| Documentation: The Application Class [Remove Frame]

The FXApp Class

The application object manages the message queue, timers, chores, GUI updating, and other system facilities. Each FOX application will have exactly one application instance. Every FOX application will start by constructing one FXApp object prior to building the GUI. Usually, the FXApp object is the last object to be deleted as well.

Using the code below, the application object will be constructed on the stack and hence is automatically destroyed when the program terminates. Also, when the application object is destroyed, all the windows and other resources it knows about are destroyed as well.

    int main(int argc,char *argv[]){

      // Make application
      FXApp application("ApplicationName","VendorName");

      // Open display
      application.init(argc,argv);

      // Make window
      MainWindow* mainwindow=new MainWindow(&application);

      // Create it
      application.create();

      // Show Window
      mainwindow->show();

      // Run
      return application.run();
      }

In the first line of code above, an application object is constructed. The constructor has two parameters, the application name, and the vendor name. The application name and vendor name are used to determine the applications registry settings.

The next line of code initializes the application object, passing in the command line arguments of the process. The application object parses its own arguments and removes them, but leaves the remaining arguments alone.

The next line creates a toplevel window, passing in a pointer to the application object.

The call to the application object's create() function realizes the entire widget tree, i.e. creates the necessary resources in the system (X Server or Windows GDI), to turn what was up till that point a collection of C++ data structures into a real-life application which is able to receive events and draw on the screen.

The final call to run() starts the toplevel event loop. A typical application will not return from this loop until the user closes the application.

Event Loops

Most GUI applications have something called an event loop or message loop. Unlike batch-oriented programs which read a datafile, perform some processing, and then produce an output file, a GUI driven application spends almost all its time in an event loop, waiting for user input, determining where that input came from, and then dispatching it to the proper function to perform some processing. Unlike batch oriented programs, these functions are typically very short, and mostly take only very little time to execute, and so the user is in complete control of the application most of the time. The events a GUI program processes can be of different types:

- Keyboard input;
- Mouse movements;
- Mouse buttons;
- Inputs from other sources (e.g. network sockets, timers, signals, and so on);
- Changes in selection and clipboard ownership;
- Drag and drop events;
- Window repaint events;
- And other things which can happen to a window.

The application object is solely responsible for coordinating all these events and dispatching them to the proper destination where they are handled.

FXApp performs delayed repaints on windows, i.e. almost all events are prioritized over repaint events so as to delay expensive redrawing as much as possible. Because of this, repainting can never fall behind more than one repaint, and needless repaints are avoided as much as possible. Also FXApp combines repaint rectangles so as to minimize the video card hardware setup and teardown time relative to the number of pixels drawn.

Event Queues

Certain devices, such as a moving mouse, can generate events faster than some programs can process them. To prevent losing events, events are commonly queued up, so programs can catch up as fast as they can. Likewise, the drawing commands a GUI program generates as its trying to draw buttons and other controls on the screen are also queued up, so that the X server (or GDI on Windows) can take its time to catch up.

Finally, the X server may have its own event queue and drawing queue, making for a total of four queues. All these queues allow for much faster performance of applications, as bigger chunks of data can be transmitted between the application and the X server, and fewer context switches of video card and cpu hardware are needed.

From the point of programming in FOX, the existence of these queues is for the most part hidden, but in a few cases some special functions are available that you may need to call:

FXApp::flush(sync)

This function flushes the output queue, i.e. the commands which have been already performed are pushed to the X server, where they are executed. If we want to make sure that the display shows the correct picture, however, just pushing the commands to the X server is not enough:- sometimes we need to make sure that these commands have been executed before we continue! When a TRUE is passed for the sync parameter, the X server is forced to execute these commands.

Sometimes, we want to check if there are any events, but continue to do some processing if no events are available. Under normal circumstances, returning to the event loop will cause our process to block until there are events; but if there is stuff we may want to do, this is of course not desirable. We have just the right solution for this problem:

FXApp::peekEvent()

This function will return TRUE if there are any events ready to process, and FALSE if there are none. The peekEvent() function can be used when we are doing a long calculation and we want to check if the user has hit the STOP button.

Types of Event Loops

There are several types of event loops supported, each of them is appropriate for certain sistuations; most commonly, application developers will however only call:

FXApp::run()

This is the top level event loop, and it will only terminate when the application is ready to call it quits. When run() finally returns, its return value is the exit value passed to stop() earlier.

FXApp::stop(code)

This function terminates the top level event loop, but also terminates all nested event loops which have been directly or indirectly invoked from this top level loop. Each nested loop is terminated with a code of zero (0), but the top level event loop is terminated with the given code.

FXApp::runOneEvent()

As the name implies, this function reads and then processes one (1) single event, and then returns. It is primarily interesting to use in combination with peekEvent(), as peekEvent() returns TRUE if there is at least one event ready to be processed.
If there is no event ready, runOneEvent() will block until there is at least one event.

FXApp::runUntil(condition)

This function processes events until the variable condition, which is passed as a reference, becomes non-zero.

Modal dialogs are dialog panels which block interaction with the rest of the application until they are completed. For example, while trying to open a file, the application is unable to interact in other way with the user until some file is selected and loaded in. In FOX, the only difference between normal dialogs and modal dialogs is how they are run:- modal dialogs are run by calling:

FXApp::runModalFor(window)

This function runs a nested or recursive invocation of the event loop, i.e. it re-enters an event loop and processes events for a while, and returns only when stopModal() or stop() is called. As long as runModalFor() is running, user-input events to all other windows, except for window and its children, are being blocked:- no interaction with other windows is possible until runModalFor() returns. When it returns, it returns the value passed to the stopModal() function, or 0 if stop() is called to terminate the application.

