feat(translation-store): add subject to inform on fallback lang use

projects/ngx-translate/src/lib/translate.service.ts

@@ -65,6 +65,10 @@ export interface FallbackLangChangeEvent {
     translations: InterpolatableTranslationObject;
 }
 
+export interface FallbackLangUseEvent {
+    lang: string;
+    missingTranslationKey: string;
+}
 /** @deprecated use `FallbackLangChangeEvent` */
 export type DefaultLangChangeEvent = FallbackLangChangeEvent;
 
@@ -217,6 +221,16 @@ export class TranslateService implements ITranslateService {
         return this.store.onFallbackLangChange;
     }
 
+    /**
+     * An Observable to listen to fallback lang use events
+     * onFallbackLangUse.subscribe((params: FallbackLangUseEvent) => {
+     *     // do something
+     * });
+     */
+    get onFallbackLangUse(): Observable<FallbackLangUseEvent> {
+        return this.store.onFallbackLangUse;
+    }
+
     /**
      * @deprecated Use onFallbackLangChange() instead
      */

projects/ngx-translate/src/lib/translate.store.ts

@@ -2,6 +2,7 @@ import { Injectable } from "@angular/core";
 import { Observable, Subject } from "rxjs";
 import {
     FallbackLangChangeEvent,
+    FallbackLangUseEvent,
     InterpolatableTranslation,
     InterpolatableTranslationObject,
     LangChangeEvent,
@@ -21,6 +22,7 @@ export class TranslateStore {
     private _onLangChange: Subject<LangChangeEvent> = new Subject<LangChangeEvent>();
     private _onFallbackLangChange: Subject<FallbackLangChangeEvent> =
         new Subject<FallbackLangChangeEvent>();
+    private _onFallbackLangUse: Subject<FallbackLangUseEvent> = new Subject<FallbackLangUseEvent>();
 
     private fallbackLang: Language | null = null;
     private currentLang!: Language;
@@ -107,6 +109,16 @@ export class TranslateStore {
         return this._onFallbackLangChange.asObservable();
     }
 
+    /**
+     * An Observable to listen to fallback lang use events
+     * onFallbackLangUse.subscribe((params: FallbackLangUseEvent) => {
+     *     // do something
+     * });
+     */
+    get onFallbackLangUse(): Observable<FallbackLangUseEvent> {
+        return this._onFallbackLangUse.asObservable();
+    }
+
     public addLanguages(languages: Language[]): void {
         this.languages = Array.from(new Set([...this.languages, ...languages]));
     }
@@ -127,6 +139,10 @@ export class TranslateStore {
             this.fallbackLang != null &&
             this.fallbackLang !== this.currentLang
         ) {
+            this._onFallbackLangUse.next({
+                lang: this.currentLang,
+                missingTranslationKey: key,
+            });
             text = this.getValue(this.fallbackLang, key);
         }
         return text;

projects/ngx-translate/src/tests/translate.store.spec.ts

@@ -9,6 +9,7 @@ import {
 import { ComponentFixture, fakeAsync, TestBed, tick, inject } from "@angular/core/testing";
 import { provideRouter, Router, RouterModule } from "@angular/router";
 import { TranslateModule, TranslateService } from "../public-api";
+import { TranslateStore } from "@ngx-translate/core";
 
 @Component({
     // eslint-disable-next-line @angular-eslint/prefer-standalone
@@ -220,4 +221,33 @@ describe("TranslateStore", () => {
             expect(translate.instant("CHILD")).toEqual("Child");
         }),
     ));
+
+    it("should not emit fallback lang use events when no translation is missing", () => {
+        const translate = TestBed.inject(TranslateService);
+
+        const spy = jasmine.createSpy("fallbackLangUseSpy");
+        const sub = translate.onFallbackLangUse.subscribe(spy);
+
+        expect(spy).toHaveBeenCalledTimes(0);
+
+        translate.instant("TEST");
+
+        expect(spy).toHaveBeenCalledTimes(0);
+        sub.unsubscribe();
+    });
+
+    it("should emit fallback lang use events when translation is missing", () => {
+        const translate = TestBed.inject(TranslateService);
+
+        const spy = jasmine.createSpy("fallbackLangUseSpy");
+        const sub = translate.onFallbackLangUse.subscribe(spy);
+        translate.setFallbackLang("en");
+
+        expect(spy).toHaveBeenCalledTimes(0);
+        translate.use("de");
+        translate.instant("TEST");
+
+        expect(spy).toHaveBeenCalledTimes(1);
+        sub.unsubscribe();
+    });
 });