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|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title>
Using the Canvas
</title>
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<tr>
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<font color="#000000" size="2">GTK+ / Gnome Application
Development</font>
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<tr>
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</div>
<div class="SECT1">
<h1 class="SECT1">
<a name="Z177">Using the Canvas</a>
</h1>
<p>
<tt class="CLASSNAME">GnomeCanvas</tt> is easy to use; this
is its virtue compared to <tt class="CLASSNAME">
GtkDrawingArea</tt> or some other low-level approach. This
section describes how to create a canvas, and work with
canvas items. It ends with a programming example.
</p>
<div class="SECT2">
<h2 class="SECT2">
<a name="SEC-CANVASPREPARE">Preparing the <tt class=
"CLASSNAME">GnomeCanvas</tt> Widget</a>
</h2>
<p>
The first decision you have to make is whether to use the
canvas in GDK mode or antialiased mode. When you create a
canvas widget, you must specify the mode you want; there
is no way to change it later. <tt class="FUNCTION">
gnome_canvas_new()</tt> creates a GDK canvas. <tt class=
"FUNCTION">gnome_canvas_new_aa()</tt> creates an
antialiased canvas. These are shown in <a href=
"z177.html#FL-CANVASCONSTRUCT">Figure 5</a>.
</p>
<p>
Sometimes it matters which visual and colormap the canvas
will use. In particular:
</p>
<ul>
<li>
<p>
In GDK mode, if you want to use the <span class=
"STRUCTNAME">GnomeCanvasImage</span> item to display
images, you must use Imlib's visual and colormap.
<span class="STRUCTNAME">GnomeCanvasImage</span> uses
Imlib to render images.
</p>
</li>
<li>
<p>
In antialiased mode, GDK's RGB buffer rendering
facilities (see <a href="z132.html#SEC-GDKRGB">the
section called <i>RGB Buffers</i> in the chapter
called <i>GDK Basics</i></a>) are used to copy the
RGB buffer to the screen. You must use the visual and
colormap from the GDK RGB module.
</p>
</li>
</ul>
<p>
To create a widget with a non-default visual and
colormap, <tt class="FUNCTION">
gtk_widget_push_visual()</tt> and <tt class="FUNCTION">
gtk_widget_push_colormap()</tt> are used. Here is the
code to create a GDK canvas that supports the image item:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget* canvas;
gtk_widget_push_visual(gdk_imlib_get_visual());
gtk_widget_push_colormap(gdk_imlib_get_colormap());
canvas = gnome_canvas_new();
gtk_widget_pop_visual();
gtk_widget_pop_colormap();
</pre>
</td>
</tr>
</table>
<p>
To create an antialiased canvas, do this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget* canvas;
gtk_widget_push_visual(gdk_rgb_get_visual());
gtk_widget_push_colormap(gdk_rgb_get_cmap());
canvas = gnome_canvas_new_aa();
gtk_widget_pop_colormap();
gtk_widget_pop_visual();
</pre>
</td>
</tr>
</table>
<div class="FIGURE">
<a name="FL-CANVASCONSTRUCT"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-CANVASCONSTRUCT.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <libgnomeui/gnome-canvas.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">GtkWidget* <tt class=
"FUNCTION">gnome_canvas_new</tt></code>(void);</code>
</p>
<p>
<code><code class="FUNCDEF">GtkWidget* <tt class=
"FUNCTION">
gnome_canvas_new_aa</tt></code>(void);</code>
</p>
</div>
<p>
<b>Figure 5. Canvas Constructors</b>
</p>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="Z178">Scroll Region</a>
</h3>
<p>
The canvas is practically infinite from a programmer's
standpoint; however, in reality your application
probably uses only a small area. When using the canvas
you must specify which region is interesting to the
user with <tt class="FUNCTION">
gnome_canvas_set_scroll_region()</tt> (<a href=
"z177.html#FL-CANVASSCROLLING">Figure 6</a>). The
scroll region is given in world coordinates. You can
query the scroll region with <tt class="FUNCTION">
gnome_canvas_get_scroll_region()</tt>.
