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:

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 [id@gtk_param_spec_expression] 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 [id@gtk_value_get_expression], to retrieve the stored GtkExpression from the GValue container, and [id@gtk_value_set_expression], to store the GtkExpression 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

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> 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.

class Expression {

protected

GtkExpression* gtkExpression;

protected

bool ownedRef;

GtkExpression* getExpressionStruct(bool transferOwnership);

void* getStruct();

this(GtkExpression* gtkExpression, bool ownedRef);

~this();

static GType getType();

ExpressionWatch bind(ObjectG target, string property, ObjectG this_);

bool evaluate(ObjectG this_, Value value);

GType getValueType();

bool isStatic();

alias doref = ref_;

Expression ref_();

void unref();

ExpressionWatch watch(ObjectG this_, GtkExpressionNotify notify, void* userData, GDestroyNotify userDestroy);

}

Constructors

this this(GtkExpression* gtkExpression, bool ownedRef): Sets our main struct and passes it to the parent class.

Destructor

~this ~this(): Undocumented in source.

Members

Aliases

doref alias doref = ref_: Undocumented in source.

Functions

bind ExpressionWatch bind(ObjectG target, string property, ObjectG this_): Bind target's property named property to self.
evaluate bool evaluate(ObjectG this_, Value value): Evaluates the given expression and on success stores the result in @value.
getExpressionStruct GtkExpression* getExpressionStruct(bool transferOwnership): Get the main Gtk struct
getStruct void* getStruct(): the main Gtk struct as a void*
getValueType GType getValueType(): Gets the GType that this expression evaluates to.
isStatic bool isStatic(): Checks if the expression is static.
ref_ Expression ref_(): Acquires a reference on the given GtkExpression.
unref void unref(): Releases a reference on the given GtkExpression.
watch ExpressionWatch watch(ObjectG this_, GtkExpressionNotify notify, void* userData, GDestroyNotify userDestroy): Installs a watch for the given expression that calls the notify function whenever the evaluation of self may have changed.

Static functions

getType GType getType()

Variables

gtkExpression GtkExpression* gtkExpression;: the main Gtk struct
ownedRef bool ownedRef;: Undocumented in source.

Examples

<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>