components/script/dom/bindings/inheritance.rs


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/* 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
        }
    }
}