/*

    Copyright (C) 2016 Apple Inc. All Rights Reserved.

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    StickyCornersViewController is a UIViewController subclass demonstrating the use of UIFieldBehavior.

*/

import UIKit

class StickyCornersViewController: UIViewController {

    // MARK: Properties

    // Dynamics.

    var animator: UIDynamicAnimator!

    var stickyBehavior: StickyCornersBehavior!

    var itemView: UIView!

    // Touch handling.

    var offset = CGPoint.zero

    let itemAspectRatio: CGFloat = 0.70

    // MARK: UIViewController

    override func viewDidLoad() {

        super.viewDidLoad()

        // Determine a reasonable item size.

        let screenBounds = UIScreen.main.bounds

        let length = floor(0.1  * max(screenBounds.width, screenBounds.height))

        /*

            Create the itemView, add a pan gesture recognizer, then add the `itemView`

            as a subview of the viewController's view.

        */

        itemView = UIView(frame: CGRect(x: 0, y: 0, width: length, height: floor(length / itemAspectRatio)))

        itemView.autoresizingMask = []

        itemView.backgroundColor = UIColor.red

        let panGestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(StickyCornersViewController.pan(_:)))

        itemView.addGestureRecognizer(panGestureRecognizer)

        view.addSubview(itemView)

        // Create a UIDynamicAnimator.

        animator = UIDynamicAnimator(referenceView: view)

        /*

            Create a StickyCornersBehavior with the itemView and a corner inset, 

            then add it to the animator.

        */

        stickyBehavior = StickyCornersBehavior(item: itemView, cornerInset: length * 0.5)

        animator.addBehavior(stickyBehavior)

    }

    override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {

        super.viewWillTransition(to: size, with: coordinator)

        // Ensure the item stays on screen during a bounds change.

        guard let corner = stickyBehavior.currentCorner else { return }

        stickyBehavior.isEnabled = false

        let bounds = CGRect(origin: CGPoint.zero, size: size)

        stickyBehavior.updateFieldsInBounds(bounds)

        coordinator.animate(alongsideTransition: { context in

            self.itemView.center = self.stickyBehavior.positionForCorner(corner)

        },

        completion: { context in

            self.stickyBehavior.isEnabled = true

        })

    }

    // MARK: Gesture Callbacks

    func pan(_ pan: UIPanGestureRecognizer) {

        var location = pan.location(in: view)

        switch pan.state {

            case .began:

                // Capture the initial touch offset from the itemView's center.

                let center = itemView.center

                offset.x = location.x - center.x

                offset.y = location.y - center.y

                // Disable the behavior while the item is manipulated by the pan recognizer.

                stickyBehavior.isEnabled = false

            case .changed:

                // Get reference bounds.

                let referenceBounds = view.bounds

                let referenceWidth = referenceBounds.width

                let referenceHeight = referenceBounds.height

                // Get item bounds.

                let itemBounds = itemView.bounds

                let itemHalfWidth = itemBounds.width / 2.0

                let itemHalfHeight = itemBounds.height / 2.0

                // Apply the initial offset.

                location.x -= offset.x

                location.y -= offset.y

                // Bound the item position inside the reference view.

                location.x = max(itemHalfWidth, location.x)

                location.x = min(referenceWidth - itemHalfWidth, location.x)

                location.y = max(itemHalfHeight, location.y)

                location.y = min(referenceHeight - itemHalfHeight, location.y)

                // Apply the resulting item center.

                itemView.center = location

            case .cancelled, .ended:

                // Get the current velocity of the item from the pan gesture recognizer.

                let velocity = pan.velocity(in: view)

                // Re-enable the stickyCornersBehavior.

                stickyBehavior.isEnabled = true

                // Add the current velocity to the sticky corners behavior.

                stickyBehavior.addLinearVelocity(velocity)

            default: ()

        }

    }

}