How can I tell if a string can be json.decode - dart

Question

My cache class
import 'dart:async';
import 'dart:convert';
import 'package:shared_preferences/shared_preferences.dart';
class CacheUtil{
static set(String key, value) async{
if(value is Map || value is List){
value = json.encode(value);
}
SharedPreferences preferences = await SharedPreferences.getInstance();
preferences.setString(key, json.encode(value));
}
static get(String key) async{
SharedPreferences preferences = await SharedPreferences.getInstance();
String data = preferences.getString(key);
return data;
}
}
In the get method ,I want to see if value can be json.decode
what should I do?

Answer 1

Just try to decode it and catch FormatException to know when it failed:
void main() {
var jsonString = '{"abc';
var decodeSucceeded = false;
try {
var decodedJSON = json.decode(jsonString) as Map<String, dynamic>;
decodeSucceeded = true;
} on FormatException catch (e) {
print('The provided string is not valid JSON');
}
print('Decoding succeeded: $decodeSucceeded');
}

Answer 2

I like something that looks like int.tryParse(data).
So I using this.
import 'dart:convert';
class Json {
static String? tryEncode(data) {
try {
return jsonEncode(data);
} catch (e) {
return null;
}
}
static dynamic tryDecode(data) {
try {
return jsonDecode(data);
} catch (e) {
return null;
}
}
}
Using like this.
void main() {
String? jsonEncode = Json.tryEncode(dataEncoded);
if (jsonEncode == null) {
print("jsonEncode is null");
} else {
print("jsonEncode is not null");
}
dynamic jsonDecode = Json.tryDecode(dataDecoded);
if (jsonDecode == null) {
print("jsonDecode is null");
} else {
print("jsonDecode is not null");
}
}
Caution: When you use Json.tryDecode( jsonEncode(null) ) that function can't tell you if this can convert to JSON or not because the result always is null. But I don't worry about this.