/*

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

    See LICENSE.txt for this sample’s licensing information

    Abstract:

    A `UISplitViewController` subclass that is used throughout the sample. Adds a method to allow its child view controllers to move focus from its master view controller to the detail view controller.

*/

import UIKit

class MenuSplitViewController: UISplitViewController {

    // MARK: Properties

    /**

        Set to true from `updateFocusToDetailViewController()` to indicate that

        the detail view controller should be the preferred focused view when

        this view controller is next queried.

    */

    private var preferDetailViewControllerOnNextFocusUpdate = false

    // MARK: UIFocusEnvironment

    override var preferredFocusEnvironments: [UIFocusEnvironment] {

        let environments: [UIFocusEnvironment]

        /*

            Check if a request has been made to move the focus to the detail

            view controller.

        */

        if preferDetailViewControllerOnNextFocusUpdate, let detailViewController = viewControllers.last {

            environments = detailViewController.preferredFocusEnvironments

            preferDetailViewControllerOnNextFocusUpdate = false

        }

        else {

            environments = super.preferredFocusEnvironments

        }

        return environments

    }

    // MARK: Focus helpers

    /**

        Called from a containing `MenuTableViewController` whenever the user

        selects a table view row in a master view controller.

    */

    func updateFocusToDetailViewController() {

        preferDetailViewControllerOnNextFocusUpdate = true

        /*

            Trigger the focus system to re-query the view hierarchy for preferred

            focused views.

        */

        setNeedsFocusUpdate()

        updateFocusIfNeeded()

    }

}