DartのFutureについて調べる

String createOrderMessage() {
  var order = fetchUserOrder();
  return 'Your order is: $order';
}

Future<String> fetchUserOrder() =>
    // Imagine that this function is more complex and slow.
    Future.delayed(
      const Duration(seconds: 2),
      () => 'Large Latte',
    );

void main() {
  print(createOrderMessage());
}

Your order is: Instance of '_Future<String>'

Future<void> fetchUserOrder() {
  // Imagine that this function is fetching user info from another service or database.
  return Future.delayed(const Duration(seconds: 2), () => print('Large Latte'));
}

void main() {
  fetchUserOrder();
  print('Fetching user order...');
}

Fetching user order...
(2秒待機)
Large Latte

Future<void> fetchUserOrder() {
// Imagine that this function is fetching user info but encounters a bug
  return Future.delayed(const Duration(seconds: 2),
      () => throw Exception('Logout failed: user ID is invalid'));
}

void main() {
  fetchUserOrder();
  print('Fetching user order...');
}

Fetching user order...
Uncaught Error: Exception: Logout failed: user ID is invalid

Future<void> main() async { ··· }

Future<String> createOrderMessage() async {
  var order = await fetchUserOrder();
  return 'Your order is: $order';
}

Future<String> fetchUserOrder() =>
    // Imagine that this function is more complex and slow.
    Future.delayed(
      const Duration(seconds: 2),
      () => 'Large Latte',
    );

Future<void> main() async {
  print(await createOrderMessage());
}

Your order is: Large Latte

Future<void> func() async {
  print("1");
  await Future.delayed(Duration(seconds: 1));
  print("2");
}

void main() {
  print("3");
  func();
  print("4");
}

3
1
4
(1秒待機)
2

Future<void> printOrderMessage() async {
  print('Awaiting user order...');
  var order = await fetchUserOrder();
  print('Your order is: $order');
}

Future<String> fetchUserOrder() {
  // Imagine that this function is more complex and slow.
  return Future.delayed(const Duration(seconds: 4), () => 'Large Latte');
}

void main() async {
  countSeconds(4);
  await printOrderMessage();
}

// You can ignore this function - it's here to visualize delay time in this example.
void countSeconds(int s) {
  for (var i = 1; i <= s; i++) {
    Future.delayed(Duration(seconds: i), () => print(i));
  }
}

Awaiting user order...
(1秒待機)
1
(1秒待機)
2
(1秒待機)
3
(1秒待機)
4
Your order is Large Latte

// Part 1
// You can call the provided async function fetchRole()
// to return the user role.
Future<String> reportUserRole() async {
  TODO('Your implementation goes here.');
}

// Part 2
// Implement reportLogins here
// You can call the provided async function fetchLoginAmount()
// to return the number of times that the user has logged in.
reportLogins() {}

// Part 1
// You can call the provided async function fetchRole()
// to return the user role.
Future<String> reportUserRole() async {
  final role = await fetchRole();
  return "User role: $role";
}

// Part 2
// Implement reportLogins here
// You can call the provided async function fetchLoginAmount()
// to return the number of times that the user has logged in.
Future<String> reportLogins() async {
  final loginAmount = await fetchLoginAmount();
  return "Total number of logins: $loginAmount";
}

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

Future<void> printOrderMessage() async {
  try {
    print('Awaiting user order...');
    var order = await fetchUserOrder();
    print(order);
  } catch (err) {
    print('Caught error: $err');
  }
}

Future<String> fetchUserOrder() {
  // Imagine that this function is more complex.
  var str = Future.delayed(
      const Duration(seconds: 4),
      () => throw 'Cannot locate user order');
  return str;
}

void main() async {
  await printOrderMessage();
}

Awaiting user order...
(4秒待機)
Caught error: Cannot locate user order

// Implement changeUsername here
changeUsername() {}

// Implement changeUsername here
Future<String> changeUsername() async {
  try {
    return await fetchNewUsername();
  } catch (err) {
    return err.toString();
  }
}

// Part 1
addHello(String user) {}

// Part 2
// You can call the provided async function fetchUsername()
// to return the username.
greetUser() {}

// Part 3
// You can call the provided async function logoutUser()
// to log out the user.
sayGoodbye() {}

// Part 1
String addHello(String user) {
  return "Hello $user";
}

// Part 2
// You can call the provided async function fetchUsername()
// to return the username.
Future<String> greetUser() async {
  final username = await fetchUsername();
  return addHello(username);
}

// Part 3
// You can call the provided async function logoutUser()
// to log out the user.
Future<String> sayGoodbye() async {
  try {
    final result = await logoutUser();
    return "$result Thanks, see you next time";
  } catch (err) {
    return err.toString();
  }
}