. Read more

Converts the given value to a String. Read more

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.