Drawing on Canvas

Generic drawing is done on a Canvas. You control what appears on this canvas by defining a draw function:

using Gtk, Graphics
c = @GtkCanvas()
win = GtkWindow(c, "Canvas")
@guarded draw(c) do widget
    ctx = getgc(c)
    h = height(c)
    w = width(c)
    # Paint red rectangle
    rectangle(ctx, 0, 0, w, h/2)
    set_source_rgb(ctx, 1, 0, 0)
    fill(ctx)
    # Paint blue rectangle
    rectangle(ctx, 0, 3h/4, w, h/4)
    set_source_rgb(ctx, 0, 0, 1)
    fill(ctx)
end
show(c)

This draw function will get called each time the window gets resized or otherwise needs to refresh its display.

Errors in the draw function can corrupt Gtk's internal state; if this happens, you have to quit julia and start a fresh session. To avoid this problem, the @guarded macro wraps your code in a try/catch block and prevents the corruption. It is especially useful when initially writing and debugging code. See further discussion about when @guarded is relevant.

Finally, Canvases have a field called mouse that allows you to easily write callbacks for mouse events:

c.mouse.button1press = @guarded (widget, event) -> begin
    ctx = getgc(widget)
    set_source_rgb(ctx, 0, 1, 0)
    arc(ctx, event.x, event.y, 5, 0, 2pi)
    stroke(ctx)
    reveal(widget)
end

This will draw a green circle on the canvas at every mouse click. Resizing the window will make them go away; they were drawn on the canvas, but they weren't added to the draw function.

Note the use of the @guarded macro here, too.