/**
 Maintains the state needed to implement your own `UITabController` replacement, without making any assumptions about
 how your UI looks or works.
 */
public final class TabItemViewControllerRouter {
    /// The tab bar items for the view controllers. You can use these when constructing your UI’s buttons
    public let tabItems: [TabItem]
    
    // MARK: - Immutable private state
    
    /// The view controllers that can be routed between
    private let viewControllers: [UIViewController]
    
    /// Mapping of tab bar items to view controllers
    private let tabItemsToViewControllers: [TabItem: UIViewController]
    
    /// The container view controller serving as the `UITabController` replacement
    private weak var parentViewController: UIViewController?
    
    /// A function for adding a view to the container view controller’s hierarchy
    private let viewHierarchyUpdater: UIView -> Void
    
    // MARK: - Mutable private state
    
    /// The currently selected view controller
    private var selectedViewController: UIViewController?
    
    // MARK: - Initialization
    
    /**
     Create a new tab bar item view controller router.
     
     - parameter viewControllers:      The view controllers that can be routed between
     - parameter parentViewController: The container view controller serving as the `UITabController` replacement
     - parameter viewHierarchyUpdater: A function for adding a view to the container view controller’s hierarchy
     
     - returns: New instance
     */
    public init(tabComponents: [TabComponent], parentViewController: UIViewController, viewHierarchyUpdater: UIView -> Void) {
        self.viewControllers = tabComponents.map { $0.viewController }
        self.parentViewController = parentViewController
        self.viewHierarchyUpdater = viewHierarchyUpdater
        
        self.tabItems = tabComponents.map { $0.tabItem }
        self.tabItemsToViewControllers = Dictionary<TabItem, UIViewController>(zip(tabItems, viewControllers))
    }
    
    /**
     Route based on a new tab bar item having been selected.
     
     - parameter tabItem: Selected tab bar item
     */
    public func selectTabItem(tabItem: TabItem) {
        guard let viewController = tabItemsToViewControllers[tabItem] else { return }
        
        selectViewController(viewController)
    }
    
    /**
     Update which view controller is the selected one.
     
     - parameter viewController: Selected view controller
     */
    private func selectViewController(viewController: UIViewController) {
        let oldValue = selectedViewController
        
        oldValue?.willMoveToParentViewController(nil)
        parentViewController?.addChildViewController(viewController)
        
        viewHierarchyUpdater(viewController.view)
        
        oldValue?.removeFromParentViewController()
        viewController.didMoveToParentViewController(parentViewController)
        
        selectedViewController = viewController
    }
}