Rapicorn Programming

A typical Rapicorn application consists of one application thread, and a number of Rapicorn windows, each of which runs its own event loop but is executed in the main application thread. Alternative execution models have been considered for Rapicorn in particular the one-thread-per-window model. But after some initial development and debugging phase, they have been disregarded for the sake of reliability and predictability. More information is provided by the paper "The Problem with Threads" by Edward A. Lee.

The usual number of steps of a GUI application written with Rapicorn are:

  1. Initialization of Rapicorn and the GUI components.
    For Rapicorn this means choosing a drawing backend, such as for example the X windowing backend via Gtk+. For backends that require a server connection (like X11), this is also when the server connection is established.
  2. Loading of the UI definitions.
    In Rapicorn, most windows and UI elements are defined (or "programmed") by writing composite definitions in XML files. After initialization, these XML files need to be loaded in order to use the GUI components. (A later example will show, that it's also possible to compile XML definitions into a Rapicorn application.)
  3. Creation of windows from the UI definitions.
    After the UI definitions have been loaded successfully, they can be instantiated. Most often, fully defined application specific windows are created from the XML definitions.
  4. Setup of application data and user command handling.
    Graphical UI programs commonly use an event based programming model, where the application reacts to user input through the graphical interface. After window creation, the application logic needs to be connected and interact with the toolkit components. This usually means setting up of one or more callback handlers to react to certain UI events, and providing the necessary data to be displayed by the application windows.
  5. Display of and interaction with the graphical interface.
    After the application dialogs have been created and setup in an application specific fashion, they are ready to be presented to the user. For this, the show() method is invoked on a window handle which causes it to be displayed on the users screen, as soon as the window's event loop starts executing. Once a window's event loop starts executing, it'll process events like drawing requests from the windowing backend, and user input events like key presses or mouse actions. If necessary, user commands setup at the previous step are processed by the application logic, by calling application callbacks from within the event loop's event handlers.
  6. Executing the event loops.
    For most GUI applications, there is no additional work to be done by the application logic, once the GUI has been started. So once all dialogs have been created and setup accordingly, the (main) application thread usually calls a toolkit function to process all GUI events until all GUI interaction has been completed, and the applications cleanup logic can be run. In Rapicorn this is accomplished by calling Application::execute_loops();.
  7. Exiting.
    A notification signal is emitted when no primary window (a window that is not a transient or child window) remains on the screen and retained events to be handled by the event loop. This signal is Application::sig_missing_primary, and if no new primary windows are present after the signal emission, Application::execute_loops() will return. Alternatively, Application::execute_loops() can be forced to return by calling Application::exit().

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