fix(TranslateService): only make one request per lang

This stops multiple requests being made for the same lang file. It will now return the same observable when making duplicate requests for the same language.

Fixes #397
Fixes #432 
Fixes #447
Fixes Greentube/localize-router#26
Fixes #462

src/translate.service.ts

@@ -47,6 +47,7 @@ export class TranslateService {
     private _currentLang: string;
     private _langs: Array<string> = [];
     private _translations: any = {};
+    private _translationRequests: any  = {};
 
     /**
      * An EventEmitter to listen to translation change events
@@ -230,7 +231,8 @@ export class TranslateService {
 
         // if this language is unavailable, ask for it
         if(typeof this.translations[lang] === "undefined") {
-            pending = this.getTranslation(lang);
+            this._translationRequests[lang] = this._translationRequests[lang] || this.getTranslation(lang);
+            pending = this._translationRequests[lang];
         }
 
         return pending;
@@ -475,6 +477,7 @@ export class TranslateService {
      * @param lang
      */
     public resetLang(lang: string): void {
+        this._translationRequests[lang] = undefined;
         this.translations[lang] = undefined;
     }
 

tests/translate.service.spec.ts

@@ -1,7 +1,7 @@
 import {Injector} from "@angular/core";
 import {TranslateService, TranslateLoader, LangChangeEvent, TranslationChangeEvent, TranslateModule} from '../index';
 import {Observable} from "rxjs/Observable";
-import {getTestBed, TestBed} from "@angular/core/testing";
+import {getTestBed, TestBed, fakeAsync, tick} from "@angular/core/testing";
 
 let translations: any = {"TEST": "This is a test"};
 class FakeLoader implements TranslateLoader {
@@ -296,4 +296,18 @@ describe('TranslateService', () => {
         expect(browserCultureLand).toBeDefined();
         expect(typeof browserCultureLand === 'string').toBeTruthy();
     });
+
+    it('should not make duplicate requests', fakeAsync(() => {
+        let getTranslationCalls = 0;
+        spyOn(translate.currentLoader, 'getTranslation').and.callFake(() => {
+            getTranslationCalls += 1;
+            return Observable.timer(1000).mapTo(Observable.of(translations));
+        });
+        translate.use('en');
+        translate.use('en');
+
+        tick(1001);
+
+        expect(getTranslationCalls).toEqual(1);
+    }));
 });