UICollisionBehavior with iOS9 seems to stop 1.5 points away from boundary: My bug or Apple's?

Question

In my app, I use UIKit Dynamics including a UICollisionBehavior to have a menu bounce when it opens and when it closes. The code I'm using for this is below. This has been working fine for iOS8. With iOS9 (including iOS9.1 beta 2 just released), however, I'm finding an odd issue. On the surface, the menu I'm bouncing with this bouncing animation wasn't full closing after opening and then closing it. Looking more closely, I find that the boundaries for the UICollisionBehavior are computed with the same values across iOS8 and iOS9.

Menu opening collision boundary: (798,330) to (1024,330) Which represents a line, on screen, where the bottom of the menu should finally rest after opening and bouncing.

Menu closing collision boundary: (798,-280) to (1024,-280) Which represents a line, off screen, where the top of the menu should finally rest after closing and bouncing.

The problem comes in iOS9 where the menu UIView doesn't actually end up resting finally at these boundaries. After opening, the menu frame looks like this with iOS9:

(798, -1.5; 226, 330) [This is: (x, y; w, h)]

and after closing, the menu frame looks like:

(798, -278.5; 226, 330)

BUT , this should actually be:

(798, 0; 226, 330) (after opening)

(798, -280; 226, 330) (after closing)

Anyone else seeing these issues with iOS9 and collision behaviors?

I'm about to put a hack in my code (search for "HACK" below), which I'll make selective for iOS9, but I really don't like these hacks!

BounceAnimation.h

//
//  BounceAnimation.h
//  Petunia
//
//  Created by Christopher Prince on 12/18/14.
//  Copyright (c) 2014 Spastic Muffin, LLC. All rights reserved.
//

// Animates an object through a straight line path, up, down, left or right until it lands, after which it bounces. This requires iOS8 or later.

#import <Foundation/Foundation.h>

@interface BounceAnimation : NSObject

// distnace is for the viewToAnimate to travel until it lands and bounces, in points. You must set this before calling run.
- (instancetype) initWithReferenceView: (UIView *) referenceView viewToAnimate: (UIView *) viewToAnimate andDistance: (CGFloat) distance;

// Direction and distance will be obtained from animateToPoint, and should be consistent with the constraints for the direction property below. I.e., the animateToPoint should be down, left, right, or up from the origin of the viewToAnimate.
- (instancetype) initWithReferenceView: (UIView *) referenceView viewToAnimate: (UIView *) viewToAnimate andFinalPoint: (CGPoint) animateToPoint;

// One shot animation. You can only call run once.
- (void) run;

// Called when animation completes, if given. Called when all the bouncing is done.
@property (nonatomic, strong) void (^completion)(void);

// Called when first contact is made with the boundary, just as the first bounce is about to begin.
@property (nonatomic, strong) void (^firstImpactCallback)(void);

// Only keeps weak references to the views passed in the init method.
@property (nonatomic, weak, readonly) UIView *referenceView;
@property (nonatomic, weak, readonly) UIView *viewToAnimate;

// Rate at which the object accelerates towards the boundary. Same units as magnitude for UIGravityBehavior. Defaults to 1.0.
@property (nonatomic) CGFloat accelerationRate;

// Defaults to 0. Units are points per second.
@property (nonatomic) CGFloat initialVelocity;

// This is a unit vector.
// dx (first component) is rightwards; e.g., dx=0, no right/left; dx=-1, is left one unit
// dy (second component) is downwards; e.g., dy=1, down one unit.
// Defaults to (0, 1), downwards.
// Right now, dx and dy can be 0, 1, or -1. One of dx and dy must be 0.
@property (nonatomic, readonly) CGVector direction;

@end

BounceAnimation.m

//
//  BounceAnimation.m
//  Petunia
//
//  Created by Christopher Prince on 12/18/14.
//  Copyright (c) 2014 Spastic Muffin, LLC. All rights reserved.
//

#import "BounceAnimation.h"
#import "Vector.h"
#import "UIDevice+Extras.h"

@interface BounceAnimation()<UIDynamicAnimatorDelegate, UICollisionBehaviorDelegate>
{
    UIDynamicAnimator *_animator;
    UIGravityBehavior *_gravityBehavior;
    UICollisionBehavior *_collision;
    UIDynamicItemBehavior *_velocity;
    CGPoint _linearVelocity;
    CGFloat _distanceInPoints;
    BOOL _calledFirstImpactCallback;
}

@property (nonatomic, weak) UIView *referenceView;
@property (nonatomic, weak) UIView *viewToAnimate;
@property (nonatomic) CGVector direction;

