swift - 无法将表达式的类型 '()' 转换为类型 'UINavigationController?'

Question

我无法解决 AppDelegate.swift 中的错误。

我收到一条消息“无法将表达式的类型‘()’转换为类型‘UINavigationController？’

有人给我建议吗？

import UIKit

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?


func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    // Override point for customization after application launch.
    window = UIWindow(frame: UIScreen.mainScreen().bounds)

    let tabBarController = UITabBarController()
    let alarmViewController = AlarmViewController(style: .Plain)
    let recorderViewController = RecorderViewController()
    let playViewController = PlayViewController()

    let tabController1 = UINavigationController(rootViewController: recorderViewController)
    tabController1?.tabBarItem = UITabBarItem(title: "Recorder", image: UIImage(named: "tabbar_microphone"), tag: 1)

    let tabController2 = UINavigationController(rootViewController: playViewController)
    tabController2?.tabBarItem = UITabBarItem(title: "Cheer me up!", image: UIImage(named: "tabbar_play"), tag: 2)

    let tabController3 = UINavigationController(rootViewController: alarmViewController!)
    tabController3?.tabBarItem = UITabBarItem(title: "Alarm", image: UIImage(named: "tabbar_alarm"), tag: 3)

    ******* here's a place I got the message 'Cannot convert the expression's type '()' to type 'UINavigationController?' *******
    tabBarController.viewControllers = [tabController1, tabController2, tabController3]

    window?.rootViewController = tabBarController
    window?.makeKeyAndVisible()
    return true
}

Answer 1

您的 tabControllerX 都是可选的(正如您似乎意识到的那样，因为您在分配 UITabBarItem 时将它们全部引用为 ? )所以你需要打开它们。考虑到您的结构，最简单的方法就是简单地更改行

tabBarController.viewControllers = [tabController1!, tabController2!, tabController3!]
// Unwrap tabControllers

我会用不同的方式来做，因为我不喜欢在我的逻辑中有可选项，而不知道它们是否为 nil，比如

if let tabController1 = UINavigationController(rootViewController: recorderViewController)
    // Now you know it's a tabController!
    tabController1.tabBarItem = UITabBarItem(title: "Recorder", image: UIImage(named: "tabbar_microphone"), tag: 1) 
    // ...
} else {
    // what are you going to do if it's nil?
}