Reduce client-side load in calculating category balances

src/app/budgets/budget-details/budget-details.component.ts

@@ -119,7 +119,7 @@ export class BudgetDetailsComponent implements OnInit, OnDestroy, Actionable {
           this.income.push(category);
           this.expectedIncome += category.amount;
         }
-        this.getCategoryBalance(category.id).subscribe(
+        this.twigsService.getCategoryBalance(category.id).subscribe(
           balance => {
             console.log(balance);
             if (category.expense) {
@@ -145,28 +145,6 @@ export class BudgetDetailsComponent implements OnInit, OnDestroy, Actionable {
     });
   }
 
-  getCategoryBalance(category: number): Observable<number> {
-    return Observable.create(subscriber => {
-      let date = new Date();
-      date.setHours(0);
-      date.setMinutes(0);
-      date.setSeconds(0);
-      date.setDate(1);  
-      this.twigsService.getTransactions(this.budget.id, category, null, date).subscribe(transactions => {
-        let balance = 0;
-        for (const transaction of transactions) {
-          if (transaction.expense) {
-            balance -= transaction.amount;
-          } else {
-            balance += transaction.amount;
-          }
-        }
-        subscriber.next(balance);
-        subscriber.complete();
-      });
-    });
-  }
-
   doAction(): void {
     this.router.navigateByUrl(this.router.routerState.snapshot.url + "/edit")
   }

src/app/shared/twigs.http.service.ts

@@ -173,6 +173,11 @@ export class TwigsHttpService implements TwigsService {
     return this.http.get<Category>(`${this.apiUrl}/categories/${id}`, this.options);
   }
 
+  getCategoryBalance(id: number): Observable<number> {
+    return this.http.get<any>(`${this.apiUrl}/categories/${id}/balance`, this.options)
+      .pipe(map(obj => obj.balance));
+  }
+
   createCategory(budgetId: number, name: string, description: string, amount: number, isExpense: boolean): Observable<Category> {
     const params = {
       'title': name,

src/app/shared/twigs.local.service.ts

@@ -153,6 +153,13 @@ export class TwigsLocalService implements TwigsService {
     });
   }
 
+  getCategoryBalance(id: number): Observable<number> {
+    return new Observable(emitter => {
+      emitter.next(20);
+      emitter.complete()
+    })
+  }
+
   createCategory(budgetId: number, name: string, description: string, amount: number, isExpense: boolean): Observable<Category> {
     return Observable.create(subscriber => {
       const category = new Category();

src/app/shared/twigs.service.ts

@@ -25,6 +25,7 @@ export interface TwigsService {
   // Categories
   getCategories(budgetId?: number, count?: number): Observable<Category[]>;
   getCategory(id: number): Observable<Category>;
+  getCategoryBalance(id: number): Observable<number>;
   createCategory(budgetId: number, name: string, description: string, amount: number, isExpense: boolean): Observable<Category>;
   updateCategory(budgetId: number, id: number, changes: object): Observable<Category>;
   deleteCategory(budgetId: number, id: number): Observable<void>;