UISnapBehavior strange jiggle - swift

Question

I am trying to use UISnapBehavior to create a tweening back animation.

I am trying to make a square draggable, then when the square is released, it tweens back to its original position. Each time it tweens back, it seems to have a strange trajectory and rather than easing back at the correct angleit seems to ease back in a sort of strange semi circle before reaching its destination.

Thank you in advance.

my code is as follows:

var animator:UIDynamicAnimator? = nil;
var ball : Ball!;
var ball2 : Ball!;
var snap: UISnapBehavior!
var gravity : UIGravityBehavior!;

var speed : Double! = 0;
var lastTouchPoint :  CGPoint!;
var attach : UIAttachmentBehavior! = nil;
var itemBehavior : UIDynamicItemBehavior!;
var snapping : UISnapBehavior! = nil;
var currentBall : Ball! = nil;


override init(frame: CGRect) {
    super.init(frame: frame);
    println( "-- initiated view --" );

    self.backgroundColor = UIColor.orangeColor();

    ball = Ball( frame: CGRect(x: 200, y: 200, width: 50, height: 50 ) );
    self.addSubview(ball);

    ball2 = Ball( frame: CGRect(x: 100, y: 100, width: 50, height: 50 ) );
    self.addSubview(ball2);

    setup()

    self.addGestureRecognizer( UIPanGestureRecognizer(target: self, action: "panHandler:") );

    self.attach = UIAttachmentBehavior(item: self, attachedToAnchor: CGPoint(x: 0, y: 0) );

}

func setup() {
    animator = UIDynamicAnimator(referenceView:self);

    var collision: UICollisionBehavior!
    collision = UICollisionBehavior(items: [ball, ball2])
    collision.translatesReferenceBoundsIntoBoundary = false;
    collision.collisionDelegate = self;
    animator?.addBehavior(collision)

    var dynamicItem : UIDynamicItemBehavior = UIDynamicItemBehavior(items: [ball, ball2] );
    dynamicItem.allowsRotation = false;
    self.animator!.addBehavior(dynamicItem);
}

func panHandler( recognizr : UIPanGestureRecognizer ) {
    switch( recognizr.state ) {
        case  UIGestureRecognizerState.Began:

            let location = recognizr.locationInView(self);
            let view : AnyObject? =  self.hitTest(location, withEvent: nil);

            if( view is Ball ) {

                currentBall = view as! Ball;

                if(  self.attach != nil ){ self.animator!.removeBehavior(attach); self.attach = nil; }


                self.attach = UIAttachmentBehavior(item: currentBall, attachedToAnchor: location );
                self.animator!.addBehavior(self.attach);
                self.attach.damping = 1;
                self.attach.length = 1;

                let dynamicItem : UIDynamicItemBehavior = UIDynamicItemBehavior(items: [currentBall] );
                dynamicItem.allowsRotation = false;

                self.animator!.addBehavior(dynamicItem);
            }
            break;

        case  UIGestureRecognizerState.Changed :

            if( currentBall != nil ) {
                let location = recognizr.locationInView(self);
                self.attach.anchorPoint = location;
            }

            break;

        default :
            println("--- snap ---");

            if( self.currentBall == nil ) {return;}

            self.animator!.removeBehavior(attach);
            self.attach = nil;

            var snap : UISnapBehavior = UISnapBehavior(item: self.currentBall, snapToPoint: self.currentBall.startingPoint );
            snap.damping = 1;

            self.animator!.addBehavior(snap);

            break;
    }
}

HuaTham · Answer 1 · 2018-04-03T10:05:42.730

It's 2018 now and I'm seeing pretty much the same movement. I think it's a bug in Apple's implementation and you and I both should file a bug report since it's the only way this will get fixed.

In my experience, this jiggle in a semi-circle movement toward the end appears more pronounced when damping is higher than 1.0.

In my example below, my gesture recognizer strictly fixes the x-axis value, so theoretically there should not be any horizontal movement.

allowsRotation is set to false.

UISnapBehavior strange jiggle - swift

1 Answers1