@end

#define INITIAL_GRAVITY_MAGNITUDE 1.0

@implementation BounceAnimation

- (void) setupWithReferenceView: (UIView *) referenceView andViewToAnimate: (UIView *) viewToAnimate;
{
    AssertIf([UIDevice ios7OrEarlier], @"Don't have at least iOS8!");

    self.referenceView = referenceView;
    self.viewToAnimate = viewToAnimate;

    _animator = [[UIDynamicAnimator alloc] initWithReferenceView:referenceView];
    _animator.delegate = self;

    _gravityBehavior = [[UIGravityBehavior alloc] initWithItems:@[viewToAnimate]];
    _gravityBehavior.magnitude = INITIAL_GRAVITY_MAGNITUDE;

    _velocity = [[UIDynamicItemBehavior alloc] initWithItems:@[viewToAnimate]];
}

// referenceView is just the view on top of which we're doing our animation. E.g., it could be self.view of a view controller. viewToAnimate must be a subview of the reference view.
- (instancetype) initWithReferenceView: (UIView *) referenceView viewToAnimate: (UIView *) viewToAnimate andDistance: (CGFloat) distanceInPoints;
{
    self = [super init];
    if (self) {
        AssertIf(distanceInPoints <= 0.0, @"Invalid distance: %f", _distanceInPoints);
        _distanceInPoints = distanceInPoints;
        [self setupWithReferenceView:referenceView andViewToAnimate:viewToAnimate];
        [self setDirection:CGVectorMake(0.0, 1.0)];
    }

    return self;
}

- (instancetype) initWithReferenceView: (UIView *) referenceView viewToAnimate: (UIView *) viewToAnimate andFinalPoint: (CGPoint) animateToPoint;
{
    self = [super init];
    if (self) {
        // Need to compute distance and direction.
        CGVector direction = [Vector subFirst:[Vector fromPoint:animateToPoint] from:[Vector fromPoint:viewToAnimate.frameOrigin]];
        SPASLogDetail(@"direction after subtraction: %@", NSStringFromCGVector(direction));

        // Special case: No direction because same start and finish.
        if (direction.dy + direction.dx == 0.0) {
            _distanceInPoints = 0.0;
        }
        else {
            direction = [Vector normalize:direction]; // vectorNormalize(direction);
            _distanceInPoints = [Vector distanceFromPoint:viewToAnimate.frameOrigin toPoint:animateToPoint];
        }

        SPASLogDetail(@"finalPoint: %@, direction: %@, distance: %f", NSStringFromCGPoint(animateToPoint), NSStringFromCGVector(direction), _distanceInPoints);

        [self setupWithReferenceView:referenceView andViewToAnimate:viewToAnimate];
        [self setDirection:direction];
    }

    return self;
}

- (void) setInitialVelocity:(CGFloat)initialVelocity;
{
    _initialVelocity = initialVelocity;
    // Only positive speeds in the velocity are relevant. Negative speeds reduce the velocity, they don't go the other direction.
    _linearVelocity =
        CGPointMake(initialVelocity * fabs(_direction.dx),
                    initialVelocity * fabs(_direction.dy));
}

- (void) setAccelerationRate:(CGFloat)accelerationRate;
{
    _accelerationRate = accelerationRate;
    _gravityBehavior.magnitude = accelerationRate;

}

// I'm only doing left, right, up, down animations because of the problem of rotating the viewToAnimate. I'm not sure I'll ever have a case where I want a rotated animated view. (Hmmm. If I want to do some kind of continuous animation, arbitrary direction with non-rotated objects could be cool!)
- (void) setDirection:(CGVector)direction;
{
    if (_collision) {
        [_animator removeBehavior:_collision];
        _collision = nil;
    }

    if (_distanceInPoints == 0.0) {
        // Why bother?
        SPASLogDetail(@"Zero distance");
        return;
    }

    // 9/24/15; HACK
    //_distanceInPoints += 1.5;

    // Since we're doing vector operations with one of the init methods above, the following seems risky!
    //AssertIf(direction.dy != 0.0 && direction.dy != -1.0 && direction.dy != 1.0, @"Invalid dy: %f", direction.dy);
    //AssertIf(direction.dx != 0.0 && direction.dx != -1.0 && direction.dx != 1.0, @"Invalid dx: %f", direction.dx);

    _direction = direction;
    _gravityBehavior.gravityDirection = direction;

    _collision = [[UICollisionBehavior alloc] initWithItems:@[self.viewToAnimate]];
    _collision.collisionDelegate = self;

