jsonEncode with async toEncodable Function Causes Exception

[yesilcimenahmet]

When using jsonEncode with an async toEncodable function, it throws an exception, even when valid data is returned. Removing the async keyword resolves the issue, indicating that jsonEncode does not properly handle asynchronous operations in toEncodable.

import 'dart:convert';

enum MyEnum { a, b, c }

Map<String, dynamic> someValues() {
  return {'name': 'name value', 'age': 99, 'enum': MyEnum.a};
}

void jsonEncodeTest() {
  final object = someValues();
  final json = jsonEncode(
    object,
    toEncodable: (nonEncodable) async {
      if (nonEncodable is MyEnum) {
        return nonEncodable.toString();
      }
      return nonEncodable;
    },
  );
  print(json);
}

void main() {
  jsonEncodeTest();
}

The async keyword in toEncodable should allow asynchronous operations, and await should work without causing an exception.

The following exception is thrown:

Exception: Converting object did not return an encodable object: Instance of 'MyEnum'
UnsupportedObject: MyEnum.a
partialResult: {"name":"name value","age":99,"enum":

Therefore, this issue also occurs in Flutter. This is a critical problem and needs to be resolved quickly.

[dart-github-bot]

Summary: jsonEncode throws an exception when the toEncodable function is asynchronous, even if it returns valid data. The issue stems from jsonEncode's inability to handle async toEncodable functions.

[mraleph]

Naturally this is working as intended: async function returns a Future which not an encodable object - so it is not a valid function to pass as toEncodable which is required to return an encodable object. Both documentation and exception you are getting is rather explicit about it.

For this to work jsonEncode itself would need to return a Future and be asynchronous. This can't be changed, as this would be too breaking. My suggestion to you would be to write a function that does necessary conversions (some of which would be asynchronous) and then uses jsonEncode to convert result to JSON.