QtAda provides two packages for each QObject subclass. First package contains declaration of the interface type and subprograms for the public operations of the class. This package supposes to be used everywhere in the application. Use of the interface type hides details of the actual class implementation from the application. This package called Qt4.<ClassName> by convention. Second package contains declaration of the tagged type and subprograms for the private operations of the class. This package supposed to be used by the application for subclassing only. This package called Qt4.<ClassName>.Directors.

Thus, when you subclassing QObject class or one of the it subclasses you must declare your own tagged type derived from the tagged type of the superclass.

Pragma Q_Object provides information about new class to the Ada-aware Meta Object Compiler. It is useful for declare custom name of the class, otherwise it can be omitted. It have following parameters:

For example, when we subclassing MyWidget from the QWidget class, we will create my_widgets.ads file for the class:

with Qt4.Widgets;
private with Qt4.Widgets.Directors;

package My_Widgets is

   type My_Widget is limited new Qt4.Widgets.Q_Widget with private;

   type My_Widget_Access is access all My_Widget'Class;

private

   type My_Widget is
     new Qt4.Widgets.Directors.Q_Widget_Director with null record;
   pragma Q_Object (My_Widget, "MyWidget");

end My_Widgets;

An ordinary subprogram must be declared for each signals and slots. The type of the first parameter of the such subprograms must be of class's tagged type or classwide type of class's tagged type or anonymous access type of tagged type or classwide tagged type.

Signals and slots subprograms must be marked for the Ada-aware Meta Object Compiler with pragma Q_Signal and Q_Slot. Both have same set of parameters:

Second parameter can be omitted, in this case name and profile will be automatically generated.

For example:

   procedure My_Slot (Self : not null access My_Widget_Impl);
   pragma Q_Slot (My_Slot, "mySlot()");

   procedure My_Signal (Self : not null access My_Widget_Impl);
   pragma Q_Signal (My_Signal, "mySignal()");

Implementation of the signal subprogram is generated by the Ada-aware Meta Object Compiler. You must declare signal subprogram as separate compilation unit in the package body:

   procedure My_Signal (Self : not null access My_Widget_Impl) is separate;

Implementation of the slot subprogram provided by the user:

   procedure My_Slot (Self : not null access My_Widget_Impl) is
   begin
      null;
   end My_Slot;

Information for the Qt Meta System is generated by the Ada-aware Meta Object Compiler. It takes the name of the class's implementation package, analyzes it and generate child package called Moc with the information for the Qt Meta System. If custom class has own signals Ada-aware Meta Object Compiler also generates it implementation in the separate compilation units.

Child package Moc generated by the Ada-aware Meta Object Compiler must be "withed" somewhere in the application code to be elaborated as part of the application. It is recommended to add it to the list of the context clauses in the body of the class's implementation package.

Every constructor for the custom class must call constructor of the superclass. It is very important to call constructor of the superclass before any others actions. Superclass's constructors are placed in the subpackage Constructors of the Qt4.<ClassName>.Directors package.

For example, follow is an implementation of the constructor subprogram for MyWidget class:

with My_Widgets.Moc; 
pragma Unreferenced (My_Widgets.Moc);

package body My_Widgets is

   ------------
   -- Create --
   ------------

   function Create return not null My_Widget_Access is
      Self : constant not null My_Widget_Access := new My_Widget;

   begin
      Qt4.Widgets.Impl.Constructors.Initialize (Self);

      --  others actions

      return Self;
   end Create;

end My_Widgets;