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This chapter describes in detail how GTK uses actions to connect activatable UI elements to callbacks. GTK inherits the underlying architecture of GAction and GMe:u for describing abstract actions and menus from the GIO library.
A GAction is essentially a way to tell the toolkit about a piece of functionality in your program, and to give it a name.
Actions are purely functional. They do not contain any presentational information.
An action has four pieces of information associated with it:
-
a name as an identifier (usually all-lowercase, untranslated English string)
-
an enabled flag indicating if the action can be activated or not (like the “sensitive” property on widgets)
-
an optional state value, for stateful actions (like a boolean for toggles)
-
an optional parameter type, used when activating the action
An action supports two operations. You can activate it, which requires passing a parameter of the correct type And you can request to change the actions state (for stateful actions) to a new state value of the correct type.
Here are some rules about an action:
-
the name is immutable (in the sense that it will never change) and it is never
NULL -
the enabled flag can change
-
the parameter type is immutable
-
the parameter type is optional: it can be
NULL -
if the parameter type is
NULLthen action activation must be done without a parameter (ie: aNULLGVariant pointer) -
if the parameter type is non-
NULLthen the parameter must have this type -
the state can change, but it cannot change type
-
if the action was stateful when it was created, it will always have a state and it will always have exactly the same type (such as boolean or string)
-
if the action was stateless when it was created, it can never have a state
-
you can only request state changes on stateful actions and it is only possible to request that the state change to a value of the same type as the existing state
An action does not have any sort of presentational information such as a label, an icon or a way of creating a widget from it.