/*

    Copyright (C) 2017 Apple Inc. All Rights Reserved.

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    A view controller that demonstrates how to add and update `UIFocusGuide`s.

*/

import UIKit

class FocusGuidesViewController: UIViewController {

    // MARK: Properties

    @IBOutlet var topLeftButton: UIButton!

    @IBOutlet var topRightButton: UIButton!

    @IBOutlet var bottomLeftButton: UIButton!

    private var focusGuide: UIFocusGuide!

    override var preferredFocusEnvironments: [UIFocusEnvironment] {

        return [topLeftButton]

    }

    // MARK: UIViewController

    override func viewDidLoad() {

        super.viewDidLoad()

        /*

            Create a focus guide to fill the gap below button 2 and to the right

            of button 3.

        */

        focusGuide = UIFocusGuide()

        view.addLayoutGuide(focusGuide)

        // Anchor the top left of the focus guide.

        focusGuide.leftAnchor.constraint(equalTo: topRightButton.leftAnchor).isActive = true

        focusGuide.topAnchor.constraint(equalTo: bottomLeftButton.topAnchor).isActive = true

        // Anchor the width and height of the focus guide.

        focusGuide.widthAnchor.constraint(equalTo: topRightButton.widthAnchor).isActive = true

        focusGuide.heightAnchor.constraint(equalTo: bottomLeftButton.heightAnchor).isActive = true

    }

    // MARK: UIFocusEnvironment

    override func didUpdateFocus(in context: UIFocusUpdateContext, with coordinator: UIFocusAnimationCoordinator) {

        super.didUpdateFocus(in: context, with: coordinator)

        /*

            Update the focus guide's `preferredFocusedView` depending on which

            button has the focus.

        */

        guard let nextFocusedView = context.nextFocusedView else { return }

        switch nextFocusedView {

            case topRightButton:

                focusGuide.preferredFocusEnvironments = [bottomLeftButton]

            case bottomLeftButton:

                focusGuide.preferredFocusEnvironments = [topRightButton]

            default:

                focusGuide.preferredFocusEnvironments = []

        }

    }

}