title: Drawing simple lines on iPhone with CoreGraphics
body: <p>I would like to draw a straight line between where a user touches the screen, and where the touch ends. i need multiple lines, for if the user repeats the touch-drag-release action, and I also need a button to clear all of the lines. So far I have this code below, but once it is called again, I receive the errors: CGContextSetStrokeColor: invalid context 0x0. This error repeats for: CGContextBeginPath, CGContextMoveToPoint, CGContextAddLineToPoint, CGContextDrawPath.</p>
  
  <p>Any ideas?</p>
  
  <pre><code>- (void)drawRect:(CGRect)rect {   
      c = UIGraphicsGetCurrentContext();
  
      CGFloat black[4] = {0, 0, 
                          0, 1};
      CGContextSetStrokeColor(c, black);
      CGContextBeginPath(c);
      CGContextMoveToPoint(c, 100, 100);
      CGContextAddLineToPoint(c, 100, 200);
      CGContextStrokePath(c);
  }
  </code></pre>