How to create our own metadata in Dart?

问题:

I want to create some own metadata for my dart codem, e.g. @table, @column, but I can't find any useful documents about it.

But I do found there are some special metadata (e.g. NgController) in angular.dart: https://github.com/angular/angular.dart/blob/master/demo/todo/web/todo.dart#L52

How to create my own metadata in Dart? Is there any documents?

回答1:

Dart supports metadata which is used to attach user defined annotations to program structures.

Metadata consists of a series of annotations, each of which begin with the character @, followed a constant expression that starts with an identifier. It is a compile time error if the expression is not one of the following:

1. A reference to a compile-time constant variable.
2. A call to a constant constructor.

Metadata can appear before a library, part header, class, typedef, type parameter, constructor, factory, function, field, parameter, or variable declaration and before an import, export or part directive.

---

So, suggested by you constants such @table, @column are very limited by functionality because they cannot hold additional information (parameters).

@DataTable("sale_orders")
class SaleOrder {
  @DataColumn("sale_order_date")
  DateTime date;
}

@table
class Product {
  @column
  String name;
}

const DataColumn column = const DataColumn();

const DataTable table = const DataTable();

class DataTable {
  final String name;

  const DataTable([this.name]);
}

class DataColumn {
  final String name;

  const DataColumn([this.name]);
}

But in any case, you choose the option that best suits your needs.

回答2:

• This is an interesting blog post about Dart annotations
  http://japhr.blogspot.co.at/2013/01/i-love-dart-annotations.html

• Here the meta data part of the Dart language specification
  https://www.dartlang.org/docs/spec/latest/dart-language-specification.html#h.d0rowtffuudf

Metadata consists of a series of annotations, each of which begin with the character @, followed a constant expression that starts with an identifier.

So you can use a class with a const constructor as annotation.

• Dart Up and Running https://www.dartlang.org/docs/dart-up-and-running/contents/ch02.html#ch02-metadata