Extension Points

/**
 * Decorate a class as a named extension point. If the decoration is not
 * present, the name of the class will be used.
 */
function extensionPoint(name: string, ...specs: BindingSpec[]): ClassDecorator;

import { extensionPoint, extensions, Getter } from "@loopback/core";

// Basic extension point
const GREETER_EXTENSION_POINT = 'greeter';

@extensionPoint(GREETER_EXTENSION_POINT)
class GreetingService {
  constructor(
    @extensions() private getGreeters: Getter<Greeter[]>
  ) {}
  
  async greet(name: string): Promise<string[]> {
    const greeters = await this.getGreeters();
    return Promise.all(
      greeters.map(greeter => greeter.greet(name))
    );
  }
}

// Extension point with binding specifications
@extensionPoint('validator', {scope: BindingScope.SINGLETON})
class ValidationService {
  constructor(
    @extensions() private getValidators: Getter<Validator[]>
  ) {}
  
  async validate(data: any): Promise<ValidationResult> {
    const validators = await this.getValidators();
    const results = await Promise.all(
      validators.map(validator => validator.validate(data))
    );
    
    return this.combineResults(results);
  }
  
  private combineResults(results: ValidationResult[]): ValidationResult {
    // Combine validation results
    return {
      isValid: results.every(r => r.isValid),
      errors: results.flatMap(r => r.errors)
    };
  }
}

/**
 * Shortcut to inject extensions for the given extension point.
 */
function extensions(
  extensionPointName?: string,
  metadata?: InjectionMetadata
): PropertyDecorator & ParameterDecorator;

import { extensionPoint, extensions, Getter } from "@loopback/core";

interface AuthenticationStrategy {
  name: string;
  authenticate(credentials: any): Promise<UserProfile | null>;
}

@extensionPoint('authentication-strategy')
class AuthenticationService {
  constructor(
    @extensions() // Extension point name inferred from class
    private getStrategies: Getter<AuthenticationStrategy[]>
  ) {}
  
  async authenticate(strategyName: string, credentials: any): Promise<UserProfile | null> {
    const strategies = await this.getStrategies();
    const strategy = strategies.find(s => s.name === strategyName);
    
    if (!strategy) {
      throw new Error(`Authentication strategy '${strategyName}' not found`);
    }
    
    return strategy.authenticate(credentials);
  }
  
  async getAvailableStrategies(): Promise<string[]> {
    const strategies = await this.getStrategies();
    return strategies.map(s => s.name);
  }
}

// Explicit extension point name
@extensionPoint('custom-processor')
class DataProcessingService {
  constructor(
    @extensions('data-processor') // Explicit extension point name
    private getProcessors: Getter<DataProcessor[]>
  ) {}
  
  async processData(data: any): Promise<any> {
    const processors = await this.getProcessors();
    let result = data;
    
    for (const processor of processors) {
      result = await processor.process(result);
    }
    
    return result;
  }
}

namespace extensions {
  /**
   * Inject a ContextView for extensions of the extension point. The view can
   * then be listened on events such as bind, unbind, or refresh to react
   * on changes of extensions.
   */
  function view(
    extensionPointName?: string,
    metadata?: InjectionMetadata
  ): PropertyDecorator & ParameterDecorator;
}

import { extensionPoint, extensions, ContextView } from "@loopback/core";

interface Plugin {
  name: string;
  version: string;
  initialize(): Promise<void>;
  destroy(): Promise<void>;
}

@extensionPoint('plugin-system')
class PluginManager {
  private initializedPlugins = new Set<string>();
  
  constructor(
    @extensions.view() private pluginsView: ContextView<Plugin>
  ) {
    // Listen for plugin additions and removals
    this.pluginsView.on('bind', this.onPluginAdded.bind(this));
    this.pluginsView.on('unbind', this.onPluginRemoved.bind(this));
  }
  
  private async onPluginAdded(event: ContextViewEvent<Plugin>): Promise<void> {
    const plugin = await event.binding.getValue(this.pluginsView.context);
    console.log(`Plugin added: ${plugin.name} v${plugin.version}`);
    
