Documentation: Timers, Signals and Input Messages

| Documentation: Timers, Signals and Input Messages [Remove Frame]

Most messages your application will receive are generated from FOX Widgets, such as buttons, sliders, and other controls. However, FOX also provides some messages which are generated from other sources.

There are four types of such messages: Timers, Chores, Signals, and Inputs.

Timer Messages

Timer messages are used so your program can receive a message after some specified interval has elapsed. This can be very useful, for example for performing an animation of some kind. Like all messages, timer messages are handled by specifying a target object which is to handle the message. When the specified time has elapsed, the object will receive a message of the type SEL_TIMEOUT, with the message ID being the one which was registered at the beginning of the time interval. The length of the time interval is expressed in milliseconds, and the interval starts at the time the callback message was registered. The message callback to the target object will be when the interval has expired.

Here's how you would program a message map entry to catch a timer message in a target object of type MyObject:

    // Message map entry of "object"
    FXDEFMAP(MyObject) MyObjectMap[]={
        FXMAPFUNC(SEL_TIMEOUT,MyObject::ID_ANIMATIONSTEP,MyObject::onAnimationStep),
      ...
    };

A timer may be registered with a specific target object and message-id identifying the object and method to be invoked on that object when the timer expires. An optional user-data pointer may be passed which is made available to the message handler when the timer message is invoked. This can be used by the handler to e.g. determine what to do. To register a timer message, you would call the function FXApp::addTimeout(), as follows:

    // Register Timer callback message for 1000 milliseconds
    MyObject* object;
    void *userdata;
    app->addTimeout(object,ID_ANIMATIONSTEP,1000,userdata);

If a timer with this particular target and message combination already exists, the interval is reset to the new time.

Timers can be removed or unregistered at any time, by calling FXApp::removeTimeout() with same target and message-id with which it was registered previously:

    // Unregister Timer callback message
    removeTimeout(object,ID_ANIMATIONSTEP);

It is OK to call removeTimeout() on a timer that has already fired. Timers in FOX are fired only once, not repeatedly; thus, to do animations the timer must be reset each time it expires, as is done below:

    // Receive a Timer callback
    long MyObject::onAnimationStep(FXObject*,FXSelector,void* ptr){
      if(continueToAnimate){

        // Do something ...

        // Restart timer for another interval, passing the user data for next time
        app->addTimeout(object,ID_ANIMATIONSTEP,1000,ptr);
        }
      return 1;
      }

It is OK to call FXApp::removeTimeout() even when the timer has already fired. A typical scenario used in FOX programs is to remove any timers that may be outstanding in the destructor of the target.
Since a timer is uniquely identified by the target and message-id, a class using timers does not need to bother with bookkeeping issues like keeping track of which timers are still in use. This is very convenient in destructors:

    // Destructor: just remove the timer, regarless of whether it was set
    MyObject::~MyObject(){
      removeTimeout(object,ID_ANIMATIONSTEP);
      }

Which makes for much cleaner code.

Timers are fired when the application returns to the event loop. In other words, handlers may be invoked a bit later than specified by the timer interval. To maintain smooth animation sequences, you might want to call gettimeofday() or some equivalent function so as to decrease the next timer interval in the sequence a little bit to correct for this effect, particularly if some of your applications are CPU intensive.

Chore Messages

Chore messages are messages which are delivered to their target object when the application is about to block for events. They are used for background tasks which are to be performed when no other, more urgent tasks need to be performed. You can use chores for housekeeping tasks in your application, or perhaps for animations. A chore will fire as soon as the event stream is exhausted and there is nothing else for the application to do, this is why it is also sometimes referred to as idle processing.

When the chore message is fired, your object will receive a message of the type SEL_CHORE, with the message ID being the one which was registered. To intercept this message, here's how you would program your message map:

    FXDEFMAP(MyObject) MyObjectMap[]={
      FXMAPFUNC(SEL_CHORE,MyObject::ID_IDLETASK,MyObject::onIdleTask),
      ...
      };

As you see, it is very similar to timer callback processing. Setting or registering a chore callback message is similar as well, and is done by calling FXApp::addChore() as shown below:

    // Register Chore callback message
    MyObject* object;
    void *userdata;
    app->addChore(object,ID_ANIMATIONSTEP,userdata);

Chores can be unregistered at any time prior to being fired, by calling FXApp::removeChore() with the same target and message-id as was used to register it in FXApp::addChore():

    // Unregister Chore callback message
    app->removeChore(object,ID_ANIMATIONSTEP);

Like timers, it is OK to remove chores that have already fired. Thus, classes which are receiving chore messages can remove them in the destructor, regardless of whether they were set or not. This can substantially simplify book keeping.

Some notes:

- Some computer graphics books describe using Motif work-proc's to perform delayed drawing for complicated 3D graphics. While FOX's Chores are indeed equivalent to Motif's work-proc's, this is unnecessary, as ALL paint operations in the FOX toolkit are already delayed.
- Repeatedly resetting the chore callback will mean that your application will never yield the CPU, because there will always be a chore ready to execute prior to blocking for event input.
- The GUI update and the Chore processing are interleaved, so that each time through the event loop, at least one GUI update callback and one Chore are always being executed. This means however, that if a Chore takes a long time, the GUI update process itself will also proceed much slower, as it will proceed in lock-step with the Chore processing.

