Data.GI.Base.Attributes

Basic attributes interface

Attributes of an object can be get, set and constructed. For types descending from GObject, properties are encoded in attributes, although attributes are slightly more general (every property of a GObject is an attribute, but we can also have attributes for types not descending from GObject).

As an example consider a button widget and a property (of the Button class, or any of its parent classes or implemented interfaces) called "label". The simplest way of getting the value of the button is to do

value <- getButtonLabel button

And for setting:

setButtonLabel button label

This mechanism quickly becomes rather cumbersome, for example for setting the "window" property in a DOMDOMWindow in WebKit:

win <- getDOMDOMWindowWindow dom

and perhaps more importantly, one needs to chase down the type which introduces the property:

setWidgetSensitive button False

There is no setButtonSensitive, since it is the Widget type that introduces the "sensitive" property.

Overloaded attributes

A much more convenient overloaded attribute resolution API is provided by this module. Getting the value of an object's attribute is straightforward:

value <- get button _label

The definition of _label is simply a Proxy encoding the name of the attribute to get:

_label = Proxy :: Proxy "label"

These proxies can be automatically generated by invoking the code generator with the -a option. The leading underscore is simply so the autogenerated identifiers do not pollute the namespace, but if this is not a concern the autogenerated names (in the autogenerated GI/Properties.hs) can be edited as one wishes.

The syntax for setting or updating an attribute is only slightly more complex. At the simplest level it is just:

set button [ _label := value ]

or for the WebKit example above

set dom [_window := win]

However as the list notation would indicate, you can set or update multiple attributes of the same object in one go:

set button [ _label := value, _sensitive := False ]

You are not limited to setting the value of an attribute, you can also apply an update function to an attribute's value. That is the function receives the current value of the attribute and returns the new value.

set spinButton [ _value :~ (+1) ]

There are other variants of these operators, (see AttrOp). :=> and :~> and like := and :~ but operate in the IO monad rather than being pure. There is also ::= and ::~ which take the object as an extra parameter.

Attributes can also be set during construction of a GObject using new

button <- new Button [_label := "Can't touch this!", _sensitive := False]

In addition for value being set/get having to have the right type, there can be attributes that are read-only, or that can only be set during construction with new, but cannot be set afterwards. That these invariants hold is also checked during compile time.

Documentation

class AttrInfo info where

Info describing an attribute.

Associated Types

type AttrAllowedOps info :: [AttrOpTag]

The operations that are allowed on the attribute.

type AttrSetTypeConstraint info :: * -> Constraint

Constraint on the value being set.

type AttrBaseTypeConstraint info :: * -> Constraint

Constraint on the type for which we are allowed to create/set/get the attribute.

type AttrGetType info

Type returned by attrGet.

type AttrLabel info :: Symbol

A string describing the attribute (for error messages).

Methods

attrGet :: AttrBaseTypeConstraint info o => proxy info -> o -> IO (AttrGetType info)

Get the value of the given attribute.

attrSet :: (AttrBaseTypeConstraint info o, AttrSetTypeConstraint info b) => proxy info -> o -> b -> IO ()

Set the value of the given attribute, after the object having the attribute has already been created.

attrConstruct :: AttrSetTypeConstraint info b => proxy info -> b -> IO (String, GValue)

Build a GValue representing the attribute.

data AttrOpTag

Possible operations on an attribute.

Constructors

AttrGet

AttrSet

AttrConstruct

data AttrOp obj tag where

Constructors for the different operations allowed on an attribute.

Constructors

(:=) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, AttrOpAllowed tag info, AttrSetTypeConstraint info b) => proxy (attr :: Symbol) -> b -> AttrOp obj tag infixr 0

(:=>) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, AttrOpAllowed tag info, AttrSetTypeConstraint info b) => proxy (attr :: Symbol) -> IO b -> AttrOp obj tag infixr 0

(:~) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, tag ~ AttrSet, AttrOpAllowed AttrSet info, AttrOpAllowed AttrGet info, AttrSetTypeConstraint info b, a ~ AttrGetType info) => proxy (attr :: Symbol) -> (a -> b) -> AttrOp obj tag infixr 0

(:~>) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, tag ~ AttrSet, AttrOpAllowed AttrSet info, AttrOpAllowed AttrGet info, AttrSetTypeConstraint info b, a ~ AttrGetType info) => proxy (attr :: Symbol) -> (a -> IO b) -> AttrOp obj tag infixr 0

(::=) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, tag ~ AttrSet, AttrOpAllowed tag info, AttrSetTypeConstraint info b) => proxy (attr :: Symbol) -> (obj -> b) -> AttrOp obj tag infixr 0

(::~) :: (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, tag ~ AttrSet, AttrOpAllowed AttrSet info, AttrOpAllowed AttrGet info, AttrSetTypeConstraint info b, a ~ AttrGetType info) => proxy (attr :: Symbol) -> (obj -> a -> b) -> AttrOp obj tag infixr 0

get :: forall info attr obj proxy. (info ~ ResolveAttribute attr obj, AttrInfo info, AttrBaseTypeConstraint info obj, AttrOpAllowed AttrGet info) => obj -> proxy (attr :: Symbol) -> IO (AttrGetType info)

Get the value of an attribute for an object.

set :: forall o. o -> [AttrOp o AttrSet] -> IO ()

Set a number of properties for some object.