</p>
<p>
To add scroll bars to the canvas, simply create a <tt
class="CLASSNAME">GtkScrolledWindow</tt> and add the
canvas to it:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
GtkWidget* sw;
sw = gtk_scrolled_window_new(NULL, NULL);
gtk_container_add(GTK_CONTAINER(sw), canvas);
</pre>
</td>
</tr>
</table>
<p>
If you want to implement scrolling via some mechanism
other than the scroll bars, you can get and set the
"scroll offsets." The scroll offsets are in canvas
pixel coordinates; they specify the top left visible
pixel. Remember that canvas pixel coordinates are
relative to the scroll region.
</p>
<div class="FIGURE">
<a name="FL-CANVASSCROLLING"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-CANVASSCROLLING.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <libgnomeui/gnome-canvas.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_set_scroll_region</tt></code>(GnomeCanvas*
<tt class="PARAMETER"><i>canvas</i></tt>, double
<tt class="PARAMETER"><i>x1</i></tt>, double <tt
class="PARAMETER"><i>y1</i></tt>, double <tt class=
"PARAMETER"><i>x2</i></tt>, double <tt class=
"PARAMETER"><i>y2</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_get_scroll_region</tt></code>(GnomeCanvas*
<tt class="PARAMETER"><i>canvas</i></tt>, double*
<tt class="PARAMETER"><i>x1</i></tt>, double* <tt
class="PARAMETER"><i>y1</i></tt>, double* <tt
class="PARAMETER"><i>x2</i></tt>, double* <tt
class="PARAMETER"><i>y2</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_scroll_to</tt></code>(GnomeCanvas* <tt
class="PARAMETER"><i>canvas</i></tt>, gint <tt
class="PARAMETER"><i>cx</i></tt>, gint <tt class=
"PARAMETER"><i>cy</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_get_scroll_offsets</tt></code>(GnomeCanvas*
<tt class="PARAMETER"><i>canvas</i></tt>, gint* <tt
class="PARAMETER"><i>cx</i></tt>, gint* <tt class=
"PARAMETER"><i>cy</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 6. Canvas Scrolling</b>
</p>
</div>
</div>
<div class="SECT3">
<h3 class="SECT3">
<a name="Z179">Zooming</a>
</h3>
<p>
The canvas gives you zooming "for free"; it is included
in the world-to-canvas and canvas-to-world coordinate
system conversions. You can set the zoom factor with
<tt class="FUNCTION">
gnome_canvas_set_pixels_per_unit()</tt> (<a href=
"z177.html#FL-CANVASZOOMING">Figure 7</a>). By default,
there ratio of pixels to canvas units is 1.0, meaning
no zoom. Specifying a value less than 1.0 means reduced
size; greater than 1.0 means increased size.
</p>
<p>
In antialiased mode, you could achieve the same visual
effect by applying a scaling affine transformation to
the root canvas group. The <span class="STRUCTNAME">
pixels_per_unit</span> member of the <span class=
"STRUCTNAME">GnomeCanvas</span> struct predates the
canvas's use of affines. Still, <tt class="FUNCTION">
gnome_canvas_set_pixels_per_unit()</tt> is a bit more
convenient than the affine transform method, and it
does work in GDK mode. (Because GDK mode uses Xlib
primitives, it's nontrivial to implement arbitrary
affine transformations; a future version of Gnome may
do so, however.)
</p>
<div class="FIGURE">
<a name="FL-CANVASZOOMING"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-CANVASZOOMING.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <libgnomeui/gnome-canvas.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_set_pixels_per_unit</tt></code>(GnomeCanvas*
<tt class="PARAMETER"><i>canvas</i></tt>, double
<tt class="PARAMETER"><i>ppu</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 7. Canvas Zooming</b>
</p>
</div>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z180">Canvas Items</a>
</h2>
<p>
Most of the time you will be interested in canvas items
rather than the canvas itself. Canvas items are typically
very easy to use, compared to widgets; none of the
standard items have any unique signals, since they are
not interactive. (Since <span class="STRUCTNAME">
GnomeCanvasItem</span> is a subclass of <span class=
"STRUCTNAME">GtkObject</span>, however, you could
certainly have an item with signals if you wanted to.)
