gi-gtk4-4.0.12: Gtk 4.x bindings

Copyright | Will Thompson and Iñaki García Etxebarria
License | LGPL-2.1
Maintainer | Iñaki García Etxebarria
Safe Haskell | None
Language | Haskell2010

GI.Gtk.Objects.Expression

Contents

- Exported types
- Methods
  - bind
  - evaluate
  - getValueType
  - isStatic
  - ref
  - unref
  - watch

Description

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 `ConstantExpression`.`new`() to looking up properties in a `GObject` (even recursively) via `propertyExpressionNew` or providing custom functions to transform and combine expressions via `closureExpressionNew`.

Here is an example of a complex expression:

### c code

     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; `expressionWatch` starts watching an expression, and `expressionWatchUnwatch` stops.

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

## GtkExpression in GObject properties

In order to use a `GtkExpression` as a `GObject` property, you must use the `paramSpecExpression` when creating a `GParamSpec` to install in the `GObject` class being defined; for instance:

### c code

    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 `valueGetExpression`, to retrieve the stored `GtkExpression` from the `GValue` container, and `valueSetExpression`, to store the `GtkExpression` into the `GValue`; for instance:

### c code

     // 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 a string that specifies the name of the object to use, an element specifying an expression to provide an object, or empty to use the `this` object.

Example:

### xml code

     <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:

### xml code

     <constant>string_filter</constant>
     <constant type='gchararray'>Hello, world</constant>

String (`type='gchararray'`) constants can be marked for translation with the `translatable=` attribute, and will then be looked up in the Builder:translationDomain when the expression is constructed.

### xml code

     <constant type='gchararray' translatable='yes'>I'm translatable!</constant>

As with other translatable strings in [type`gtk`.Builder], constants can also have a context and/or translation comment:

### xml code

     <constant type='gchararray'
               translatable='yes'
               context='example'
               comments='A sample string'>I'm translatable!</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:

### xml code

     <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:

### xml code

     <object class='GtkLabel'>
       <binding name='label'>
         <lookup name='string'>some_other_object</lookup>
       </binding>
     </object>

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.

Constructors

Expression (ManagedPtr Expression) |

#### Instances

Instances details

Eq Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression Methods (==) :: Expression -> Expression -> Bool Source # (/=) :: Expression -> Expression -> Bool Source #
BoxedPtr Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression Methods boxedPtrCopy :: Expression -> IO Expression Source # boxedPtrFree :: Expression -> IO () Source #
ManagedPtrNewtype Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression Methods toManagedPtr :: Expression -> ManagedPtr Expression Source #
TypedObject Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression Methods glibType :: IO GType Source #
HasParentTypes Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression
IsGValue (Maybe Expression) Source # | Convert `Expression` to and from `GValue`. See `toGValue` and `fromGValue`.
Instance details Defined in GI.Gtk.Objects.Expression Methods gvalueGType_ :: IO GType Source # gvalueSet_ :: Ptr GValue -> Maybe Expression -> IO () Source # gvalueGet_ :: Ptr GValue -> IO (Maybe Expression) Source #
type ParentTypes Expression Source # |
Instance details Defined in GI.Gtk.Objects.Expression type ParentTypes Expression = '[] :: [Type]

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

Instances details

(BoxedPtr o, TypedObject o, IsDescendantOf Expression o) => IsExpression o Source # |
Instance details 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

Expand

#### Methods

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

#### Getters

getValueType.

#### Setters

None.

## bind

expressionBind Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b, IsObject c) |
=> a | `self`: a `GtkExpression`
-> 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 `GtkExpressionWatch`

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.

Note that this function takes ownership of `self`. If you want to keep it around, you should `expressionRef` it beforehand.

## evaluate

expressionEvaluate Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b) |
=> a | `self`: a `GtkExpression`
-> Maybe b | `this_`: the this argument for the evaluation
-> GValue | `value`: an empty `GValue`
-> 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 `NULL`. In that case `value` will remain empty and `FALSE` will be returned.

## getValueType

expressionGetValueType Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a) |
=> a | `self`: a `GtkExpression`
-> 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 Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a) |
=> a | `self`: a `GtkExpression`
-> 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 Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a) |
=> a | `self`: a `GtkExpression`
-> m Expression | Returns: the `GtkExpression` with an additional reference

Acquires a reference on the given `GtkExpression`.

## unref

expressionUnref Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a) |
=> a | `self`: a `GtkExpression`
-> m () |

Releases a reference on the given `GtkExpression`.

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

## watch

expressionWatch Source #

Arguments

:: (HasCallStack, MonadIO m, IsExpression a, IsObject b) |
=> a | `self`: a `GtkExpression`
-> 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.

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.