    await plugin.initialize();
    this.initializedPlugins.add(plugin.name);
  }
  
  private async onPluginRemoved(event: ContextViewEvent<Plugin>): Promise<void> {
    const plugin = await event.binding.getValue(this.pluginsView.context);
    console.log(`Plugin removed: ${plugin.name}`);
    
    if (this.initializedPlugins.has(plugin.name)) {
      await plugin.destroy();
      this.initializedPlugins.delete(plugin.name);
    }
  }
  
  async getActivePlugins(): Promise<Plugin[]> {
    return this.pluginsView.values();
  }
}

namespace extensions {
  /**
   * Inject an array of resolved extension instances for the extension point.
   * The list is a snapshot of registered extensions when the injection is
   * fulfilled. Extensions added or removed afterward won't impact the list.
   */
  function list(
    extensionPointName?: string,
    metadata?: InjectionMetadata
  ): PropertyDecorator & ParameterDecorator;
}

import { extensionPoint, extensions } from "@loopback/core";

interface Middleware {
  name: string;
  priority: number;
  execute(context: RequestContext, next: () => Promise<void>): Promise<void>;
}

@extensionPoint('middleware')
class MiddlewareChain {
  constructor(
    @extensions.list() private middlewareList: Middleware[]
  ) {
    // Sort middleware by priority
    this.middlewareList.sort((a, b) => a.priority - b.priority);
  }
  
  async executeChain(context: RequestContext): Promise<void> {
    let index = 0;
    
    const next = async (): Promise<void> => {
      if (index < this.middlewareList.length) {
        const middleware = this.middlewareList[index++];
        await middleware.execute(context, next);
      }
    };
    
    await next();
  }
  
  getMiddlewareNames(): string[] {
    return this.middlewareList.map(m => m.name);
  }
}

/**
 * Register an extension for the given extension point to the context
 */
function addExtension(
  context: Context,
  extensionPointName: string,
  extensionClass: Constructor<unknown>,
  options?: BindingFromClassOptions
): Binding<unknown>;

/**
 * A factory function to create binding template for extensions of the given
 * extension point
 */
function extensionFor(
  ...extensionPointNames: string[]
): BindingTemplate;

/**
 * A factory function to create binding filter for extensions of a named
 * extension point
 */
function extensionFilter(
  ...extensionPointNames: string[]
): BindingFilter;

import { 
  addExtension, 
  extensionFor, 
  extensionFilter,
  Context,
  Application 
} from "@loopback/core";

// Extension implementations
class EmailNotificationProvider implements NotificationProvider {
  async send(message: string, recipient: string): Promise<void> {
    console.log(`Email to ${recipient}: ${message}`);
  }
}

class SmsNotificationProvider implements NotificationProvider {
  async send(message: string, recipient: string): Promise<void> {
    console.log(`SMS to ${recipient}: ${message}`);
  }
}

class SlackNotificationProvider implements NotificationProvider {
  async send(message: string, channel: string): Promise<void> {
    console.log(`Slack to ${channel}: ${message}`);
  }
}

const app = new Application();
const NOTIFICATION_EXTENSION_POINT = 'notification-provider';

// Register extensions using addExtension
addExtension(
  app,
  NOTIFICATION_EXTENSION_POINT,
  EmailNotificationProvider,
  { name: 'email-provider' }
);

addExtension(
  app,
  NOTIFICATION_EXTENSION_POINT,
  SmsNotificationProvider,
  { name: 'sms-provider' }
);

// Register extension using binding template
app.bind('notification.slack')
  .toClass(SlackNotificationProvider)
  .apply(extensionFor(NOTIFICATION_EXTENSION_POINT));

// Find all extensions using filter
const extensionBindings = app.find(extensionFilter(NOTIFICATION_EXTENSION_POINT));
console.log('Found extensions:', extensionBindings.map(b => b.key));

import { extensionFor, addExtension } from "@loopback/core";

// Extension that implements multiple interfaces
class LoggingAuditProvider implements Logger, AuditTrail {
  // Logger interface
  log(level: string, message: string): void {
    console.log(`[${level}] ${message}`);
  }
  
  // AuditTrail interface
  recordEvent(event: AuditEvent): void {
    console.log(`Audit: ${event.action} by ${event.user}`);
  }
}

const app = new Application();

