How do you namespace a Dart class?

Question

How do you create a namespace for a Dart class? I come from a C# background, where one would just use namespace SampleNamespace { }.

How do you achieve the same in Dart?

Charades

Answer 1

Dart doesn't have the concept of namespaces, but instead it has libraries. You can consider a library to be sort of equivalent to a namespace, in that a library can be made of multiple files, and contain multiple classes and functions.

Privacy in Dart is also at the library, rather than the class level (anything prefixed with an underscore is private to that library).

An example of defining a library (using the example of a utilities library:

// utilities.dart
library utilities; // being the first statement in the library file

You can make other files part of the same library by using the part keyword. Part files are only used to help organize your code; you can put all your classes in a single library file, or split them among several part files (or part files and the library file) - it has no effect on the execution. It is stylistic to put the main library file in a parent folder, and part files in a src/ folder.

Expanding the example to show Part files.

// utilities.dart
library utilities;

part "src/string_utils.dart";
part "src/date_utils.dart";

Those part files then link back to the library they are part of by using the part of statement:

// src/string_utils.dart
part of utilities;

// functions and classes
String reverseString(s) => // implementation ....

String _stringBuilder(strings) => // a private (to the library) function, 
                                  // indicated by the leading underscore

//... snip other classes and functions

Now that you have a library containing a function, you can make use of that library elsewhere by importing the library:

 // my_app.dart;
 import "path/to/library/utilities.dart";

 main() {
   var reversed = reverseString("Foo");
   // _stringBulider(["a","b"]); // won't work - this function is 
                                 // only visible inside the library
 }

If you want to alias your library to avoid clashes (where you might import two libraries, both containing a reverseString() function, you use the as keyword:

 // my_app.dart;
 import "path/to/library/utilities.dart";
 import "some/other/utilities.dart" as your_utils;

 main() {
   var reversed = reverseString("Foo"); 
   var your_reversed_string = your_utils.reverseString("Bar");
 }

The import statement also makes use of packages, as imported by pub, Dart's package manager, so you can also host your library on github or elsewhere, and reference your library as so:

 // my_app.dart;
 import "package:utilities/utilities.dart";

 main() {
   var reversed = reverseString("Foo");        
 }

The pub dependency is defined in a pubspec.yaml file, which tells pub where to find the library. You can find out more at pub.dartlang.org

It is important to note that only the library file can:

• contain import statements. Part files cannot.
• contain the library keyword. Part files cannot.
• contain part files. Part files cannot.

One final point is that a runnable app file can (and is likely to be) a library file, and can also be made of part files

 // my_app.dart;
 library my_app;

 import "package:utilities/utilities.dart";

 part "src/edit_ui.dart";
 part "src/list_ui.dart";
 part "src/foo.dart";

 main() {
   var reversed = reverseString("Foo");    
   showEditUi(); // perhaps defined in edit_ui.dart....?
 }

Crack

It should be noted that using the part directive is discouraged. "Note: You may have heard of the part directive, which allows you to split a library into multiple Dart files. We recommend that you avoid using part and create mini libraries instead." -- dartlang.org/guides/libraries/create-library-packages – Araujo

part is now the reference for code-generations tooling instead – Hereditable

can you please explain how your long story about dart's library is the solution for namespace? based on en.wikipedia.org/wiki/Namespace i still can't see it – Brannan

@jerinho.com - you mustve missed the part that said: " is an abstract container or environment created to hold a logical grouping of ...". so the way a class is a container to group data logically, a namespace is used to group classes logically. which is what he is showing us how to do in dart. – Orinasal

Answer 2

The easiest way that I've found to create a namespace in Dart is this:

Say you have the files a.dart and b.dart containing the classes Apple and Banana respectively. Create a file called my_namespace.dart. In this example, it's residing in the same folder as the other two files. Export all the files that you want under your namespace from the my_namespace.dart file:

export 'a.dart';
export 'b.dart';

Then wherever you would like to use the exported code from these two files, use this:

import 'my_namespace.dart' as my_namespace;

// you can now access the classes under the same namespace:

final myApple = my_namespace.Apple();
final myBanana = my_namespace.Banana();

Another way to do this, which removes the need of the intermediary file my_namespace.dart, is to have several import statements with the same alias:

import 'a.dart' as my_namespace;
import 'b.dart' as my_namespace;

// you can once again access the classes under the same namespace:

final myApple = my_namespace.Apple();
final myBanana = my_namespace.Banana();

I prefer the first method because I don't have to repeat the multiple import statements whenever I need to use a class under the namespace.

Of course the imported and exported files do not need to be in the same folder, but having the files under the same namespace in the same folder would probably be more convenient.

Lotic

I like the first option two. I wish you could export 'a.dart' as a. this would allow for nested namespacing. you could import 'my_namespace.dart' as my_namespace; my_namespace.a.App(); is there anyway to do that? – Eleazar