Do I use Strategy pattern correctly in my dart code? - dart

Question

I'm trying to apply a strategy pattern in my dart code, but I don't know why the overridden method in child class doesn't get called.
Below is an example push notification message that will be sent to mobile devices, the data in "content" node can be different depends on the "type" value. In order to deserialize the message correctly, I created classes as below,
{
"priority": "high",
"to": "123342"
"notification": {
"body": "this is a body",
"title": "this is a title"
},
"data": {
"click_action": "FLUTTER_NOTIFICATION_CLICK",
"status": "done",
"type": "Order",
"content" : {
"order_id" : "[order_id]"
}
},
}
class Message<T extends BaseDataContent> {
String to;
String priority;
Notification notification;
Data<T> data;
Message({this.to, this.priority, this.notification, this.data});
Message.fromJson(Map<String, dynamic> json) {
to = json['to'];
priority = json['priority'];
notification = json['notification'] != null
? new Notification.fromJson(json['notification'])
: null;
data = json['data'] != null
? new Data.fromJson(json['data'])
: null;
}
}
class Notification {
String title;
String body;
Notification({this.title, this.body});
Notification.fromJson(Map<String, dynamic> json) {
title = json['title'];
body = json['body'];
}
}
class Data<T extends BaseDataContent> {
String click_action;
String status;
String type;
T content;
Data({this.click_action, this.status, this.type, this.content});
Data.fromJson(Map<String, dynamic> json) {
click_action = json['click_action'];
status = json['status'];
type = json['type'];
content = json['content'] != null?
this.content.deserializeContent(json['content'])
: null;
}
}
abstract class BaseDataContent{
BaseDataContent deserializeContent(Map<String, dynamic> jsonString);
}
class OrderMessageContent extends BaseDataContent {
int orderId;
OrderMessageContent({this.orderId}) : super();
#override
OrderMessageContent deserializeContent(Map<String, dynamic> jsonString){
///Deserialize Content json.
}
}
To test my code, I wrote some demo code as below
String json = '{"priority": "high", "to": "343434", "data":{"click_action":"343434","content":{"order_id" : "1234"}}}';
var jsonResponse = jsonDecode(json);
var result = Message<OrderMessageContent>.fromJson(jsonResponse);
The code is failed when it reaches to line
this.content.deserializeContent(json['content'])
The error message is "NoSuchMethodError: the method deserializeContent was called on null. Receiver:null". I don't know why deserializeContent method in OrderMessageContent doesn't get called, please help.
Thanks.

Answer 1

In Data constructor you called a method on a null object, where you called deserializeContent. You should either make that method static or initialize content first.
Data.fromJson(Map<String, dynamic> json) {
click_action = json['click_action'];
status = json['status'];
type = json['type'];
content = json['content'] != null?
this.content.deserializeContent(json['content'])
: null;
}