Dart metadata and how to create custom metadata

Dart metadata :

A metadata starts with an @ character followed by the name of the metadata. Metadata is used to provide extra information in code. For example, if you override one method, @override is used above the overridden method name. This is a metadata annotation. We can create one annotation in dart.

Metadata is created using a compile-time constant or call to a constant constructor.

Example of dart metadata :

Mostly used dart metadata are deprecated and override. For example :

class MainClass {
  getClassname() {
    return 'MainClass';
  }
}

class SubClass extends MainClass {
  
  getClassname() {
    return super.getClassname();
  }
}

SubClass class is extending MainClass and it is overriding the getClassname method. Here, @override is metadata.

Metadata can be used before a class, typedef, constructor, factory, parameter, field, function, library, type parameter, variable declaration or before an import or export directive.

Custom metadata :

As I have explained above, we can use one constant constructor to create one custom metadata. For example :

class Student {
  final String name;
  final int age;

  const Student(this.name, this.age);
}

This class has one constant constructor that takes two parameters. We can use this as metadata like below :

('Alex', 20)

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