Null safety in Flutter

2 min readJul 6, 2021

The null assertion operator(!)

int? couldNull = -1;

int? couldNullFunc() => -2;
List<int?> listHoldsNulls = [0, null, 2];

void main() {
  int a = couldNull!; // Hey dart, it could be null but just trust me it isn't
  int b = couldNullFunc()!.abs(); // Ter hamaagui ee
  int c = listHoldsNulls.first!; // I know what I'm doing
}

The required keyword

int addThreeValues({
  required int first,
  required int second,
  int third = 0,
}) {
  return first + second + third;
}

void main() {
  final sum = addThreeValues(
    first: 1,
    second: 5,
// third: 3,
  );
}

Type promotion via null checks

void main() {
  Object a = 'This is a string';
  if (a is String) {
    print(a.length);
  }
  String? text;
  if (DateTime.now().hour < 12) {
    text = 'a';
  } else {
    text = 'b';
  }
  print(text.length);
  print(getLength('This is a string'));
}

int getLength(String? str) {
  if (str == null) {
    throw 'Hey, that string was null';
// return 0;
  }
  return str.length;
}

Object properties aren’t promotable

import 'dart:math';

class StrProvider {
  String? get value =>
      Random()
          .nextBool() ? 'string' : null;
  String? b = 'Hello';
}

class StrProviderFactory {
  static StrProvider getProvider() => StrProvider();
}

void printStr(String str) => print(str);

void main() {
  final provider = StrProvider();
  final str = provider.value;
  if (str == null) {
    print('null');
  } else {
    print('not null');
    printStr(str);
  }
  if (provider.value == null) {
    print('null');
  } else {
    print('not null');
    printStr(provider.value!);
  }
  if (provider.b == null) {
    print('null');
  } else {
    print('not null');
    printStr(provider.b!);
  }
  final pr = StrProviderFactory.getProvider();
  final s = pr.value;
  if (s == null) {
    print('null');
  } else {
    print('not null');
    printStr(s);
  }
}

The late keyword

class Meal {
  late String desc; // Trust me dart desc will be fine. it will never null
  void setDesc(String str) {
    desc = str;
  }
}

void main() {
  final meal = Meal();
  meal.setDesc('Tuna fish');
  print(meal.desc);
}

late circular references

class Team {
  late final Coach coach;
}

class Coach {
  late final Team team;
}

void main() {
  final team = Team();
  final coach = Coach();
  team.coach = coach;
  coach.team = team;
}

Late and lazy

int _computeValue() {
  print('In _computeValue…');
  return 3;
}

class CachedValueProvider {
// final _cache = _computeValue(); // Immadiately
  late final _cache = _computeValue(); // Just delay, it is not really null safety related
  int get value => _cache;
}

void main() {
  print('Calling constructor…');
  var provider = CachedValueProvider();
  print('Getting value…');
  print('The value is ${provider.value}!');
}

Null safety in Flutter

Written by Jagaa