The DrawingArea Widget
The DrawingArea widget is a blank window that gives you the freedom to create any graphic you desire. Along with that freedom comes the responsibility to draw on the widget. When a widget is first shown, or when it is covered and then uncovered again it needs to redraw itself. Most widgets have code to do this, but the DrawingArea does not, allowing you to write your own draw function to determine how the contents of the widget will be drawn. This is done by setting a draw function with a call to the set_draw_func() member function.
You can draw very sophisticated shapes using Cairo, but the methods to do so are quite basic. Cairo provides methods for drawing straight lines, curved lines, and arcs (including circles). These basic shapes can be combined to create more complex shapes and paths which can be filled with solid colors, gradients, patterns, and other things. In addition, Cairo can perform complex transformations, do compositing of images, and render antialiased text.
Although Cairo can render text, it's not meant to be a replacement for Pango. Pango is a better choice if you need to perform more advanced text rendering such as wrapping or ellipsizing text. Drawing text with Cairo should only be done if the text is part of a graphic.
In this section of the tutorial, we'll cover the basic Cairo drawing model, describe each of the basic drawing elements in some detail (with examples), and then present a simple application that uses Cairo to draw a custom clock widget.