One of the clients wanted in the app a little feature for a menu system, something like a karusel, where specific menu items will be scrolled from side to side with a touch in an infinite loop.

I found out that it can be possible to do with a simple tweak to UIScrollView, handing the

scrollViewDidEndDecelerating from the UIScrollViewDelegate.

Guys from Monotouch team made the great job on this (as usual) and we have the delegate already available via built-in events, in this case DecelerationEnded.

So lets have a look at the implementation of the UIViewController class of some view, note we are adding the UI from the code, not from the nib file, just for simplicity. View has UIScrollView item and loads some images, last image is placed as the first one, then all images in the order and then first image as the last one.

Then the event for DecelerationEnded is handled to actually swap the position (fast - no animation) so user does not find out. For added more touch, the paging is enabled and of course the scroller is hidden, so it is not visible to the user where in the scrolling position he actually is.

If you ever needed to set the status bar hidden in Monotouch App, and wanted to do it properly for code that runs backward compatible on OS 3.X while developing on iOS SDK 4.X and also for newer devices, here is the answer:

if (UIApplication.SharedApplication.RespondsToSelector (new Selector ("setStatusBarHidden: withAnimation:")))
	UIApplication.SharedApplication.SetStatusBarHidden (true, UIStatusBarAnimation.Fade);
else
	UIApplication.SharedApplication.SetStatusBarHidden (true, true);
 

Few weeks ago I wrote a post about how to handle tap gesture on iOS prior gesture recognizers. In Objective-C there is simple trick with [self.nextResponder ...] and then [NSObject cancelPreviousPerformRequestWithTarget ...].

In MonoTouch NextResponder on Controller object is read only for some reason, and therefore cannot be used to implement this technique in a simple way. So a bit more coding is needed.

It is a common issue in MonoTouch that function performSelector with delay attribute is not working out of the box, you need to fiddle with the NSRunLoop as per the comment from Geoff Norton from Novell:

PerformSelector with delay works fine, given that you understand its
restrictions.  It only works on threads with a NSRunLoop, and is completed
async.

So what to do, and do not want to or cannot get into the NSRunLoop "issue"?

Sometimes the image provided by the UIImagePicker is not the right size, and putting it to the UIImageView is not enough as we do need to know the size it was being fit to.

When implementing touch and gesture based interaction in your application, for example the swipe effect, you had to handle the TouchesMoved, TouchesBegan, TouchesEnded and TouchesCancelled events and handle the location of the finger and last location and so on, based on the quality of the algorithm it worked or not.

With the new SDK and with the great support from MonoTouch team, its current version of MonoTouch implements the UIGestureRecognizer and UIGestureRecognizerDelegate and related classes - UITapGestureRecognizer, UISwipeGestureRecognizer, UIRotationGestureRecognizer, UIPinchGestureRecognizer, UILongPressGestureRecognizer, UIPanGestureRecognizer. Lets have a look and few simple steps to take advantage of these common gesture implementations on the iPhone/iPad. For the demonstration we will use the swipe gesture.

First we need to have a view controller and define a Selector for the gesture recognizer. More on Selectors check the iPhone SDK, in general it is a special form of message. In MonoTouch Selectors can be consumed (fired from MonoTouch code) or exposed, being available to SDK functions and classes to be called from within.

Lets have a look at the typical implementation:

1
2
3
4
5
6
7
8
9
10
11
12
13

public partial class MyViewController : UIViewController
	{
		public static Selector MySelector
		{
			get 
			{
				return new Selector("HandleSwipe");
			}
		}
///
/// and other class definition
///
	}

Please note that the Selector is defined as a static access property returning the identification of the selector via its name (HandleSwipe).

Next we need to define the gesture recognizer and its delegate and assign it to the view. We do this in the ViewDidLoad event of the controller implementation class.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16

// define the gesture recognizer 
MonoTouch.UIKit.UISwipeGestureRecognizer sgr = new MonoTouch.UIKit.UISwipeGestureRecognizer();
 
// add the target to it, we put the instance itself of the controller 
// and the class instance selector
sgr.AddTarget(this, MainComicsViewController.MySelector);

// add the swipe direction, there are 4 of them (left, right, up, down). If other than one swipe is 
// needed then more recognizers must be defined and added to the view - each for the direction
sgr.Direction = UISwipeGestureRecognizerDirection.Left; 

// also assign the delegate
sgr.Delegate = new SwipeRecognizerDelegate();
 
// and last, add the recognizer to this view to take actions
this.View.AddGestureRecognizer(sgr);

Then we need our delegate class for the swipe recognizer:

1
2
3
4
5
6
7

public class SwipeRecognizerDelegate : MonoTouch.UIKit.UIGestureRecognizerDelegate
	{
		public override bool ShouldReceiveTouch(UIGestureRecognizer recognizer, UITouch touch)
		{
			return true;
		}
	}

And last and finally our handler for the swipe event:

1
2
3
4
5
6
7
8

[Export("HandleSwipe")]
public void HandleSwipe(UISwipeGestureRecognizer recognizer)
{
	// get the point of the swipe action
	PointF point = recognizer.LocationInView(this.View);
 
	// TODO: do something with the swipe
}

Well, that is all and we are done.

Happy coding!