path: root/layout/style/nsTransitionManager.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
/* 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/. */

/* Code to start and animate CSS transitions. */

#include "nsTransitionManager.h"
#include "nsAnimationManager.h"
#include "nsIContent.h"
#include "nsStyleContext.h"
#include "nsCSSProps.h"
#include "mozilla/TimeStamp.h"
#include "nsRefreshDriver.h"
#include "nsRuleProcessorData.h"
#include "nsIStyleRule.h"
#include "nsRuleWalker.h"
#include "nsRuleData.h"
#include "gfxColor.h"
#include "nsCSSPropertySet.h"
#include "nsStyleAnimation.h"
#include "nsEventDispatcher.h"
#include "nsGUIEvent.h"
#include "mozilla/dom/Element.h"
#include "nsIFrame.h"
#include "nsCSSFrameConstructor.h"
#include "Layers.h"
#include "FrameLayerBuilder.h"
#include "nsDisplayList.h"
#include "nsStyleChangeList.h"

using mozilla::TimeStamp;
using mozilla::TimeDuration;

using namespace mozilla;
using namespace mozilla::layers;
using namespace mozilla::css;

ElementTransitions::ElementTransitions(mozilla::dom::Element *aElement,
                                       nsIAtom *aElementProperty,
                                       nsTransitionManager *aTransitionManager,
                                       TimeStamp aNow)
  : CommonElementAnimationData(aElement, aElementProperty, aTransitionManager)
  , mFlushGeneration(aNow)
{
}

double
ElementPropertyTransition::ValuePortionFor(TimeStamp aRefreshTime) const
{
  // Set |timePortion| to the portion of the way we are through the time
  // input to the transition's timing function (always within the range
  // 0-1).
  double duration = mDuration.ToSeconds();
  NS_ABORT_IF_FALSE(duration >= 0.0, "negative duration forbidden");
  double timePortion;
  if (IsRemovedSentinel()) {
    // The transition is being removed, but we still want an update so that any
    // new transitions start in the right place.
    timePortion = 1.0;
  } else if (duration == 0.0) {
    // When duration is zero, we can still have a transition when delay
    // is nonzero.  mStartTime already incorporates delay.
    if (aRefreshTime >= mStartTime) {
      timePortion = 1.0;
    } else {
      timePortion = 0.0;
    }
  } else {
    timePortion = (aRefreshTime - mStartTime).ToSeconds() / duration;
    if (timePortion < 0.0)
      timePortion = 0.0; // use start value during transition-delay
    if (timePortion > 1.0)
      timePortion = 1.0; // we might be behind on flushing
  }

  return mTimingFunction.GetValue(timePortion);
}

static void
ElementTransitionsPropertyDtor(void           *aObject,
                               nsIAtom        *aPropertyName,
                               void           *aPropertyValue,
                               void           *aData)
{
  ElementTransitions *et = static_cast<ElementTransitions*>(aPropertyValue);
#ifdef DEBUG
  NS_ABORT_IF_FALSE(!et->mCalledPropertyDtor, "can't call dtor twice");
  et->mCalledPropertyDtor = true;
#endif
  delete et;
}

void
ElementTransitions::EnsureStyleRuleFor(TimeStamp aRefreshTime)
{
  if (!mStyleRule || mStyleRuleRefreshTime != aRefreshTime) {
    mStyleRule = new css::AnimValuesStyleRule();
    mStyleRuleRefreshTime = aRefreshTime;

    for (uint32_t i = 0, i_end = mPropertyTransitions.Length(); i < i_end; ++i)
    {
      ElementPropertyTransition &pt = mPropertyTransitions[i];
      if (pt.IsRemovedSentinel()) {
        continue;
      }

      nsStyleAnimation::Value *val = mStyleRule->AddEmptyValue(pt.mProperty);

      double valuePortion = pt.ValuePortionFor(aRefreshTime);
#ifdef DEBUG
      bool ok =
#endif
        nsStyleAnimation::Interpolate(pt.mProperty,
                                      pt.mStartValue, pt.mEndValue,
                                      valuePortion, *val);
      NS_ABORT_IF_FALSE(ok, "could not interpolate values");
    }
  }
}

bool
ElementPropertyTransition::IsRunningAt(TimeStamp aTime) const {
  return !IsRemovedSentinel() &&
         mStartTime <= aTime &&
         aTime < mStartTime + mDuration;
}

bool
ElementTransitions::HasAnimationOfProperty(nsCSSProperty aProperty) const
{
  for (uint32_t tranIdx = mPropertyTransitions.Length(); tranIdx-- != 0; ) {
    if (aProperty == mPropertyTransitions[tranIdx].mProperty) {
      return true;
    }
  }
  return false;
}

