id	title	brief	sdk	dateupdated
92FBA356-314E-4971-8E64-8F31379422D1	Drag and Rotate an Image	This recipe shows how to use gesture recognizers to drag and rotate an image.	title url UIRotateGestureRecognizer http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIRotateGestureRecognizer_Class/Reference/Reference.html title url UIPanGestureRecognizer http://developer.apple.com/library/ios/#documentation/UIKit/Reference/UIPanGestureRecognizer_Class/Reference/Reference.html	2017-06-09

Recipe

The following code assigns a UIRotationGestureRecognizer and a UIPanGestureRecognizer to a UIImageView, with implementations that enable image dragging and rotation via touch:

UIRotationGestureRecognizer rotateGesture;
UIPanGestureRecognizer panGesture;
UIImageView imageView;

public override void ViewDidLoad ()
{
  base.ViewDidLoad ();

  nfloat r = 0;
  nfloat dx = 0;
  nfloat dy = 0;

  using (var image = UIImage.FromFile ("monkey.png")) {
    imageView = new UIImageView (image){Frame = new CoreGraphics.CGRect (new PointF(0,0), image.Size)};
    imageView.UserInteractionEnabled = true;
    View.AddSubview (imageView);
  }

  rotateGesture = new UIRotationGestureRecognizer (() => {
    if ((rotateGesture.State == UIGestureRecognizerState.Began || rotateGesture.State == UIGestureRecognizerState.Changed) && (rotateGesture.NumberOfTouches == 2)) {

      imageView.Transform = CGAffineTransform.MakeRotation (rotateGesture.Rotation + r);
    } else if (rotateGesture.State == UIGestureRecognizerState.Ended) {
      r += rotateGesture.Rotation;
    }
  });

  panGesture = new UIPanGestureRecognizer (() => {
    if ((panGesture.State == UIGestureRecognizerState.Began || panGesture.State == UIGestureRecognizerState.Changed) && (panGesture.NumberOfTouches == 1)) {

      var p0 = panGesture.LocationInView (View);

      if (dx == 0)
        dx = p0.X - imageView.Center.X;

      if (dy == 0)
        dy = p0.Y - imageView.Center.Y;

      var p1 = new CGPoint (p0.X - dx, p0.Y - dy);

      imageView.Center = p1;
    } else if (panGesture.State == UIGestureRecognizerState.Ended) {
      dx = 0;
      dy = 0;
    }
  });

  imageView.AddGestureRecognizer (panGesture);
  imageView.AddGestureRecognizer (rotateGesture);

  View.BackgroundColor = UIColor.White;
}

When the user touches the image with a single touch, the code in the UIPanGestureRecognizer moves the image to track the touch point as it changes. Likewise, when the user touches the image with two fingers and rotates, the image rotates as well thanks to the code in the UIRotationGestureRecognizer.

Additional Information

Gesture recognizers allow touch support to be encapsulated in reusable classes that can be applied to any UIView, such as the UIImageView shown in this recipe. There are several different types of gesture recognizers provided by iOS. Additionally, iOS allows custom recognizers to be defined.