Drawing Area

The Drawing Area is the main component of the Editor module. It is a GTK drawing area which is described in Appendix B. There are four important events which Drawing Area handles:

• expose_event calls function named on_drawingarea_expose_event. It is to redraw visible part of Drawing Area e.g. when Drawing Area becomes visible.
• button_press_event calls on_drawingarea_button_press_event function. It is executed every time when the user presses a mouse button when a mouse pointer is within the Drawing Area. This function is used to create, to select and to delete graphical objects depending on the pointer behavior which is controlled by the Editor toolbar.
• button_release_event calls on_drawingarea_button_release_event function. Similar to the previous action; it is called when the user releases a mouse button within the Drawing Area. It is used to notify where to move or where to link a graphical object.
• motion_notify_event calls on_drawingarea_motion_notify_event function. It is called when a mouse pointer passes through the Drawing Area. It is used to move graphical objects and link them one another. Selection between moving and linking depends on where a mouse pointer is. If it is on a connection area of a graphical object then Osiris makes a connection if it is elsewhere within an object then the object is moved.

Dealing with many graphical objects forces to arrange them in such a way that helps to manipulate them. Doubly-linked lists are chosen to arrange a graphical layer. Each object is a separate element in the list and each element in the list has information about where is the next and the previous element. Such a structure allows to keep all objects together using dynamic memory allocation and provides an easy way to perform operations on a separate object or on a group of objects.

What is more, doubly-linked lists technology is fully supported by the GLib library described in Appendix B which is very helpful.

There are several support functions to manipulate graphical objects e.g. create, move or draw, and are declared in the header file main.h in the Osiris source code tree. They all use doubly-linked lists from Glib library.