Touch Events and UIResponder - iOS Programming: The Big Nerd Ranch Guide - page 548

Line *line = [linesInProcess objectForKey:v];
CGContextMoveToPoint(context, [line begin].x, [line begin].y);
CGContextAddLineToPoint(context, [line end].x, [line end].y);
CGContextStrokePath(context);
}
}
Finally, write a method that will clear the collections and redraw the view in
TouchDrawView.m .
- (void)clearAll
{
// Clear the collections
[linesInProcess removeAllObjects];
[completeLines removeAllObjects];
// Redraw
[self setNeedsDisplay];
}