Description

Gnome::Gtk4::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 Gnome::GObject::Object 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 .newexpression() in class Gnome::Gtk4::ConstantExpression to looking up properties in a Gnome::GObject::Object (even recursively) via .newexpression() in class Gnome::Gtk4::PropertyExpression or providing custom functions to transform and combine expressions via .newexpression() in class Gnome::Gtk4::ClosureExpression.

Here is an example of a complex expression:

when evaluated with this being a Gnome::Gtk4::ListItem, it will obtain the "item" property from the Gnome::Gtk4::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, Gnome::Gtk4::DropDown is evaluating a Gnome::Gtk4::Expression to obtain strings from the items in its model that it can then use to match against the contents of its search entry. Gnome::Gtk4::StringFilter is using a Gnome::Gtk4::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 Gnome::Gtk4::N-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() in class Gnome::Gtk4::N-ExpressionWatch stops.

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

GtkExpression in GObject properties

In order to use a Gnome::Gtk4::Expression as a Gnome::GObject::Object property, you must use the .param-spec-expression() when creating a Gnome::GObject::ParamSpec to install in the Gnome::GObject::Object class being defined; for instance:

When implementing the GObjectClass.set_property` and GObjectClass.get_property` virtual functions, you must use .value-get-expression(), to retrieve the stored Gnome::Gtk4::Expression from the Gnome::GObject::N-Value container, and .value-set-expression(), to store the Gnome::Gtk4::Expression into the Gnome::GObject::N-Value; for instance:

GtkExpression in .ui files

Gnome::Gtk4::Builder has support for creating expressions. The syntax here can be used where a Gnome::Gtk4::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 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:

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 Gnome::Gtk4::ListItems. See Gnome::Gtk4::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:

To create a closure expression, use the `<closure>` element. The function attribute specifies what function to use for the closure, and the type attribute specifies its return type. The content of the element contains the expressions for the parameters. For instance:

To create a property binding, use the `<binding>` element in place of where a `<property>` tag would ordinarily be used. The name and object attributes are supported. The name attribute is required, and pertains to the applicable property name. The object attribute is optional. If provided, it will use the specified object as the this object when the expression is evaluated. Here is an example in which the label property of a Gnome::Gtk4::Label is bound to the string property of another arbitrary object:

Class initialization

new

:native-object

Create an object using a native object from elsewhere. See also Gnome::N::TopLevelSupportClass.

multi method new ( N-Object :$native-object! )

Methods

bind

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 .ref() it beforehand.

method bind ( gpointer $target, Str $property, gpointer $this --> N-Object )

• $target; the target object to bind to.

• $property; name of the property on target to bind to.

• $this; the this argument for the evaluation of self.

Return value; a Gnome::Gtk4::N-ExpressionWatch.

evaluate

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

The Gnome::GObject::Expression of value will be the type given by .get-value-type().

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

method evaluate ( gpointer $this, N-Object $value --> Bool )

• $this; the this argument for the evaluation.

• $value; an empty Gnome::GObject::N-Value

Return value; True if the expression could be evaluated.

get-value-type

Gets the Gnome::GObject::Expression that this expression evaluates to.

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

method get-value-type (--> GType )

Return value; The type returned from .evaluate().

is-static

Checks if the expression is static.

A static expression will never change its result when .evaluate() is called on it with the same arguments.

That means a call to .watch() is not necessary because it will never trigger a notify.

method is-static (--> Bool )

Return value; True if the expression is static.

ref

Acquires a reference on the given Gnome::Gtk4::Expression.

method ref (--> N-Object )

Return value; the Gnome::Gtk4::Expression with an additional reference.

unref

Releases a reference on the given Gnome::Gtk4::Expression.

If the reference was the last, the resources associated to the self are freed.

method unref ( )

watch This function is not yet available

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.

method watch ( gpointer $this, &notify, gpointer $user-data, … --> N-Object )

• $this; the this argument to watch.

• &notify; callback to invoke when the expression changes. Tthe function must be specified with following signature; :( gpointer $user-data ).

• $user-data; user data to pass to the notify callback.

• user-destroy; destroy notify for user_data. Note that each argument must be specified as a type followed by its value!

Return value; 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 .unwatch() in class Gnome::Gtk4::N-ExpressionWatch. You should call .ref() in class Gnome::Gtk4::N-ExpressionWatch if you want to keep the watch around..