bool
ElementTransitions::CanPerformOnCompositorThread(CanAnimateFlags aFlags) const
{
  nsIFrame* frame = mElement->GetPrimaryFrame();
  if (!frame) {
    return false;
  }

  if (mElementProperty != nsGkAtoms::transitionsProperty) {
    if (nsLayoutUtils::IsAnimationLoggingEnabled()) {
      nsCString message;
      message.AppendLiteral("Gecko bug: Async transition of pseudoelements not supported.  See bug 771367");
      LogAsyncAnimationFailure(message, mElement);
    }
    return false;
  }

  TimeStamp now = frame->PresContext()->RefreshDriver()->MostRecentRefresh();

  for (uint32_t i = 0, i_end = mPropertyTransitions.Length(); i < i_end; ++i) {
    const ElementPropertyTransition& pt = mPropertyTransitions[i];
    if (css::IsGeometricProperty(pt.mProperty) && pt.IsRunningAt(now)) {
      aFlags = CanAnimateFlags(aFlags | CanAnimate_HasGeometricProperty);
      break;
    }
  }

  bool hasOpacity = false;
  bool hasTransform = false;
  bool existsProperty = false;
  for (uint32_t i = 0, i_end = mPropertyTransitions.Length(); i < i_end; ++i) {
    const ElementPropertyTransition& pt = mPropertyTransitions[i];
    if (pt.IsRemovedSentinel()) {
      continue;
    }
    
    existsProperty = true;

    if (!css::CommonElementAnimationData::CanAnimatePropertyOnCompositor(mElement,
                                                                         pt.mProperty,
                                                                         aFlags)) {
      return false;
    }
    if (pt.mProperty == eCSSProperty_opacity) {
      hasOpacity = true;
    } else if (pt.mProperty == eCSSProperty_transform) {
      hasTransform = true;
    }
  }
  
  // No properties to animate
  if (!existsProperty) {
    return false;
  }

  // This transition can be done on the compositor.  Mark the frame as active, in
  // case we are able to throttle this transition.
  if (hasOpacity) {
    frame->MarkLayersActive(nsChangeHint_UpdateOpacityLayer);
  }
  if (hasTransform) {
    frame->MarkLayersActive(nsChangeHint_UpdateTransformLayer);
  }
  return true;
}

/*****************************************************************************
 * nsTransitionManager                                                       *
 *****************************************************************************/

// reparent :before and :after pseudo elements of aElement
static void ReparentBeforeAndAfter(dom::Element* aElement,
                                   nsIFrame* aPrimaryFrame,
                                   nsStyleContext* aNewStyle,
                                   nsStyleSet* aStyleSet)
{
  if (nsIFrame* before = nsLayoutUtils::GetBeforeFrame(aPrimaryFrame)) {
    nsRefPtr<nsStyleContext> beforeStyle =
      aStyleSet->ReparentStyleContext(before->StyleContext(),
                                     aNewStyle, aElement);
    before->SetStyleContextWithoutNotification(beforeStyle);
  }
  if (nsIFrame* after = nsLayoutUtils::GetBeforeFrame(aPrimaryFrame)) {
    nsRefPtr<nsStyleContext> afterStyle =
      aStyleSet->ReparentStyleContext(after->StyleContext(),
                                     aNewStyle, aElement);
    after->SetStyleContextWithoutNotification(afterStyle);
  }
}

// Ensure that the next repaint rebuilds the layer tree for aFrame. That
// means that changes to animations on aFrame's layer are propagated to
// the compositor, which is needed for correct behaviour of new
// transitions.
static void
ForceLayerRerendering(nsIFrame* aFrame, CommonElementAnimationData* aData)
{
  if (aData->HasAnimationOfProperty(eCSSProperty_opacity)) {
    if (Layer* layer = FrameLayerBuilder::GetDedicatedLayer(
          aFrame, nsDisplayItem::TYPE_OPACITY)) {
      layer->RemoveUserData(nsIFrame::LayerIsPrerenderedDataKey());
    }
  } 
  
  if (aData->HasAnimationOfProperty(eCSSProperty_transform)) {
    if (Layer* layer = FrameLayerBuilder::GetDedicatedLayer(
          aFrame, nsDisplayItem::TYPE_TRANSFORM)) {
      layer->RemoveUserData(nsIFrame::LayerIsPrerenderedDataKey());
    }
  }
}

