Ios 使用MDCTabBarViewController时的空白视图控制器-Swift 4.2

我在使用MDCTabBarViewController时遇到问题，因为所有的ViewController都显示为空白（例如：使用故事板或通过

view.addSubview（）

）添加视图），但似乎唯一受影响的是ViewController的背景 （例如：

view.backgroundColor=UIColor.green

）

选项卡视图控制器：

import UIKit
import MaterialComponents

class TabViewController: MDCTabBarViewController {

override func viewDidLoad() {
    super.viewDidLoad()

    //view.backgroundColor = MDCPalette.grey.tint100
    loadTabBar()
}

func loadTabBar() {

    let firstVC = FeedViewController()
    firstVC.tabBarItem = UITabBarItem(title: "Feed", image: #imageLiteral(resourceName: "feed"), tag: 0)

    let secondVC = TableViewController()
    secondVC.tabBarItem =  UITabBarItem(title: "Timetable", image: #imageLiteral(resourceName: "table"), tag: 1)

    let thirdVC = ToDoViewController()
    thirdVC.tabBarItem = UITabBarItem(title: "To-Do", image: #imageLiteral(resourceName: "todo"), tag: 2)

    let viewControllersArray = [firstVC, secondVC, thirdVC]
    viewControllers = viewControllersArray

    let childVC = viewControllers.first
    selectedViewController = childVC

    tabBar?.delegate = self

    tabBar?.items = [firstVC.tabBarItem,
                     secondVC.tabBarItem ,
                     thirdVC.tabBarItem]

    tabBar?.selectedItem = tabBar?.items.first

    tabBar?.backgroundColor = MDCPalette.lightBlue.tint500
    tabBar?.selectedItemTintColor = .white
    tabBar?.unselectedItemTintColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.3)
    tabBar?.inkColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.1)
    tabBar?.alignment = .justified


}

override func tabBar(_ tabBar: MDCTabBar, didSelect item: UITabBarItem) {

    switch item.tag {
    case 0:
        print("feed")
        tabBar.backgroundColor = MDCPalette.lightBlue.tint500
        selectedViewController = viewControllers[0]
    case 1:
        print("table")
        tabBar.backgroundColor = MDCPalette.purple.tint500
        selectedViewController = viewControllers[1]
    case 2:
        print("todo")
        tabBar.backgroundColor = MDCPalette.teal.tint500
        selectedViewController = viewControllers[2]
    default:
        print("feed")
        tabBar.backgroundColor = MDCPalette.lightBlue.tint500
        selectedViewController = viewControllers[0]
    }

}


}

import UIKit

class FeedViewController: UIViewController {

@IBOutlet weak var test: UILabel!

override func viewDidLoad() {
    super.viewDidLoad()

    view.backgroundColor = UIColor.green
    print(test.text ?? "unable to fetch")
}

} 

所有FeedViewController、TableViewController和ToDoController都是新创建的文件，其中没有代码

当

view.backgroundColor=UIColor.green

添加标签时

view.addSubview（标签）

提前谢谢大家

更新：

当通过故事板添加视图并尝试访问它时，应用程序崩溃，表示标签为零

FeedViewController：

import UIKit
import MaterialComponents

class TabViewController: MDCTabBarViewController {

override func viewDidLoad() {
    super.viewDidLoad()

    //view.backgroundColor = MDCPalette.grey.tint100
    loadTabBar()
}

func loadTabBar() {

    let firstVC = FeedViewController()
    firstVC.tabBarItem = UITabBarItem(title: "Feed", image: #imageLiteral(resourceName: "feed"), tag: 0)

    let secondVC = TableViewController()
    secondVC.tabBarItem =  UITabBarItem(title: "Timetable", image: #imageLiteral(resourceName: "table"), tag: 1)

    let thirdVC = ToDoViewController()
    thirdVC.tabBarItem = UITabBarItem(title: "To-Do", image: #imageLiteral(resourceName: "todo"), tag: 2)

    let viewControllersArray = [firstVC, secondVC, thirdVC]
    viewControllers = viewControllersArray

    let childVC = viewControllers.first
    selectedViewController = childVC

    tabBar?.delegate = self

    tabBar?.items = [firstVC.tabBarItem,
                     secondVC.tabBarItem ,
                     thirdVC.tabBarItem]

    tabBar?.selectedItem = tabBar?.items.first

    tabBar?.backgroundColor = MDCPalette.lightBlue.tint500
    tabBar?.selectedItemTintColor = .white
    tabBar?.unselectedItemTintColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.3)
    tabBar?.inkColor = UIColor(red: 0/255, green: 0/255, blue: 0/255, alpha: 0.1)
    tabBar?.alignment = .justified


}

override func tabBar(_ tabBar: MDCTabBar, didSelect item: UITabBarItem) {

    switch item.tag {
    case 0:
        print("feed")
        tabBar.backgroundColor = MDCPalette.lightBlue.tint500
        selectedViewController = viewControllers[0]
    case 1:
        print("table")
        tabBar.backgroundColor = MDCPalette.purple.tint500
        selectedViewController = viewControllers[1]
    case 2:
        print("todo")
        tabBar.backgroundColor = MDCPalette.teal.tint500
        selectedViewController = viewControllers[2]
    default:
        print("feed")
        tabBar.backgroundColor = MDCPalette.lightBlue.tint500
        selectedViewController = viewControllers[0]
    }

}


}

import UIKit

class FeedViewController: UIViewController {

@IBOutlet weak var test: UILabel!

override func viewDidLoad() {
    super.viewDidLoad()

    view.backgroundColor = UIColor.green
    print(test.text ?? "unable to fetch")
}

} 

错误：致命错误：在展开可选值时意外发现nil

请尝试以下操作：

override func viewDidLoad() {
    super.viewDidLoad()

    view?.backgroundColor = UIColor.green
    print(test?.text ?? "unable to fetch")
}