ios - 火力地堡， swift : `runTransactionBlock()` returns null

Question

每次我运行 runTransactionBlock 它都会给我 null，尽管那个位置有一个节点:-

FIRMutableData (top-most transaction) (null)

在行:- print(totalPost)

func updateTotalNoOfPost(){


    let prntRef = FIRDatabase.database().reference().child("TotalPosts")
    prntRef.observeSingleEventOfType(.Value, withBlock: {(totalSnap) in

        if totalSnap.exists(){

            prntRef.child("noOfTotalPost").runTransactionBlock({ (totalPost: FIRMutableData) -> FIRTransactionResult in

                print(FIRAuth.auth()!.currentUser!.uid)//Giving me correct userID 
                print(totalPost)//<Null>
                print(prntRef.child("noOfTotalPost"))//Giving me correct path to that node 

                totalPost.value = totalPost.value as! Int + 1
                return FIRTransactionResult.successWithValue(totalPost)
                }, andCompletionBlock: { (err, TF, snap) in

                    print(err?.localizedDescription)
                    print(TF)
                    print(snap!.value)
                    FIRTransactionResult.abort()// Am i using this right?
            })
        }else{

            prntRef.setValue(["noOfTotalPost": 1])

        }
    })
}

我的 JSON 树结构是这样的:-

 {
  "TotalPosts" : {
    "noOfTotalPost" : 1
     },
   "Users" : {
    "FBLXPOxBomakPCQuDTilGC7Becu2" : {...},
     "1DASWPOxBomakPCdasd1d123au6" : {...},...}}

我正在为我的数据库使用默认的安全规则:-

      {
       "rules": {
          ".read": "auth != null",
          ".write": "auth != null"

        }
      }

我只想在每次任何用户创建帖子时增加一个 noOfTotalPost 子节点的值:-

我已经有了使用 observeSingleEventOfType 的解决方法。:

    let prntRef = FIRDatabase.database().reference().child("TotalPosts")
    prntRef.child("noOfTotalPost").observeSingleEventOfType(.Value, withBlock: {(totalSnap) in

        if totalSnap.exists(){

            if let tNo = totalSnap.value as? Int{

               prntRef.child("noOfTotalPost").setValue(tNo+1)//Working Fine

             }
          })
        }else{

            prntRef.setValue(["noOfTotalPost": 1])

        }
    })

但无法弄清楚为什么 runTransactionBlock 不起作用!

Answer 1

正如@Frank 在评论中所说，runTransactionBloack 最初返回 NSNull 是预期的行为，但如果该位置已经有一个值，它会再次触发。如果在更新值时发生冲突，它将再次触发。

代码:-

func updateTotalNoOfPost(completionBlock : (() -> Void)){


    let prntRef = FIRDatabase.database().reference().child("TotalPosts")

    prntRef.child("noOfTotalPost").runTransactionBlock({ (noOfPosts) -> FIRTransactionResult in
        if let totalPost = noOfPosts.value as? Int{

            noOfPosts.value = totalPost + 1
            return FIRTransactionResult.successWithValue(noOfPosts)
        }else{

            return FIRTransactionResult.successWithValue(noOfPosts)

        }
        }, andCompletionBlock: {(error,completion,snap) in

                print(error?.localizedDescription)
                print(completion)
                print(snap)
            if !completion {

                print("The value wasn't able to Update")
                }else{

                completionBlock()
            }
    })

}

基本概念:-

Data in transaction is null