Expression#
- class Expression(*args, **kwargs)#
Subclasses: CClosureExpression
, ClosureExpression
, ConstantExpression
, ObjectExpression
, PropertyExpression
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
evaluate
for evaluating an expression.
Various methods for defining expressions exist, from simple constants via
new
to looking up properties in a GObject
(even recursively) via new
or providing
custom functions to transform and combine expressions via
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 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
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 GtkExpression
as a GObject
property, you must use the
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 value_get_expression
, to retrieve the
stored GtkExpression
from the GValue
container, and 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 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:
<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
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:
<constant>string_filter</constant>
<constant type='gchararray'>Hello, world</constant>
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:
<closure type='gchararray' function='combine_args_somehow'>
<constant type='gchararray'>File size:</constant>
<lookup type='GFile' name='size'>myfile</lookup>
</closure>
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 GtkLabel
is bound to the string
property of another arbitrary
object:
<object class='GtkLabel'>
<binding name='label'>
<lookup name='string'>some_other_object</lookup>
</binding>
</object>
Methods#
- class Expression
- bind(target: Object, property: str, this_: Object | None = None) ExpressionWatch #
Bind
target
’s property namedproperty
toself
.The value that
self
evaluates to is set via:func:`~gi.repository.GObject.GObject.Object.set`
ontarget
. This is repeated wheneverself
changes to ensure that the object’s property stays synchronized withself
.If
self
’s evaluation fails,target
’sproperty
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 shouldref
it beforehand.- Parameters:
target – the target object to bind to
property – name of the property on
target
to bind tothis – the this argument for the evaluation of
self
- evaluate(this_: Object | None, value: Any) bool #
Evaluates the given expression and on success stores the result in
value
.The
GType
ofvalue
will be the type given byget_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 casevalue
will remain empty andFALSE
will be returned.- Parameters:
this – the this argument for the evaluation
value – an empty
GValue
- get_value_type() type #
Gets the
GType
that this expression evaluates to.This type is constant and will not change over the lifetime of this expression.
- is_static() bool #
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.
- watch(this_: Object | None, notify: Callable[[...], None], *user_data: Any) ExpressionWatch #
Watch the given
expression
for changes.The
notify
function will be called whenever the evaluation ofself
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, thenotify
will be invoked.- Parameters:
this – the
this
argument to watchnotify – callback to invoke when the expression changes
user_data – user data to pass to the
notify
callback