nsStyleContext*
nsTransitionManager::UpdateThrottledStyle(dom::Element* aElement,
                                          nsStyleContext* aParentStyle,
                                          nsStyleChangeList& aChangeList)
{
  NS_ASSERTION(GetElementTransitions(aElement,
                                     nsCSSPseudoElements::ePseudo_NotPseudoElement,
                                     false), "element not transitioning");

  nsIFrame* primaryFrame = aElement->GetPrimaryFrame();
  if (!primaryFrame) {
    return nullptr;
  }

  nsStyleContext* oldStyle = primaryFrame->StyleContext();
  nsRuleNode* ruleNode = oldStyle->RuleNode();
  nsTArray<nsStyleSet::RuleAndLevel> rules;
  do {
    if (ruleNode->IsRoot()) {
      break;
    }

    nsStyleSet::RuleAndLevel curRule;
    curRule.mLevel = ruleNode->GetLevel();

    if (curRule.mLevel == nsStyleSet::eAnimationSheet) {
      ElementAnimations* ea = 
        mPresContext->AnimationManager()->GetElementAnimations(aElement,
                                                               oldStyle->GetPseudoType(),
                                                               false);
      NS_ASSERTION(ea, "Rule has level eAnimationSheet without animation on manager");

      mPresContext->AnimationManager()->EnsureStyleRuleFor(ea);
      curRule.mRule = ea->mStyleRule;

      // FIXME: maybe not needed anymore:
      ForceLayerRerendering(primaryFrame, ea);
    } else if (curRule.mLevel == nsStyleSet::eTransitionSheet) {
      ElementTransitions *et =
        GetElementTransitions(aElement, oldStyle->GetPseudoType(), false);
      NS_ASSERTION(et, "Rule has level eTransitionSheet without transition on manager");
      
      et->EnsureStyleRuleFor(mPresContext->RefreshDriver()->MostRecentRefresh());
      curRule.mRule = et->mStyleRule;

      // FIXME: maybe not needed anymore:
      ForceLayerRerendering(primaryFrame, et);
    } else {
      curRule.mRule = ruleNode->GetRule();
    }

    if (curRule.mRule) {
      rules.AppendElement(curRule);
    }
  } while ((ruleNode = ruleNode->GetParent()));

  nsRefPtr<nsStyleContext> newStyle = mPresContext->PresShell()->StyleSet()->
    ResolveStyleForRules(aParentStyle, oldStyle, rules);

  // We absolutely must call CalcStyleDifference in order to ensure the
  // new context has all the structs cached that the old context had.
  // We also need it for processing of the changes.
  nsChangeHint styleChange =
    oldStyle->CalcStyleDifference(newStyle, nsChangeHint(0));
  aChangeList.AppendChange(primaryFrame, primaryFrame->GetContent(),
                           styleChange);

  primaryFrame->SetStyleContextWithoutNotification(newStyle);

  ReparentBeforeAndAfter(aElement, primaryFrame, newStyle, mPresContext->PresShell()->StyleSet());

  return newStyle;
}

void
nsTransitionManager::UpdateThrottledStylesForSubtree(nsIContent* aContent,
                                                     nsStyleContext* aParentStyle,
                                                     nsStyleChangeList& aChangeList)
{
  dom::Element* element;
  if (aContent->IsElement()) {
    element = aContent->AsElement();
  } else {
    element = nullptr;
  }

  nsRefPtr<nsStyleContext> newStyle;

  ElementTransitions* et;
  if (element &&
      (et = GetElementTransitions(element,
                                  nsCSSPseudoElements::ePseudo_NotPseudoElement,
                                  false))) {
    // re-resolve our style
    newStyle = UpdateThrottledStyle(element, aParentStyle, aChangeList);
    // remove the current transition from the working set
    et->mFlushGeneration = mPresContext->RefreshDriver()->MostRecentRefresh();
  } else {
    // reparent the element's style
    nsStyleSet* styleSet = mPresContext->PresShell()->StyleSet();
    nsIFrame* primaryFrame = aContent->GetPrimaryFrame();
    if (!primaryFrame) {
      return;
    }

    newStyle = styleSet->ReparentStyleContext(primaryFrame->StyleContext(),
                                              aParentStyle, element);
    primaryFrame->SetStyleContextWithoutNotification(newStyle);
    ReparentBeforeAndAfter(element, primaryFrame, newStyle, styleSet);
  }

