Graphics Reference
In-Depth Information
to
True
, which says that we've processed the button click, and no other part of our
code needs to do anything about it.
A more complex example of event handling is demonstrated by the handler
for a change in the value of the slider, namely
slider1change
, which is shown
in Listing 4.7. As you can see, we print a message indicating the new value
(
e.NewValue
) that was passed in, and then adjust the
x
-coordinate of the triangle's
first point. Rather than removing and reinserting the point, we simply reassign the
entire
Points
array, since there are only three points. This causes the triangle to
be marked as “changed,” which provokes a redisplay of the entire canvas, with the
result that as we adjust the slider with the mouse, the top of the triangle moves
fromsidetoside.
Listing 4.7: The
slider1change
method that moves a triangle vertex.
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2
3
4
5
6
7
8
9
10
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14
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void slider1change(
object sender,
RoutedPropertyChangedEventArgs<double> e)
{
Debug.Print(
"Slider changed, ready = "
+ ready +
", and val = "
+ e.NewValue +
".
\
n"
);
e.Handled = true;
if (ready)
{
PointCollection p = myTriangle.Points.Clone();
Debug.Print(myTriangle.Points.ToString());
p[0].X = e.NewValue;
myTriangle.Points = p;
}
}
What about the flag called
ready
in lines 19 and 50 of Listing 4.5, and line 8
of Listing 4.7? Well, in the course of creating the
Window1
in our application,
WPF creates the slider and sets its initial value to the predetermined value in
the XAML code. That in turn raises a
ValueChanged
event,
6
which makes WPF
invoke the
slider1change
method. But all this happens in the course of the
InitializeComponent
process, so the triangle has not yet been created. If we try
to alter the coordinates of one of its
Point
s, that'll be a bug. We therefore ignore
all events like this until the
Window1
is ready, as indicated by the
ready
flag.
Inline Exercise 4.5:
Modify the test bed so that it displays a diamond-shaped
object rather than a triangle, and make the slider adjust the
x
-coordinate of both
the top and bottom vertices at the same time (and in the same direction). Add
a second slider, and make it vary the
y
-coordinate of the left and right vertices
of the diamond too.
The test bed also supports several kinds of geometry besides polygons. These
include dots, which are like points but are displayed and have a tool-tip that says
where they're located; arrows (used for depicting vectors) and quivers, which are
collections of arrows; segments, which are line segments between either points or
6. The value has changed from “undefined” to the specified initial value.
