ios - Firebase 元素在调用、iOS、Swift 中可能不存在

Question

我正在使用 firebase 数据库和存储来保存图像及其描述。目前，我将其编码为具有 8 个图像和 8 个描述。我正在尝试弄清楚如何获得它，以便如果用户只选择 4 张图片并输入 4 个描述，它就可以将其回调。它与 8 一起工作正常，但是当我仅添加 4 并选择要调用的表时，它会崩溃并出现错误，即第五张照片变量不包含任何内容。

下面是我用来调用表数据的代码及其崩溃的地方。

func configureCell(post: Post) {
    self.post = post
    self.carImages = []

    if post.imageUrl1 != "" {
        self.carImages.append(post.imageUrl1)
    }
    if post.imageUrl2 != ""{
        self.carImages.append(post.imageUrl2)
    }
    if post.imageUrl3 != "" {
        self.carImages.append(post.imageUrl3)
    }
    if post.imageUrl4 != "" {
        self.carImages.append(post.imageUrl4)
    }
    if post.imageUrl5 != "" {
        print ("Tony: \(post.imageUrl5)")
        self.carImages.append(post.imageUrl5)
    }
    if post.imageUrl6 != "" {
        self.carImages.append(post.imageUrl6)
    }
    if post.imageUrl7 != "" {
        self.carImages.append(post.imageUrl7)
    }
    if post.imageUrl8 != "" {
        self.carImages.append(post.imageUrl8)
    }

崩溃发生在 imageUrl5

import Foundation
import Firebase

class Post {




private var _imageUrl1: String!
private var _imageUrl2: String!
private var _imageUrl3: String!
private var _imageUrl4: String!
private var _imageUrl5: String!
private var _imageUrl6: String!
private var _imageUrl7: String!
private var _imageUrl8: String!

private var _postKey: String!
private var _postRef: DatabaseReference!
private var _photoInfo1: String!
private var _photoInfo2: String!
private var _photoInfo3: String!
private var _photoInfo4: String!
private var _photoInfo5: String!
private var _photoInfo6: String!
private var _photoInfo7: String!
private var _photoInfo8: String!




var profileImageUrl: String {
    return _profileImageUrl
}

var imageUrl1: String {
    return _imageUrl1
}
var imageUrl2: String {
    return _imageUrl2
}
var imageUrl3: String {
    return _imageUrl3
}
var imageUrl4: String {
    return _imageUrl4
}
var imageUrl5: String {
    return _imageUrl5
}
var imageUrl6: String {
    return _imageUrl6
}   
var imageUrl7: String {
    return _imageUrl7
}   
var imageUrl8: String {
    return _imageUrl8
}


var postKey: String {
    return _postKey
}



var photoInfo1: String {
    return _photoInfo1
}
var photoInfo2: String {
    return _photoInfo2
}
var photoInfo3: String {
    return _photoInfo3
}
var photoInfo4: String {
    return _photoInfo4
}
var photoInfo5: String {
    return _photoInfo5
}
var photoInfo6: String {
    return _photoInfo6
}
var photoInfo7: String {
    return _photoInfo7
}
var photoInfo8: String {
    return _photoInfo8
}

init(imageUrl1: String, imageUrl2: String, imageUrl3: String, imageUrl4: String, imageUrl5: String, imageUrl6: String, imageUrl7: String, imageUrl8: String, photoInfo1: String, photoInfo2: String, photoInfo3: String, photoInfo4: String, photoInfo5: String, photoInfo6: String, photoInfo7: String, photoInfo8: String) {


    self._imageUrl1 = imageUrl1
    self._imageUrl2 = imageUrl2
    self._imageUrl3 = imageUrl3
    self._imageUrl4 = imageUrl4
    self._imageUrl5 = imageUrl5
    self._imageUrl6 = imageUrl6
    self._imageUrl7 = imageUrl7
    self._imageUrl8 = imageUrl8

    self._photoInfo1 = photoInfo1
    self._photoInfo2 = photoInfo2
    self._photoInfo3 = photoInfo3
    self._photoInfo4 = photoInfo4
    self._photoInfo5 = photoInfo5
    self._photoInfo6 = photoInfo6
    self._photoInfo7 = photoInfo7
    self._photoInfo8 = photoInfo8

}

init(postKey: String, postData: Dictionary<String, AnyObject>) {
    self._postKey = postKey



    if let imageUrl1 = postData["imageUrl1"] as? String {
        self._imageUrl1 = imageUrl1
    }
    if let imageUrl2 = postData["imageUrl2"] as? String {
        self._imageUrl2 = imageUrl2
    }
    if let imageUrl3 = postData["imageUrl3"] as? String {
        self._imageUrl3 = imageUrl3
    }
    if let imageUrl4 = postData["imageUrl4"] as? String {
        self._imageUrl4 = imageUrl4
    }
    if let imageUrl5 = postData["imageUrl5"] as? String {
        self._imageUrl5 = imageUrl5
    }
    if let imageUrl6 = postData["imageUrl6"] as? String {
        self._imageUrl6 = imageUrl6
    }
    if let imageUrl7 = postData["imageUrl7"] as? String {
        self._imageUrl7 = imageUrl7
    }
    if let imageUrl8 = postData["imageUrl8"] as? String {
        self._imageUrl8 = imageUrl8
    }


    if let photoInfo1 = postData["photoInfo1"] as? String {
        self._photoInfo1 = photoInfo1
    }
    if let photoInfo2 = postData["photoInfo2"] as? String {
        self._photoInfo2 = photoInfo2
    }
    if let photoInfo3 = postData["photoInfo3"] as? String {
        self._photoInfo3 = photoInfo3
    }
    if let photoInfo4 = postData["photoInfo4"] as? String {
        self._photoInfo4 = photoInfo4
    }
    if let photoInfo5 = postData["photoInfo5"] as? String {
        self._photoInfo5 = photoInfo5
    }
    if let photoInfo6 = postData["photoInfo6"] as? String {
        self._photoInfo6 = photoInfo6
    }
    if let photoInfo7 = postData["photoInfo7"] as? String {
        self._photoInfo7 = photoInfo7
    }
    if let photoInfo8 = postData["photoInfo8"] as? String {
        self._photoInfo8 = photoInfo8
    }

    _postRef = DataService.ds.REF_POSTS.child(_postKey)

}
}

Answer 1

也许通过一个例子来回答

我们有一个带有私有(private)变量的post类，它是可选的——这意味着它可以包含一个值或者可以是nil。这是用字符串表示的吗？

当外部 imageUrl1 被访问时，它可能会返回一个值或者可能是 nil，又是 ?表示。

class Post{
    private var _imageUrl1: String?

    var imageUrl1: String? {
        return _imageUrl1
    }

    init(url: String?) {  //this could be nil!
        self._imageUrl1 = url
    }
}

let aPost = Post(url: "testUrl")

if aPost.imageUrl1 != nil {
    print(aPost.imageUrl1!)
}

//or the preferred way which unwraps the optional automatically
if let aPostUrl = aPost.imageUrl1 {
    print(aPostUrl)
} else {
    print("it was nil")
}

//here's a nil example
let bPost = Post(url: nil)
if let bPostUrl = bPost.imageUrl1 {
    print(bPostUrl)
} else {
    print("it was nil")  //prints it was nil
}