Struct gtk4::Expression

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#[repr(transparent)]
pub struct Expression { /* private fields */ }
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 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 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§

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impl Expression

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pub fn as_ptr(&self) -> *mut GtkExpression

Return the inner pointer to the underlying C value.

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pub unsafe fn from_glib_ptr_borrow<'a>( ptr: *const *const GtkExpression ) -> &'a Self

Borrows the underlying C value.

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impl Expression

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pub const NONE: Option<&'static Expression> = None

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pub fn bind( &self, target: &impl IsA<Object>, property: &str, this_: Option<&impl IsA<Object>> ) -> ExpressionWatch

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

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pub fn value_type(&self) -> Type

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()

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pub fn is_static(&self) -> 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.

Returns

TRUE if the expression is static

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pub fn watch<P: Fn() + 'static>( &self, this_: Option<&impl IsA<Object>>, notify: P ) -> ExpressionWatch

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.

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.

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impl Expression

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pub fn upcast(self) -> Self

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pub fn upcast_ref(&self) -> &Self

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pub fn is<E: IsExpression>(&self) -> bool

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pub fn downcast<E: IsExpression>(self) -> Result<E, Expression>

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pub fn downcast_ref<E: IsExpression>(&self) -> Option<&E>

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pub fn type_(&self) -> Type

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pub fn evaluate(&self, this: Option<&impl IsA<Object>>) -> Option<Value>

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pub fn evaluate_as<V: for<'b> FromValue<'b> + 'static, T: IsA<Object>>( &self, this: Option<&T> ) -> Option<V>

Similar to Self::evaluate but panics if the value is of a different type.

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pub fn chain_property<T: IsA<Object>>( &self, property_name: &str ) -> PropertyExpression

Create a PropertyExpression that looks up for property_name with self as parameter. This is useful in long chains of Expressions.

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pub fn chain_closure<R>(&self, closure: RustClosure) -> ClosureExpressionwhere R: ValueType,

Create a ClosureExpression from a glib::Closure with self as the second parameter and R as the return type. The return type is checked at run-time and must always be specified. This is useful in long chains of Expressions when using the [glib::closure!] macro.

Note that the first parameter will always be the this object bound to the expression. If None is passed as this then the type of the first parameter must be Option<glib::Object> otherwise type checking will panic.

use gtk::prelude::*;
use gtk::glib;
use glib::{closure, Object};

let button = gtk::Button::new();
button.set_label("Hello");
let label = button
    .property_expression("label")
    .chain_closure::<String>(closure!(|_: Option<Object>, label: &str| {
        format!("{} World", label)
    }))
    .evaluate_as::<String, _>(gtk::Widget::NONE);
assert_eq!(label.unwrap(), "Hello World");
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pub fn chain_closure_with_callback<F, R>(&self, f: F) -> ClosureExpressionwhere F: Fn(&[Value]) -> R + 'static, R: ValueType,

Create a ClosureExpression with self as the second parameter. This is useful in long chains of Expressions.

Trait Implementations§

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impl AsRef<Expression> for ClosureExpression

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fn as_ref(&self) -> &Expression

Converts this type into a shared reference of the (usually inferred) input type.
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impl AsRef<Expression> for ConstantExpression

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fn as_ref(&self) -> &Expression

Converts this type into a shared reference of the (usually inferred) input type.
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impl AsRef<Expression> for ObjectExpression

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fn as_ref(&self) -> &Expression

Converts this type into a shared reference of the (usually inferred) input type.
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impl AsRef<Expression> for PropertyExpression

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fn as_ref(&self) -> &Expression

Converts this type into a shared reference of the (usually inferred) input type.
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impl Clone for Expression

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fn clone(&self) -> Self

Returns a copy of the value. Read more
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fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source. Read more
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impl Debug for Expression

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fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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impl Display for Expression

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fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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impl From<Expression> for Value

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fn from(e: Expression) -> Self

Converts to this type from the input type.
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impl<'a> FromValue<'a> for Expression

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type Checker = GenericValueTypeOrNoneChecker<Expression>

Value type checker.
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unsafe fn from_value(value: &'a Value) -> Self

Get the contained value from a Value. Read more
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impl HasParamSpec for Expression

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type ParamSpec = ParamSpecExpression

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type SetValue = Expression

Preferred value to be used as setter for the associated ParamSpec.
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type BuilderFn = fn(_: &'a str) -> ParamSpecExpressionBuilder<'a>

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fn param_spec_builder() -> Self::BuilderFn

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impl StaticType for Expression

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fn static_type() -> Type

Returns the type identifier of Self.
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impl ToValue for Expression

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fn to_value(&self) -> Value

Convert a value to a Value.
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fn value_type(&self) -> Type

Returns the type identifer of self. Read more
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impl ToValueOptional for Expression

