Gnome Developers' Information
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# Getting started with the canvas

This section presents a simple example of using the canvas to display some items and manipulate them. We will create a blank canvas and provide a button that the user can click on to insert random items in the canvas. If the user double-clicks on an item with mouse button 1, the color of that item will be changed. The user can also move items around by pressing mouse button 1 and dragging. In addition, items may be deleted by pressing mouse button 3 over them. Items will get a wide outline when the mouse passes over them, and will return to a thin outline when the mouse leaves them.

We will present the program in small sections and explain each of them separately.

Example 1. Creating the main window and the canvas

Here we create a window for the canvas example and put a canvas widget into it. we also create buttons that let the user insert a new item in the canvas and exit the program. We also define the auxiliary handlers for the clicked signal of the Exit button and the delete_event signal of the main window.

    #include <gnome.h>

    /* This defines the size of the canvas, in pixels */

    #define CANVAS_SIZE 300

    /* Prototypes for the functions we will define later */

    static void add_object_clicked (GtkWidget *button, gpointer data);
    static void exit_clicked (GtkWidget *widget, gpointer data);
    static gint delete_event (GtkWidget *widget, GdkEvent *event, gpointer data);

    int
    main (int argc, char **argv)
    {
    	GtkWidget *window;
    	GtkWidget *vbox;
    	GtkWidget *frame;
    	GtkWidget *canvas;
    	GtkWidget *hbox;
    	GtkWidget *button;

    	gnome_init ("canvas-example", "1.0", argc, argv);

    	/* Create the main window and the main vbox */

    	window = gtk_window_new (GTK_WINDOW_TOPLEVEL);

    	vbox = gtk_vbox_new (FALSE, 0);
    	gtk_container_add (GTK_CONTAINER (window), vbox);
    	gtk_widget_show (vbox);

    	/* Create a frame for the canvas and the canvas itself */

    	frame = gtk_frame_new (NULL);
    	gtk_frame_set_shadow_type (GTK_FRAME (frame), GTK_SHADOW_IN);
    	gtk_box_pack_start (GTK_BOX (vbox), frame, TRUE, TRUE, 0);
    	gtk_widget_show (frame);

    	canvas = gnome_canvas_new ();
    	gtk_widget_set_usize (canvas, CANVAS_SIZE, CANVAS_SIZE);
    	gnome_canvas_set_scroll_region (GNOME_CANVAS (canvas), 0.0, 0.0, CANVAS_SIZE, CANVAS_SIZE);

    	gtk_container_add (GTK_CONTAINER (frame), canvas);
    	gtk_widget_show (canvas);

    	/* Create the hbox for the buttons */

    	hbox = gtk_hbox_new (TRUE, 0);
    	gtk_box_pack_start (GTK_BOX (vbox), hbox, FALSE, FALSE, 0);
    	gtk_widget_show (hbox);

    	/* Create the button used to add objects -- we pass the canvas to the callback
    	 * in the user data.
    	 */

    	button = gtk_button_new_with_label ("Add an object");
    	gtk_signal_connect (GTK_OBJECT (button), "clicked",
    			    (GtkSignalFunc) add_object_clicked,
    			    canvas);
    	gtk_box_pack_start (GTK_BOX (hbox), button, TRUE, TRUE, 0);
    	gtk_widget_show (button);

    	/* Create the button used to exit the program -- we pass the main window to the callback in
    	 * the user data.
    	 */

    	button = gtk_button_new_with_label ("Exit");
    	gtk_signal_connect (GTK_OBJECT (button), "clicked",
    			    (GtkSignalFunc) exit_clicked,
    			    window);
    	gtk_box_pack_start (GTK_BOX (hbox), button, TRUE, TRUE, 0);
    	gtk_widget_show (button);

    	/* Connect to the delete_event signal and Run the application */

    	gtk_signal_connect (GTK_OBJECT (window), "delete_event",
    			    (GtkSignalFunc) delete_event,
    			    NULL);

    	gtk_widget_show (window);
    	gtk_main ();
    	return 0;
    }

    /* Callback for the clicked signal of the Exit button */
    static void
    exit_clicked (GtkWidget *widget, gpointer data)
    {
    	gtk_widget_destroy (GTK_WIDGET (data)); /* the user data points to the main window */
    	gtk_main_quit ();
    }

    /* Callback for the delete_event signal of the main application window */
    static gint
    delete_event (GtkWidget *widget, GdkEvent *event, gpointer data)
    {
    	gtk_widget_destroy (widget); /* destroy the main window */
    	gtk_main_quit ();
    	return TRUE;
    }

As you can see, a new canvas is created using the `gnome_canvas_new()` function. Then we set the starting size of the canvas window and the extents of the scrolling region using the `gtk_widget_set_usize()` and `gnome_canvas_set_scroll_region()` functions, respectively. We will discuss the meaning of the scrolling region later; for now, we just set it to go from the origin to the canvas size in pixels — we will be using a canvas zoom factor of 1:1, that is, one pixel for each unit.