The <span class="STRUCTNAME">GnomeCanvasItem</span> base
class has a single signal, <span class="SYMBOL">
"event"</span>, which is used to convey all types of
event. The <span class="SYMBOL">"event"</span> signal has
no default handler; canvas items do not respond to events
unless you connect handlers of your own. <a href=
"z177.html#FL-CANVASITEMS">Figure 8</a> lists all the
useful functions for working with the <span class=
"STRUCTNAME">GnomeCanvasItem</span> base class.
</p>
<p>
To create a canvas item, you use the generic <tt class=
"FUNCTION">gnome_canvas_item_new()</tt> (or <tt class=
"FUNCTION">gnome_canvas_item_newv()</tt>). This function
accepts the group to place the item in, the <span class=
"STRUCTNAME">GtkType</span> of the <span class=
"STRUCTNAME">GnomeCanvasItem</span> subclass to create,
and finally a NULL-terminated list of arguments to set.
The argument list is purely for convenience, so you don't
have to call <tt class="FUNCTION">
gnome_canvas_item_set()</tt> immediately. <tt class=
"FUNCTION">gnome_canvas_item_new()</tt> creates a new
instance of the type with <tt class="FUNCTION">
gtk_type_new()</tt>, adds the item to its <span class=
"STRUCTNAME">GnomeCanvasGroup</span>, and schedules it to
be redrawn.
</p>
<p>
To destroy an item and remove it from the canvas, simply
call <tt class="FUNCTION">gtk_object_destroy()</tt>. You
can also use the standard reference counting mechanism
with canvas items.
</p>
<p>
You can set an item's affine using <tt class="FUNCTION">
gnome_canvas_item_affine_absolute()</tt>, or compose a
new affine with the item's existing affine using <tt
class="FUNCTION">
gnome_canvas_item_affine_relative()</tt>. These functions
can be used to translate, scale, or rotate a canvas item
(however, scaling and rotation only work in antialiased
mode).
</p>
<p>
Items in a group are normally stacked in the order you
add them, with the most recently-added item "on top" and
the oldest item on the bottom. You can manipulate the
stacking order with <tt class="FUNCTION">
gnome_canvas_item_raise()</tt> and <tt class="FUNCTION">
gnome_canvas_item_lower()</tt>. These move an item up or
down by the given number of positions. It is safe to pass
in a too-large value for <span class="STRUCTNAME">
positions</span>; the item will be moved as far as
possible and no more. You can also request that an item
is moved to one extreme or the other, using <tt class=
"FUNCTION">gnome_canvas_item_raise_to_top()</tt> and <tt
class="FUNCTION">gnome_canvas_item_lower_to_bottom</tt>.
</p>
<p>
Items can be shown and hidden; hidden items are not
rendered by the canvas and do not receive events. All
items are visible by default. The routines are <tt class=
"FUNCTION">gnome_canvas_item_show()</tt> and <tt class=
"FUNCTION">gnome_canvas_item_hide()</tt>.
</p>
<p>
Reparenting a canvas item is straightforward; the only
rule is that the new group must be on the same canvas as
the old group.
</p>
<p>
<tt class="FUNCTION">gnome_canvas_item_grab_focus()</tt>
is analagous to <tt class="FUNCTION">
gtk_widget_grab_focus()</tt>; it sends all key events to
the item with the grab. It also sends focus change events
to the item (when the item gains or loses the focus).
