Flutter : Avoid using `forEach` with a function literal

Question

Hi everyone this is my whole method :

Future<void> init() async {
    FirebaseAuth.instance.userChanges().listen((user) {
      if (user != null) {
        _loginState = ApplicationLoginState.loggedIn;
        _guestBookSubscription = FirebaseFirestore.instance
            .collection('guestbook')
            .orderBy('timestamp', descending: true)
            .limit(3)
            .snapshots()
            .listen((snapshot) {
          _guestBookMessages = [];
          snapshot.docs.forEach((document) {
            _guestBookMessages.add(
              GuestBookMessage(
                name: document.data()['name'] as String,
                message: document.data()['text'] as String,
              ),
            );
          });
          notifyListeners();
        });
      } else {
        _loginState = ApplicationLoginState.loggedOut;
        _guestBookMessages = [];
        _guestBookSubscription?.cancel();
      }
      notifyListeners();
    });
  }

the part that dart complains about is this one :

snapshot.docs.forEach((document) {
            _guestBookMessages.add(
              GuestBookMessage(
                name: document.data()['name'] as String,
                message: document.data()['text'] as String,
              ),
            );
          });

how can I change this method without ruining the whole functionality ? Im just looking for a way that makes dart happy . I appreciate your help in advance.

Munsif Ali · Accepted Answer · 2025-05-07 05:07:44Z

54

AVOID using forEach with a function literal.

AVOID:

snapshot.docs.forEach((document) {
  ...
});

GOOD:

for (final document in snapshot.docs) {
  // Rest of your code
}

edited May 7 at 5:07

Munsif Ali

8,0127 gold badges35 silver badges67 bronze badges

answered Jun 22, 2021 at 16:52

Sifat Amin

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6 Comments

Konstantin Kozirev Over a year ago

@BugsHappen probably because it will make the code look way more complex. It is recommended to either do like Sifat proposed above, or I think even better is to create another function that you will supply to forEach method. It is described here: link Quote: "If you want to invoke some already existing function on each element, forEach() is fine." So define a new function and use it in forEach.

princess_hacker Over a year ago

Could you explain why either is good/bad?

Brandon Over a year ago

Agreed, the "good/bad" terminology is unhelpful. I think that in most languages it is more performant to use a loop rather than call an anonymous function repeatedly. This is because functions come with the overhead of allocating and cleaning up a stack, whereas a loop gets optimized into more "flat" code by the compiler. I assume that is the case here as well, and why Dart issues a warning.

Vladimir Demirev Over a year ago

It is ridiculous tbh. They should improve their compiler/interpreter instead of giving such recommendations.

lolelo Over a year ago

The latter is less readable in my opinion

|

Sittiphan Sittisak · Accepted Answer · 2022-11-08 06:48:40Z

Using like data.forEach(function); example:

void main() async {
  List<int> list = [1, 2, 3, 4, 5];

  //recommended
  list.forEach(showSquareNumbers);

  //not recommended
  list.forEach((int number) => showSquareNumbers(number));
  list.forEach((int number) {
    showSquareNumbers(number);
  });
}

void showSquareNumbers(int data) {
  print("$data * $data = ${data * data}");
}

This is my opinion.

I think the forEach seems more complicated than the for loop and the forEach can't use continue and break (return is available but not a thing happens when using return).

void test(List data) {
  data.forEach((element) {
    print(element);
    if(element) return;
  });

  for (var element in data) {
    print(element);
    if(element) continue;
    if(element) break;
    if(element) return;
  }
}

I think we should use the for instead of the forEach loop when your forEach loop seems like this code below because the for loop have many options more than forEach as I said.

  data.forEach((element) {
    print(element)
  });
  //or
  data.forEach((element) => print(element));

I think the forEach loop is used for short code (easy to understand) and when you want to do something you don't care about the result like this code (using with Function(dynamic)).

void test(List data) {
  void showInTerminal(e) {
    print("data is $e");
  }
  data.forEach(showInTerminal);
  
  Function(dynamic) function = showInTerminal;
  data.forEach(function);
}

Make sure the data type and function(type) are the same.

