UIRotationGestureRecognizer 判斷旋轉方向

//
//  UIRotationGestureRecognizer.h
//  UIKit
//
//  Copyright (c) 2009-2014 Apple Inc. All rights reserved.
//

#import <CoreGraphics/CoreGraphics.h>
#import <UIKit/UIGestureRecognizer.h>

NS_ASSUME_NONNULL_BEGIN

// Begins:  when two touches have moved enough to be considered a rotation
// Changes: when a finger moves while two fingers are down
// Ends:    when both fingers have lifted

NS_CLASS_AVAILABLE_IOS(3_2) __TVOS_PROHIBITED @interface UIRotationGestureRecognizer : UIGestureRecognizer

@property (nonatomic)          CGFloat rotation;            // rotation in radians
@property (nonatomic,readonly) CGFloat velocity;            // velocity of the pinch in radians/second

@end

NS_ASSUME_NONNULL_END

You may set the rotation value to an arbitrary value; however, setting the rotation resets the velocity.

The rotation value is a single value that varies over time. It is not the delta value from the last time that the rotation was reported. Apply the rotation value to the state of the view when the gesture is first recognized—do not concatenate the value each time the handler is called.

可以通過rotation與上一次rotation相減的差值進行判斷