</p>
<p>
Canvas items can grab and ungrab the mouse pointer just
as a <span class="STRUCTNAME">GdkWindow</span> can; the
arguments to <tt class="FUNCTION">
gnome_canvas_item_grab()</tt> are exactly analagous to
those of <tt class="FUNCTION">gdk_pointer_grab()</tt>
(see <a href="cha-gdk.html">the chapter called <i>GDK
Basics</i></a>). While a canvas item has the pointer
grabbed, no other item receives events. Behind the
scenes, <tt class="CLASSNAME">GnomeCanvas</tt> uses <tt
class="FUNCTION">gdk_pointer_grab()</tt> to implement <tt
class="FUNCTION">gnome_canvas_item_grab()</tt>, so an
item grabbing the mouse away from other items implies the
canvas grabbing the mouse away from other widgets.
</p>
<p>
The visual properties of canvas items are manipulated
almost entirely via object arguments. If you skipped <a
href="cha-objects.html">the chapter called <i>The GTK+
Object and Type System</i></a>, go back and read the
section on object arguments now. Two functions are used
to set canvas item properties: <tt class="FUNCTION">
gnome_canvas_item_set()</tt> and <tt class="FUNCTION">
gnome_canvas_item_setv()</tt>. These are almost but not
quite equivalent to <tt class="FUNCTION">
gtk_object_set()</tt> and <tt class="FUNCTION">
gtk_object_setv()</tt>---they set object arguments in the
same way, but they also mark the canvas item to be
redrawn. So you should prefer them to the <span class=
"STRUCTNAME">GtkObject</span> variants. (This is
something of a design bug, and future canvas versions
will most likely allow you to use <tt class="FUNCTION">
gtk_object_set()</tt>.)
</p>
<p>
<tt class="FUNCTION">
gnome_canvas_item_request_update()</tt> marks the canvas
item as "dirty" and queues it to be redrawn. Internally,
the canvas uses a one-shot idle function to perform
redraws; that is, it waits until no more GTK+ events are
pending, then redraws itself a single time. It does this
by installing an idle function with <tt class="FUNCTION">
gtk_idle_add()</tt> and removing it after it runs once.
Thus <tt class="FUNCTION">
gnome_canvas_item_request_update()</tt> can be called
many times without creating an efficiency problem---it
pretty much does nothing at all if an update is already
pending.
</p>
<div class="FIGURE">
<a name="FL-CANVASITEMS"></a>
<div class="FUNCSYNOPSIS">
<a name="FL-CANVASITEMS.SYNOPSIS"></a>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="FUNCSYNOPSISINFO">
#include <libgnomeui/gnome-canvas.h>
</pre>
</td>
</tr>
</table>
<p>
<code><code class="FUNCDEF">GnomeCanvasItem* <tt
class="FUNCTION">
gnome_canvas_item_new</tt></code>(GnomeCanvasGroup*
<tt class="PARAMETER"><i>parent</i></tt>, GtkType <tt
class="PARAMETER"><i>type</i></tt>, const gchar* <tt
class="PARAMETER"><i>first_arg_name</i></tt>, <tt
class="PARAMETER"><i>...</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">GnomeCanvasItem* <tt
class="FUNCTION">
gnome_canvas_item_newv</tt></code>(GnomeCanvasGroup*
<tt class="PARAMETER"><i>parent</i></tt>, GtkType <tt
class="PARAMETER"><i>type</i></tt>, guint <tt class=
"PARAMETER"><i>nargs</i></tt>, GtkArg* <tt class=
"PARAMETER"><i>args</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_set</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, const gchar*
<tt class="PARAMETER"><i>first_arg_name</i></tt>, <tt
class="PARAMETER"><i>...</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_setv</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, guint <tt
class="PARAMETER"><i>nargs</i></tt>, GtkArg* <tt
class="PARAMETER"><i>args</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_affine_relative</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, const double
<tt class="PARAMETER"><i>affine[6]</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_affine_absolute</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, const double
<tt class="PARAMETER"><i>affine[6]</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_raise</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, int <tt
class="PARAMETER"><i>positions</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_lower</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, int <tt
