feat: add config changes stream

lib/config.service.ts

@@ -5,10 +5,12 @@ import fs from 'fs';
 import get from 'lodash/get';
 import has from 'lodash/has';
 import set from 'lodash/set';
+import { Subject } from 'rxjs';
 import {
   CONFIGURATION_TOKEN,
   VALIDATED_ENV_PROPNAME,
 } from './config.constants';
+import { ConfigChangeEvent } from './interfaces/config-change-event.interface';
 import { NoInferType, Path, PathValue } from './types';
 
 /**
@@ -53,6 +55,7 @@ export class ConfigService<
   }
 
   private readonly cache: Partial<K> = {} as any;
+  private readonly _changes$ = new Subject<ConfigChangeEvent>();
   private _isCacheEnabled = false;
   private envFilePaths: string[] = [];
 
@@ -62,6 +65,14 @@ export class ConfigService<
     private readonly internalConfig: Record<string, any> = {},
   ) {}
 
+  /**
+   * Returns a stream of configuration changes.
+   * Each event contains the attribute path, the old value and the new value.
+   */
+  get changes$() {
+    return this._changes$.asObservable();
+  }
+
   /**
    * Get a configuration value (either custom configuration or process environment variable)
    * based on property path (you can use dot notation to traverse nested object, e.g. "database.host").
@@ -208,6 +219,7 @@ export class ConfigService<
    * @param value
    */
   set<T = any>(propertyPath: KeyOf<K>, value: T): void {
+    const oldValue = this.get(propertyPath);
     set(this.internalConfig, propertyPath, value);
 
     if (typeof propertyPath === 'string') {
@@ -218,6 +230,12 @@ export class ConfigService<
     if (this.isCacheEnabled) {
       this.setInCacheIfDefined(propertyPath, value);
     }
+
+    this._changes$.next({
+      path: propertyPath as string,
+      oldValue,
+      newValue: value,
+    });
   }
   /**
    * Sets env file paths from `config.module.ts` to parse.

lib/interfaces/config-change-event.interface.ts

@@ -0,0 +1,10 @@
+/**
+ * Represents a change in the configuration object.
+ * Dispatched when one updates the configuration object through the `ConfigService#set` method.
+ * @publicApi
+ */
+export interface ConfigChangeEvent<OldValue = any, NewValue = any> {
+  path: string;
+  oldValue: OldValue;
+  newValue: NewValue;
+}

lib/interfaces/index.ts

@@ -1,2 +1,3 @@
+export * from './config-change-event.interface';
 export * from './config-factory.interface';
 export * from './config-module-options.interface';