docs: updated Plugins Development.md

docs/Plugins Development.md

@@ -19,6 +19,54 @@ Fastify decorators provides several hooks:
 - `appReady` - executes when Fastify decorators done initialization
 - `appDestroy` - executes before Fastify instance destroyed
 
+### Class loader
+
+"Class loader" is a function used to instantiate Controller/RequestHandler/Service/etc.
+It accepts class itself and scope.
+
+where:
+
+- class itself could be Controller/RequestHandler or any other class type defined in the plugin (f.e. Service)
+- scope is where this class were requested - FastifyInstance or FastifyRequest.
+
+Registration of class loader SHOULD be done on `appInit` stage by decorating FastifyInstance.
+
+*example*:
+
+```typescript
+import { CLASS_LOADER, createInitializationHook } from 'fastify-decorators/plugins';
+
+createInitializationHook('appInit', (fastify) => {
+  fastify.decorate(CLASS_LOADER, (clazz: new () => any, _scope) => new clazz)
+})
+```
+
+### Handlers, Hooks and Error Handlers
+
+In addition to normal Lifecycle hooks,
+Fastify Decorators provides interfaces and symbols to interact with registered handlers and hooks.
+
+This functionality can be used to implicitly decorate methods
+
+*Example: log arguments when any hook is called*:
+
+```typescript
+import { FastifyInstance } from 'fastify';
+import { createInitializationHook, Registrable, hasHooks, HOOKS, IHook } from 'fastify-decorators/plugins';
+
+createInitializationHook('beforeControllerCreation', (fastifyInstance: FastifyInstance, target: Registrable) => {
+  if (hasHooks(target)) {
+    for (const hook of target[HOOKS]) { // hook has type IHook
+      const _method = target[hook.handlerName];
+      target[hook.handlerName] = function logged(...args: any) {
+        console.log('Hook called with args: ', args)
+        return _method.apply(this, args)
+      }
+    }
+  }
+})
+```
+
 ### Fastify Lifecycle:
 
 Please refer to [Fastify docs]