servo/components/style/invalidation/element/element_wrapper.rs

/* 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 wrapper over an element and a snapshot, that allows us to selector-match
//! against a past state of the element.

use {Atom, CaseSensitivityExt, LocalName, Namespace};
use dom::TElement;
use element_state::ElementState;
use selector_parser::{NonTSPseudoClass, PseudoElement, SelectorImpl, Snapshot, SnapshotMap, AttrValue};
use selectors::Element;
use selectors::attr::{AttrSelectorOperation, CaseSensitivity, NamespaceConstraint};
use selectors::matching::{ElementSelectorFlags, LocalMatchingContext, MatchingContext};
use selectors::matching::RelevantLinkStatus;
use std::cell::Cell;
use std::fmt;

/// In order to compute restyle hints, we perform a selector match against a
/// list of partial selectors whose rightmost simple selector may be sensitive
/// to the thing being changed. We do this matching twice, once for the element
/// as it exists now and once for the element as it existed at the time of the
/// last restyle. If the results of the selector match differ, that means that
/// the given partial selector is sensitive to the change, and we compute a
/// restyle hint based on its combinator.
///
/// In order to run selector matching against the old element state, we generate
/// a wrapper for the element which claims to have the old state. This is the
/// ElementWrapper logic below.
///
/// Gecko does this differently for element states, and passes a mask called
/// mStateMask, which indicates the states that need to be ignored during
/// selector matching. This saves an ElementWrapper allocation and an additional
/// selector match call at the expense of additional complexity inside the
/// selector matching logic. This only works for boolean states though, so we
/// still need to take the ElementWrapper approach for attribute-dependent
/// style. So we do it the same both ways for now to reduce complexity, but it's
/// worth measuring the performance impact (if any) of the mStateMask approach.
pub trait ElementSnapshot : Sized {
    /// The state of the snapshot, if any.
    fn state(&self) -> Option<ElementState>;

    /// If this snapshot contains attribute information.
    fn has_attrs(&self) -> bool;

    /// The ID attribute per this snapshot. Should only be called if
    /// `has_attrs()` returns true.
    fn id_attr(&self) -> Option<Atom>;

    /// Whether this snapshot contains the class `name`. Should only be called
    /// if `has_attrs()` returns true.
    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool;

    /// A callback that should be called for each class of the snapshot. Should
    /// only be called if `has_attrs()` returns true.
    fn each_class<F>(&self, F)
        where F: FnMut(&Atom);

    /// The `xml:lang=""` or `lang=""` attribute value per this snapshot.
    fn lang_attr(&self) -> Option<AttrValue>;
}

/// A simple wrapper over an element and a snapshot, that allows us to
/// selector-match against a past state of the element.
#[derive(Clone)]
pub struct ElementWrapper<'a, E>
    where E: TElement,
{
    element: E,
    cached_snapshot: Cell<Option<&'a Snapshot>>,
    snapshot_map: &'a SnapshotMap,
}

impl<'a, E> ElementWrapper<'a, E>
    where E: TElement,
{
    /// Trivially constructs an `ElementWrapper`.
    pub fn new(el: E, snapshot_map: &'a SnapshotMap) -> Self {
        ElementWrapper {
            element: el,
            cached_snapshot: Cell::new(None),
            snapshot_map: snapshot_map,
        }
    }

    /// Gets the snapshot associated with this element, if any.
    pub fn snapshot(&self) -> Option<&'a Snapshot> {
        if !self.element.has_snapshot() {
            return None;
        }

        if let Some(s) = self.cached_snapshot.get() {
            return Some(s);
        }

        let snapshot = self.snapshot_map.get(&self.element);
        debug_assert!(snapshot.is_some(), "has_snapshot lied!");

        self.cached_snapshot.set(snapshot);

        snapshot
    }

    /// Returns the states that have changed since the element was snapshotted.
    pub fn state_changes(&self) -> ElementState {
        let snapshot = match self.snapshot() {
            Some(s) => s,
            None => return ElementState::empty(),
        };

        match snapshot.state() {
            Some(state) => state ^ self.element.get_state(),
            None => ElementState::empty(),
        }
    }

    /// Returns the value of the `xml:lang=""` (or, if appropriate, `lang=""`)
    /// attribute from this element's snapshot or the closest ancestor
    /// element snapshot with the attribute specified.
    fn get_lang(&self) -> Option<AttrValue> {
        let mut current = self.clone();
        loop {
            let lang = match self.snapshot() {
                Some(snapshot) if snapshot.has_attrs() => snapshot.lang_attr(),
                _ => current.element.lang_attr(),
            };
            if lang.is_some() {
                return lang;
            }
            match current.parent_element() {
                Some(parent) => current = parent,
                None => return None,
            }
        }
    }
}

impl<'a, E> fmt::Debug for ElementWrapper<'a, E>
    where E: TElement,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        // Ignore other fields for now, can change later if needed.
        self.element.fmt(f)
    }
}

impl<'a, E> Element for ElementWrapper<'a, E>
    where E: TElement,
{
    type Impl = SelectorImpl;

    fn match_non_ts_pseudo_class<F>(&self,
                                    pseudo_class: &NonTSPseudoClass,
                                    context: &mut LocalMatchingContext<Self::Impl>,
                                    relevant_link: &RelevantLinkStatus,
                                    _setter: &mut F)
                                    -> bool
        where F: FnMut(&Self, ElementSelectorFlags),
    {
        // Some pseudo-classes need special handling to evaluate them against
        // the snapshot.
        match *pseudo_class {
            #[cfg(feature = "gecko")]
            NonTSPseudoClass::MozAny(ref selectors) => {
                use selectors::matching::matches_complex_selector;
                return selectors.iter().any(|s| {
                    matches_complex_selector(s.iter(), self, context, _setter)
                })
            }