Example 2. Creating random items in the canvas

Here we define the callback function `add_object_clicked()` that is used by the "Add an object" button. When the user presses this button, a rectangle or an ellipse will be created using random coordinates.

    /* Prototype for the item event handler */

    static gint item_event (GnomeCanvasItem *item, GdkEvent *event, gpointer data);

    /* Callback for the clicked signal of the Add object button.  It creates a rectangle or an ellipse
     * at a random position.  The canvas comes in the user data pointer.
     */
    static void
    add_object_clicked (GtkWidget *button, gpointer data)
    {
    	GnomeCanvas *canvas;
    	GnomeCanvasItem *item;
    	guint type;
    	int x1, y1, x2, y2;
    	int tmp;

    	canvas = GNOME_CANVAS (data);

    	/* Compute some random coordinates, with the condition that (x1 <= x2) and (y1 <= y2), and
    	 * ensure that the objects are not too small.
    	 */

    	x1 = rand () % CANVAS_SIZE;
    	y1 = rand () % CANVAS_SIZE;
    	x2 = rand () % CANVAS_SIZE;
    	y2 = rand () % CANVAS_SIZE;

    	if (x1 > x2) {
    		tmp = x1;
    		x1 = x2;
    		x2 = tmp;
    	}

    	if (y1 > y2) {
    		tmp = y1;
    		y1 = y2;
    		y2 = tmp;
    	}

    	if ((x2 - x1) < 10)
    		x2 += 10;

    	if ((y2 - y1) < 10)
    		y2 += 10;

    	/* Pick a type for the item randomly */

    	if (rand () & 1)
    		type = gnome_canvas_rect_get_type ();
    	else
    		type = gnome_canvas_ellipse_get_type ();

    	/* Create the item and make it white with black outline by default.  Also,
    	 * connect to its event signal so that we can know when events get to the
    	 * item.*/

    	item = gnome_canvas_item_new (gnome_canvas_root (canvas),
    				      type,
    				      "x1", (double) x1,
    				      "y1", (double) y1,
    				      "x2", (double) x2,
    				      "y2", (double) y2,
    				      "fill_color", "white",
    				      "outline_color", "black",
    				      "width_units", 1.0,
    				      NULL);
    	gtk_signal_connect (GTK_OBJECT (item), "event",
    			    (GtkSignalFunc) item_event,
    			    NULL);
    }

In this function, first we compute some random coordinates for the corners of the rectangle or ellipse that we will create. We have to ensure that x1 and y1 are less than or equal to x2 and y2, respectively.

New items are created and inserted into the canvas using the `gnome_canvas_item_new()` function. The first parameter to this function specifies the canvas item group that will act as the new item's parent. For this simple example, we insert all the items inside the toplevel canvas group, the root item, which we obtain by calling the `gnome_canvas_root()` function.

The second parameter to `gnome_canvas_item_new()` specifies the type of item we want to create. This is simply the Gtk type identifier used by the class of the item you want to create. We pick randomly between `gnome_canvas_rect_get_type()` and `gnome_canvas_ellipse_get_type()`, and then we pass this value to the `gnome_canvas_item_new()` function.

The following parameters are optional, and are a list of key/value pairs that specify which arguments to set for that particular item. Internally these are handled using the Gtk object argument system. The simplest way of using these is, for each argument you want to set, pass a string with the name of the argument, and then the value you want to set for that argument.

| You must pass in values with the correct type! Remember that the C compiler cannot warn you about incorrect types when you are using a variable argument list. In this example, we must use doubles for the coordinates of the corners of the rectangle or ellipse. Colors are passed using a string with a valid X color specification, and the width in units is passed as a double. Please look at the reference part of the GnomeCanvas documentation for detailed information on the argument types supported by each canvas item.

We pass NULL as the last argument to indicate that we have no more arguments to set for this item.

Finally, we connect to the "event" signal of the item so that we can be notified when the item receives events from the mouse.

