6.10 Metadata

Several constructs can be attributed with custom metadata:

  • class and enum declarations
  • Class fields
  • Enum constructors
  • Expressions

These metadata information can be obtained at runtime through the haxe.rtti.Meta API:

import haxe.rtti.Meta;

class MyClass {
  @range(1, 8)
  var value:Int;

  static function method() {}

class Main {
  static public function main() {
    // { author : ["Nicolas"] }
    // [1,8]
    // { broken: null }

We can easily identify metadata by the leading @ character, followed by the metadata name and, optionally, by a number of comma-separated constant arguments enclosed in parentheses.

  • Class MyClass has an author metadata with a single String argument "Nicolas", as well as a :keep metadata without arguments.
  • The member variable value has a range metadata with two Int arguments 1 and 8.
  • The static method method has a broken metadata without arguments.

The main method accesses these metadata values using their API. The output reveals the structure of the obtained data:

  • There is a field for each metadata, with the field name being the metadata name.
  • The field values correspond to the metadata arguments. If there are no arguments, the field value is null. Otherwise the field value is an array with one element per argument.

Allowed values for metadata arguments are:

Compile-time Metadata

Metadata starting with :, such as @:keep, is available at compile time only; it is omitted at runtime. It may be used by macros or by the Haxe compiler itself. Unlike runtime metadata, arguments to compile-time metadata can be any valid expression.

Built-in Compiler Metadata

An exhaustive list of all defined metadata can be obtained by running haxe --help-metas from command line.

since Haxe 4.0.0

Prior to Haxe 4, metadata names had to be valid identifiers. Starting in Haxe 4, metadata names can consist of multiple identifiers separated by . symbols. This change was primarily intended to make it easier to organize compile-time metadata. Runtime metadata with such a name can only be accessed via dynamic access.

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