    CGPoint startBoundary;
    CGPoint endBoundary;

#define SMALL_VALUE 0.05
    BOOL (^closeToZero)(CGFloat) = ^(CGFloat value) {
        if (value > -SMALL_VALUE && value < SMALL_VALUE) {
            return YES;
        }
        else {
            return NO;
        }
    };

    if (closeToZero(direction.dx)) {
        // Vertical motion.
        CGFloat yBoundary = direction.dy * _distanceInPoints + self.viewToAnimate.frameY;
        if (direction.dy > 0.0) {
            // If we're going down, then we need to add the height of the self.viewToAnimate to our boundary. This is because the origin coords are in the *upper*, left of the viewToAnimate.
            yBoundary += self.viewToAnimate.frameHeight;
        }

        startBoundary = CGPointMake(self.viewToAnimate.frameX, yBoundary);
        endBoundary = CGPointMake(self.viewToAnimate.frameX + self.viewToAnimate.frameWidth, yBoundary);
    }
    else {
        // Horizontal motion.
        CGFloat xBoundary = direction.dx * _distanceInPoints + self.viewToAnimate.frameX;
        if (direction.dx > 0.0) {
            // If we're going to the right, then we need to add the width of the self.viewToAnimate to our boundary. This is because the origin coords are in the upper, *left* of the viewToAnimate.
            xBoundary += self.viewToAnimate.frameWidth;
        }

        startBoundary = CGPointMake(xBoundary, self.viewToAnimate.frameY);
        endBoundary = CGPointMake(xBoundary, self.viewToAnimate.frameY + self.viewToAnimate.frameHeight);
    }

    SPASLog(@"startBoundary: %@, endBoundary: %@", NSStringFromCGPoint(startBoundary), NSStringFromCGPoint(endBoundary));

    [_collision addBoundaryWithIdentifier:@"barrier"
                                fromPoint:startBoundary
                                  toPoint:endBoundary];

    [_animator addBehavior:_collision];
}

- (void) run;
{
    if (_collision) {
        [_animator addBehavior:_gravityBehavior];
        [_velocity addLinearVelocity:_linearVelocity forItem:self.viewToAnimate];
        [_animator addBehavior:_velocity];
    }
    else {
        if (self.completion) {
            self.completion();
        }
    }
}

#pragma mark - UIDynamicAnimatorDelegate methods

- (void)dynamicAnimatorDidPause:(UIDynamicAnimator*)animator;
{
    if (self.completion) {
        self.completion();
    }
}

#pragma mark -

#pragma mark - UICollisionBehaviorDelegate methods

// This isn't the method that gets called in our case.
//- (void)collisionBehavior:(UICollisionBehavior*)behavior beganContactForItem:(id <UIDynamicItem>)item1 withItem:(id <UIDynamicItem>)item2 atPoint:(CGPoint)p;

- (void)collisionBehavior:(UICollisionBehavior*)behavior beganContactForItem:(id <UIDynamicItem>)item withBoundaryIdentifier:(id <NSCopying>)identifier atPoint:(CGPoint)p;
{
    if (!_calledFirstImpactCallback) {
        _calledFirstImpactCallback = YES;
        if (self.firstImpactCallback) {
            self.firstImpactCallback();
        }
    }
}

#pragma mark -


@end

Answer 1

I'm working in Swift, and I am seeing the same behavior as you do with 1.5 points offset. It seems to be unrelated to the resolution of the device (1x, 2x or 3x), all have 1.5 points offset.

However, using the the path-based API, the problem seems to not be there anymore:

let collisionBehavior: UICollisionBehavior = ...
let topLeft: CGPoint = ...
let bottomLeft: CGPoint = ...

let path = CGPathCreateMutable()
CGPathMoveToPoint(path, nil, topLeft.x, topLeft.y)
CGPathAddLineToPoint(path, nil, bottomLeft.x, bottomLeft.y)
let bezierPath = UIBezierPath(CGPath: path)
collisionBehavior.addBoundaryWithIdentifier("myID", forPath: bezierPath)

Converting this to Objective-C is easy enough:

UICollisionBehavior *collisionBehavior = ...
CGPoint topLeft = ...
CGPoint bottomLeft = ...

CGMutablePathRef path = CGPathCreateMutable();
CGPathMoveToPoint(path, nil, topLeft.x, topLeft.y);
CGPathAddLineToPoint(path, nil, bottomLeft.x, bottomLeft.y);
UIBezierPath *bezierPath = [UIBezierPath bezierPathWithCGPath: path];
[collisionBehavior addBoundaryWithIdentifier: @"myID" forPath: path];