useValue抽象服務

可將物件實體抽換掉服務類別，可以透過建立間諜ＳＰＹ物件模擬測試情境

src/app/app.module.ts

@@ -9,7 +9,10 @@ import { OrderDiscountService } from './order-discount.service';
 @NgModule({
   imports: [BrowserModule, FormsModule],
   declarations: [AppComponent],
-  providers: [{ provide: OrderService, useExisting: OrderDiscountService },OrderDiscountService],
+  providers: [{
+    provide: OrderService,
+    useValue:{computeTotal:()=>100},
+  },],
 
   bootstrap: [AppComponent],
 })