ParentVC.swift

import UIKit

class ParentVC: UIViewController {

    enum LeftItem {
        case none
        case backSystemDefault
        case backGeneral
        case backCustom(image: UIImage?, title: String?, leftSpaceCloseToDefault:Bool)
        case backFn(visibilityFn:(() -> Bool), backFn:(() -> ()))
        case backCustomFn(image: UIImage?, title: String?, leftSpaceCloseToDefault:Bool, visibilityFn:(() -> Bool), backFn:(() -> ()))
        case customView(view: UIView)
    }
    
    var leftItem: LeftItem = LeftItem.backGeneral {
        didSet {
            self.updateNavigationBarUI()
        }
    }
    
    lazy var backButtonImage = UIImage(named: "back_button")?.imageWithRenderingMode(UIImageRenderingMode.AlwaysOriginal)

    func navigationControllerPop() {
        self.navigationController?.popViewControllerAnimated(true)
    }

    func createEmptyButton() -> UIBarButtonItem {
        return UIBarButtonItem(title: "", style: UIBarButtonItemStyle.Plain, target: self, action: nil)
    }
    
    func createCustomBackButton(image: UIImage?, title: String?, leftSpaceCloseToDefault:Bool, backFn: (() -> ())) -> [UIBarButtonItem] {
        var arr: [UIBarButtonItem] = []
        if leftSpaceCloseToDefault {
            let negativeSpacer = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.FixedSpace, target: nil, action: nil)
            negativeSpacer.width = -8
            arr.append(negativeSpacer)
        }
        if let image = image,
            title = title {
            let button = BlockBarButtonItem(image: image, title: title, actionHandler: backFn)
            arr.append(button)
        } else {
            if let image = image {
                let imageButton = BlockBarButtonItem(image: image, actionHandler: backFn)
                imageButton.title = title
                arr.append(imageButton)
            }
            if let title = title {
                let titleButton = BlockBarButtonItem(title: title, actionHandler: backFn)
                arr.append(titleButton)
            }
        }
        return arr
    }

    func createBackButtonGeneral() -> [UIBarButtonItem] {
        return createCustomBackButton(backButtonImage, title: nil, leftSpaceCloseToDefault: true, backFn: navigationControllerPop)
    }

    func createBackButtonWithFn(backFn: (() -> ()) ) -> [UIBarButtonItem] {
        return createCustomBackButton(backButtonImage, title: nil, leftSpaceCloseToDefault: true, backFn: backFn)
    }
    
    func updateNavigationBarUI() {
        self.navigationItem.backBarButtonItem = createEmptyButton()
        self.navigationItem.leftBarButtonItem = nil
        self.navigationItem.leftBarButtonItems = nil
        self.navigationItem.hidesBackButton = true
        switch self.leftItem {
        case .none:
            break;
        case .backSystemDefault:
            self.navigationItem.backBarButtonItem = nil
            self.navigationItem.hidesBackButton = false
        case .backGeneral:
            if (self.navigationController?.viewControllers.count > 1) {
                self.navigationItem.leftBarButtonItems = createBackButtonGeneral()
            }
        case .backFn(let visibilityFn, let backFn):
            if (visibilityFn()) {
                self.navigationItem.leftBarButtonItems = createBackButtonWithFn(backFn)
            }
        case .backCustom(let image, let title, let leftSpaceCloseToDefault):
            if (self.navigationController?.viewControllers.count > 1) {
                self.navigationItem.leftBarButtonItems = createCustomBackButton(image, title: title, leftSpaceCloseToDefault: leftSpaceCloseToDefault, backFn: navigationControllerPop)
            }
        case .backCustomFn(let image, let title, let leftSpaceCloseToDefault, let visibilityFn, let backFn):
            if (visibilityFn()) {
                self.navigationItem.leftBarButtonItems = createCustomBackButton(image, title: title, leftSpaceCloseToDefault: leftSpaceCloseToDefault, backFn: backFn)
            }
        case .customView(let view):
            self.navigationItem.leftBarButtonItem = UIBarButtonItem(customView: view)
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()

        self.updateNavigationBarUI()
    }
}