Update README.md

README.md

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 # SnapDrag
 Snap behavior in UIScrollView in Swift
+
+Needed this to answer a StackOverflow question:<br>
+[UIAnimator's UISnapBehavior possible with UIScrollview?](http://stackoverflow.com/q/38465498/1378447)
+
+###Question
+What I am Trying to Achieve
+
+UIScrollView to 'snap' to certain point while the user is dragging the scroll view. However, the scrolling has to resume from snapped position without the user having to lift the touch.
+
+Apple seems to achieve this in Photo Editing in their iOS Photos App. (See screenshot below)
+
+<p align="center"><img src="http://i.stack.imgur.com/1k57B.png"/></p>
+
+###Answer
+I've tried to mimic iOS Photos app. Here is my logic:
+
+    // CALCULATE A CONTENT OFFSET FOR SNAPPING POINT 
+    let snapPoint = CGPoint(x: 367, y: 0)  
+
+    // CHANGE THESE VALUES TO TEST
+    let minDistanceToSnap = 7.0
+    let minVelocityToSnap = 25.0
+    let minDragDistanceToReleaseSnap = 7.0
+    let snapDuringDecelerating = false
+
+
+This kind of scrolling needs 3 stages
+
+    enum SnapState {
+    case willSnap
+    case didSnap
+    case willRelease
+    }
+
+ 1. `willSnap:` Default state. Decide when to snap. Compare `contentOffset distance from SnapPoint with minDistanceToSnap` and `scrollview velocity with minVelocityToSnap`. Change to `didSnap` state.
+ 2. `didSnap:` Manually `setContentOffset` to a provided `contextOffset(snapPoint)`. Calculate `dragDistance` on `scrollView`. If user drag more than a certain distance (`minDragDistanceToReleaseSnap`) change to `willRelease` state.
+ 3. `willRelease:` Change to `willSnap` state again if `distance scroll from snapPoint` is more than `minDistanceToSnap`.
+
+<br>
+         
+    extension ViewController: UIScrollViewDelegate {
+        func scrollViewDidScroll(scrollView: UIScrollView) {
+            switch(snapState) {
+                case .willSnap:
+                    let distanceFromSnapPoint = distance(between: scrollView.contentOffset, and: snapPoint)
+                    let velocity = scrollView.panGestureRecognizer.velocityInView(view)
+                    let velocityDistance = distance(between: velocity, and: CGPointZero)
+                    if distanceFromSnapPoint <= minDistanceToSnap && velocityDistance <= minVelocityToSnap && (snapDuringDecelerating || velocityDistance > 0.0) {
+                        startSnapLocaion = scrollView.panGestureRecognizer.locationInView(scrollView)
+                        snapState = .didSnap
+                    }
+                case .didSnap:
+                    scrollView.setContentOffset(snapPoint, animated: false)
+                    var dragDistance = 0.0
+                    let location = scrollView.panGestureRecognizer.locationInView(scrollView)
+                    dragDistance = distance(between: location, and: startSnapLocaion)
+                    if dragDistance > minDragDistanceToReleaseSnap  {
+                        startSnapLocaion = CGPointZero
+                        snapState = .willRelease
+                    }
+                case .willRelease:
+                    let distanceFromSnapPoint = distance(between: scrollView.contentOffset, and: snapPoint)
+                    if distanceFromSnapPoint > minDistanceToSnap {
+                        snapState = .willSnap
+                    }
+            }
+        }
+    }
+
+
+Helper function
+
+    func distance(between point1: CGPoint, and point2: CGPoint) -> Double {
+        return Double(hypotf(Float(point1.x - point2.x), Float(point1.y - point2.y)))
+    }
+
+