Defining custom exceptions

Defining custom exceptions

You can create custom exception types in your backend to throw specific errors and react to them accordingly in your Flutter app.

Using custom exceptions

Below is an example of how to define a custom exception. You can create exceptions inside your celest/lib/exceptions/ folder.

class BadNameException implements Exception {
  const BadNameException(this.message);
  final String message;

You can then throw these exceptions in your Celest Functions whenever needed as shown below.

import 'package:celest_backend/exceptions/bad_name_exception.dart';
Future<String> sayHello(String name) async {
  // Perform custom validation
  if (name.isEmpty) {
    throw const BadNameException('Name cannot be empty');
  return 'Hello, $name';

In your Flutter app, the same BadNameException type will be thrown by the generated client if an error occurs.

import 'package:celest_backend/client.dart';
Future<String> getGreeting(String name) async {
  try {
    return await celest.functions.greeting.sayHello(name);
  // Catch the exception type defined in your backend
  } on BadNameException catch (e) {
    print('Uh oh! Could not greet $name: $e');

Writing custom serialization logic

Just like custom model types, custom exceptions can define their own serialization logic. This is useful when you have existing types you want to make use of which are not serializable by default.

Next steps

You have now learned how to create and throw custom exceptions in your Celest Functions. We have additional guides to teach you about managing environment variables and creating tests for your functions.