FactoryService使用

可以使用useFactory設定條件，來使用不同的Service

src/app/app.module.ts

@@ -4,16 +4,24 @@ import { FormsModule } from '@angular/forms';
 
 import { AppComponent } from './app.component';
 import { OrderService } from './order.service';
-import { OrderDiscountService } from './order-discount.service';
+import { OrderAnniversaryService } from './order-anniversary.service';
 
 @NgModule({
   imports: [BrowserModule, FormsModule],
   declarations: [AppComponent],
-  providers: [{
+  providers: [
+    {
       provide: OrderService,
-    useValue:{computeTotal:()=>100},
-  },],
-
+      useFactory: () => {
+        const today = new Date(2020, 9, 3);
+        if (today.getMonth() === 9) {
+          return new OrderAnniversaryService();
+        } else {
+          return new OrderService();
+        }
+      },
+    },
+  ],
   bootstrap: [AppComponent],
 })
 export class AppModule {}

src/app/order-anniversary.service.ts

@@ -0,0 +1,18 @@
+import { Injectable } from '@angular/core';
+
+import { OrderDetail } from './order-detail';
+
+@Injectable({
+  providedIn: 'root',
+})
+export class OrderAnniversaryService {
+  details: OrderDetail[] = [];
+
+  constructor() {}
+
+  computeTotal(): number {
+    return this.details
+      .map((d) => d.PurchaseCount * d.UnitPrice * 0.8)
+      .reduce((act, curr) => act + curr, 0);
+  }
+}