//error
void test(List<Map> data) {
  void showInTerminal(String e) {
    print("data is $e");
  }
  data.forEach(showInTerminal);
}

//success
void test(List<Map> data) {
  void showInTerminal(Map e) {
    print("data is $e");
  }
  data.forEach(showInTerminal);
}

I code like this. I think it's easy to read.

void test() {
  dataList.forEach(removeData);
  fileList.forEach(removeFile);
}

Mathis Fouques · Accepted Answer · 2024-01-28 15:41:28Z

4

There is a debate below the accepted answer, about readability of the recommended lint rules at stake here. This is the rule in question causing the problem : https://dart.dev/tools/linter-rules/avoid_function_literals_in_foreach_calls

You can disable it by modifying your analysis_options.yaml file :

linter:
  rules:
    avoid_function_literals_in_foreach_calls: false

answered Jan 28, 2024 at 15:41

Mathis Fouques

2441 silver badge6 bronze badges

Comments

Mukus · Accepted Answer · 2025-05-07 00:47:15Z

It is because of how closures behave.

I took the default example in dartpad and slightly modified.

Try this example in dartpad.

void main() {
  for (var i = 0; i < 10; i++) {
    print('hello ${i + 1}');
  }
  print('');
  
  List<int> numbers = [0,1,2,3,4,5,6,7,8,9];
  
  numbers.forEach((i) => print('hello ${i + 1}')); 
  
}

The result here will be the same as you are not returning from either loop.

Result -

hello 1
hello 2
hello 3
hello 4
hello 5
hello 6
hello 7
hello 8
hello 9
hello 10

hello 1
hello 2
hello 3
hello 4
hello 5
hello 6
hello 7
hello 8
hello 9
hello 10

But as soon as you return from the loop, see the behaviour change.

void main() {

  List<int> numbers = [0,1,2,3,4,5,6,7,8,9];  
  
  for (var i = 0; i < 10; i++) {
     var j = 5;
    print('hello ${i + 1}');
     if(i == j) {
        print('for loop inside if');
       return; ///// ---> 1 
     }
  }
  print('');  
  
  
  numbers.forEach((i) { 
    var j = 5;
    print('hello ${i + 1}');
     if(i == j) {
       print('foreach inside if');
       return;  ///// ---> 2
     }
  });  
  
  
}

Result -

hello 1
hello 2
hello 3
hello 4
hello 5
hello 6
for loop inside if

As soon as you return from the for loop, you return from the main method. When you return from the forEach you will only break out of that if block and it will continue to run forEach.

Now see how it works when I switch the order.

void main() {

  List<int> numbers = [0,1,2,3,4,5,6,7,8,9];  
  
  numbers.forEach((i) { 
    var j = 5;
    print('hello ${i + 1}');
     if(i == j) {
       print('foreach inside if');
       return;  ///// ---> 2
     }
  });  
  
  print('');  
  
  for (var i = 0; i < 10; i++) {
     var j = 5;
    print('hello ${i + 1}');
     if(i == j) {
        print('for loop inside if');
       return; ///// ---> 1 
     }
  }  
}

The result is -

hello 1
hello 2
hello 3
hello 4
hello 5
hello 6
foreach inside if
hello 7
hello 8
hello 9
hello 10

hello 1
hello 2
hello 3
hello 4
hello 5
hello 6
for loop inside if

Contrary to what Dart suggests, depending on what you are trying to achieve, you may want to use one or the other.

iDecode · Accepted Answer · 2022-01-17 07:36:00Z

-6

Use await for:

await for (final document in snapshot.docs) {
  // Your code...
}

edited Jan 17, 2022 at 7:36

answered Jun 22, 2021 at 16:49

iDecode

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3 Comments

delaram Over a year ago

thanks for your attention . i tried your solution but here the method 'listen' isn't avaliable to use

iDecode Over a year ago

@delaram Sorry I mistakenly wrote .docs in the code. I've updated it though.

Mukus May 7 at 0:08

Why use await for instead of foreach or for loop?

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