Target Meta Available since v0.7

The targetMeta() decorator offers the ability to associate metadata directly with a class instance or class method reference. This can be useful in situations when you do not know the class that owns the metadata.

Behind the scene, targetMeta() uses the meta() decorator and stores a reference to the target that is decorated inside a WeakMap.

• Supported Elements
• Class Instance
• Method Reference
• Inheritance
  • Static Methods

Supported Elements

Unlike the meta() decorator, targetMeta() only supports the following elements:

• class
• method

Class Instance

The following shows how to define target meta for a class and retrieve it.

import { targetMeta, getTargetMeta } from '@aedart/support/meta';

@targetMeta('description', { type: 'Search Service', alias: 'Location Sercher' })
class LocationSearcherService {}

const instance = new LocationSearcherService();

// ...later in your application...
getTargetMeta(instance, 'description')?.type; // Search Service

Method Reference

The following shows how to define target meta for a class method and retrieve it.

import { targetMeta, getTargetMeta } from '@aedart/support/meta';

class LocationSearcherService {

    @targetMeta('dependencies', [ 'httpClient' ])    
    search(apiClient) {}
}

const instance = new LocationSearcherService();

// ...later in your application...
getTargetMeta(instance.search, 'dependencies'); // [ 'httpClient' ]

Inheritance

Target meta is automatically inherited by subclasses and can also be overwritten, similar to that of the meta() decorator.

Example: classes

import {targetMeta, getTargetMeta} from '@aedart/support/meta';

@meta('service_alias', 'locationSearcher')
class Service {}

class CitySearcher extends Service {}

const instance = new CitySearcher();

// ...later in your application...
getTargetMeta(instance, 'service_alias'); // locationSearcher

Example: methods

import {targetMeta, getTargetMeta} from '@aedart/support/meta';

class Service {

    @targetMeta('dependencies', [ 'countrySearchApiClient' ])
    search(apiClient) {
        // ...not shown...
    }
}

class CountrySearcher extends Service {
    // ... not method overwrite here...
}

class CitySearcher extends Service {

    @targetMeta('dependencies', [ 'citySearchApiClient' ])
    search(apiClient) {
        // ...not shown...
    }
}

const instanceA = new Service();
const instanceB = new CountrySearcher();
const instanceC = new CitySearcher();

// ...later in your application...
getTargetMeta(instanceA.search, 'dependencies'); // [ 'countrySearchApiClient' ]
getTargetMeta(instanceB.search, 'dependencies'); // [ 'countrySearchApiClient' ]
getTargetMeta(instanceC.search, 'dependencies'); // [ 'citySearchApiClient' ]

Static Methods

Inheritance for static methods works a bit differently. By default, any subclass will automatically inherit target metadata, even for static methods. However, if you overwrite the given static method, the metadata is lost.

Limitation

When a static method is overwritten, the parent's "target" metadata cannot be obtained due to a general limitation of the meta() decorator. The decorator has no late this binding available to the overwritten static method. This makes it impossible to associate the overwritten static method with metadata from the parent.

Example: inheritance for static methods

import {targetMeta, getTargetMeta} from '@aedart/support/meta';

class Service {

    @targetMeta('dependencies', [ 'xmlClient' ])
    static search(client) {
        // ...not shown...
    }
}

class CountrySearcher extends Service {
    // ... not method overwrite here...
}

class CitySearcher extends Service {
    
    // Overwite of static method - target meta is lost
    static search(client) {}
}

// ...later in your application...
getTargetMeta(CountrySearcher.search, 'dependencies'); // [ 'xmlClient' ]
getTargetMeta(CitySearcher.search, 'dependencies'); // undefined

To overcome the above shown issue, you can use the inheritTargetMeta() decorator. It forces the static method to "copy" metadata from its parent, if available.

Example: force inheritance for static methods

import {
    targetMeta,
    getTargetMeta,
    inheritTargetMeta
} from '@aedart/support/meta';

class Service {

    @targetMeta('dependencies', [ 'xmlClient' ])
    static search(client) {
        // ...not shown...
    }
}

class CountrySearcher extends Service {
    // ... not method overwrite here...
}

class CitySearcher extends Service {
    
    @inheritTargetMeta()
    static search(client) {}
}

// ...later in your application...
getTargetMeta(CountrySearcher.search, 'dependencies'); // [ 'xmlClient' ]
getTargetMeta(CitySearcher.search, 'dependencies'); // [ 'xmlClient' ]