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Many widgets, like buttons, do all their drawing themselves. You just tell them the label you want to see, and they figure out what font to use, draw the button outline and focus rectangle, etc. Sometimes, it is necessary to do some custom drawing. In that case, a GtkDrawingArea might be the right widget to use. It offers a canvas on which you can draw by setting its draw function.
The contents of a widget often need to be partially or fully
redrawn, e.g. when another window is moved and uncovers part of
the widget, or when the window containing it is resized. It is
also possible to explicitly cause a widget to be redrawn, by
calling gtk_widget_queue_draw(). GTK takes care of most of the
details by providing a ready-to-use cairo context to the draw
function.
The following example shows how to use a draw function with GtkDrawingArea. It is a bit more complicated than the previous examples, since it also demonstrates input event handling with event controllers.
Create a new file with the following content named
example-4.c.
#include <gtk/gtk.h>
/* Surface to store current scribbles */
static cairo_surface_t *surface = NULL;
static void
clear_surface (void)
{
cairo_t *cr;
cr = cairo_create (surface);
cairo_set_source_rgb (cr, 1, 1, 1);
cairo_paint (cr);
cairo_destroy (cr);
}
/* Create a new surface of the appropriate size to store our scribbles */
static void
resize_cb (GtkWidget *widget,
int width,
int height,
gpointer data)
{
if (surface)
{
cairo_surface_destroy (surface);
surface = NULL;
}
if (gtk_native_get_surface (gtk_widget_get_native (widget)))
{
surface = gdk_surface_create_similar_surface (gtk_native_get_surface (gtk_widget_get_native (widget)),
CAIRO_CONTENT_COLOR,
gtk_widget_get_width (widget),
gtk_widget_get_height (widget));
/* Initialize the surface to white */
clear_surface();
}
}
/* Redraw the screen from the surface. Note that the draw
* callback receives a ready-to-be-used cairo_t that is already
* clipped to only draw the exposed areas of the widget
*/
static void
draw_cb (GtkDrawingArea *drawing_area,
cairo_t *cr,
int width,
int height,
gpointer data)
{
cairo_set_source_surface (cr, surface, 0, 0);
cairo_paint (cr);
}
/* Draw a rectangle on the surface at the given position */
static void
draw_brush (GtkWidget *widget,
double x,
double y)
{
cairo_t *cr;
/* Paint to the surface, where we store our state */
cr = cairo_create (surface);
cairo_rectangle (cr, x - 3, y - 3, 6, 6);
cairo_fill (cr);
cairo_destroy (cr);
/* Now invalidate the drawing area. */
gtk_widget_queue_draw (widget);
}
static double start_x;
static double start_y;
static void
drag_begin (GtkGestureDrag *gesture,
double x,
double y,
GtkWidget *area)
{
start_x = x;
start_y = y;
draw_brush (area, x, y);
}
static void
drag_update (GtkGestureDrag *gesture,
double x,
double y,
GtkWidget *area)
{
draw_brush (area, start_x + x, start_y + y);
}
static void
drag_end (GtkGestureDrag *gesture,
double x,
double y,
GtkWidget *area)
{
draw_brush (area, start_x + x, start_y + y);
}
static void
pressed (GtkGestureClick *gesture,
int n_press,
double x,
double y,
GtkWidget *area)
{
clear_surface();
gtk_widget_queue_draw (area);
}
static void
close_window (void)
{
if (surface)
cairo_surface_destroy (surface);
}
static void
activate (GtkApplication *app,
gpointer user_data)
{
GtkWidget *window;
GtkWidget *frame;
GtkWidget *drawing_area;
GtkGesture *drag;
GtkGesture *press;
window = gtk_application_window_new (app);
gtk_window_set_title (GTK_WINDOW (window), "Drawing Area");
g_signal_connect (window, "destroy", G_CALLBACK (close_window), NULL);
frame = gtk_frame_new (NULL);
gtk_window_set_child (GTK_WINDOW (window), frame);
drawing_area = gtk_drawing_area_new();
/* set a minimum size */
gtk_widget_set_size_request (drawing_area, 100, 100);
gtk_frame_set_child (GTK_FRAME (frame), drawing_area);
gtk_drawing_area_set_draw_func (GTK_DRAWING_AREA (drawing_area), draw_cb, NULL, NULL);
g_signal_connect_after (drawing_area, "resize", G_CALLBACK (resize_cb), NULL);
drag = gtk_gesture_drag_new();
gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (drag), GDK_BUTTON_PRIMARY);
gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (drag));
g_signal_connect (drag, "drag-begin", G_CALLBACK (drag_begin), drawing_area);
g_signal_connect (drag, "drag-update", G_CALLBACK (drag_update), drawing_area);
g_signal_connect (drag, "drag-end", G_CALLBACK (drag_end), drawing_area);
press = gtk_gesture_click_new();
gtk_gesture_single_set_button (GTK_GESTURE_SINGLE (press), GDK_BUTTON_SECONDARY);
gtk_widget_add_controller (drawing_area, GTK_EVENT_CONTROLLER (press));
g_signal_connect (press, "pressed", G_CALLBACK (pressed), drawing_area);
gtk_widget_show (window);
}
int
main (int argc,
char **argv)
{
GtkApplication *app;
int status;
app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
status = g_application_run (G_APPLICATION (app), argc, argv);
g_object_unref (app);
return status;
}
You can compile the program above with GCC using:
gcc `pkg-config --cflags gtk4` -o example-4 example-4.c `pkg-config --libs gtk4`