adding inertia to a UIPanGestureRecognizer

Question

I am trying to move a sub view across the screen which works, but i also want to add inertia or momentum to the object.
My UIPanGestureRecognizer code that i already have is below.

Thanks in advance.

UIPanGestureRecognizer *panGesture = [[UIPanGestureRecognizer alloc] initWithTarget:self       action:@selector(handlePan:)];
[self addGestureRecognizer:panGesture];

(void)handlePan:(UIPanGestureRecognizer *)recognizer
{

    CGPoint translation = [recognizer translationInView:self.superview];
    recognizer.view.center = CGPointMake(recognizer.view.center.x + translation.x,
                                     recognizer.view.center.y + translation.y);
    [recognizer setTranslation:CGPointMake(0, 0) inView:self.superview];

    if (recognizer.state == UIGestureRecognizerStateEnded) {
        [self.delegate card:self.tag movedTo:self.frame.origin];
    }
}

Again thanks.

Answer 1

I took the inspiration from the accepted answer's implementation. Here is a Swift 5.1 version.

---

Logic:

• You need to calculate the angle changes with the velocity at which the pan gesture ended and keep rotating the wheel in an endless timer until the velocity wears down because of deceleration rate.
• Keep decreasing the current velocity in every iteration of the timer with some factor (say, 0.9).
• Keep a lower limit on the velocity to invalidate the timer and complete the deceleration process.

---

Main function used to calculate deceleration:

// deceleration behaviour constants (change these for different deceleration rates)
private let timerDuration = 0.025
private let decelerationSmoothness = 0.9
private let velocityToAngleConversion = 0.0025

private func animateWithInertia(velocity: Double) {
    _ = Timer.scheduledTimer(withTimeInterval: self.timerDuration, repeats: true) { [weak self] timer in
        guard let this = self else {
            return
        }
        let concernedVelocity = this.currentVelocity == 0.0 ? velocity : this.currentVelocity
        let newVelocity = concernedVelocity * this.decelerationSmoothness
        this.currentVelocity = newVelocity
        var angleTraversed = newVelocity * this.velocityToAngleConversion * this.maximumRotationAngleInCircle
        if !this.isClockwiseRotation {
            angleTraversed *= -1
        }
        // exit condition
        if newVelocity < 0.1 {
            timer.invalidate()
            this.currentVelocity = 0.0
        } else {
            this.traverseAngularDistance(angle: angleTraversed)
        }
    }
}

---

Full working code with helper functions, extensions and usage of aforementioned function in the handlePanGesture function.

// deceleration behaviour constants (change these for different deceleration rates)
private let timerDuration = 0.025
private let decelerationSmoothness = 0.9
private let velocityToAngleConversion = 0.0025

private let maximumRotationAngleInCircle = 360.0

private var currentRotationDegrees: Double = 0.0 {
    didSet {
        if self.currentRotationDegrees > self.maximumRotationAngleInCircle {
            self.currentRotationDegrees = 0
        }
        if self.currentRotationDegrees < -self.maximumRotationAngleInCircle {
            self.currentRotationDegrees = 0
        }
    }
}
private var previousLocation = CGPoint.zero
private var currentLocation = CGPoint.zero
private var velocity: Double {
    let xFactor = self.currentLocation.x - self.previousLocation.x
    let yFactor = self.currentLocation.y - self.previousLocation.y
    return Double(sqrt((xFactor * xFactor) + (yFactor * yFactor)))
}

private var currentVelocity = 0.0
private var isClockwiseRotation = false

@objc private func handlePanGesture(panGesture: UIPanGestureRecognizer) {
    let location = panGesture.location(in: self)

    if let rotation = panGesture.rotation {
        self.isClockwiseRotation = rotation > 0
        let angle = Double(rotation).degrees
        self.currentRotationDegrees += angle
        self.rotate(angle: angle)
    }

    switch panGesture.state {
    case .began, .changed:
        self.previousLocation = location
    case .ended:
        self.currentLocation = location
        self.animateWithInertia(velocity: self.velocity)
    default:
        print("Fatal State")
    }
}

