## 10.1 Control-structure of XPCE/Prolog applications

This section deals with the control-structure of interactive applications written in XPCE/Prolog . Interactive graphical applications are very different from terminal oriented applications. Terminal oriented applications often have a top level control structure of the form:

    go :-
            initialise,
            main_loop.

    main_loop :-
            present_question,
            read_answer(Answer),
            process_answer(Answer),
            main_loop.

This schema is often refined with sub-loops dealing with question/answers in a specific context.

Many interactive graphical applications present various UI components simultaneously: the user is free on which component s/he wants to operate next. The users actions (keyboard-typing, mouse movement, and mouse-clicking) must be related to the correct UI component and interpreted accordingly in the application. This interpretation is much more complex than the interpretation of a stream of ASCII characters typed by the user.

### 10.1.1 Event-driven applications

One approach is to write a main-loop that reads events, locates the UI-component referred to and executes the appropriate actions. This loop, which must take care of repaint-requests, various local feedback procedures (changing the mouse-cursor, inverting objects, etc.), is complicated. The approach taken by most graphical programming systems including XPCE , is to move this loop into the infra-structure (i.e. into the XPCE kernel). The application programmer creates the desired UI-components and supplies code fragments that will be called by the main-loop when a certain event happens. This control-structure is called event-driven control. Consider a button:

    1 ?- new(B, button(hello,
                       message(@pce, write_ln, hello))),
         send(B, open).

In this example the application creates and displays a button UI component and associates a code fragment (the message) to be executed when the button is pressed. The XPCE kernel will loop through the main event-loop. For each event it will locate the UI component that should handle the event. When the button has recognised a `click' it will execute the code fragment attached to it. This behaviour is part of the definition of class button.

It is clear that this approach relieves the application programmer of many of the complications associated with event-processing. As a consequence, the `main-loop' of a XPCE application is no longer in the application itself, but in the XPCE kernel. Below is an outline of the control structure of a XPCE/Prolog application:

    go :-
            initialise_database,
            create_ui_components.

    handle_help_pressed :-
            create_help_window.

    handle_solve :-
            solve_the_problem,
            create_solution_window.

    ...

The predicate go will exit after it has initialised the application and created the UI components. Assuming the application window has a button invoking the predicate handle_help_pressed, XPCE will call this predicate when the user presses the help button.

#### 10.1.1.1 Keeping control

The application code often wants to wait for the user to finish an interaction. In section 4.4, we have seen a simple way of programming this using `frame`<-`confirm'. In this section, we will provide some other options.

Message Queue One possibility is to fall back to the XPCE 1 and 2 compatibility, where @prolog implements a queue of messages. @prolog is an instance of class host. The relevant methods are:

host ->call_back: bool
  The default is @on. In this case, a message to @prolog is translated into a predicate call on the Prolog engine. If @off, a message is appended to the `host`<-`messages' queue.
host ->catch_all: Selector:name, Arg:any...
  If `<-`call_back equals @on, use the Selector to determine the predicate to call, and the arguments to construct the argument vector for the predicate. Call the predicate and succeed or fail according to success or failure of the Prolog predicate.

  If `<-`call_back equals @off, create a message of the form

  > message(@prolog, Selector, Arg ...)

  and append this message to the `<-`messages queue.

host <-message:
  message Return the `<-`head of the `<-`messages queue. If the queue is empty, ensure `<-`call_back is (temporary) set to @off, and dispatch events using `@display`->`dispatch' as long as the `<-`messages queue is empty.

Note that it is possible to create multiple instances of class host, to realise multiple message queues. It is not desirable to modify the @prolog host object, as other code may rely on the `<-`call_back properties of @prolog.

> Warning During normal operation, event processing guards the objects created that are not assigned to any other object and destroys all such objects after the event has completely been processed (see section E. Using the host message queue mechanism, the Prolog programmer becomes responsible for such objects. For example, the message object returned should be discarded using `object`->`done' after processing.

Explicit dispatching An alternative to the above, and the `frame`<-`confirm' mechanism is to dispatch the events yourself. This is realised using `send(@display, dispatch)`, described below. This mechanism is the base of all the others. It should be used to realise different interaction schemas than the default callback schema.

display ->dispatch:
  Process events and return on any of the following conditions
  - event has been processed
    Either a normal event, a timer or an input stream has been processed. The method fails in this case.
  - timeout
    The timeout (see `display_manager`->`dispatch') has expired. The method fails in this case.
  - Input on the console
    There is input from the Prolog window. The message succeeds in this case.

  For example, the following processes events in call-back style until the fact quit/0 is in the Prolog database:

      :- dynamic
              quit/0.

      process_to_quit :-
              repeat,
                  send(@display, dispatch),
              quit, !.

### 10.1.2 XPCE and existing applications

Due to the different control-regime described in the previous section, traditional terminal oriented applications are not easily transformed into XPCE/Prolog graphical applications. Depending on the application, there are two ways to proceed.

The first is to keep the existing control-regime. This implies that the questions asked on the terminal will be replaced by modal dialog windows. The main loop will be:

    go :-
            initialise_database,
            create_dialog(Dialog).

    main_loop(Dialog) :-
            fill_dialog_with_next_question(Dialog),
            send(Dialog, fit),
            get(Dialog, confirm, Answer),
            process_answer(Answer),
            main_loop(Dialog).

This example reuses the same dialog window for all questions. It is trivial to change this loop to use a new dialog window for each question. Output from the program may be presented in other windows. The approach does not exploit the potentially larger freedom for the user that is possible in graphical user interfaces.

If the application could be viewed as a number of commands operating on some data-structure and this data-structure is stored on the Prolog heap using assert/1 or recorda/2 one could consider rewriting the toplevel control and provide a more flexible interface.