GI.Gtk.Objects.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 expressionEvaluate for evaluating an expression.

Various methods for defining expressions exist, from simple constants via gtk_constant_expression_new() to looking up properties in a Object (even recursively) via propertyExpressionNew or providing custom functions to transform and combine expressions via closureExpressionNew.

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, DropDown 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. StringFilter 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. expressionWatch starts watching an expression, and expressionWatchUnwatch stops.

Watches can be created for automatically updating the property of an object, similar to GObject's Binding mechanism, by using expressionBind.

GtkExpression in GObject properties

In order to use a Expression as a Object property, you must use the paramSpecExpression when creating a ParamSpec to install in the Object 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 ObjectClass.set_property() and ObjectClass.get_property() virtual functions, you must use valueGetExpression, to retrieve the stored Expression from the Value container, and valueSetExpression, to store the Expression into the Value; 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 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: > > 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.

Example: > > constantstring_filter/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.

Example: > > type='gchararray' function='combine_args_somehow' > type='gchararray'File size:/constant > type='GFile' name='size'myfile/lookup > /closure

Exported types

newtype Expression

Memory-managed wrapper type.

Constructors

Expression (ManagedPtr Expression)

Instances

Eq Expression

Defined in GI.Gtk.Objects.Expression

Methods

(==) :: Expression -> Expression -> Bool

(/=) :: Expression -> Expression -> Bool

BoxedPtr Expression

Defined in GI.Gtk.Objects.Expression

Methods

boxedPtrCopy :: Expression -> IO Expression

boxedPtrFree :: Expression -> IO ()

ManagedPtrNewtype Expression

Defined in GI.Gtk.Objects.Expression

Methods

toManagedPtr :: Expression -> ManagedPtr Expression

TypedObject Expression

Defined in GI.Gtk.Objects.Expression

Methods

glibType :: IO GType

HasParentTypes Expression

Defined in GI.Gtk.Objects.Expression

IsGValue (Maybe Expression)

Convert Expression to and from GValue. See toGValue and fromGValue.

Defined in GI.Gtk.Objects.Expression

Methods

gvalueGType_ :: IO GType

gvalueSet_ :: Ptr GValue -> Maybe Expression -> IO ()

gvalueGet_ :: Ptr GValue -> IO (Maybe Expression)

type ParentTypes Expression

Defined in GI.Gtk.Objects.Expression

type ParentTypes Expression = '[] :: [Type]

class (BoxedPtr o, TypedObject o, IsDescendantOf Expression o) => IsExpression o

Type class for types which can be safely cast to Expression, for instance with toExpression.

Instances

(BoxedPtr o, TypedObject o, IsDescendantOf Expression o) => IsExpression o

Defined in GI.Gtk.Objects.Expression

toExpression :: (MonadIO m, IsExpression o) => o -> m Expression

Cast to Expression, for types for which this is known to be safe. For general casts, use castTo.

Methods

Click to display all available methods, including inherited ones

Methods

bind, evaluate, isStatic, ref, unref, watch.

Getters

getValueType.

Setters

None.

bind

expressionBind

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b, IsObject c)

=> a

self: a Expression

-> b

target: the target object to bind to

-> Text

property: name of the property on target to bind to

-> Maybe c

this_: the this argument for the evaluation of self

-> m ExpressionWatch

Returns: a 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 expressionRef it beforehand.

evaluate

expressionEvaluate

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b)

=> a

self: a Expression

-> Maybe b

this_: the this argument for the evaluation

-> GValue

value: an empty Value

-> m Bool

Returns: True if the expression could be evaluated

Evaluates the given expression and on success stores the result in value. The GType of value will be the type given by expressionGetValueType.

It is possible that expressions cannot be evaluated - for example when the expression references objects that have been destroyed or set to Nothing. In that case value will remain empty and False will be returned.

getValueType

expressionGetValueType

Arguments

:: (HasCallStack, MonadIO m, IsExpression a)

=> a

self: a Expression

-> m GType

Returns: The type returned from expressionEvaluate

Gets the GType that this expression evaluates to. This type is constant and will not change over the lifetime of this expression.

isStatic

expressionIsStatic

Arguments

:: (HasCallStack, MonadIO m, IsExpression a)

=> a

self: a Expression

-> m Bool

Returns: True if the expression is static

Checks if the expression is static.

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

That means a call to expressionWatch is not necessary because it will never trigger a notify.

ref

expressionRef

Arguments

:: (HasCallStack, MonadIO m, IsExpression a)

=> a

self: a Expression

-> m Expression

Returns: the Expression with an additional reference

Acquires a reference on the given Expression.

unref

expressionUnref

Arguments

:: (HasCallStack, MonadIO m, IsExpression a)

=> a

self: a Expression

-> m ()

Releases a reference on the given Expression.

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

watch

expressionWatch

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b)

=> a

self: a Expression

-> Maybe b

this_: the this argument to watch

-> ExpressionNotify

notify: callback to invoke when the expression changes

-> m ExpressionWatch

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 expressionWatchUnwatch. You should call expressionWatchRef if you want to keep the watch around.

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.