class="PARAMETER"><i>positions</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_raise_to_top</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_lower_to_bottom</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_show</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_hide</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_reparent</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>,
GnomeCanvasGroup* <tt class="PARAMETER"><i>
new_group</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_grab_focus</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">int <tt class="FUNCTION">
gnome_canvas_item_grab</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, unsigned int
<tt class="PARAMETER"><i>event_mask</i></tt>,
GdkCursor* <tt class="PARAMETER"><i>cursor</i></tt>,
guint32 <tt class="PARAMETER"><i>
etime</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_ungrab</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, guint32 <tt
class="PARAMETER"><i>etime</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_get_bounds</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>, double* <tt
class="PARAMETER"><i>x1</i></tt>, double* <tt class=
"PARAMETER"><i>y1</i></tt>, double* <tt class=
"PARAMETER"><i>x2</i></tt>, double* <tt class=
"PARAMETER"><i>y2</i></tt>);</code>
</p>
<p>
<code><code class="FUNCDEF">void <tt class=
"FUNCTION">
gnome_canvas_item_request_update</tt></code>(GnomeCanvasItem*
<tt class="PARAMETER"><i>item</i></tt>);</code>
</p>
</div>
<p>
<b>Figure 8. Using <span class="STRUCTNAME">
GnomeCanvasItem</span></b>
</p>
</div>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z181">Canvas Items and Events</a>
</h2>
<p>
The standard Gnome canvas items have only one signal,
<span class="SYMBOL">"event"</span>, which is emitted for
<i class="EMPHASIS">all</i> types of event. The canvas
widget preprocesses all GDK events that it receives, and
forwards some of them to canvas items. It also sythesizes
certain events. Remember that X sends events only to X
windows (<span class="STRUCTNAME">GdkWindow</span>s), and
canvas items do not have an associated <span class=
"STRUCTNAME">GdkWindow</span>. Thus the canvas widget
must act as intermediary. Here are some of the actions it
takes:
</p>
<ul>
<li>
<p>
Coordinates are automatically converted to canvas
world coordinates. For example, if a canvas item
receives an event of type <span class="STRUCTNAME">
GDK_BUTTON_PRESS</span>, the <span class=
"STRUCTNAME">x</span> and <span class="STRUCTNAME">
y</span> fields of the event will be in world
coordinates. (The raw event was received on the
canvas's <span class="STRUCTNAME">GdkWindow</span>
and thus had window coordinates.)
</p>
</li>
<li>
<p>
Enter/leave events are synthesized for canvas items
as the mouse pointer moves across the canvas.
</p>
</li>
<li>
<p>
Events are propagated up the canvas item hierarchy,
until some item's <span class="SYMBOL">"event"</span>
signal handler returns <span class="STRUCTNAME">
TRUE</span>. This works just as it does with <tt
class="CLASSNAME">GtkWidget</tt>; events are first
sent to the bottommost or leaf canvas item, and
eventually make it up to the root item.
</p>
</li>
<li>
<p>
Only user-generated events are sent to canvas items;
many events you might expect to receive on a <span
class="STRUCTNAME">GdkWindow</span>, such as expose
and configure events, are not forwarded to canvas
items.
</p>
</li>
</ul>
<p>
The canvas does this work behind the scenes, so item
events work intuitively and much like normal GDK events.
</p>
<p>
A canvas item event callback looks like this:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
static gint
item_event_callback(GnomeCanvasItem* item,
GdkEvent* event,
gpointer data)
{
switch (event->type) {
case GDK_BUTTON_PRESS:
break;
case GDK_MOTION_NOTIFY:
break;
case GDK_BUTTON_RELEASE:
break;
default:
break;
}
/* Returning FALSE propagates the event to parent items;
* returning TRUE ends event propagation.
*/
return FALSE;
}
</pre>
</td>
</tr>
</table>
<p>
Of course, a real callback would probably examine the
contents of the event and take some action in response to
some of them.