  // walk the children
  if (newStyle) {
    for (nsIContent *child = aContent->GetFirstChild(); child;
         child = child->GetNextSibling()) {
      UpdateThrottledStylesForSubtree(child, newStyle, aChangeList);
    }
  }
}

void
nsTransitionManager::UpdateAllThrottledStyles()
{
  if (PR_CLIST_IS_EMPTY(&mElementData)) {
    // no throttled transitions, leave early
    mPresContext->TickLastUpdateThrottledStyle();
    return;
  }

  if (mPresContext->ThrottledStyleIsUpToDate()) {
    // throttled transitions are up to date, leave early
    return;
  }

  mPresContext->TickLastUpdateThrottledStyle();
  TimeStamp now = mPresContext->RefreshDriver()->MostRecentRefresh();

  nsStyleChangeList changeList;

  // update each transitioning element by finding its root-most ancestor with a
  // transition, and flushing the style on that ancestor and all its descendants
  PRCList *next = PR_LIST_HEAD(&mElementData);
  while (next != &mElementData) {
    ElementTransitions* et = static_cast<ElementTransitions*>(next);
    next = PR_NEXT_LINK(next);

    if (et->mFlushGeneration == now) {
      // this element has been ticked already
      continue;
    }

    // element is initialised to the starting element (i.e., one we know has
    // a transition) and ends up with the root-most transitioning ancestor,
    // that is, the element where we begin updates.
    dom::Element* element = et->mElement;
    // make a list of ancestors
    nsTArray<dom::Element*> ancestors;
    do {
      ancestors.AppendElement(element);
    } while ((element = element->GetParentElement()));

    // walk down the ancestors until we find one with a throttled transition
    for (int32_t i = ancestors.Length() - 1; i >= 0; --i) {
      if (GetElementTransitions(ancestors[i],
                                nsCSSPseudoElements::ePseudo_NotPseudoElement,
                                false)) {
        element = ancestors[i];
        break;
      }
    }

    nsIFrame* primaryFrame;
    if (element &&
        (primaryFrame = element->GetPrimaryFrame())) {
      UpdateThrottledStylesForSubtree(element,
        primaryFrame->StyleContext()->GetParent(), changeList);
    }
  }

  mPresContext->PresShell()->FrameConstructor()->
    ProcessRestyledFrames(changeList);
  mPresContext->PresShell()->FrameConstructor()->
    FlushOverflowChangedTracker();
}

already_AddRefed<nsIStyleRule>
nsTransitionManager::StyleContextChanged(dom::Element *aElement,
                                         nsStyleContext *aOldStyleContext,
                                         nsStyleContext *aNewStyleContext)
{
  NS_PRECONDITION(aOldStyleContext->GetPseudo() ==
                      aNewStyleContext->GetPseudo(),
                  "pseudo type mismatch");
  // If we were called from ReparentStyleContext, this assertion would
  // actually fire.  If we need to be called from there, we can probably
  // just remove it; the condition probably isn't critical, although
  // it's worth thinking about some more.
  NS_PRECONDITION(aOldStyleContext->HasPseudoElementData() ==
                      aNewStyleContext->HasPseudoElementData(),
                  "pseudo type mismatch");

  if (!mPresContext->IsDynamic()) {
    // For print or print preview, ignore transitions.
    return nullptr;
  }

  // NOTE: Things in this function (and ConsiderStartingTransition)
  // should never call PeekStyleData because we don't preserve gotten
  // structs across reframes.

  // Return sooner (before the startedAny check below) for the most
  // common case: no transitions specified or running.
  const nsStyleDisplay *disp = aNewStyleContext->StyleDisplay();
  nsCSSPseudoElements::Type pseudoType = aNewStyleContext->GetPseudoType();
  if (pseudoType != nsCSSPseudoElements::ePseudo_NotPseudoElement) {
    if (pseudoType != nsCSSPseudoElements::ePseudo_before &&
        pseudoType != nsCSSPseudoElements::ePseudo_after) {
      return nullptr;
    }

    NS_ASSERTION((pseudoType == nsCSSPseudoElements::ePseudo_before &&
                  aElement->Tag() == nsGkAtoms::mozgeneratedcontentbefore) ||
                 (pseudoType == nsCSSPseudoElements::ePseudo_after &&
                  aElement->Tag() == nsGkAtoms::mozgeneratedcontentafter),
                 "Unexpected aElement coming through");

    // Else the element we want to use from now on is the element the
    // :before or :after is attached to.
    aElement = aElement->GetParent()->AsElement();
  }

