When to use part/part of versus import/export in Dart?

Solution 1:

update 2018/03

part and part of is used more and more for code generation scenarios recently (instead of deprecated transformers) and unlikely to go away anytime soon.

Packages like built_value, json_serializable, and many others depend on it.

Discouraged is only the patter where all files of a package are tied together to a single library by having one library file and all other files being part of that library.

original

In Dart, private members are accessible within the same library. With import you import a library and can access only its public members. With part/part of you can split one library into several files and private members are accessible for all code within these files.

see clarifications to below paragraph in above update

Using part / part of is discouraged and the Dart team is considering getting rid of it. I assume they will introduce something like "friend" (https://github.com/dart-lang/sdk/issues/22841), where two libraries can access each other's private members as an alternative before they discontinue part / part of (maybe in a future Dart version).

Solution 2:

Let's suppose we have a Dart library called mylib, whose file is lib/mylib.dart.

library mylib;

// Definitions

That library can be included in the main.dart file as

import 'package:mypackage/mylib.dart';

When you create a new library and use other libraries you want to make available automatically when using your package, then you use export:

library mylib;

export 'otherlib.dart';

// Definitions

You can use the show keyword to import/export only some parts of a library (like a class or something).

You are using the part of directive wrong here. You can't use both library and part of, which is used to specify the contents that belong to a library. For example, you can split your library file in more than one file (the parts):

Suppose we have in the file mylib.dart:

library mylib;

part 'src/class1.part';
// More parts

And then we have in another file src/class1.part the part specified in mylib.dart

part of mylib;

class Class1 { 
  /* ... */
}