drawRect not responding - iphone

Question

I am trying to draw a pentagon with a set of points storing in a NSArrary, however, the view is empty without any error...
-(void)prepareVertex:(int)noOfVertex
{
polygonPoint=[NSMutableArray arrayWithCapacity:noOfVertex];
for(int i=0;i<noOfVertex;i++)
{
CGPoint point=CGPointMake(sin((2*M_PI)*i/noOfVertex),(cos(2*M_PI)*i/noOfVertex));
[polygonPoint addObject:[NSValue valueWithCGPoint:point]];
NSValue *tempVal=[polygonPoint objectAtIndex:i];
CGPoint tempPoint=[tempVal CGPointValue];
NSLog(#"%f,%f",tempPoint.x,tempPoint.y);
}
}
- (void)drawRect:(CGRect)rect
{
[self prepareVertex:5];
for (int i=0; i<5; i++) {
NSValue *tempVal=[polygonPoint objectAtIndex:i];
CGPoint tempPoint=[tempVal CGPointValue];
}
CGContextRef context=UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 5);
CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
CGContextMoveToPoint(context, [[polygonPoint objectAtIndex:0] CGPointValue].x, [[polygonPoint objectAtIndex:0] CGPointValue].x);
for (int i=1; i<5; i++) {
CGPoint point=[[polygonPoint objectAtIndex:i] CGPointValue];
CGContextAddLineToPoint(context,point.x, point.y);
}
CGContextStrokePath(context);
}
Can anyone tell me whats going on??

Answer 1

Well, the first think to note is that you use black to stroke the polygon, so if your view background is black too, you can't see any thing.
Then, and I think this is the real problem, sin(x) and cos(x) are always between -1 and 1, so the points you generate in:
CGPoint point=CGPointMake(sin((2*M_PI)*i/noOfVertex),(cos(2*M_PI)*i/noOfVertex));
are all located in the rectangle CGrectMake(-1, -1, 2, 2). And that area of your view is very probably hidden by the status bar.
So, if the coordinates you generated are what you are looking for are correct, you may try to remove the status bar or change the coordinates of your view. But I think What you should really do is change the previous line into something like this:
CGFloat x = centerPoint.x + radius * sin(2*M_PI*i/noOfVertex);
CGFloat y = centerPoint.y + radius * cos(2*M_PI*i/noOfVertex);
CGPoint point = CGPointMake(x, y);

Answer 2

It draws something in the upper left corner of the view. The figure is just too small to see easily. That's because the values in polygonPoint (btw you should at least call the array polygonPoints) are between -1.0 and 1.0. You have to translate your points to the center of the view and scale the size accordingly.
Something like
- (void)drawRect:(CGRect)rect {
[self prepareVertex:5];
CGContextRef context=UIGraphicsGetCurrentContext();
CGContextSetLineWidth(context, 5);
CGContextSetStrokeColorWithColor(context, [UIColor blackColor].CGColor);
CGFloat x = self.center.x + self.bounds.size.width * 0.5 * [[polygonPoint objectAtIndex:0] CGPointValue].x;
CGFloat y = self.center.y + self.bounds.size.height * 0.5 * [[polygonPoint objectAtIndex:0] CGPointValue].y;
NSLog(#"Point %f/%f", x, y);
CGContextMoveToPoint(context, x, y);
for (int i=1; i<5; i++) {
CGPoint point=[[polygonPoint objectAtIndex:i] CGPointValue];
x = self.center.x + self.bounds.size.width * 0.5 * point.x;
y = self.center.y + self.bounds.size.height * 0.5 * point.y;
NSLog(#"Point %f/%f", x, y);
CGContextAddLineToPoint(context, x, y);
};
CGContextStrokePath(context);
}
does the trick.