Struct gtk4::Expression[][src]

pub struct Expression(_);
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 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

  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, 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; 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 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",
                             "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 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()...
  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

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> 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.

Example:

  <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>

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

Implementations

Gets the GType that this expression evaluates to.

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

Returns

The type returned from evaluate()

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.

Returns

TRUE if the expression is static

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.

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

Returns

a ExpressionWatch

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

The GType of value will be the type given by 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.

this_

the this argument for the evaluation

value

an empty GValue

Returns

TRUE if the expression could be evaluated

Installs a watch for the given expression that calls the notify function 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.

this_

the this argument to watch

notify

callback to invoke when the expression changes

Returns

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.

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

Value type checker.

Get the contained value from a Value. Read more

Feeds this value into the given Hasher. Read more

Feeds a slice of this type into the given Hasher. Read more

This method returns an Ordering between self and other. Read more

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

Restrict a value to a certain interval. Read more

This method tests for self and other values to be equal, and is used by ==. Read more

This method tests for !=.

This method returns an ordering between self and other values if one exists. Read more

This method tests less than (for self and other) and is used by the < operator. Read more

This method tests less than or equal to (for self and other) and is used by the <= operator. Read more

This method tests greater than (for self and other) and is used by the > operator. Read more

This method tests greater than or equal to (for self and other) and is used by the >= operator. 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)

recently added

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