Assertions in Dart and Flutter Tests: Error Matchers

In Flutter testing, it's essential to handle errors gracefully and verify that they occur as expected. Flutter provides a set of error matchers to assert specific types of errors. In this article, we'll explore various error matchers such as isArgumentError, isCastError, isConcurrentModificationError, isCyclicInitializationError, isException, isFormatException, isNoSuchMethodError, isNullThrownError, isRangeError, isStateError, isUnimplementedError, along with explanations and examples for each.

isArgumentError

isArgumentError asserts that an error is an ArgumentError, which occurs when an invalid argument is provided to a function or method.

Example:

test('Throw ArgumentError for invalid argument', () {
  expect(() => throw ArgumentError(), throwsA(isArgumentError));
});

isConcurrentModificationError

isConcurrentModificationError asserts that an error is a ConcurrentModificationError, which occurs when a collection is modified while an iteration is in progress.

Example:

test('Throw ConcurrentModificationError for concurrent modification', () {
  var list = [1, 2, 3];
  expect(() {
    for (var item in list) {
      list.add(4); // Modify the list while iterating
    }
  }, throwsA(isConcurrentModificationError));
});

isException

isException asserts that an error is an Exception, a superclass for all exceptions.

Example:

test('Throw Exception for general exception', () {
  expect(() => throw Exception('Custom exception'), throwsA(isException));
});

isFormatException

isFormatException checks if an error is a FormatException, which occurs when a string cannot be parsed into the expected format.

Example:

test('Throw FormatException for invalid string format', () {
  expect(() => int.parse('abc'), throwsA(isFormatException));
});

isNoSuchMethodError

isNoSuchMethodError asserts that an error is a NoSuchMethodError, which occurs when a non-existent method or property is accessed.

Example:

test('Throw NoSuchMethodError for accessing non-existent method', () {
  dynamic car;
  expect(() => car.nonExistentMethod(), throwsA(isNoSuchMethodError));
});

isRangeError

isRangeError asserts that an error is a RangeError, which occurs when a value is outside the valid range for a method or operation.

Example:

test('Throw RangeError for value out of range', () {
  expect(() => List<int>.filled(-1, 0), throwsA(isRangeError));
});

isStateError

isStateError checks if an error is a StateError, which occurs when a method is invoked at an illegal or inappropriate time or state.

Example:

test('Throw StateError for illegal state', () {
  expect(() => [].firstWhere((element) => false), throwsA(isStateError));
});

isUnimplementedError

isUnimplementedError asserts that an error is an UnimplementedError, which occurs when a method or operation is not implemented.

Example:

test('Throw UnimplementedError for unimplemented method', () {
  expect(() => throw UnimplementedError(), throwsA(isUnimplementedError));
});

Conclusion

These error matchers provide valuable tools for handling and verifying errors in Flutter tests. By leveraging isArgumentError, isCastError, isConcurrentModificationError, isCyclicInitializationError, isException, isFormatException, isNoSuchMethodError, isNullThrownError, isRangeError, isStateError, and isUnimplementedError, developers can ensure their applications behave as expected under various error conditions.