Make Subscription cancellation fail-fast

vision-api/src/main/java/me/sparky983/vision/Subscribers.java

@@ -25,7 +25,9 @@ public Subscription subscribe(final T subscriber) {
             @Override
             public void cancel() {
 
-                subscribers.remove(this);
+                if (subscribers.remove(this) == null) {
+                    throw new IllegalStateException("Subscription has already been cancelled");
+                }
             }
 
             @Override

vision-api/src/main/java/me/sparky983/vision/Subscription.java

@@ -12,6 +12,7 @@ public interface Subscription {
      * Cancels this {@code Subscription}.
      *
      * @since 0.1
+     * @throws IllegalStateException if this {@code Subscription} has already been cancelled.
      */
     void cancel();
 

vision-api/src/test/java/me/sparky983/vision/ButtonTests.java

@@ -423,6 +423,18 @@ void testSubscriberThrowsException() {
             verifyNoMoreInteractions(subscriber); // exception should not be called again
         }
 
+        @Test
+        void testCancelSubscriptionWhenSubscriptionIsAlreadyCancelled() {
+
+            final Button button = Button.button().type(ItemType.STONE);
+
+            final Subscription subscription = button.subscribe(mock());
+
+            subscription.cancel();
+
+            assertThrows(IllegalStateException.class, subscription::cancel);
+        }
+
         @Test
         void testCancelSubscription() {
 
@@ -818,6 +830,18 @@ void testSubscriberThrowsException() {
             verifyNoMoreInteractions(subscriber); // exception should not be called again
         }
 
+        @Test
+        void testCancelSubscriptionWhenSubscriptionIsAlreadyCancelled() {
+
+            final Button button = Button.of(ItemType.STONE);
+
+            final Subscription subscription = button.subscribe(mock());
+
+            subscription.cancel();
+
+            assertThrows(IllegalStateException.class, subscription::cancel);
+        }
+
         @Test
         void testCancelSubscription() {
 

vision-api/src/test/java/me/sparky983/vision/ChestTests.java

@@ -335,6 +335,18 @@ void testSubscriberThrowsException() {
         verifyNoMoreInteractions(subscriber);
     }
 
+    @Test
+    void testCancelSubscriptionWhenSubscriptionIsAlreadyCancelled() {
+
+        final Gui gui = Gui.chest().build();
+
+        final Subscription subscription = gui.subscribe(mock());
+
+        subscription.cancel();
+
+        assertThrows(IllegalStateException.class, subscription::cancel);
+    }
+
     @Test
     void testCancelSubscription() {
 

vision-api/src/test/java/me/sparky983/vision/DropperTest.java

@@ -295,6 +295,18 @@ void testSubscriberThrowsException() {
         verifyNoMoreInteractions(subscriber);
     }
 
+    @Test
+    void testCancelSubscriptionWhenSubscriptionIsAlreadyCancelled() {
+
+        final Gui gui = Gui.dropper().build();
+
+        final Subscription subscription = gui.subscribe(mock());
+
+        subscription.cancel();
+
+        assertThrows(IllegalStateException.class, subscription::cancel);
+    }
+
     @Test
     void testCancelSubscription() {
 

vision-api/src/test/java/me/sparky983/vision/HopperTests.java

@@ -295,6 +295,18 @@ void testSubscriberThrowsException() {
         verifyNoMoreInteractions(subscriber);
     }
 
+    @Test
+    void testCancelSubscriptionWhenSubscriptionIsAlreadyCancelled() {
+
+        final Gui gui = Gui.hopper().build();
+
+        final Subscription subscription = gui.subscribe(mock());
+
+        subscription.cancel();
+
+        assertThrows(IllegalStateException.class, subscription::cancel);
+    }
+
     @Test
     void testCancelSubscription() {