</p>
</div>
<div class="SECT2">
<h2 class="SECT2">
<a name="Z182">A Canvas Example</a>
</h2>
<p>
This section gives a brief example program, demonstrating
the user of the canvas. It does not explain the
particulars of the canvas items being created; see <a
href="sec-itemreference.html">the section called <i>
Standard Canvas Item Reference</i></a> for that. <a href=
"z177.html#FIG-CANVAS-EXAMPLE">Figure 9</a> shows the
example program in action. You can drag canvas items
around the screen with the left mouse button; clicking an
item with the Shift key held down destroys it.
</p>
<div class="FIGURE">
<a name="FIG-CANVAS-EXAMPLE"></a>
<p>
<img src="figures/canvas-example.png">
</p>
<p>
<b>Figure 9. Simple <tt class="CLASSNAME">
GnomeCanvas</tt> program</b>
</p>
</div>
<p>
Here is the code to create an antialiased canvas. Notice
the call to <tt class="FUNCTION">gdk_rgb_init()</tt>;
notice that the canvas's scroll region is set; finally,
notice that the GdkRGB colormap and visual are pushed
when creating the canvas.
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
#include <gnome.h>
static gint delete_event_cb(GtkWidget* window, GdkEventAny* e, gpointer data);
static void create_canvas_items(GtkWidget* canvas);
int
main(int argc, char* argv[])
{
GtkWidget* window;
GtkWidget* sw;
GtkWidget* canvas;
gnome_init("canvas-example", "0.0", argc, argv);
gdk_rgb_init();
window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
gtk_window_set_title(GTK_WINDOW(window), "Canvas Example");
gtk_window_set_policy(GTK_WINDOW(window), TRUE, TRUE, TRUE);
gtk_signal_connect(GTK_OBJECT(window),
"delete_event",
GTK_SIGNAL_FUNC(delete_event_cb),
NULL);
sw = gtk_scrolled_window_new(NULL, NULL);
gtk_scrolled_window_set_policy(GTK_SCROLLED_WINDOW(sw),
GTK_POLICY_AUTOMATIC,
GTK_POLICY_AUTOMATIC);
gtk_widget_push_visual(gdk_rgb_get_visual());
gtk_widget_push_colormap(gdk_rgb_get_cmap());
canvas = gnome_canvas_new_aa();
gtk_widget_pop_colormap();
gtk_widget_pop_visual();
gnome_canvas_set_scroll_region(GNOME_CANVAS(canvas), 0, 0, 600, 450);
create_canvas_items(canvas);
gtk_container_add(GTK_CONTAINER(sw), canvas);
gtk_container_add(GTK_CONTAINER(window), sw);
gtk_window_set_default_size(GTK_WINDOW(window), 300, 300);
gtk_widget_show_all(window);
gtk_main();
return 0;
}
static gint
delete_event_cb(GtkWidget* window, GdkEventAny* e, gpointer data)
{
gtk_main_quit();
return FALSE;
}
</pre>
</td>
</tr>
</table>
<p>
Once the canvas has been created, the program adds some
items to it, and connects a simple callback to the item's
<span class="SYMBOL">"event"</span> signal. Here's the
code:
</p>
<table border="0" bgcolor="#E0E0E0" width="100%">
<tr>
<td>
<pre class="PROGRAMLISTING">
static gint
item_event(GnomeCanvasItem *item, GdkEvent *event, gpointer data)
{
static double x, y;
double new_x, new_y;
GdkCursor *fleur;
static int dragging;
double item_x, item_y;
item_x = event->button.x;
item_y = event->button.y;
gnome_canvas_item_w2i(item->parent, &item_x, &item_y);
switch (event->type)
{
case GDK_BUTTON_PRESS:
switch(event->button.button)
{
case 1:
if (event->button.state & GDK_SHIFT_MASK)
{
gtk_object_destroy(GTK_OBJECT(item));