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fn to_value_optional(s: Option<&Self>) -> Value

Convert an Option to a Value.
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impl ValueType for Expression

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type Type = Expression

Type to get the Type from. Read more

Auto Trait Implementations§

Blanket Implementations§

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impl<T> Any for Twhere T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more
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impl<T> Borrow<T> for Twhere T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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impl<T> BorrowMut<T> for Twhere T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GPtrArray> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_num_as_vec( ptr: *const GPtrArray, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_container_num_as_vec( _: *const GPtrArray, _: usize ) -> Vec<T, Global>

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unsafe fn from_glib_full_num_as_vec( _: *const GPtrArray, _: usize ) -> Vec<T, Global>

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GSList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_num_as_vec( ptr: *const GSList, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_container_num_as_vec( _: *const GSList, _: usize ) -> Vec<T, Global>

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unsafe fn from_glib_full_num_as_vec( _: *const GSList, _: usize ) -> Vec<T, Global>

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_num_as_vec( ptr: *mut GList, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_container_num_as_vec( ptr: *mut GList, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_full_num_as_vec( ptr: *mut GList, num: usize ) -> Vec<T, Global>

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GPtrArray> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_num_as_vec( ptr: *mut GPtrArray, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_container_num_as_vec( ptr: *mut GPtrArray, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_full_num_as_vec( ptr: *mut GPtrArray, num: usize ) -> Vec<T, Global>

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impl<T> FromGlibContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GSList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_num_as_vec( ptr: *mut GSList, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_container_num_as_vec( ptr: *mut GSList, num: usize ) -> Vec<T, Global>

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unsafe fn from_glib_full_num_as_vec( ptr: *mut GSList, num: usize ) -> Vec<T, Global>

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GPtrArray> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_as_vec(ptr: *const GPtrArray) -> Vec<T, Global>

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unsafe fn from_glib_container_as_vec(_: *const GPtrArray) -> Vec<T, Global>

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unsafe fn from_glib_full_as_vec(_: *const GPtrArray) -> Vec<T, Global>

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *const GSList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_as_vec(ptr: *mut GList) -> Vec<T, Global>

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unsafe fn from_glib_container_as_vec(ptr: *mut GList) -> Vec<T, Global>

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unsafe fn from_glib_full_as_vec(ptr: *mut GList) -> Vec<T, Global>

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GPtrArray> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_as_vec(ptr: *mut GPtrArray) -> Vec<T, Global>

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unsafe fn from_glib_container_as_vec(ptr: *mut GPtrArray) -> Vec<T, Global>

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unsafe fn from_glib_full_as_vec(ptr: *mut GPtrArray) -> Vec<T, Global>

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impl<T> FromGlibPtrArrayContainerAsVec<<T as GlibPtrDefault>::GlibType, *mut GSList> for Twhere T: GlibPtrDefault + FromGlibPtrNone<<T as GlibPtrDefault>::GlibType> + FromGlibPtrFull<<T as GlibPtrDefault>::GlibType>,

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unsafe fn from_glib_none_as_vec(ptr: *mut GSList) -> Vec<T, Global>

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unsafe fn from_glib_container_as_vec(ptr: *mut GSList) -> Vec<T, Global>

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unsafe fn from_glib_full_as_vec(ptr: *mut GSList) -> Vec<T, Global>

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impl<T, U> Into<U> for Twhere U: From<T>,

const: unstable · source§

fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T> IntoClosureReturnValue for Twhere T: Into<Value>,

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impl<T> Property for Twhere T: HasParamSpec,

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type Value = T

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impl<T> PropertyGet for Twhere T: HasParamSpec,

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type Value = T

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fn get<R, F>(&self, f: F) -> Rwhere F: Fn(&<T as PropertyGet>::Value) -> R,

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impl<T> StaticTypeExt for Twhere T: StaticType,

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fn ensure_type()

Ensures that the type has been registered with the type system.
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impl<T> ToOwned for Twhere T: Clone,

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type Owned = T

The resulting type after obtaining ownership.
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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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impl<T> ToString for Twhere T: Display + ?Sized,

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default fn to_string(&self) -> String

Converts the given value to a String. Read more
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impl<T> TransparentType for Twhere T: TransparentPtrType,

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impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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type Error = Infallible

The type returned in the event of a conversion error.
const: unstable · source§

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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impl<T> TryFromClosureReturnValue for Twhere T: for<'a> FromValue<'a> + StaticType + 'static,

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impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
const: unstable · source§

fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.
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impl<'a, T, C, E> FromValueOptional<'a> for Twhere T: FromValue<'a, Checker = C>, C: ValueTypeChecker<Error = ValueTypeMismatchOrNoneError<E>>, E: Error + Send + 'static,