Drawing simple lines on iPhone with CoreGraphics

Question

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.

Any ideas?

- (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);
}

Answer 1

The complete code is as below.

/* Set the color that we want to use to draw the line */ 
[[UIColor brownColor] set];
/* Get the current graphics context */ 
CGContextRef currentContext =UIGraphicsGetCurrentContext();
/* Set the width for the line */
CGContextSetLineWidth(currentContext,5.0f);
/* Start the line at this point */ 
CGContextMoveToPoint(currentContext,50.0f, 10.0f);
/* And end it at this point */ 
CGContextAddLineToPoint(currentContext,100.0f, 200.0f);
/* Use the context's current color to draw the line */
CGContextStrokePath(currentContext);