Signal Messages

Signal messages are generated when certain asynchronous events happen. On most systems, these events are generated in the form of POSIX signals. The POSIX signal facility is available on most systems to which FOX has been ported, although non-POSIX [e.g. BSD) signals should work also.

You can use Signal messages to allow FOX objects to receive signals and process them. For example, you could register a signal handler for SIGINT, so that an application may be closed down properly when the user hits `^C` on the controlling terminal. Another use might be to register a handler to catch the SIGFPE during a computation, so a warning panel can be popped for a divide by zero, and perhaps gracefully save the user's data rather than core dumping.

When a Signal message is sent, your target object will receive a message of the type SEL_SIGNAL with the ID being the one specified when the callback message was registered:

    // Message map entry of "object"
    FXDEFMAP(MyObject) MyObjectMap[]={
      FXMAPFUNC(SEL_SIGNAL,MyObject::ID_INTERRUPT,MyObject::onCleanUpAndQuit),
      ...
      };

A signal handler can be added by calling FXApp::addSignal(). There are two methods to deliver a signal to the application: synchronously, and asynchronously (immediately).

Synchronous or non-immediatesignals are held until the application returns to the event loop, and then dispatched to the application. Thus, in most cases, the normal flow of computation in the application will not be interrupted, and your signal callback message handler can assume that all data structures are in a consistent state. Relatively harmless signals such as SIGINT are best handled synchronously.

Asynchronous or immediate signals are dispatched to the target object immediately. Since the regular processing of your application may have been interrupted by the signal, you will have to exercise extreme caution in the handler, as data structures may be partically complete. The immediate signal handlers are best reserved for last-ditch efforts, such as cleaning up after a SIGSEGV or SIGBUS, when a grave error has occured but there may be a chance to perhaps recover some of the user's data.

    // Register Signal callback message
    app->addSignal(SIGINT,myobject,ID_INTERRUPT,FALSE,flags);

The flags are set as for POSIX signal handling facilities, pleace confer your man pages for sigaction(2).
To remove the signal handler callback message and restore the default signal handling action, you can call FXApp::removeSignal() as follows:

    // Unregister Signal callback message
    app->removeSignal(SIGINT);

Input Messages

Input messages allow your programs to receive inputs from other sources than the GUI. Input messages can for example be used to watch sockets, pipes, and a host of other synchronization objects [if available on your machine].

Writing networked applications, such as e.g. a chat program, involves watching inputs from a number of different sources. You could have your program continuously check all these inputs for activity in a timer callback, but it is far more efficient to register an input source and yield the CPU until there is something going on.

Fortunately, most operating systems provide such a facility, and FOX can take advantage of this:

- On UNIX, the select() system call is used. The select mechanism allow a group of file descriptors representing sockets, pipes, and [where supported] asynchronous files to be watched for activity. In fact, the connection to the display, i.e. the GUI is just one of the file descriptors that can be watched. Please consult your UNIX select(2) man pages for more information about this system call.
- On WIndows NT, the MsgWaitForMultipleObjects() system call is used. This system call waits for GUI messages, as well as any number of synchronization objects, such as asynchronous files, sockets, pipes, event objects, mutexes, and even directories. More information about this can be found on the Microsoft Developer Network CD's, or on their on-line version of MSDN.

To register a callback message for an input source, you can call FXApp::addInput(). The callback message will remain registered even even after it has fired, unlike for Timers and Chores which are automatically removed after being fired once.

When a synchronization object becomes signaled, a message of the type SEL_IO_READ, SEL_IO_WRITE, or SEL_IO_EXCEPT will be sent to the target object, with the ID being the one specified in addInput(). You can intercept these messages as follows:

    // Message map entry of "object"
    FXDEFMAP(MyObject) MyObjectMap[]={
      FXMAPFUNC(SEL_IO_READ,MyObject::ID_ACCEPT,MyObject::onAcceptConnectionFromTheNet),
      FXMAPFUNC(SEL_IO_READ,MyObject::ID_SOCKET,MyObject::onReceivedInputFromTheNet),
      FXMAPFUNC(SEL_IO_WRITE,MyObject::ID_SOCKET,MyObject::onSendOutputToTheNet),
      FXMAPFUNC(SEL_IO_EXCEPT,MyObject::ID_SOCKET,MyObject::onDealWithExcept),
      ...
      };

In this example, a server type application may be creating a socket (socket(2)), and listen for incoming connections. When an incoming connection is received the callback handler onAcceptConnectionFromTheNet`()` presumably verifies the request and calls accept (accept(2)) and registers another handler to deal with incoming or outgoing data, and exceptional conditions.

You can register a input handler by calling FXApp::addInput().

    // Accept the connection
    socket=accept(...);

    // Register input callback message
    app->addInput(socket,INPUT_READ|INPUT_WRITE|INPUT_EXCEPT,myobject,ID_SOCKET);

Passing INPUT_READ|INPUT_WRITE|INPUT_EXCEPT will register the same callback message handler ID for all three types of I/O activities.
To remove a callback message handler, you can call FXApp::removeInput() as follows:

    // Unregister input callback message
    app->removeInput(socket,INPUT_WRITE);

This will remove the callback message ID for I/O output. It is usually a good idea for output, because the file descriptor will remain signaled as long as there is buffering to accept more outgoing data.
You would add the INPUT_WRITE back only when buffers get full [when the other party is tardy processing the data you're sending, lets say].

Copyright © 1997-2005 Jeroen van der Zijp