[O] Optimize count unread for overall-course

src/pages/overall/overall-course/overall-course.ts

@@ -11,20 +11,14 @@ export default class OverallCourse extends Vue
 {
     @Prop({required: true}) course: Course;
 
-    private unread: number = -1;
+    get unreadAssignments(): Assignment[]
-    private unreadAssignments: Assignment[] = [];
-
-    /**
-     * Count the number of unread assignments with cache
-     */
-    countUnread(): number
     {
-        if (this.unread == -1)
+        return this.course.assignments.filter(a => a.unread);
-        {
+    }
-            this.unreadAssignments = this.course.assignments.filter(a => a.unread);
+
-            return this.unread = this.unreadAssignments.length;
+    get unread(): number
-        }
+    {
-        else return this.unread;
+        return this.unreadAssignments.length;
     }
 
     /**