Extract all invariant assert to private assert* methods

For readability.

Author: Jeroen de Graaf

src/Domain/Course/ChangeCourseCapacity.php

@@ -42,16 +42,24 @@ public function changeCapacity(int $capacity): void
         /*
          * Protect invariants (business rules).
          */
-        if (!isset($this->courseId)) {
-            throw CourseNotFoundException::create();
-        }
+        $this->assertCourseExists();
 
         /*
          * Apply events when all business rules are met.
          */
         $this->apply(new CourseCapacityChangedEvent((string) $this->courseId, $capacity));
     }
 
+    /**
+     * @throws CourseNotFoundException
+     */
+    private function assertCourseExists(): void
+    {
+        if (!isset($this->courseId)) {
+            throw CourseNotFoundException::create();
+        }
+    }
+
     /*
      * Change internal state by subscribing to relevant domain events for any of the domain tags,
      * so that this use case can apply its business rules.

src/Domain/StudentToCourseSubscription/SubscribeStudentToCourse.php

@@ -57,30 +57,59 @@ public function subscribe(): void
         /*
          * Protect invariants (business rules).
          */
+        $this->assertCourseExists();
+        $this->assertStudentExists();
+        $this->assertCourseHasSpotsAvailable();
+        $this->assertStudentCanSubscribeToMoreCourses();
+
+        /*
+         * Apply events when all business rules are met.
+         */
+        $this->apply(new StudentSubscribedToCourseEvent((string) $this->courseId, (string) $this->studentId));
+
+        if ($this->totalCountSubscriptionsForCourse >= $this->courseCapacity) {
+            $this->apply(new CourseFullyBookedEvent((string) $this->courseId));
+        }
+    }
+
+    /**
+     * @throws CourseNotFoundException
+     */
+    private function assertCourseExists(): void
+    {
         if (!isset($this->courseId)) {
             throw CourseNotFoundException::create();
         }
+    }
 
+    /**
+     * @throws StudentNotFoundException
+     */
+    private function assertStudentExists(): void
+    {
         if (!isset($this->studentId)) {
             throw StudentNotFoundException::create();
         }
+    }
 
+    /**
+     * @throws CourseCannotAcceptMoreStudentsException
+     */
+    private function assertCourseHasSpotsAvailable(): void
+    {
         if ($this->totalCountSubscriptionsForCourse >= $this->courseCapacity) {
             throw CourseCannotAcceptMoreStudentsException::create();
         }
+    }
 
+    /**
+     * @throws StudentCannotSubscribeToMoreCoursesException
+     */
+    private function assertStudentCanSubscribeToMoreCourses(): void
+    {
         if ($this->totalCountSubscriptionsForStudent >= 10) {
             throw StudentCannotSubscribeToMoreCoursesException::create();
         }
-
-        /*
-         * Apply events when all business rules are met.
-         */
-        $this->apply(new StudentSubscribedToCourseEvent((string) $this->courseId, (string) $this->studentId));
-
-        if ($this->totalCountSubscriptionsForCourse >= $this->courseCapacity) {
-            $this->apply(new CourseFullyBookedEvent((string) $this->courseId));
-        }
     }
 
     /*

src/Domain/StudentToCourseSubscription/UnsubscribeStudentFromCourse.php

@@ -51,18 +51,33 @@ public function unsubscribe(): void
         /*
          * Protect invariants (business rules).
          */
+        $this->assertCourseExists();
+        $this->assertStudentExists();
+
+        /*
+         * Apply events when all business rules are met.
+         */
+        $this->apply(new StudentUnsubscribedFromCourseEvent((string) $this->courseId, (string) $this->studentId));
+    }
+
+    /**
+     * @throws CourseNotFoundException
+     */
+    private function assertCourseExists(): void
+    {
         if (!isset($this->courseId)) {
             throw CourseNotFoundException::create();
         }
+    }
 
+    /**
+     * @throws StudentNotFoundException
+     */
+    private function assertStudentExists(): void
+    {
         if (!isset($this->studentId)) {
             throw StudentNotFoundException::create();
         }
-
-        /*
-         * Apply events when all business rules are met.
-         */
-        $this->apply(new StudentUnsubscribedFromCourseEvent((string) $this->courseId, (string) $this->studentId));
     }
 
     /*