}
else
{
x = item_x;
y = item_y;
fleur = gdk_cursor_new(GDK_FLEUR);
gnome_canvas_item_grab(item,
GDK_POINTER_MOTION_MASK |
GDK_BUTTON_RELEASE_MASK,
fleur,
event->button.time);
gdk_cursor_destroy(fleur);
dragging = TRUE;
}
break;
default:
break;
}
break;
case GDK_MOTION_NOTIFY:
if (dragging && (event->motion.state & GDK_BUTTON1_MASK))
{
new_x = item_x;
new_y = item_y;
gnome_canvas_item_move(item, new_x - x, new_y - y);
x = new_x;
y = new_y;
}
break;
case GDK_BUTTON_RELEASE:
gnome_canvas_item_ungrab(item, event->button.time);
dragging = FALSE;
break;
default:
break;
}
return FALSE;
}
static void
setup_item(GnomeCanvasItem *item)
{
gtk_signal_connect(GTK_OBJECT(item), "event",
(GtkSignalFunc) item_event,
NULL);
}
static void
create_canvas_items(GtkWidget* canvas)
{
GnomeCanvasPoints* points;
GnomeCanvasGroup* group;
GnomeCanvasItem* item;
double affine[6];
group = gnome_canvas_root(GNOME_CANVAS(canvas));
/* A polygon */
points = gnome_canvas_points_new(14);
points->coords[0] = 270.0;
points->coords[1] = 330.0;
points->coords[2] = 270.0;
points->coords[3] = 430.0;
points->coords[4] = 390.0;
points->coords[5] = 430.0;
points->coords[6] = 390.0;
points->coords[7] = 330.0;
points->coords[8] = 310.0;
points->coords[9] = 330.0;
points->coords[10] = 310.0;
points->coords[11] = 390.0;
points->coords[12] = 350.0;
points->coords[13] = 390.0;
points->coords[14] = 350.0;
points->coords[15] = 370.0;
points->coords[16] = 330.0;
points->coords[17] = 370.0;
points->coords[18] = 330.0;
points->coords[19] = 350.0;
points->coords[20] = 370.0;
points->coords[21] = 350.0;
points->coords[22] = 370.0;
points->coords[23] = 410.0;
points->coords[24] = 290.0;
points->coords[25] = 410.0;
points->coords[26] = 290.0;
points->coords[27] = 330.0;
item = gnome_canvas_item_new(group,
gnome_canvas_polygon_get_type (),
"points", points,
"fill_color", "tan",
"outline_color", "black",
"width_units", 3.0,
NULL);
setup_item(item);
gnome_canvas_points_unref(points);
/* Translate the polygon */
art_affine_translate(affine, -150.0, -300.0);
gnome_canvas_item_affine_relative(item, affine);
/* A translucent rectangle */
setup_item (gnome_canvas_item_new (group,
gnome_canvas_rect_get_type(),
"x1", 90.0,
"y1", 40.0,
"x2", 180.0,
"y2", 100.0,
"fill_color_rgba", 0x3cb37180,
"outline_color", "black",
"width_units", 4.0,
NULL));
/* A translucent ellipse */
setup_item (gnome_canvas_item_new (group,
gnome_canvas_ellipse_get_type(),
"x1", 210.0,
"y1", 80.0,
"x2", 280.0,
"y2", 140.0,
"fill_color_rgba", 0x5f9ea080,
"outline_color", "black",
"width_pixels", 0,
NULL));
/* Create ellipses arranged in a line; they're manipulated as a
single item. */
group =
GNOME_CANVAS_GROUP (gnome_canvas_item_new (group,
gnome_canvas_group_get_type(),
"x", 0.0,
"y", 0.0,
NULL));
setup_item(GNOME_CANVAS_ITEM(group));
{
double xpos = 20.0;
while (xpos < 300.0)
{
gnome_canvas_item_new(group,
gnome_canvas_ellipse_get_type(),
"x1", xpos,
"y1", 100.0,
"x2", xpos + 10.0,
"y2", 110.0,
"fill_color_rgba", 0x0000FFFF,
"outline_color_rgba", 0xFF,
NULL);
xpos += 15.0;
}
}
}
</pre>
</td>
</tr>
</table>
</div>
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