Hit detection between a line and a UIView?

Question

Hi all,
I'm trying to figure out if a UIView I've got on the screen is intersecting a line I'm drawing.
Here's the code for drawRect:
-(void)drawRect:(CGRect)rect
// Get the current graphics context and clear it
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextClipToRect(ctx, CGRectMake(0, 0, 240, 320));
CGContextClearRect(ctx, rect);
// The line style
CGContextSetLineWidth(ctx, 3.0);
CGContextSetLineCap(ctx, kCGLineCapRound);
CGContextSetRGBStrokeColor(ctx, 1.0, 1.0, 1.0, 1.0);
// Create the line
CGContextMoveToPoint(ctx, source.x, source.y);
CGContextAddLineToPoint(ctx, dest.x, dest.y);
// Draw the line
CGContextDrawPath(ctx, kCGPathStroke);
CGContextStrokePath(ctx);
That draws a line from and to wherever I need it, but when I draw it over a UIView that's already on the screen, I need to know if the centre point of the UIView is on that line. So, if you have a line going from 0,0 to 10,10, you have points at 1,1 2,2 3,3 ... 10,10. If the centre point of the UIView is at, say, 3,3, it would be on that line.
I've seen use of the CGContextPathContainsPoint method, but I can't get it working - my line doesn't draw. Here's the code for drawRect:
-(void)drawRect:(CGRect)rect
// Get the current graphics context and clear it
CGContextRef ctx = UIGraphicsGetCurrentContext();
CGContextClipToRect(ctx, CGRectMake(0, 0, 240, 320));
CGContextClearRect(ctx, rect);
// The line style
CGContextSetLineWidth(ctx, 3.0);
CGContextSetLineCap(ctx, kCGLineCapRound);
CGContextSetRGBStrokeColor(ctx, 1.0, 1.0, 1.0, 1.0);
// Create the path
CGMutablePathRef linePath = CGPathCreateMutable();
// Move to the source point and add a line to the destination point
CGPathMoveToPoint(linePath, NULL, source.x, source.y);
CGPathAddLineToPoint(linePath, NULL, dest.x, dest.y);
// Draw the line
CGPathDrawingMode(kCGPathStroke);
CGContextDrawPath(ctx, kCGPathStroke);
CGContextStrokePath(ctx);
// Save the graphics state before doing the hit detection
CGContextSaveGState(ctx);
CGPathRef path = linePath;
CGContextAddPath(ctx, path);
if(CGContextPathContainsPoint(ctx, baddie, kCGPathStroke)) {
NSLog(@"Yes");
CGContextRestoreGState(ctx);
I'm clearly doing something wrong here, but I can't quite figure it out.
Any help much appreciated!

Answer 1

I'm tearing my hair out with this. This is simple maths!!! Every point I put into the formula works fine, but when it's in code it fails. I think it's something to do with the frame I'm using...
This is set in the init method:
CGRect frame = CGRectMake(40.0, 60.0, 240.0, 320.0);
The source and destination of the line I'm drawing, and the point I want to check (myPoint) are passed in and set up as follows (in their respective setter methods):
source = CGPointMake(pos.x - 40.0, pos.y - 60.0);
dest = CGPointMake(pos.x - 40.0, pos.y - 60.0);
myPoint = CGPointMake(pos.x - 40.0, pos.y - 60.0);
They have 40 and 60 pixels removed as the co-ords I'm passing in are the location on the iPhone screen (320x480). I have to subtract those values to get the top-left corner of the frame I'm drawing into, i.e. 40, 60 on the screen is actually 0, 0 of the frame.
Now I draw the line in the drawRect method:
CGMutablePathRef linePath = CGPathCreateMutable();
CGPathMoveToPoint(linePath, NULL, source.x, source.y);
CGPathAddLineToPoint(linePath, NULL, dest.x, dest.y);
And here's where I check whether the point has touched the line:
float m = (source.y - dest.y) / (source.x - dest.x);
float y = (m * (source.x - myPoint.x)) + myPoint.y;
if((y >= source.y - 3) && (y <= source.y + 3) && (myPoint.x >= source.x) && (myPoint.x <= dest.x)) // ±3 pixels for tolerance
NSLog(@"Point hit the line!");
What am I doing wrong?!
The whole premise of my game rests on this one simple little problem. I've spent two weeks writing the rest of it, and a week writing THIS one part! Grrr.
Any help much appreciated. If I can't get this done within a week, pop along to London and see a grown man cry.