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 [ method@Gtk.Expression.evaluate] for evaluating an expression.

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

Here is an example of a complex expression:

```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

``` 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 [struct@Gtk.ExpressionWatch], which will cause a callback to be called whenever the value of the expression may have changed; [method@Gtk.Expression.watch] starts watching an expression, and [ method@Gtk.ExpressionWatch.unwatch] stops.

Watches can be created for automatically updating the property of an object, similar to GObject's `GBinding` mechanism, by using [ method@Gtk.Expression.bind].

GtkExpression in GObject properties

In order to use a `GtkExpression` as a `GObject` property, you must use the [func@Gtk.param_spec_expression] when creating a `GParamSpec` to install in the `GObject` class being defined; for instance:

```c 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 [ func@Gtk.value_get_expression], to retrieve the stored `GtkExpression` from the `GValue` container, and [func@Gtk.value_set_expression], to store the `GtkExpression` into the `GValue`; for instance:

```c // 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:

```xml <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 `GtkListItem`s. See [class@Gtk.BuilderListItemFactory] 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:

```xml <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:

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