Modified Teehan+Lax UIDynamics Snippet.

gistfile1.m

-(void)setupContentViewControllerAnimatorProperties {
    NSAssert(self.animator == nil, @"Animator is not nil – setupContentViewControllerAnimatorProperties likely called twice.");
    
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self];
    self.animator.delegate = self;
    
    UICollisionBehavior *collisionBehavior = [[UICollisionBehavior alloc] initWithItems:@[self.overlayView]];
        // Need to create a boundary that lies to the left off of the right edge of the screen.
    [collisionBehavior setTranslatesReferenceBoundsIntoBoundaryWithInsets:UIEdgeInsetsMake(0, -280, 0, 0)];
    [self.animator addBehavior:collisionBehavior];
    
    self.gravityBehavior = [[UIGravityBehavior alloc] initWithItems:@[self.overlayView]];
    self.gravityBehavior.gravityDirection = CGVectorMake(1, 0);
    [self.animator addBehavior:self.gravityBehavior];
    
    self.pushBehavior = [[UIPushBehavior alloc] initWithItems:@[self.overlayView] mode:UIPushBehaviorModeInstantaneous];
    self.pushBehavior.magnitude = 1.0f;
    self.pushBehavior.angle = 0.0f;
    [self.animator addBehavior:self.pushBehavior];
    
    UIDynamicItemBehavior *itemBehaviour = [[UIDynamicItemBehavior alloc] initWithItems:@[self.overlayView]];
    itemBehaviour.elasticity = 0.45f;
    [self.animator addBehavior:itemBehaviour];
}


- (void)overlayWasPanned:(UIPanGestureRecognizer *)gestureRecognizer {

    CGPoint location = [gestureRecognizer locationInView:self];
    location.y = CGRectGetMidY(self.bounds);
    
    if (gestureRecognizer.state == UIGestureRecognizerStateBegan) {
        [self.animator removeBehavior:self.gravityBehavior];
        
        self.attachmentBehavior = [[UIAttachmentBehavior alloc] initWithItem:self.overlayView attachedToAnchor:location];
        [self.animator addBehavior:self.attachmentBehavior];
    }
    else if (gestureRecognizer.state == UIGestureRecognizerStateChanged) {
        self.attachmentBehavior.anchorPoint = location;
    }
    else if (gestureRecognizer.state == UIGestureRecognizerStateEnded) {
        [self.animator removeBehavior:self.attachmentBehavior], self.attachmentBehavior = nil;
        
        CGPoint velocity = [gestureRecognizer velocityInView:self];
        
        if (velocity.x < 0) {
                // Open menu
            self.trayOpen = YES;
            
            velocity.x = -20;
            
            self.gravityBehavior.gravityDirection = CGVectorMake(-1, 0);
        }
        else {
                // Close menu
            self.trayOpen = NO;
            
            velocity.x = 20;
            
            self.gravityBehavior.gravityDirection = CGVectorMake(1, 0);
        }
        
        [self.animator addBehavior:self.gravityBehavior];
        
        self.pushBehavior.pushDirection = CGVectorMake(velocity.x, 0);
        self.pushBehavior.active = YES;
    }
}