Wednesday, April 14, 2010

Gesture recoginization in iPhone SDK

For many magazine applications running on iPad, you easily see that the application detect your gestures for interactions on the content: rotate, swipe right to left (or left to right), tap. If we have to handle these gestures manually by code, it is not efficient.

So, Apple tries to make our developer life easier to breath by providing us a built-in mechanism to detect these common gestures on the latest iPhone SDK version.

You can find these APIs from UIGestureRecognizer class.
"A gesture-recognizer object (or, simply, a gesture recognizer) decouples the logic for recognizing a gesture and acting on that recognition. When one of these objects recognizes a common gesture or, in some cases, a change in the gesture, it sends an action message to each designated target object" - from Apple API documentation.

This class derives to:
1. UITapGestureRecognizer: presents the gesture that p fingers tapped the screen q times consecutively.
  • Input properties: numberOfTaps (q), numberOfFingers (p)
  • Output properties: location, touches
2. UISwipeGestureRecognizer: represents the gesture that p fingers tapped the screen q times consecutively.

3. UIRotationGestureRecognizer: looks for rotation gestures involving two touches. When the user moves the fingers opposite each other in a circular motion, the underlying view should rotate in a corresponding direction and speed.

4. UIPinchGestureRecognizer: represents the gesture that 2 fingers moving towards or away from a center point. This gesture is usually used for scaling a view.
  • Input properties: scaleThreshold
  • Output properties: scale, velocity, anchorPoint, transform
scaleThreshold is the critical scale the fingers must first move apart/together before this recognizer could fire.

5. UILongPressGestureRecognizer: represents the gesture that p fingers moved/stayed on the screen continuously for T seconds.
  • Input properties: numberOfFingers (p), delay (T), allowableMovement
  • Output properties: touches, centroid, startPoint
6. UIDragRecognizer: represents the gesture that 1 or more fingers moving on the screen for a distance of d. Optionally it may also be constrained to moving only within an angle of θ ± Δθ.
  • Input properties: minimumDistance (d), angle (θ), maximumDeviation (Δθ), restrictsToAngle
  • Output properties: touch, startPosition, startAngle
Note that the angles are in radians.
7. UIPanGestureRecognizer: represents the gesture that 1 or more fingers moving on the screen for a relatively large distance. This gesture is usually used for scrolling, and is used by UIScroller and UIScrollView.
  • Input properties: directionalLockEnabled
  • Output properties: offset, velocity, transform
8. UITapAndAHalfRecognizer: represents the gesture that 1 finger tapped the screen q times consecutively, and then moved/stayed on the screen.
  • Input properties: numberOfFullTaps (q), allowableMovement
  • Output properties: touch

An example for how to use these API:
UISwipeGestureRecognizer *swipeRecognizer = [[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(DidSwipe:)] autorelease];

By default this line of code detect the swipes from left to right.

If you want to detect a swipe from right to left add the below line after the above line:
swipeRecognizer.direction = UISwipeGestureRecognizerDirectionLeft;

On 1 view, you can add many gesture recognizers to handle different interactions by using below function
[self.view addGestureRecognizer:swipeRecognizer];

After that, just implement the [DidSwipe] method we already registered above:
- (void) DidSwipe:(UISwipeGestureRecognizer*)sender{
   Do something...

+ iPhone SDK 3.2 API document

Happy coding!