许多小部件，如按钮，都自己做所有的绘图。你只要告诉他们你想要看到的标签，他们弄清楚要使用的字体，绘制按钮轮廓和焦点矩形等。有时，有必要做一些自定义的绘图。在这种情况下，GtkDrawingArea 可能是正确使用的小部件。它提供一个画布，您可以通过连接到“绘制”信号绘制。 小部件的内容通常需要部分或全部重新绘制，例如，当另一个窗口被移动并发现部分小部件时，或者当包含它的窗口被调整大小时。也可以通过调用gtk_widget_queue_draw()或其变体来明确地引起部分或全部小部件的重绘 。GTK +通过为:: draw信号处理程序提供一个即用的开罗上下文来处理大部分细节。 以下示例显示了一个:: draw信号处理程序。它比以前的例子要复杂一点，因为它也通过:: button-press和:: motion-notification处理程序来演示输入事件处理。
示例：响应输入进行绘图
创建一个名为test.c的以下内容的新文件。
#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 gboolean configure_event_cb (GtkWidget *widget, GdkEventConfigure *event, gpointer data) { if (surface) cairo_surface_destroy (surface); surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget), CAIRO_CONTENT_COLOR, gtk_widget_get_allocated_width (widget), gtk_widget_get_allocated_height (widget)); /* Initialize the surface to white */ clear_surface (); /* We've handled the configure event, no need for further processing. */ return TRUE; } /* Redraw the screen from the surface. Note that the ::draw * signal receives a ready-to-be-used cairo_t that is already * clipped to only draw the exposed areas of the widget */ static gboolean draw_cb (GtkWidget *widget, cairo_t *cr, gpointer data) { cairo_set_source_surface (cr, surface, 0, 0); cairo_paint (cr); return FALSE; } /* Draw a rectangle on the surface at the given position */ static void draw_brush (GtkWidget *widget, gdouble x, gdouble 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 affected region of the drawing area. */ gtk_widget_queue_draw_area (widget, x - 3, y - 3, 6, 6); } /* Handle button press events by either drawing a rectangle * or clearing the surface, depending on which button was pressed. * The ::button-press signal handler receives a GdkEventButton * struct which contains this information. */ static gboolean button_press_event_cb (GtkWidget *widget, GdkEventButton *event, gpointer data) { /* paranoia check, in case we haven't gotten a configure event */ if (surface == NULL) return FALSE; if (event->button == GDK_BUTTON_PRIMARY) { draw_brush (widget, event->x, event->y); } else if (event->button == GDK_BUTTON_SECONDARY) { clear_surface (); gtk_widget_queue_draw (widget); } /* We've handled the event, stop processing */ return TRUE; } /* Handle motion events by continuing to draw if button 1 is * still held down. The ::motion-notify signal handler receives * a GdkEventMotion struct which contains this information. */ static gboolean motion_notify_event_cb (GtkWidget *widget, GdkEventMotion *event, gpointer data) { /* paranoia check, in case we haven't gotten a configure event */ if (surface == NULL) return FALSE; if (event->state & GDK_BUTTON1_MASK) draw_brush (widget, event->x, event->y); /* We've handled it, stop processing */ return TRUE; } static void close_window (void) { if (surface) cairo_surface_destroy (surface); gtk_main_quit (); } static void activate (GtkApplication *app, gpointer user_data) { GtkWidget *window; GtkWidget *frame; GtkWidget *drawing_area; 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); gtk_container_set_border_width (GTK_CONTAINER (window), 8); frame = gtk_frame_new (NULL); gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN); gtk_container_add (GTK_CONTAINER (window), frame); drawing_area = gtk_drawing_area_new (); /* set a minimum size */ gtk_widget_set_size_request (drawing_area, 100, 100); gtk_container_add (GTK_CONTAINER (frame), drawing_area); /* Signals used to handle the backing surface */ g_signal_connect (drawing_area, "draw", G_CALLBACK (draw_cb), NULL); g_signal_connect (drawing_area,"configure-event", G_CALLBACK (configure_event_cb), NULL); /* Event signals */ g_signal_connect (drawing_area, "motion-notify-event", G_CALLBACK (motion_notify_event_cb), NULL); g_signal_connect (drawing_area, "button-press-event", G_CALLBACK (button_press_event_cb), NULL); /* Ask to receive events the drawing area doesn't normally * subscribe to. In particular, we need to ask for the * button press and motion notify events that want to handle. */ gtk_widget_set_events (drawing_area, gtk_widget_get_events (drawing_area) | GDK_BUTTON_PRESS_MASK | GDK_POINTER_MOTION_MASK); gtk_widget_show_all (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; }效果图