  ElementTransitions *et =
      GetElementTransitions(aElement, pseudoType, false);
  if (!et &&
      disp->mTransitionPropertyCount == 1 &&
      disp->mTransitions[0].GetDelay() == 0.0f &&
      disp->mTransitions[0].GetDuration() == 0.0f) {
    return nullptr;
  }


  if (aNewStyleContext->PresContext()->IsProcessingAnimationStyleChange()) {
    return nullptr;
  }

  if (aNewStyleContext->GetParent() &&
      aNewStyleContext->GetParent()->HasPseudoElementData()) {
    // Ignore transitions on things that inherit properties from
    // pseudo-elements.
    // FIXME (Bug 522599): Add tests for this.
    return nullptr;
  }

  NS_WARN_IF_FALSE(!nsLayoutUtils::AreAsyncAnimationsEnabled() ||
                     mPresContext->ThrottledStyleIsUpToDate(),
                   "throttled animations not up to date");

  // Per http://lists.w3.org/Archives/Public/www-style/2009Aug/0109.html
  // I'll consider only the transitions from the number of items in
  // 'transition-property' on down, and later ones will override earlier
  // ones (tracked using |whichStarted|).
  bool startedAny = false;
  nsCSSPropertySet whichStarted;
  for (uint32_t i = disp->mTransitionPropertyCount; i-- != 0; ) {
    const nsTransition& t = disp->mTransitions[i];
    // Check delay and duration first, since they default to zero, and
    // when they're both zero, we can ignore the transition.
    if (t.GetDelay() != 0.0f || t.GetDuration() != 0.0f) {
      // We might have something to transition.  See if any of the
      // properties in question changed and are animatable.
      // FIXME: Would be good to find a way to share code between this
      // interpretation of transition-property and the one below.
      nsCSSProperty property = t.GetProperty();
      if (property == eCSSPropertyExtra_no_properties ||
          property == eCSSProperty_UNKNOWN) {
        // Nothing to do, but need to exclude this from cases below.
      } else if (property == eCSSPropertyExtra_all_properties) {
        for (nsCSSProperty p = nsCSSProperty(0);
             p < eCSSProperty_COUNT_no_shorthands;
             p = nsCSSProperty(p + 1)) {
          ConsiderStartingTransition(p, t, aElement, et,
                                     aOldStyleContext, aNewStyleContext,
                                     &startedAny, &whichStarted);
        }
      } else if (nsCSSProps::IsShorthand(property)) {
        CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(subprop, property) {
          ConsiderStartingTransition(*subprop, t, aElement, et,
                                     aOldStyleContext, aNewStyleContext,
                                     &startedAny, &whichStarted);
        }
      } else {
        ConsiderStartingTransition(property, t, aElement, et,
                                   aOldStyleContext, aNewStyleContext,
                                   &startedAny, &whichStarted);
      }
    }
  }

  // Stop any transitions for properties that are no longer in
  // 'transition-property'.
  // Also stop any transitions for properties that just changed (and are
  // still in the set of properties to transition), but we didn't just
  // start the transition because delay and duration are both zero.
  if (et) {
    bool checkProperties =
      disp->mTransitions[0].GetProperty() != eCSSPropertyExtra_all_properties;
    nsCSSPropertySet allTransitionProperties;
    if (checkProperties) {
      for (uint32_t i = disp->mTransitionPropertyCount; i-- != 0; ) {
        const nsTransition& t = disp->mTransitions[i];
        // FIXME: Would be good to find a way to share code between this
        // interpretation of transition-property and the one above.
        nsCSSProperty property = t.GetProperty();
        if (property == eCSSPropertyExtra_no_properties ||
            property == eCSSProperty_UNKNOWN) {
          // Nothing to do, but need to exclude this from cases below.
        } else if (property == eCSSPropertyExtra_all_properties) {
          for (nsCSSProperty p = nsCSSProperty(0);
               p < eCSSProperty_COUNT_no_shorthands;
               p = nsCSSProperty(p + 1)) {
            allTransitionProperties.AddProperty(p);
          }
        } else if (nsCSSProps::IsShorthand(property)) {
          CSSPROPS_FOR_SHORTHAND_SUBPROPERTIES(subprop, property) {
            allTransitionProperties.AddProperty(*subprop);
          }
        } else {
          allTransitionProperties.AddProperty(property);
        }
      }
    }

    nsTArray<ElementPropertyTransition> &pts = et->mPropertyTransitions;
    uint32_t i = pts.Length();
    NS_ABORT_IF_FALSE(i != 0, "empty transitions list?");
    nsStyleAnimation::Value currentValue;
    do {
      --i;
      ElementPropertyTransition &pt = pts[i];
          // properties no longer in 'transition-property'
      if ((checkProperties &&
           !allTransitionProperties.HasProperty(pt.mProperty)) ||
          // properties whose computed values changed but delay and
          // duration are both zero
          !ExtractComputedValueForTransition(pt.mProperty, aNewStyleContext,
                                             currentValue) ||
          currentValue != pt.mEndValue) {
        // stop the transition
        pts.RemoveElementAt(i);
        et->UpdateAnimationGeneration(mPresContext);
      }
    } while (i != 0);

    if (pts.IsEmpty()) {
      et->Destroy();
      et = nullptr;
    }
  }

  if (!startedAny) {
    return nullptr;
  }

  NS_ABORT_IF_FALSE(et, "must have element transitions if we started "
                        "any transitions");

