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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/. */
//! A shareable mutable container for the DOM.
use dom::bindings::trace::JSTraceable;
use js::jsapi::{JSTracer};
use util::task_state;
use util::task_state::{SCRIPT, IN_GC};
use std::cell::{BorrowState, RefCell, Ref, RefMut};
/// A mutable field in the DOM.
///
/// This extends the API of `core::cell::RefCell` to allow unsafe access in
/// certain situations, with dynamic checking in debug builds.
#[derive(Clone)]
pub struct DOMRefCell<T> {
value: RefCell<T>,
}
// Functionality specific to Servo's `DOMRefCell` type
// ===================================================
impl<T> DOMRefCell<T> {
/// Return a reference to the contents.
///
/// For use in the layout task only.
#[allow(unsafe_code)]
pub unsafe fn borrow_for_layout<'a>(&'a self) -> &'a T {
debug_assert!(task_state::get().is_layout());
&*self.value.as_unsafe_cell().get()
}
/// Borrow the contents for the purpose of GC tracing.
///
/// This succeeds even if the object is mutably borrowed,
/// so you have to be careful in trace code!
#[allow(unsafe_code)]
pub unsafe fn borrow_for_gc_trace<'a>(&'a self) -> &'a T {
// FIXME: IN_GC isn't reliable enough - doesn't catch minor GCs
// https://github.com/servo/servo/issues/6389
//debug_assert!(task_state::get().contains(SCRIPT | IN_GC));
&*self.value.as_unsafe_cell().get()
}
/// Borrow the contents for the purpose of script deallocation.
///
#[allow(unsafe_code)]
pub unsafe fn borrow_for_script_deallocation<'a>(&'a self) -> &'a mut T {
debug_assert!(task_state::get().contains(SCRIPT));
&mut *self.value.as_unsafe_cell().get()
}
/// Is the cell mutably borrowed?
///
/// For safety checks in debug builds only.
pub fn is_mutably_borrowed(&self) -> bool {
self.value.borrow_state() == BorrowState::Writing
}
/// Attempts to immutably borrow the wrapped value.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple
/// immutable borrows can be taken out at the same time.
///
/// Returns `None` if the value is currently mutably borrowed.
///
/// # Panics
///
/// Panics if this is called off the script thread.
pub fn try_borrow<'a>(&'a self) -> Option<Ref<'a, T>> {
debug_assert!(task_state::get().is_script());
match self.value.borrow_state() {
BorrowState::Writing => None,
_ => Some(self.value.borrow()),
}
}
/// Mutably borrows the wrapped value.
///
/// The borrow lasts until the returned `RefMut` exits scope. The value
/// cannot be borrowed while this borrow is active.
///
/// Returns `None` if the value is currently borrowed.
///
/// # Panics
///
/// Panics if this is called off the script thread.
pub fn try_borrow_mut<'a>(&'a self) -> Option<RefMut<'a, T>> {
debug_assert!(task_state::get().is_script());
match self.value.borrow_state() {
BorrowState::Unused => Some(self.value.borrow_mut()),
_ => None,
}
}
}
impl<T: JSTraceable> JSTraceable for DOMRefCell<T> {
fn trace(&self, trc: *mut JSTracer) {
unsafe {
(*self).borrow_for_gc_trace().trace(trc)
}
}
}
// Functionality duplicated with `core::cell::RefCell`
// ===================================================
impl<T> DOMRefCell<T> {
/// Create a new `DOMRefCell` containing `value`.
pub fn new(value: T) -> DOMRefCell<T> {
DOMRefCell {
value: RefCell::new(value),
}
}
/// Immutably borrows the wrapped value.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple
/// immutable borrows can be taken out at the same time.
///
/// # Panics
///
/// Panics if this is called off the script thread.
///
/// Panics if the value is currently mutably borrowed.
pub fn borrow<'a>(&'a self) -> Ref<'a, T> {
self.try_borrow().expect("DOMRefCell<T> already mutably borrowed")
}
/// Mutably borrows the wrapped value.
///
/// The borrow lasts until the returned `RefMut` exits scope. The value
/// cannot be borrowed while this borrow is active.
///
/// # Panics
///
/// Panics if this is called off the script thread.
///
/// Panics if the value is currently borrowed.
pub fn borrow_mut<'a>(&'a self) -> RefMut<'a, T> {
self.try_borrow_mut().expect("DOMRefCell<T> already borrowed")
}
}
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