FirebaseAPIExample.swift

// Instead of this ...

ref.observeSingleEvent(of: .value) { snapshot in
    guard snapshot.exists() else {
        /* HANDLE ERROR */
    }
    guard let value = snapshot.value else {
        /* HANDLE ERROR */
    }
    guard let product = Product(usingMyCustomJSONConversion: value) else {
        /* HANDLE ERROR */
    }
}
// ... and this
let json: Any = product.myCustomJSONEncoding()
ref.setValue(json)

// ... instead do this ...

struct Product: Decodable { ... }

ref.observeSingleEvent(of: .value) { (result: DecodeResult<Product>) -> Void in
  // Use result or handle error    
}

// ... and this
try ref.setValue(product)