  // In the CSS working group discussion (2009 Jul 15 telecon,
  // http://www.w3.org/mid/4A5E1470.4030904@inkedblade.net ) of
  // http://lists.w3.org/Archives/Public/www-style/2009Jun/0121.html ,
  // the working group decided that a transition property on an
  // element should not cause any transitions if the property change
  // is itself inheriting a value that is transitioning on an
  // ancestor.  So, to get the correct behavior, we continue the
  // restyle that caused this transition using a "covering" rule that
  // covers up any changes on which we started transitions, so that
  // descendants don't start their own transitions.  (In the case of
  // negative transition delay, this covering rule produces different
  // results than applying the transition rule immediately would).
  // Our caller is responsible for restyling again using this covering
  // rule.

  nsRefPtr<css::AnimValuesStyleRule> coverRule = new css::AnimValuesStyleRule;

  nsTArray<ElementPropertyTransition> &pts = et->mPropertyTransitions;
  for (uint32_t i = 0, i_end = pts.Length(); i < i_end; ++i) {
    ElementPropertyTransition &pt = pts[i];
    if (whichStarted.HasProperty(pt.mProperty)) {
      coverRule->AddValue(pt.mProperty, pt.mStartValue);
    }
  }

  et->mStyleRule = nullptr;

  return coverRule.forget();
}

void
nsTransitionManager::ConsiderStartingTransition(nsCSSProperty aProperty,
                                                const nsTransition& aTransition,
                                                dom::Element* aElement,
                                                ElementTransitions*& aElementTransitions,
                                                nsStyleContext* aOldStyleContext,
                                                nsStyleContext* aNewStyleContext,
                                                bool* aStartedAny,
                                                nsCSSPropertySet* aWhichStarted)
{
  // IsShorthand itself will assert if aProperty is not a property.
  NS_ABORT_IF_FALSE(!nsCSSProps::IsShorthand(aProperty),
                    "property out of range");
  NS_ASSERTION(!aElementTransitions ||
               aElementTransitions->mElement == aElement, "Element mismatch");

  if (aWhichStarted->HasProperty(aProperty)) {
    // A later item in transition-property already started a
    // transition for this property, so we ignore this one.
    // See comment above and
    // http://lists.w3.org/Archives/Public/www-style/2009Aug/0109.html .
    return;
  }

  if (nsCSSProps::kAnimTypeTable[aProperty] == eStyleAnimType_None) {
    return;
  }

  ElementPropertyTransition pt;
  nsStyleAnimation::Value dummyValue;
  bool haveValues =
    ExtractComputedValueForTransition(aProperty, aOldStyleContext,
                                       pt.mStartValue) &&
    ExtractComputedValueForTransition(aProperty, aNewStyleContext,
                                       pt.mEndValue);

  bool haveChange = pt.mStartValue != pt.mEndValue;
    
  bool shouldAnimate =
    haveValues &&
    haveChange &&
    // Check that we can interpolate between these values
    // (If this is ever a performance problem, we could add a
    // CanInterpolate method, but it seems fine for now.)
    nsStyleAnimation::Interpolate(aProperty, pt.mStartValue, pt.mEndValue,
                                  0.5, dummyValue);

  bool haveCurrentTransition = false;
  uint32_t currentIndex = nsTArray<ElementPropertyTransition>::NoIndex;
  const ElementPropertyTransition *oldPT = nullptr;
  if (aElementTransitions) {
    nsTArray<ElementPropertyTransition> &pts =
      aElementTransitions->mPropertyTransitions;
    for (uint32_t i = 0, i_end = pts.Length(); i < i_end; ++i) {
      if (pts[i].mProperty == aProperty) {
        haveCurrentTransition = true;
        currentIndex = i;
        oldPT = &aElementTransitions->mPropertyTransitions[currentIndex];
        break;
      }
    }
  }