Example 3. Defining behavior for the canvas items

Here we define the event handler for items in our canvas. Canvas items receive events just as X windows do. In our sample event handler, the user can drag items around using button 1. Items may be deleted by pressing button 3 on them. If the user double-clicks on an item using mouse button 1, then that item's color will be changed randomly. Finally, the width of an item's outline will change depending on whether the mouse is inside or outside the item.

    /* Prototype for the function that changes the item's color randomly */

    static void change_item_color (GnomeCanvasItem *item);

    /* Callback for the event signal of the canvas items.  If the user drags the item with button 1,
     * then it will move appropriately.  If the user double clicks on the item, then its color will
     * change randomly.  If the user clicks on an item with button 3, then the item will be destroyed.
     * When the mouse enters an item, its outline width will be set to 3 units.  When the mouse leaves
     * an item, its border width will be set to 1 unit.
     */

    static gint
    item_event (GnomeCanvasItem *item, GdkEvent *event, gpointer data)
    {
    	static double x, y; /* used to keep track of motion coordinates */
    	double new_x, new_y;

    	switch (event->type) {
    	case GDK_BUTTON_PRESS:
    		if (event->button.button == 1) {
    			/* Remember starting position */
    			x = event->button.x;
    			y = event->button.y;
    			return TRUE;
    		} else if (event->button.button == 3) {
    			/* Destroy the item */
    			gtk_object_destroy (GTK_OBJECT (item));
    			return TRUE;
    		}
    		break;

    	case GDK_2BUTTON_PRESS:
    		if (event->button.button == 1) {
    			/* Change the item's color */
    			change_item_color (item);
    			return TRUE;
    		}
    		break;

    	case GDK_MOTION_NOTIFY:
    		if (event->button.state & GDK_BUTTON1_MASK) {
    			/* Get the new position and move by the difference */

    			new_x = event->motion.x;
    			new_y = event->motion.y;

    			gnome_canvas_item_move (item, new_x - x, new_y - y);

    			x = new_x;
    			y = new_y;
    			return TRUE;
    		}
    		break;

    	case GDK_ENTER_NOTIFY:
    		/* Make the outline wide */
    		gnome_canvas_item_set (item,
    				       "width_units", 3.0,
    				       NULL);
    		return TRUE;

    	case GDK_LEAVE_NOTIFY:
    		/* Make the outline thin */
    		gnome_canvas_item_set (item,
    				       "width_units", 1.0,
    				       NULL);
    		return TRUE;

    	default:
    		break;
    	}

    	return FALSE;
    }

    /* Utility function to randomly change the fill color of a canvas item */
    static void
    change_item_color (GnomeCanvasItem *item)
    {
    	static const char *color_specs[] = {
    		"red",
    		"yellow",
    		"green",
    		"cyan",
    		"blue",
    		"magenta"
    	};

    	int n;

    	/* Pick a random color from the list */

    	n = rand () % (sizeof (color_specs) / sizeof (color_specs[0]));

    	gnome_canvas_item_set (item,
    			       "fill_color", color_specs[n],
    			       NULL);
    }

The "event" signal for canvas items is very similar to the "event" signal in Gtk widgets. Handlers for this signal take in a pointer to the relevant item, a pointer to the event that the item received, and the normal user data pointer. As with the "event" signal for widgets, handlers return FALSE if they did not process the event or if they want further processing to be done on it, or TRUE if they did handle the event and they do not want any further processing to be done on it.

Our sample event handler is a switch statement that selects among the different event types. We have the following cases:

- For a single button press with button 1, we remember this initial mouse position for if the user decides to drag the item around.

  For a single button press with button 3, we destroy the item. This will remove the item from the canvas and free its memory.

- For double-clicks with button 1, we call a function that randomly changes the color of the item.

- For motion events, we only handle them if button 1 is down; this means that the user is dragging with the button down. We compute the deltas between the new and the old mouse coordinates, and move the item by that amount. This is not the best way to do dragging of items for all applications, but it is good enough for our simple example.

- For enter and leave notify events, we simply change the outline width of the item. The outline will be made 3.0 units thick when the mouse enters the item, and it will be made 1.0 units thick when the mouse leaves the item.

As with all event handlers, we return TRUE when we have handled the event, or FALSE if we did not process it.

Note the use of the `gnome_canvas_item_set()`. It is used to change the values of the item's arguments. It is similar in use to `gnome_canvas_item_new()`, and it takes in the item for which we want to change attributes, and a NULL-terminated list of key/value pairs for the new attributes.

In the following sections, we will explain the canvas internals in detail.

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