move Castable into dom::bindings::inheritance

components/script/dom/bindings/inheritance.rs

@@ -0,0 +1,34 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use dom::bindings::conversions::get_dom_class;
+use dom::bindings::conversions::{DerivedFrom, IDLInterface};
+use dom::bindings::utils::Reflectable;
+use std::mem;
+
+/// A trait to hold the cast functions of IDL interfaces that either derive
+/// or are derived from other interfaces.
+pub trait Castable: IDLInterface + Reflectable + Sized {
+    /// Check whether a DOM object implements one of its deriving interfaces.
+    fn is<T>(&self) -> bool where T: DerivedFrom<Self> {
+        let class = unsafe {
+            get_dom_class(self.reflector().get_jsobject().get()).unwrap()
+        };
+        T::derives(class)
+    }
+
+    /// Cast a DOM object upwards to one of the interfaces it derives from.
+    fn upcast<T>(&self) -> &T where T: Castable, Self: DerivedFrom<T> {
+        unsafe { mem::transmute(self) }
+    }
+
+    /// Cast a DOM object downwards to one of the interfaces it might implement.
+    fn downcast<T>(&self) -> Option<&T> where T: DerivedFrom<Self> {
+        if self.is::<T>() {
+            Some(unsafe { mem::transmute(self) })
+        } else {
+            None
+        }
+    }
+}