However, it is quite possible, and in fact common, that one modal dialog invokes another. In that case, only the most recently invoked dialog can be interacted with.

FXApp::runModalWhileShown(window)

The function runModalWhileShown() runs until either stopModal() is called or the specified window becomes hidden. This is of interest when running popup menus or other temporary windows.

FXApp::stopModal(window,code)

Calling stopModal() causes the modal event loop with the matching window to terminate with code. However, stopModal() also causes all modal event loops which are nested more deeply to terminate with code zero (0).

FXApp::isModal(window)

This function returns TRUE if a modal loop is in effect for the given window.

Modal dialogs are always run with runModalFor(). Because it is so common to construct a dialog on the stack, run it modally, and then process the inputs thus obtained, there is a convenience member function FXDialogBox::execute() which calls create(), show(), and then runModalFor() in turn. The FXDialogBox also understands several messages, for example ID_ACCEPT, ID_CANCEL, and SEL_CLOSE which call stopModal() returning a code 1, 0, and 0 respectively. The return code 0 for FXDialogBox::execute() should never cause any action to be performed as it is the code returned when the dialog box is cancelled or closed!!

GUI Updating

As has been discussed before, the event loop blocks when no events are available. Prior to blocking, all windows which have been marked as dirty are repainted. One other thing that happens just prior to blocking is the GUI update process, which periodically refreshes all widgets by interrogating the application state. Each GUI update takes only a little time, yet the total number of widgets may be large. The GUI update is peformed just prior to blocking, so at the time it is being performed there is really nothing else for the application process to do (or it would be doing it!). The GUI update is normally started by calling:

FXApp::refresh()

This function reschedules another GUI update to be performed in the future.

FXApp::forceRefresh()

Calling this function will cause an immediate GUI update pass to be performed. Unlike the normal GUI update, which takes place unobstrusively one and a time, prior to blocking, forceRefresh() will not return until all windows have been refreshed. It is therefore quite expensive, and should be used only when strictly necessary.

The GUI update has no impact on the perceived speed of an application because between each pair of GUI updates performed, a check for events is performed.

Visuals

An FXVisual is a description of the pixel organization on the display. For example, some high-quality graphics cards can do 24 bits per pixel; other graphics cards may be limited to 16, 15 bits per pixel, or even just 8 bits/pixel. FOX programs can even work on 4 bit/pixel (VGA mode) and 1 bit/pixel (monochrome), although it wouldn't be as much fun!

Besides depth (number of bits/pixel), there are also other characteristics which come into play describing the pixel organization, such as colormaps, and wether or not a colormap can be written or not, and the byte and bit organization.

Colormaps are commonly used on 8-bit/pixel systems. Most hardware only supports one hardware colormap, and this must be shared among all programs on the display. Because legacy toolkits such as Motif do not deal with full colormaps very gracefully, FOX applications deliberately do not try to grab all 256 colors from the colormap, but only 125 colors. Images and Icons are dithered to get the best possible rendering given the number of colors available.

You can improve the look of your program quite easily, however, as the maximum number of colors which the default visual tries to allocate can be easily changed using a command line argyment; for example, "myapp -maxcolors 256" will start myapp in such a way as to attempt to acquire all 256 colors from the colormap.

Because other programs may already be running, the desired gamut of colors may not be available. If the exact color can not be obtained, FOX will try to find the closest color available and use that.

Normally, the FXVisual a window uses is copied from its parent. You can change this visual for each window, or you can call:

FXApp::setDefaultVisual(visual)

This function will change the default visual to be used for all toplevel windows; the child-windows will simply inherit the visual from their parent.

Alternatively, the maximum number of colors can also be set via the registry, using the maxcolors key under [SETTINGS] any number less than or equal to 256 colors may be specified, FXVisual will determine the best gamut to pick from the allowable number of colors.

Wait Cursors

Sometimes, an application needs to undertake a long task, such as e.g. loading a big file. In such cases, it is good form to present the user with some feedback to indicate that the application may be temporarily unresponsive. Common ways to do this are progress bars and changing the cursor shape to a stopwatch, or hourglass, or something like that.FXApp supports this by means of the following functions:

FXApp::beginWaitCursor()

This will change the cursor shape for all windows to the stopwatch cursor, or the cursor designated by setWaitCursor(). Calls to beginWaitCursor() and endWaitCursor() can be nested in a stack-like fashion, with only the first call to beginWaitCursor() and last call to endWaitCursor() actually changing the cursor shape.

FXApp::endWaitCursor()

A matching call to endWaitCursor() will restore the original cursor for each window.

FXApp::setWaitCursor(cursor)

This will change the cursor shape used in during a beginWaitCursor() endWaitCursor() pair.

FXApp::getWaitCursor()

This returns the current FXCursor being used as hourglass or stopwatch cursor.

The beginWaitCursor() and endWaitCursor() calls can be nested pairwise, so that a functions which bracket a long calculation by means of a beginWaitCursor/endWaitCursor pair can call upon each other.

Drag Types

Exchanging data via the Primary Selection, the Clipboard, or by means of Drag and Drop requires that all applications agree with the type of data being exchanged.
This is done by registering a Drag Type. In most cases, the name being registered should be a mime-type, such as "text/plain" or "image/gif".

Manipulating drag types is done with the following API's:

FXApp::registerDragType(name)

This will register a new drag type based on the mime type name.

FXApp::getDragTypeName(dragtype)

Obtain the name of a previously registered drag type.

Copyright © 1997-2003 Jeroen van der Zijp