            // :dir is implemented in terms of state flags, but which state flag
            // it maps to depends on the argument to :dir.  That means we can't
            // just add its state flags to the NonTSPseudoClass, because if we
            // added all of them there, and tested via intersects() here, we'd
            // get incorrect behavior for :not(:dir()) cases.
            //
            // FIXME(bz): How can I set this up so once Servo adds :dir()
            // support we don't forget to update this code?
            #[cfg(feature = "gecko")]
            NonTSPseudoClass::Dir(ref s) => {
                use invalidation::element::invalidation_map::dir_selector_to_state;
                let selector_flag = dir_selector_to_state(s);
                if selector_flag.is_empty() {
                    // :dir() with some random argument; does not match.
                    return false;
                }
                let state = match self.snapshot().and_then(|s| s.state()) {
                    Some(snapshot_state) => snapshot_state,
                    None => self.element.get_state(),
                };
                return state.contains(selector_flag);
            }

            // For :link and :visited, we don't actually want to test the element
            // state directly.  Instead, we use the `relevant_link` to determine if
            // they match.
            NonTSPseudoClass::Link => {
                return relevant_link.is_unvisited(self, context.shared);
            }
            NonTSPseudoClass::Visited => {
                return relevant_link.is_visited(self, context.shared);
            }

            #[cfg(feature = "gecko")]
            NonTSPseudoClass::MozTableBorderNonzero => {
                if let Some(snapshot) = self.snapshot() {
                    if snapshot.has_other_pseudo_class_state() {
                        return snapshot.mIsTableBorderNonzero();
                    }
                }
            }

            #[cfg(feature = "gecko")]
            NonTSPseudoClass::MozBrowserFrame => {
                if let Some(snapshot) = self.snapshot() {
                    if snapshot.has_other_pseudo_class_state() {
                        return snapshot.mIsMozBrowserFrame();
                    }
                }
            }

            // :lang() needs to match using the closest ancestor xml:lang="" or
            // lang="" attribtue from snapshots.
            NonTSPseudoClass::Lang(ref lang_arg) => {
                return self.element.match_element_lang(Some(self.get_lang()), lang_arg);
            }

            _ => {}
        }

        let flag = pseudo_class.state_flag();
        if flag.is_empty() {
            return self.element.match_non_ts_pseudo_class(pseudo_class,
                                                          context,
                                                          relevant_link,
                                                          &mut |_, _| {})
        }
        match self.snapshot().and_then(|s| s.state()) {
            Some(snapshot_state) => snapshot_state.intersects(flag),
            None => {
                self.element.match_non_ts_pseudo_class(pseudo_class,
                                                       context,
                                                       relevant_link,
                                                       &mut |_, _| {})
            }
        }
    }

    fn match_pseudo_element(&self,
                            pseudo_element: &PseudoElement,
                            context: &mut MatchingContext)
                            -> bool
    {
        self.element.match_pseudo_element(pseudo_element, context)
    }

    fn is_link(&self) -> bool {
        self.element.is_link()
    }

    fn parent_element(&self) -> Option<Self> {
        self.element.parent_element()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }

    fn first_child_element(&self) -> Option<Self> {
        self.element.first_child_element()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }

    fn last_child_element(&self) -> Option<Self> {
        self.element.last_child_element()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }

    fn prev_sibling_element(&self) -> Option<Self> {
        self.element.prev_sibling_element()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }

    fn next_sibling_element(&self) -> Option<Self> {
        self.element.next_sibling_element()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }

    fn is_html_element_in_html_document(&self) -> bool {
        self.element.is_html_element_in_html_document()
    }

    fn get_local_name(&self) -> &<Self::Impl as ::selectors::SelectorImpl>::BorrowedLocalName {
        self.element.get_local_name()
    }

    fn get_namespace(&self) -> &<Self::Impl as ::selectors::SelectorImpl>::BorrowedNamespaceUrl {
        self.element.get_namespace()
    }

    fn attr_matches(&self,
                    ns: &NamespaceConstraint<&Namespace>,
                    local_name: &LocalName,
                    operation: &AttrSelectorOperation<&AttrValue>)
                    -> bool {
        match self.snapshot() {
            Some(snapshot) if snapshot.has_attrs() => {
                snapshot.attr_matches(ns, local_name, operation)
            }
            _ => self.element.attr_matches(ns, local_name, operation)
        }
    }

    fn has_id(&self, id: &Atom, case_sensitivity: CaseSensitivity) -> bool {
        match self.snapshot() {
            Some(snapshot) if snapshot.has_attrs() => {
                snapshot.id_attr().map_or(false, |atom| case_sensitivity.eq_atom(&atom, id))
            }
            _ => self.element.has_id(id, case_sensitivity)
        }
    }

    fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
        match self.snapshot() {
            Some(snapshot) if snapshot.has_attrs() => {
                snapshot.has_class(name, case_sensitivity)
            }
            _ => self.element.has_class(name, case_sensitivity)
        }
    }

    fn is_empty(&self) -> bool {
        self.element.is_empty()
    }

    fn is_root(&self) -> bool {
        self.element.is_root()
    }

    fn pseudo_element_originating_element(&self) -> Option<Self> {
        self.element.closest_non_native_anonymous_ancestor()
            .map(|e| ElementWrapper::new(e, self.snapshot_map))
    }
}