标准化社交媒体分享方法中的“致命错误:在打开可选值时意外发现nil”

Question

我正试图在我的SocialMediaSharingDelegate中创建一种标准化的社交媒体分享方法。就像我在facebookShare方法中所做的那样，指定一个单独的社交媒体服务非常好用。因此，为了扩展这一点，我创建了一个具有可能的服务类型的枚举，shareOn方法接受任何有效的服务名称来执行共享。

这看起来很简单。但是，当我运行代码并单击调用shareOn方法的按钮时，我得到一个运行时错误。在让serviceViewController...行上出现错误"fatal error: unexpectedly nil while an Optional value“。就在守卫口供之后。

如果我在该行将socialMediaService.associatedService()替换为SLServiceTypeFacebook，也可以很好地工作，这很奇怪，因为在guard语句中使用socialMediaService.associatedService()没有问题。

目前，我只能为每个服务创建单独的函数，但就代码重用而言，这并不理想。

import Foundation
import UIKit
import Social

enum SocialMediaEnum : String {
    case Twitter = "SLServiceTypeTwitter"
    case Facebook = "SLServiceTypeFacebook"

    // This internal enum function returns the SLServiceType associated with the current enum value
    func associatedService() -> String {
        var serviceName: String

        switch self {
        case .Twitter:
            serviceName = "SLServiceTypeTwitter"
        case .Facebook:
            serviceName = "SLServiceTypeFacebook"
        }
        return serviceName
    }
}

class socialMediaSharing : SocialMediaSharingDelegate {

    func shareOn(let socialMediaService: SocialMediaEnum, sender: subclassedUIButton?, currentViewController: UIViewController) {
        guard SLComposeViewController.isAvailableForServiceType(socialMediaService.associatedService())  else {
            let alert = UIAlertController(title: "Accounts", message: "Please login to a Twitter account to tweet.", preferredStyle: UIAlertControllerStyle.Alert)
            alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: nil))
            currentViewController.presentViewController(alert, animated: true, completion: nil)
            return
        }

        let serviceViewController :SLComposeViewController = SLComposeViewController(forServiceType: socialMediaService.associatedService())

        if let imageUrl = sender?.urlString {
            if let url:NSURL = NSURL(string: imageUrl  as String) {
                serviceViewController.addURL(url)
            }
        }
        currentViewController.presentViewController(serviceViewController, animated: true, completion: nil)
    }

    func facebookShare(sender: subclassedUIButton?, currentViewController: UIViewController) {
        guard SLComposeViewController.isAvailableForServiceType(SLServiceTypeFacebook)  else {
            let alert = UIAlertController(title: "Accounts", message: "Please login to a Facebook account to share.", preferredStyle: UIAlertControllerStyle.Alert)
            alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.Default, handler: nil))
            currentViewController.presentViewController(alert, animated: true, completion: nil)
            return
        }

        let shareToFacebook :SLComposeViewController = SLComposeViewController(forServiceType: SLServiceTypeFacebook)
        if let imageUrl = sender?.urlString {
            if let url:NSURL = NSURL(string: imageUrl  as String) {
                shareToFacebook.addURL(url)
            }
        }
        currentViewController.presentViewController(shareToFacebook, animated: true, completion: nil)
    }

}

Answer 1

SLServiceTypeTwitter和SLServiceTypeFacebook是关键字，因此在switch语句中不应该有引号。它们实际持有的值是“com.apple.social.(服务名称)”。