无法将'__NSDictionaryM'（0x1111152b0）类型的值转换为'FIRDataSnapshot'Firebase Swift 3

Question

我试图从Firebase数据库中读取嵌套数据结构，但是当[String：AnyObject]类型的对象可能为nil时，我不知道如何管理这种情况。 当调用readFeesCleaner(callback_)时，它会抛出错误。

  func readFeesCleaner(callback: @escaping ((_ feesCleaner: FeesCleaner) -> Void)) {

 dbRef.child("FeesCleaner").child(self.uidOfTextField!).observeSingleEvent(of: .value, with: { (snapshot: FIRDataSnapshot) in

        guard !(snapshot.value is NSNull) else {
            return
        }

       //throws error: signal SIGABRTCould not cast value of type '__NSDictionaryM' (0x1111152b0) to 'FIRDataSnapshot' (0x10ef16d18).
            let feesCleanersReceived = FeesCleaner(snapshot: (snapshot.value)! as! FIRDataSnapshot)
                callback(feesCleanersReceived)

    }) { (error:Error) in
        print(#line, "\(error.localizedDescription)")
    }
 } 


struct FeesCleaner {

   var outstandingFees: AnyObject!
   var timeStampFeesSaved: [String:AnyObject]!
   var backgroundCheck: AnyObject!

    init(
       outstandingFees: AnyObject? = nil, //value might not exist when reading
       timeStampFeesSaved: [String:AnyObject]? = nil,// value might not exist when reading
       backgroundCheck: AnyObject) {

       self.outstandingFees = outstandingFees
       self.timeStampFeesSaved = timeStampFeesSaved
       self.backgroundCheck = backgroundCheck   
  }//end of init

    //read data here
     [full struct data here][1]
      https://gist.github.com/bibscy/dc48f7107459379e045a50fdbbc35335


}//end of struct

Answer 1

这里有很多问题。第一：

当[String：AnyObject]类型的对象可能为nil时，如何管理大小写。

您已使用先前的声明处理了这一点，并指出您也可以添加

if snapshot.exists == false {return}

第二：你必须正确处理选项 - 如果var可能是零，你需要代码来处理这种情况而不是通过它。如果你强行打开一个可选项，你基本上就是说它肯定永远不会是零，所以基本上不要这样做。

一个修复可能是简单地将快照作为DataSnapshot传递，然后一次拉出一个属性;如果它们存在，则指定它们，如果未设置为0或nil或其他占位符。

在Firebase闭包内有类似的东西：

let feesCleanersReceived = FeesCleaner(withSnapshot: snapshot)

然后你的结构如下：注意我们正在利用零合并算子，??

struct FeesCleanerStruct {
    var outstandingFees: String?
    var timeStampFeesSaved: String?

    init(withSnapshot: DataSnapshot) {
        let dict = withSnapshot.value as! [String: Any]
        self.outstandingFees = dict["outstandingFees"] as? String ?? "0.0"
        self.timeStampFeesSaved = dict["timeStampFeesSaved"] as? String ?? "0"
    }
}