how to split dart class in flutter?

Question

I did the following test, but it doesn't work:

//main.dart
class Test
{
  static const   a = 10;
  final b = 20;
  final c = a+1;

}

//part.dart
part of 'main.dart';
class Test
{
  final d = a +1;   //<---undefined name 'a'
} 

I would like to split the class in flutter tutorial into multiple files. For example: _buildSuggestions in a separate file, _buildRow in a separate file, etc.

update:

my solution:

before:

//main.dart
class RandomWordsState extends State<RandomWords> {
{
    final _var1;
    final _var2;
    @override
    Widget build(BuildContext context) {
      ...
      body: _buildList(),
    );

    Widget _buildList() { ... }
    Widget _buildRow() { ... }
}

after:

//main.dart
import 'buildlist.dart';
class RandomWordsState extends State<RandomWords> {
{
    final var1;
    final var2;
    @override
    Widget build(BuildContext context) {
      ...
      body: buildList(this),
    );

}

//buildlist.dart
import 'main.dart';

  Widget buildList(RandomWordsState obj) {
     ... obj.var1 ...
  }

Answer 1

I am faced with same problem. My variant based on extensions:

page.dart

part 'section.dart';

class _PageState extends State<Page> {
    build(BuildContext context) {
        // ...
        _buildSection(context);
        // ...
    }
}

section.dart

part of 'page.dart';

extension Section on _PageState {
    _buildSection(BuildContext context) {
        // ...
    }
}

Answer 2

Dart doesn't support partial classes. part and part of are to split a library into multiple files, not a class.

Private (identifiers starting with _) in Dart is per library which is usually a *.dart file.

main.dart

part 'part.dart';

class Test {
  /// When someone tries to create an instance of this class
  /// Create an instance of _Test instead
  factory Test() = _Test;

  /// private constructor that can only be accessed within the same library
  Test._(); 

  static const   a = 10;
  final b = 20;
  final c = a+1;
}

part.dart

part of 'main.dart';
class _Test extends Test {
  /// private constructor can only be called from within the same library
  /// Call the private constructor of the super class
  _Test() : super._();

  /// static members of other classes need to be prefixed with
  /// the class name, even when it is the super class
  final d = Test.a +1;   //<---undefined name 'a'
} 

A similar pattern is used in many code-generation scenarios in Dart like in

• https://pub.dartlang.org/packages/built_value
• https://pub.dartlang.org/packages/built_redux
• https://pub.dartlang.org/packages/json_serializable

and many others.

Answer 3

I just extend it with extension keyword like Swift.

// class_a.dart
class ClassA {}

// class_a+feature_a.dart
import 'class_a.dart';    

extension ClassA_FeatureA on ClassA {
  String separatedFeatureA() {
    // do your job here
  }
}

Please ignore the coding conventions, it's just a sample.

Answer 4

I learn from tutorial way, select a class name and use Refactor menu, choose Move XXXClass to file, it will auto generate new file and insert import to your code.

tutorial: https://codelabs.developers.google.com/codelabs/flutter-codelab-first?hl=zh-cn#4

IDE: vscode

Open Refactor shortcut (Mac): CMD + .