Firebase numChildren（）在本机反应中始终返回0

Question

我试图从firebase数据库中获取子计数。而且每次执行代码时，我都会得到0而不是实际计数作为输出，或者是否有其他方法可以从数据库中获取子代计数。

this.itemsRef = firebase.database().ref('sample').child((new Date(), 'dd-mmm-yyyy').toLowercase()).child('dataContainer')

this.itemsRef.on('value', snapShot => {
     this.state.count = snapShot.numChildren()
}) 

JSON数据：

  "sample" : {
  "20-jun-2018" : {
    "dataContainer" : {
      "1" : {
        "header" : "sample",
        "price" : 2.33
      },
      "-LFTL4hUnVr4WbZRMeBU" : {
        "header" : "item",
        "imageLink" : "https://firebasestorage.googleapis.com/v0/b/annam-6e679.appspot.com/o/Specials%2Fshawaramas?alt=media&token=a7907c48-005d-4d52-bb6e-26e97bda0c4e",
        "price" : "5.65"
      }
    }
  },
  "21-jun-2018" : {
    "dataContainer" : [ null, {
      "header" : "widget",
      "imageLink" : "https://firebasestorage.googleapis.com/v0/b/annam-6e679.appspot.com/o/Specials%2FMasala%20Dosai?alt=media&token=a103c1be-37a1-4ca3-9da8-5f87d5c870a8",
      "price" : "8.68"
    }, {
      "header" : "images"
    } ]
  },
  "22-jun-2018" : {
    "dataContainer" : [ null, {
      "header" : "sample data",
      "imageLink" : "https://firebasestorage.googleapis.com/v0/b/annam-6e679.appspot.com/o/Specials%2FDosai?alt=media&token=1191f1bb-6035-46b0-b24c-7b18bef65036",
      "price" : "4.56"
    }, {
      "header" : "data"
    } ]
  }
}