UIViewControllerAnimatedTransitioning only works every other time? - ios

Question

I have been at this for hours and cant understand what i am doing wrong. I have rigged up custom tab bar controller transitions by conforming to UITabBarControllerDelegate as described in my previous Swift: Problems with custom UIView.transition?
I don't use the normal storyboard tab bar buttons, I switch selectedIndex programmatically. My problem is that only with this implemented:
func tabBarController(_ tabBarController: UITabBarController, animationControllerForTransitionFrom fromVC: UIViewController, to toVC: UIViewController) -> UIViewControllerAnimatedTransitioning? {
let animator = ModalTransition()
animator.fromView = fromVC.view
animator.toView = toVC.view
return animator
}
the animations AND switching of index occurs only every other time. I have custom buttons to switch the index and every other time, nothing happens when I click the switch button. Here is my animation:
//
// ModalTransition.swift
// Adventures In Design
//
// Created by Skylar Thomas on 8/28/17.
//
import UIKit
class ModalTransition: NSObject, UIViewControllerAnimatedTransitioning {
weak var transitionContext: UIViewControllerContextTransitioning?
var fromView = UIView()
var toView = UIView()
var duration = 1.1
func transitionDuration(using transitionContext: UIViewControllerContextTransitioning?) -> TimeInterval {
return 1
}
func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
let containerView = transitionContext.containerView
containerView.addSubview(toView)
containerView.sendSubview(toBack: toView)
print("ANIMATING")
UIView.animate(withDuration: duration, delay: 0.0, usingSpringWithDamping: 0.5, initialSpringVelocity: 0.0, options: .curveLinear, animations: {
self.fromView.center.y += 900
}, completion: {
finished in
//only works every OTHER click
transitionContext.completeTransition(!transitionContext.transitionWasCancelled)
self.fromView.center.y -= 900
})
}
}
What is causing this? Is it something

Answer 1

I think you are not assigning your toView and FromView. Try something like this
func animateTransition(using transitionContext: UIViewControllerContextTransitioning) {
let fromViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.from)!
let toViewController = transitionContext.viewController(forKey: UITransitionContextViewControllerKey.to)!
let fromView = fromViewController.view
let toView = toViewController.view
let container = transitionContext.containerView
container.addSubview(toView!)
// Replace your animations here
toView?.frame = transitionContext.finalFrame(for: toViewController)
toView?.alpha = 0
let duration = self.transitionDuration(using: transitionContext)
UIView.animate(withDuration: duration, delay: 0, options: .curveEaseInOut, animations: {
toView?.alpha = 1
fromView?.alpha = 0
}, completion: { finished in
toView?.alpha = 1.0
fromView?.alpha = 1
fromView?.removeFromSuperview()
transitionContext.completeTransition(true)
})
}