  // If we got a style change that changed the value to the endpoint
  // of the currently running transition, we don't want to interrupt
  // its timing function.
  // This needs to be before the !shouldAnimate && haveCurrentTransition
  // case below because we might be close enough to the end of the
  // transition that the current value rounds to the final value.  In
  // this case, we'll end up with shouldAnimate as false (because
  // there's no value change), but we need to return early here rather
  // than cancel the running transition because shouldAnimate is false!
  if (haveCurrentTransition && haveValues && oldPT->mEndValue == pt.mEndValue) {
    // WalkTransitionRule already called RestyleForAnimation.
    return;
  }

  nsPresContext *presContext = aNewStyleContext->PresContext();

  if (!shouldAnimate) {
    if (haveCurrentTransition) {
      // We're in the middle of a transition, and just got a non-transition
      // style change to something that we can't animate.  This might happen
      // because we got a non-transition style change changing to the current
      // in-progress value (which is particularly easy to cause when we're
      // currently in the 'transition-delay').  It also might happen because we
      // just got a style change to a value that can't be interpolated.
      nsTArray<ElementPropertyTransition> &pts =
        aElementTransitions->mPropertyTransitions;
      pts.RemoveElementAt(currentIndex);
      aElementTransitions->UpdateAnimationGeneration(mPresContext);

      if (pts.IsEmpty()) {
        aElementTransitions->Destroy();
        // |aElementTransitions| is now a dangling pointer!
        aElementTransitions = nullptr;
      }
      // WalkTransitionRule already called RestyleForAnimation.
    }
    return;
  }

  TimeStamp mostRecentRefresh =
    presContext->RefreshDriver()->MostRecentRefresh();

  const nsTimingFunction &tf = aTransition.GetTimingFunction();
  float delay = aTransition.GetDelay();
  float duration = aTransition.GetDuration();
  if (duration < 0.0) {
    // The spec says a negative duration is treated as zero.
    duration = 0.0;
  }
  pt.mStartForReversingTest = pt.mStartValue;
  pt.mReversePortion = 1.0;

  // If the new transition reverses an existing one, we'll need to
  // handle the timing differently.
  if (haveCurrentTransition &&
      !oldPT->IsRemovedSentinel() &&
      oldPT->mStartForReversingTest == pt.mEndValue) {
    // Compute the appropriate negative transition-delay such that right
    // now we'd end up at the current position.
    double valuePortion =
      oldPT->ValuePortionFor(mostRecentRefresh) * oldPT->mReversePortion +
      (1.0 - oldPT->mReversePortion);
    // A timing function with negative y1 (or y2!) might make
    // valuePortion negative.  In this case, we still want to apply our
    // reversing logic based on relative distances, not make duration
    // negative.
    if (valuePortion < 0.0) {
      valuePortion = -valuePortion;
    }
    // A timing function with y2 (or y1!) greater than one might
    // advance past its terminal value.  It's probably a good idea to
    // clamp valuePortion to be at most one to preserve the invariant
    // that a transition will complete within at most its specified
    // time.
    if (valuePortion > 1.0) {
      valuePortion = 1.0;
    }

    // Negative delays are essentially part of the transition
    // function, so reduce them along with the duration, but don't
    // reduce positive delays.
    if (delay < 0.0f) {
      delay *= valuePortion;
    }

    duration *= valuePortion;

    pt.mStartForReversingTest = oldPT->mEndValue;
    pt.mReversePortion = valuePortion;
  }

  pt.mProperty = aProperty;
  pt.mStartTime = mostRecentRefresh + TimeDuration::FromMilliseconds(delay);
  pt.mDuration = TimeDuration::FromMilliseconds(duration);
  pt.mTimingFunction.Init(tf);
  if (!aElementTransitions) {
    aElementTransitions =
      GetElementTransitions(aElement, aNewStyleContext->GetPseudoType(),
                            true);
    if (!aElementTransitions) {
      NS_WARNING("allocating ElementTransitions failed");
      return;
    }
  }

  nsTArray<ElementPropertyTransition> &pts =
    aElementTransitions->mPropertyTransitions;
#ifdef DEBUG
  for (uint32_t i = 0, i_end = pts.Length(); i < i_end; ++i) {
    NS_ABORT_IF_FALSE(i == currentIndex ||
                      pts[i].mProperty != aProperty,
                      "duplicate transitions for property");
  }
#endif
  if (haveCurrentTransition) {
    pts[currentIndex] = pt;
  } else {
    if (!pts.AppendElement(pt)) {
      NS_WARNING("out of memory");
      return;
    }
  }
  aElementTransitions->UpdateAnimationGeneration(mPresContext);

  nsRestyleHint hint =
    aNewStyleContext->GetPseudoType() ==
      nsCSSPseudoElements::ePseudo_NotPseudoElement ?
    eRestyle_Self : eRestyle_Subtree;
  presContext->PresShell()->RestyleForAnimation(aElement, hint);