// Register for multiple extension points
app.bind('logging-audit-provider')
  .toClass(LoggingAuditProvider)
  .apply(extensionFor('logger', 'audit-trail'));

// Or using addExtension (requires multiple calls)
const context = app;
addExtension(context, 'logger', LoggingAuditProvider);
addExtension(context, 'audit-trail', LoggingAuditProvider);

import { 
  extensionPoint, 
  extensions, 
  extensionFilter,
  Context,
  ContextView 
} from "@loopback/core";

// Service that manages multiple extension points
class ExtensionManager {
  constructor(
    @inject.context() private context: Context
  ) {}
  
  async getExtensionsForPoint(extensionPointName: string): Promise<any[]> {
    const filter = extensionFilter(extensionPointName);
    const view = new ContextView(this.context, filter);
    return view.values();
  }
  
  async getAllExtensionPoints(): Promise<string[]> {
    const bindings = this.context.find(binding => 
      binding.tagMap[CoreTags.EXTENSION_POINT]
    );
    
    return bindings.map(binding => 
      binding.tagMap[CoreTags.EXTENSION_POINT]
    ).filter(Boolean);
  }
  
  async getExtensionInfo(): Promise<ExtensionInfo[]> {
    const extensionPoints = await this.getAllExtensionPoints();
    const info: ExtensionInfo[] = [];
    
    for (const point of extensionPoints) {
      const extensions = await this.getExtensionsForPoint(point);
      info.push({
        extensionPoint: point,
        extensionCount: extensions.length,
        extensions: extensions.map(ext => ext.constructor.name)
      });
    }
    
    return info;
  }
}

interface ExtensionInfo {
  extensionPoint: string;
  extensionCount: number;
  extensions: string[];
}

import { 
  extensionPoint, 
  extensions, 
  ContextView,
  BindingFilter 
} from "@loopback/core";

interface Handler {
  name: string;
  priority: number;
  condition?: (context: any) => boolean;
  handle(data: any): Promise<any>;
}

@extensionPoint('request-handler')
class RequestProcessor {
  constructor(
    @extensions.view() private handlersView: ContextView<Handler>
  ) {}
  
  async processRequest(data: any, context: any): Promise<any> {
    const handlers = await this.getApplicableHandlers(context);
    
    // Sort by priority
    handlers.sort((a, b) => a.priority - b.priority);
    
    let result = data;
    for (const handler of handlers) {
      console.log(`Processing with handler: ${handler.name}`);
      result = await handler.handle(result);
    }
    
    return result;
  }
  
  private async getApplicableHandlers(context: any): Promise<Handler[]> {
    const allHandlers = await this.handlersView.values();
    
    return allHandlers.filter(handler => 
      !handler.condition || handler.condition(context)
    );
  }
  
  async getHandlerInfo(): Promise<Array<{name: string, priority: number}>> {
    const handlers = await this.handlersView.values();
    return handlers.map(h => ({ name: h.name, priority: h.priority }));
  }
}

// Conditional handler example
class AuthHandler implements Handler {
  name = 'auth-handler';
  priority = 1;
  
  condition(context: any): boolean {
    return context.requiresAuth === true;
  }
  
  async handle(data: any): Promise<any> {
    // Authentication logic
    return { ...data, authenticated: true };
  }
}

class ValidationHandler implements Handler {
  name = 'validation-handler';
  priority = 0; // Run first
  
  async handle(data: any): Promise<any> {
    // Validation logic
    if (!data.isValid) {
      throw new Error('Validation failed');
    }
    return data;
  }
}

interface InjectionMetadata {
  optional?: boolean;
  asProxyWithInterceptors?: boolean;
  bindingComparator?: BindingComparator;
}

type BindingSpec = 
  | BindingFromClassOptions
  | BindingTemplate
  | { [tag: string]: any };

interface BindingFromClassOptions {
  name?: string;
  namespace?: string;
  type?: string;
  defaultScope?: BindingScope;
}

type BindingTemplate<T = unknown> = (binding: Binding<T>) => void;

type BindingFilter = (binding: Readonly<Binding<unknown>>) => boolean;

type Constructor<T = {}> = new (...args: any[]) => T;