lib/json: add hook to customize the deserialization of plain JSON objects

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --git a/lib/json/serialization.nit b/lib/json/serialization.nit
index 35ade36..3707b19 100644 (file)
--- a/lib/json/serialization.nit
+++ b/lib/json/serialization.nit
@@ -90,7 +90,7 @@ module serialization
 
 import ::serialization::caching
 private import ::serialization::engine_tools
-private import static
+import static
 
 # Serializer of Nit objects to Json string.
 class JsonSerializer
@@ -287,12 +287,17 @@ class JsonDeserializer
                                        end
                                end
 
-                               if not object.keys.has("__class") then
+                               var class_name = object.get_or_null("__class")
+                               if class_name == null then
+                                       # Fallback to custom heuristic
+                                       class_name = class_name_heuristic(object)
+                               end
+
+                               if class_name == null then
                                        errors.add new Error("Serialization Error: JSON object declaration does not declare a `__class`.")
                                        return object
                                end
 
-                               var class_name = object["__class"]
                                if not class_name isa String then
                                        errors.add new Error("Serialization Error: JSON object declaration declares a non-string `__class`.")
                                        return object
@@ -393,6 +398,54 @@ class JsonDeserializer
                errors.clear
                return convert_object(root)
        end
+
+       # User customizable heuristic to get the name of the Nit class to deserialize `json_object`
+       #
+       # This method is called only when deserializing an object without the metadata `__class`.
+       # Return the class name as a `String` when it can be inferred.
+       # Return `null` when the class name cannot be found.
+       #
+       # If a valid class name is returned, `json_object` will then be deserialized normally.
+       # So it must contain the attributes of the corresponding class, as usual.
+       #
+       # ~~~nitish
+       # class MyData
+       #     serialize
+       #
+       #     var data: String
+       # end
+       #
+       # class MyError
+       #     serialize
+       #
+       #     var error: String
+       # end
+       #
+       # class MyJsonDeserializer
+       #     super JsonDeserializer
+       #
+       #     redef fun class_name_heuristic(json_object)
+       #     do
+       #         if json_object.keys.has("error") then return "MyError"
+       #         if json_object.keys.has("data") then return "MyData"
+       #         return null
+       #     end
+       # end
+       #
+       # var json = """{"data": "some other data"}"""
+       # var deserializer = new MyJsonDeserializer(json)
+       # var deserialized = deserializer.deserialize
+       # assert deserialized isa MyData
+       #
+       # json = """{"error": "some error message"}"""
+       # deserializer = new MyJsonDeserializer(json)
+       # deserialized = deserializer.deserialize
+       # assert deserialized isa MyError
+       # ~~~
+       protected fun class_name_heuristic(json_object: JsonObject): nullable String
+       do
+               return null
+       end
 end
 
 redef class Serializable