Copyright | Will Thompson Iñaki García Etxebarria and Jonas Platte |
---|---|
License | LGPL-2.1 |
Maintainer | Iñaki García Etxebarria |
Safe Haskell | Safe-Inferred |
Language | Haskell2010 |
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
Synopsis
- newtype Expression = Expression (ManagedPtr Expression)
- class (BoxedPtr o, TypedObject o, IsDescendantOf Expression o) => IsExpression o
- toExpression :: (MonadIO m, IsExpression o) => o -> m Expression
- expressionBind :: (HasCallStack, MonadIO m, IsExpression a, IsObject b, IsObject c) => a -> b -> Text -> Maybe c -> m ExpressionWatch
- expressionEvaluate :: (HasCallStack, MonadIO m, IsExpression a, IsObject b) => a -> Maybe b -> GValue -> m Bool
- expressionGetValueType :: (HasCallStack, MonadIO m, IsExpression a) => a -> m GType
- expressionIsStatic :: (HasCallStack, MonadIO m, IsExpression a) => a -> m Bool
- expressionRef :: (HasCallStack, MonadIO m, IsExpression a) => a -> m Expression
- expressionUnref :: (HasCallStack, MonadIO m, IsExpression a) => a -> m ()
- expressionWatch :: (HasCallStack, MonadIO m, IsExpression a, IsObject b) => a -> Maybe b -> ExpressionNotify -> m ExpressionWatch
Exported types
newtype Expression Source #
Memory-managed wrapper type.
Expression (ManagedPtr Expression) |
Instances
Eq Expression Source # | |
Defined in GI.Gtk.Objects.Expression (==) :: Expression -> Expression -> Bool # (/=) :: Expression -> Expression -> Bool # | |
BoxedPtr Expression Source # | |
Defined in GI.Gtk.Objects.Expression boxedPtrCopy :: Expression -> IO Expression boxedPtrFree :: Expression -> IO () | |
ManagedPtrNewtype Expression Source # | |
Defined in GI.Gtk.Objects.Expression toManagedPtr :: Expression -> ManagedPtr Expression | |
TypedObject Expression Source # | |
Defined in GI.Gtk.Objects.Expression | |
HasParentTypes Expression Source # | |
Defined in GI.Gtk.Objects.Expression | |
IsGValue (Maybe Expression) Source # | Convert |
Defined in GI.Gtk.Objects.Expression gvalueGType_ :: IO GType gvalueSet_ :: Ptr GValue -> Maybe Expression -> IO () gvalueGet_ :: Ptr GValue -> IO (Maybe Expression) | |
type ParentTypes Expression Source # | |
Defined in GI.Gtk.Objects.Expression |
class (BoxedPtr o, TypedObject o, IsDescendantOf Expression o) => IsExpression o Source #
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 Source # | |
Defined in GI.Gtk.Objects.Expression |
toExpression :: (MonadIO m, IsExpression o) => o -> m Expression Source #
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
bind
:: (HasCallStack, MonadIO m, IsExpression a, IsObject b, IsObject c) | |
=> a |
|
-> b |
|
-> Text |
|
-> Maybe c |
|
-> m ExpressionWatch | Returns: a |
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
:: (HasCallStack, MonadIO m, IsExpression a, IsObject b) | |
=> a |
|
-> Maybe b |
|
-> GValue |
|
-> m Bool | Returns: |
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 Source #
:: (HasCallStack, MonadIO m, IsExpression a) | |
=> a |
|
-> m GType | Returns: The type returned from |
Gets the GType
that this expression evaluates to. This type
is constant and will not change over the lifetime of this expression.
isStatic
:: (HasCallStack, MonadIO m, IsExpression a) | |
=> a |
|
-> m Bool | Returns: |
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
:: (HasCallStack, MonadIO m, IsExpression a) | |
=> a |
|
-> m Expression | Returns: the |
Acquires a reference on the given Expression
.
unref
:: (HasCallStack, MonadIO m, IsExpression a) | |
=> a |
|
-> m () |
Releases a reference on the given Expression
.
If the reference was the last, the resources associated to the self
are
freed.
watch
:: (HasCallStack, MonadIO m, IsExpression a, IsObject b) | |
=> a |
|
-> Maybe b |
|
-> ExpressionNotify |
|
-> 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
|
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.