如何在帮助程序类中使用DataSnapshot对象？

Question

我打算使用一个类从Firebase数据库检索条目。那里的方法包括例如getAllProducts() -> [Product]和getProductsByCategory(category:) -> [Product]。

在这些方法中，我从Firebase调用observeSingleEvent()方法以在特定路径上获取快照-可以正常工作。

现在，这样我就不必在上述方法之一中创建产品对象时重复我自己了：

class FirebaseFactory {
self.ref.child("products").observeSingleEvent(of: .value) { (snapshot) in
    var products = [Product]()
    for product in snapshot.children.allObjects as! [DataSnapshot] {
        let productToAdd = self.createProduct(product)   
    }
}

助手类createProduct()如下所示：

func createProduct(productSnapshot: DataSnapshot) -> Product? {
        let identifier = productSnapshot.key
        if let ean = productSnapshot.childSnapshot(forPath: "ean").value as? Int,
            let name = productSnapshot.childSnapshot(forPath: "name").value as? String,
            let category = productSnapshot.childSnapshot(forPath: "category").value as? String,
            let productCategory = Category(rawValue: category){
            let product = Product(identifier: identifier, category: productCategory, ean: ean, name: name)
            return product
        }
        return nil
    }

现在，即使for-Loop中的产品对象的类型为DataSnapshot，而我的助手函数也希望它作为参数，Xcode会向我显示失败

'DataSnapshot' is not convertible to 'FirebaseFactory'，

其中FirebaseFactory是我的自定义类，所有这些方法都位于...

类的概述在这里（省略了更多方法）：

class FirebaseFactory {
  /// Captures all available products in an array
  func getAvailableProducts(completion: @escaping ([Product]) -> Void) {
    self.ref.child("products").observeSingleEvent(of: .value) { (snapshot) in
      var products = [Product]()
      for product in snapshot.children.allObjects as! [DataSnapshot] {
        let productToAdd = self.createProduct(product)
        products.append(productToAdd)
      }
    }
    completion(products)
  }

  /// Helper function to create a product from the Firebase Node
  func createProduct(productSnapshot: DataSnapshot) -> Product? {
    let identifier = productSnapshot.key
    if let ean = productSnapshot.childSnapshot(forPath: "ean").value as? Int,
      let name = productSnapshot.childSnapshot(forPath: "name").value as? String,
      let category = productSnapshot.childSnapshot(forPath: "category").value as? String,
      let productCategory = Category(rawValue: category){
        let product = Product(identifier: identifier, category: productCategory, ean: ean, name: name)
        return product
    }
    return nil
  }
}

知道这里发生了什么吗？