private func animateWithInertia(velocity: Double) {
    _ = Timer.scheduledTimer(withTimeInterval: self.timerDuration, repeats: true) { [weak self] timer in
        guard let this = self else {
            return
        }
        let concernedVelocity = this.currentVelocity == 0.0 ? velocity : this.currentVelocity
        let newVelocity = concernedVelocity * this.decelerationSmoothness
        this.currentVelocity = newVelocity
        var angleTraversed = newVelocity * this.velocityToAngleConversion * this.maximumRotationAngleInCircle
        if !this.isClockwiseRotation {
            angleTraversed *= -1
        }
        if newVelocity < 0.1 {
            timer.invalidate()
            this.currentVelocity = 0.0
            this.selectAtIndexPath(indexPath: this.nearestIndexPath, shouldTransformToIdentity: true)
        } else {
            this.traverseAngularDistance(angle: angleTraversed)
        }
    }
}

private func traverseAngularDistance(angle: Double) {
    // keep the angle in -360.0 to 360.0 range
    let times = Double(Int(angle / self.maximumRotationAngleInCircle))
    var newAngle = angle - times * self.maximumRotationAngleInCircle
    if newAngle < -self.maximumRotationAngleInCircle {
        newAngle += self.maximumRotationAngleInCircle
    }
    self.currentRotationDegrees += newAngle
    self.rotate(angle: newAngle)
}

Extensions being used in above code:

extension UIView {
    func rotate(angle: Double) {
        self.transform = self.transform.rotated(by: CGFloat(angle.radians))
    }
}

extension Double {
    var radians: Double {
        return (self * Double.pi)/180
    }

    var degrees: Double {
        return (self * 180)/Double.pi
    }
}

Answer 2

Have a look at RotationWheelAndDecelerationBehaviour. there is an example for how to do the deceleration for both linear panning and rotational movement. Trick is to see what is the velocity when user ends the touch and continue in that direction with a small deceleration.

Answer 3

Well, I'm not a pro but, checking multiple answers, I managed to make my own code with which I am happy.

Please tell me how to improve it and if there are any bad practices I used.

- (IBAction)handlePan:(UIPanGestureRecognizer *)recognizer {

CGPoint translatedPoint = [recognizer translationInView:self.postViewContainer];
CGPoint velocity = [recognizer velocityInView:recognizer.view];

float bottomMargin = self.view.frame.size.height - containerViewHeight;
float topMargin = self.view.frame.size.height - scrollViewHeight;

if ([recognizer state] == UIGestureRecognizerStateChanged) {

    newYOrigin = self.postViewContainer.frame.origin.y + translatedPoint.y;

    if (newYOrigin <= bottomMargin  && newYOrigin >= topMargin) {
        self.postViewContainer.center = CGPointMake(self.postViewContainer.center.x, self.postViewContainer.center.y + translatedPoint.y);
    }
    [recognizer setTranslation:CGPointMake(0, 0) inView:self.postViewContainer];
}

if ([recognizer state] == UIGestureRecognizerStateEnded) {

    __block float newYAnimatedOrigin = self.postViewContainer.frame.origin.y + (velocity.y / 2.5);

    if (newYAnimatedOrigin <= bottomMargin && newYAnimatedOrigin >= topMargin) {
        [UIView animateWithDuration:1.2 delay:0
                            options:UIViewAnimationOptionCurveEaseOut
                         animations:^ {
                             self.postViewContainer.center = CGPointMake(self.postViewContainer.center.x, self.postViewContainer.center.y + (velocity.y / 2.5));
                         }
                         completion:^(BOOL finished) {
                             [self.postViewContainer setFrame:CGRectMake(0, newYAnimatedOrigin, self.view.frame.size.width, self.view.frame.size.height - newYAnimatedOrigin)];
                         }
         ];
    }
    else {
        [UIView animateWithDuration:0.6 delay:0
                            options:UIViewAnimationOptionCurveEaseOut
                         animations:^ {
                             if (newYAnimatedOrigin > bottomMargin) {
                                 self.postViewContainer.center = CGPointMake(self.postViewContainer.center.x, bottomMargin + self.postViewContainer.frame.size.height / 2);
                             }

                             if (newYAnimatedOrigin < topMargin) {
                                 self.postViewContainer.center = CGPointMake(self.postViewContainer.center.x, topMargin + self.postViewContainer.frame.size.height / 2);
                             }
                         }
                         completion:^(BOOL finished) {
                             if (newYAnimatedOrigin > bottomMargin)
                                 [self.postViewContainer setFrame:CGRectMake(0, bottomMargin, self.view.frame.size.width, scrollViewHeight)];

                             if (newYAnimatedOrigin < topMargin)
                                 [self.postViewContainer setFrame:CGRectMake(0, topMargin, self.view.frame.size.width, scrollViewHeight)];
                         }
         ];
    }
}

}

I have used two different animation, one is the default inertia one and the other if for when the user flings the containerView with high velocity.

It works well under iOS 7.