UIImagePickerController 致命错误：在展开可选值时意外发现 nil

Question

我有以下代码：

func startCameraFromViewController(viewController: UIViewController, withDelegate delegate:
protocol<UIImagePickerControllerDelegate, UINavigationControllerDelegate>) -> Bool {

    if (UIImagePickerController.isSourceTypeAvailable(.Camera) == false) {
        return false
    }

    let cameraController = UIImagePickerController()
    cameraController.sourceType = .Camera
    cameraController.mediaTypes = [kUTTypeMovie as String]
    cameraController.allowsEditing = false
    cameraController.delegate = delegate

    presentViewController(cameraController, animated: true, completion: nil)
    return true    
}

它的调用方式如下：

func recordVideoButtonTapped() {
    startCameraFromViewController(self, withDelegate: self)
}

但由于某种原因，我收到以下错误

致命错误：在展开可选值时意外发现 nil

在这一行：

 presentViewController(cameraController, animated: true, completion: nil)

请帮忙调试。

这个问题不同于

What does "fatal error: unexpectedly found nil while unwrapping an Optional value" mean?

这是一个与解包选项相关的更普遍的问题

这个问题专门针对 UIImagePickerController

Answer 1

我使用的是 Xcode 6.4，它运行良好：

import UIKit
import MobileCoreServices

class FirstViewController: UIViewController, UIImagePickerControllerDelegate, UINavigationControllerDelegate {

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    func startCameraFromViewController(viewController: UIViewController, withDelegate delegate:
        protocol<UIImagePickerControllerDelegate, UINavigationControllerDelegate>) -> Void {

            if (UIImagePickerController.isSourceTypeAvailable(.Camera) == false) {
                println("fail")
                //return false
            }

            let cameraController = UIImagePickerController()
            cameraController.sourceType = .Camera
            cameraController.mediaTypes = [kUTTypeMovie as String]
            cameraController.allowsEditing = false
            cameraController.delegate = delegate

            presentViewController(cameraController, animated: true, completion: nil)
            println("ok")
            //return true
    }

    @IBAction func ToSecondPressed(sender: AnyObject) {
        startCameraFromViewController(self, withDelegate: self)  
    }
}