Build Your Game: Vector Actors and Particles - Beginning iOS6 Games Development - page 165

To actually change the color of the pixels of the UIView being drawn, we call CGContextFillRect
passing in our context and the CGRect backgroundArea . To draw the foreground rectangle, we repeat
the process, but the width of our top rectangle, healthArea , is calculated based the value of the
property percent . This indicates that every time the percent of the HealthBar changes, we have
to redraw the HealthBar . To make sure this happens, we have to provide our own implementation
of setPercent: instead of relying on the default implementation that would be provided by a
synthesized property. Listing 7-12 shows the custom implementation of setPercent: for the class
HealthBar .
Listing 7-12. HealthBar.m (setPercent:)
-(void)setPercent:(float)aPercent{
if (percent != aPercent){
percent = aPercent;
if (percent < 0){
percent = 0;
}
if (percent > 1){
percent = 1;
}
[self setNeedsViewUpdated:YES];
}
setPercent: . First we check to see if the values of
percent (the backing variable for the property) and the passed in value aPercent have different values.
If they do, we set percent to aPercent and we normalize the values by ensuring it is between the
values 0 and 1. Lastly, we call setNeedsViewUpdate: with YES . This ensures that the HealthBar will be
redrawn during the next step in the animation. Let's now take a look at how the Bullet class is drawn.
Drawing the Bullet Class
We have looked at the class HealthBar and inspected how it is drawn. We only used a single
drawing function, CGContextFillRect , to create our visual effect. Of course, there are many, many
more drawing functions that allow you to create any visual effect you want. We are not going to
cover all of these functions, but there is fantastic documentation available online. That being said,
let's take a look at the drawing implementation of the class Bullet . See Listing 7-13.
Listing 7-13. Bullet.m (drawRect:WithContext:InRect:)
-(void)drawActor:(Actor*)anActor WithContext:(CGContextRef)context InRect:(CGRect)rect{
CGContextAddEllipseInRect(context, rect);
CGContextClip(context);
CGFloat locations[2];
locations[0] = 0.0;
locations[1] = 1.0;