绘制

许多插件，比如buttons，自己就做了它们所有的绘制工作。比如你仅仅需要告诉它们你想看到的标签、你想它们使用的字体、绘制按钮的轮廓和焦点矩形。有时候，有必要做些自定义的绘制。在这种情况下，一个 GtkDrawingArea控件可能是正确的选择，这个控件提供了一个画布，在这个画布上你可以绘制并且将其连接到”draw“信号。

控件的内容常常需要被部分或者全部重新绘制。比如，当另一个窗口控件被移动并且露出控件的一部分，或者当包含它的窗口重新调整大小时，也会导致控件的部分或者全部被重新绘制。通过调用 gtk_widget_queue_draw()或者它的变体，GTK+提供一个现成的cairo给绘制信号从而实现众多细节。

下面的程序将会展示一个绘制信号句柄。这个例子比之前的略微复杂，因为它也通过button_press和motion_notify句柄显示出输入活动。

drawingpng

Example 3. Drawing in response to input

新建一个名为 example-3.c 的文件，写入如下内容：

  1. #include <gtk/gtk.h>
  3. /* Surface to store current scribbles */
  4. static cairo_surface_t *surface = NULL;
  6. static void clear_surface (void)
  7. {
  8.     cairo_t *cr;
  10.     cr = cairo_create (surface);
  12.     cairo_set_source_rgb (cr, 1, 1, 1);
  13.     cairo_paint (cr);
  15.     cairo_destroy (cr);
  17. }
  19. /* Create a new surface of the appropriate size to store our scribbles */
  20. static gboolean
  21. configure_event_cb (GtkWidget         *widget,
  22.                     GdkEventConfigure *event,
  23.                     gpointer           data)
  24. {
  25.     if (surface)
  26.     cairo_surface_destroy (surface);
  28.     surface = gdk_window_create_similar_surface (gtk_widget_get_window (widget),
  29.                                                  CAIRO_CONTENT_COLOR,
  30.                                                  gtk_widget_get_allocated_width (widget),
  31.                                                  gtk_widget_get_allocated_height (widget));
  33.     /* Initialize the surface to white */
  34.     clear_surface ();
  36.     /* We've handled the configure event, no need for further processing. */
  37.     return TRUE;
  39. }
  41. /* Redraw the screen from the surface. Note that the ::draw
  42. *  * signal receives a ready-to-be-used cairo_t that is already
  43. *   * clipped to only draw the exposed areas of the widget
  44. *    */
  45. static gboolean draw_cb (GtkWidget *widget,
  46.          cairo_t   *cr,
  47.          gpointer   data)
  48. {
  49.     cairo_set_source_surface (cr, surface, 0, 0);
  50.     cairo_paint (cr);
  52.     return FALSE;
  54. }
  56.  /* Draw a rectangle on the surface at the given position */
  57.  static void draw_brush (GtkWidget *widget,
  58.              gdouble    x,
  59.              gdouble    y)
  60.  {
  61.      cairo_t *cr;
  63.      /* Paint to the surface, where we store our state */
  64.      cr = cairo_create (surface);
  66.      cairo_rectangle (cr, x - 3, y - 3, 6, 6);
  67.      cairo_fill (cr);
  69.      cairo_destroy (cr);
  71.      /* Now invalidate the affected region of the drawing area. */
  72.      gtk_widget_queue_draw_area (widget, x - 3, y - 3, 6, 6);
  74.  }
  76.  /* Handle button press events by either drawing a rectangle
  77.  * or clearing the surface, depending on which button was pressed.
  78.  * The ::button-press signal handler receives a GdkEventButton
  79.  * struct which contains this information.
  80.  */
  81.  static gboolean button_press_event_cb (GtkWidget      *widget,
  82.                         GdkEventButton *event,
  83.                         gpointer        data)
  84.  {
  85.      /* paranoia check, in case we haven't gotten a configure event */
  86.      if (surface == NULL)
  87.      return FALSE;
  89.      if (event->button == GDK_BUTTON_PRIMARY)
  90.      {
  91.          draw_brush (widget, event->x, event->y);
  92.      }
  93.      else if (event->button == GDK_BUTTON_SECONDARY)
  94.      {
  95.          clear_surface ();
  96.          gtk_widget_queue_draw (widget);
  98.      }
  100.      /* We've handled the event, stop processing */
  101.      return TRUE;
  103.  }
  105.  /* Handle motion events by continuing to draw if button 1 is
  106.  * still held down. The ::motion-notify signal handler receives
  107.  * a GdkEventMotion struct which contains this information.
  108.  */
  109.  static gboolean
  110.  motion_notify_event_cb (GtkWidget      *widget,
  111.                          GdkEventMotion *event,
  112.                          gpointer        data)
  113.  {
  114.      /* paranoia check, in case we haven't gotten a configure event */
  115.      if (surface == NULL)
  116.      return FALSE;
  118.      if (event->state & GDK_BUTTON1_MASK)
  119.      draw_brush (widget, event->x, event->y);
  121.      /* We've handled it, stop processing */
  122.      return TRUE;
  124.  }
  126.   static void close_window (void)
  127.   {
  128.       if (surface)
  129.       cairo_surface_destroy (surface);
  131.       gtk_main_quit ();
  133.   }
  135.   int main (int   argc, char *argv[])
  136.   {
  137.       GtkWidget *window;
  138.       GtkWidget *frame;
  139.       GtkWidget *da;
  141.       gtk_init (&argc, &argv);
  143.       window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  144.       gtk_window_set_title (GTK_WINDOW (window), "Drawing Area");
  146.       g_signal_connect (window, "destroy", G_CALLBACK (close_window), NULL);
  148.       gtk_container_set_border_width (GTK_CONTAINER (window), 8);
  150.       frame = gtk_frame_new (NULL);
  151.       gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN);
  152.       gtk_container_add (GTK_CONTAINER (window), frame);
  154.       da = gtk_drawing_area_new ();
  155.       /* set a minimum size */
  156.       gtk_widget_set_size_request (da, 100, 100);
  158.       gtk_container_add (GTK_CONTAINER (frame), da);
  160.       /* Signals used to handle the backing surface */
  161.       g_signal_connect (da, "draw",
  162.                         G_CALLBACK (draw_cb), NULL);
  163.       g_signal_connect (da,"configure-event",
  164.                         G_CALLBACK (configure_event_cb), NULL);
  166.       /* Event signals */
  167.       g_signal_connect (da, "motion-notify-event",
  168.                         G_CALLBACK (motion_notify_event_cb), NULL);
  169.       g_signal_connect (da, "button-press-event",
  170.                         G_CALLBACK (button_press_event_cb), NULL);
  172.       /* Ask to receive events the drawing area doesn't normally
  173.       * subscribe to. In particular, we need to ask for the
  174.       * button press and motion notify events that want to handle.
  175.       */
  176.       gtk_widget_set_events (da, gtk_widget_get_events (da)
  177.                              | GDK_BUTTON_PRESS_MASK
  178.                              | GDK_POINTER_MOTION_MASK);
  180.       gtk_widget_show_all (window);
  182.       gtk_main ();
  184.       return 0;
  185.   }

然后在终端输入以下命令用GCC编译程序：

  1. gcc `pkg-config --cflags gtk+-3.0` -o example-3 example-3.c `pkg-config --libs gtk+-3.0`