Make World::register* functions idempotent (amethyst#96)

Previously repeated calls to `register` (and friends) would stomp over
the existing storage for the component type.

Now repeated calls will silently do nothing.

src/world.rs

@@ -249,9 +249,14 @@ impl<C> World<C>
         }
     }
     /// Registers a new component type and id pair.
+    ///
+    /// Does nothing if the type and id pair was already registered.
     pub fn register_w_comp_id<T: Component>(&mut self, comp_id: C) {
-        let any = RwLock::new(MaskedStorage::<T>::new());
-        self.components.insert((comp_id, TypeId::of::<T>()), Box::new(any));
+        self.components.entry((comp_id, TypeId::of::<T>()))
+            .or_insert_with(|| {
+                let any = RwLock::new(MaskedStorage::<T>::new());
+                Box::new(any)
+            });
     }
     /// Unregisters a component type and id pair.
     pub fn unregister_w_comp_id<T: Component>(&mut self, comp_id: C) -> Option<MaskedStorage<T>> {
@@ -382,6 +387,8 @@ impl World<()> {
     }
 
     /// Registers a new component type.
+    ///
+    /// Does nothing if the component type was already registered.
     pub fn register<T: Component>(&mut self) {
         self.register_w_comp_id::<T>(())
     }

tests/tests.rs

@@ -299,3 +299,23 @@ fn dynamic_component() {
     let c = w.read_w_comp_id::<CompBool>(2).get(e).unwrap().0;
     assert_eq!(c, true);
 }
+
+#[test]
+fn register_idempotency() {
+    // Test that repeated calls to `register` do not silently
+    // stomp over the existing storage, but instead silently do nothing.
+    let mut w = specs::World::new();
+    w.register::<CompInt>();
+
+    let e = w.create_now()
+        .with::<CompInt>(CompInt(10))
+        .build();
+
+    // At the time this test was written, a call to `register`
+    // would blindly plough ahead and stomp the existing storage, so...
+    w.register::<CompInt>();
+
+    // ...this would end up trying to unwrap a `None`.
+    let i = w.read::<CompInt>().get(e).unwrap().0;
+    assert_eq!(i, 10);
+}