feat:增加消息功能

lib/models/ImMessage.dart

@@ -1,3 +1,5 @@
+import 'dart:convert';
+
 class ImMessage {
   final String type;
   final String from;
@@ -11,10 +13,27 @@ class ImMessage {
     required this.content,
   });
 
+  factory ImMessage.fromJsonString(String json) {
+    return ImMessage.fromJson(jsonDecode(json));
+  }
+
+  factory ImMessage.fromJson(Map<String, dynamic> json) {
+    return ImMessage(
+      type: json['type'],
+      from: json['from'],
+      to: json['to'],
+      content: json['content'],
+    );
+  }
+
   Map<String, dynamic> toJson() => {
     'type': type,
     'from': from,
     'to': to,
     'content': content,
   };
+
+  String toJsonString() {
+    return jsonEncode(toJson());
+  }
 }