Question

I was using the drawing code from this tutorial to let someone draw in a small box: http://www.ifans.com/forums/showthread.php?t=132024

The default code draws on the entire screen, how do you limit the drawing to a small box on the screen?

Answer 1

In this examples the touches and the drawing happen in the main view of the view controller:

UIGraphicsBeginImageContext(self.view.frame.size);
[drawImage.image drawInRect:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height)];

etc…

So if you change the frame of that view to a smaller size, the drawing should be limited to the smaller size automatically.

Or you could “move” the touch recognition and the drawing to a another smaller subview:

1. Create a new smaller subview.
2. Add it to the main view.
3. Add a UITapGestureRecognizer to the small view
4. Handle the touches in the UITapGestureRecognizer action and do the drawing on the subview.

===== EDIT =====

Here is how to do it:

- (void)viewDidLoad{
   [super viewDidLoad];
   self.view.backgroundColor = [UIColor grayColor];

   canvas = [[UIView alloc] initWithFrame:CGRectMake(200.0, 200.0, 300.0, 300.0)];
   canvas.backgroundColor = [UIColor whiteColor];
   [self.view addSubview:canvas];

   UIImageView *iv = [[UIImageView alloc] initWithImage:nil];
   iv.frame = canvas.frame;
   [self.view addSubview:iv];
   self.drawImage = iv;
   [iv release];

   UIPanGestureRecognizer *panRec = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(userPanned:)];
   [canvas addGestureRecognizer:panRec];    
   [panRec release];
}


- (void)userPanned:(UIPanGestureRecognizer *)recognizer {
   CGPoint touchPoint = [recognizer locationInView:canvas];

   if (recognizer.state == UIGestureRecognizerStateBegan) lastTouchPoint = touchPoint;

      if (CGRectContainsPoint(CGRectMake(0.0, 0.0, canvas.frame.size.width, canvas.frame.size.height), touchPoint)) {
         UIGraphicsBeginImageContext(canvas.frame.size);
         [drawImage.image drawInRect:CGRectMake(0.0, 0.0, canvas.frame.size.width, canvas.frame.size.height)];
         CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);
         CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 5.0);
         CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), 1.0, 0.0, 0.0, 1.0);
         CGContextBeginPath(UIGraphicsGetCurrentContext());
         CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastTouchPoint.x, lastTouchPoint.y);
         CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), touchPoint.x, touchPoint.y);
         CGContextStrokePath(UIGraphicsGetCurrentContext());
         drawImage.image = UIGraphicsGetImageFromCurrentImageContext();
         UIGraphicsEndImageContext();

         lastTouchPoint = touchPoint;
     }
}

canvas, lastTouchPoint and drawImage are iVars.

I did not implement the doubleTap and Tap routines to clear the image and to draw a point, but this should be enough to get you started.