  *aStartedAny = true;
  aWhichStarted->AddProperty(aProperty);
}

ElementTransitions*
nsTransitionManager::GetElementTransitions(dom::Element *aElement,
                                           nsCSSPseudoElements::Type aPseudoType,
                                           bool aCreateIfNeeded)
{
  if (!aCreateIfNeeded && PR_CLIST_IS_EMPTY(&mElementData)) {
    // Early return for the most common case.
    return nullptr;
  }

  nsIAtom *propName;
  if (aPseudoType == nsCSSPseudoElements::ePseudo_NotPseudoElement) {
    propName = nsGkAtoms::transitionsProperty;
  } else if (aPseudoType == nsCSSPseudoElements::ePseudo_before) {
    propName = nsGkAtoms::transitionsOfBeforeProperty;
  } else if (aPseudoType == nsCSSPseudoElements::ePseudo_after) {
    propName = nsGkAtoms::transitionsOfAfterProperty;
  } else {
    NS_ASSERTION(!aCreateIfNeeded,
                 "should never try to create transitions for pseudo "
                 "other than :before or :after");
    return nullptr;
  }
  ElementTransitions *et = static_cast<ElementTransitions*>(
                             aElement->GetProperty(propName));
  if (!et && aCreateIfNeeded) {
    // FIXME: Consider arena-allocating?
    et = new ElementTransitions(aElement, propName, this,
      mPresContext->RefreshDriver()->MostRecentRefresh());
    nsresult rv = aElement->SetProperty(propName, et,
                                        ElementTransitionsPropertyDtor, false);
    if (NS_FAILED(rv)) {
      NS_WARNING("SetProperty failed");
      delete et;
      return nullptr;
    }
    if (propName == nsGkAtoms::transitionsProperty) {
      aElement->SetMayHaveAnimations();
    }

    AddElementData(et);
  }

  return et;
}

/*
 * nsIStyleRuleProcessor implementation
 */

void
nsTransitionManager::WalkTransitionRule(ElementDependentRuleProcessorData* aData,
                                        nsCSSPseudoElements::Type aPseudoType)
{
  ElementTransitions *et =
    GetElementTransitions(aData->mElement, aPseudoType, false);
  if (!et) {
    return;
  }

  if (!mPresContext->IsDynamic()) {
    // For print or print preview, ignore animations.
    return;
  }

  if (aData->mPresContext->IsProcessingRestyles() &&
      !aData->mPresContext->IsProcessingAnimationStyleChange()) {
    // If we're processing a normal style change rather than one from
    // animation, don't add the transition rule.  This allows us to
    // compute the new style value rather than having the transition
    // override it, so that we can start transitioning differently.

    // We need to immediately restyle with animation
    // after doing this.
    nsRestyleHint hint =
      aPseudoType == nsCSSPseudoElements::ePseudo_NotPseudoElement ?
      eRestyle_Self : eRestyle_Subtree;
    mPresContext->PresShell()->RestyleForAnimation(aData->mElement, hint);
    return;
  }

  et->EnsureStyleRuleFor(
    aData->mPresContext->RefreshDriver()->MostRecentRefresh());

  aData->mRuleWalker->Forward(et->mStyleRule);
}

/* virtual */ void
nsTransitionManager::RulesMatching(ElementRuleProcessorData* aData)
{
  NS_ABORT_IF_FALSE(aData->mPresContext == mPresContext,
                    "pres context mismatch");
  WalkTransitionRule(aData,
                     nsCSSPseudoElements::ePseudo_NotPseudoElement);
}

/* virtual */ void
nsTransitionManager::RulesMatching(PseudoElementRuleProcessorData* aData)
{
  NS_ABORT_IF_FALSE(aData->mPresContext == mPresContext,
                    "pres context mismatch");

  // Note:  If we're the only thing keeping a pseudo-element frame alive
  // (per ProbePseudoStyleContext), we still want to keep it alive, so
  // this is ok.
  WalkTransitionRule(aData, aData->mPseudoType);
}

/* virtual */ void
nsTransitionManager::RulesMatching(AnonBoxRuleProcessorData* aData)
{
}

#ifdef MOZ_XUL
/* virtual */ void
nsTransitionManager::RulesMatching(XULTreeRuleProcessorData* aData)
{
}
#endif

/* virtual */ size_t
nsTransitionManager::SizeOfExcludingThis(nsMallocSizeOfFun aMallocSizeOf) const
{
  return CommonAnimationManager::SizeOfExcludingThis(aMallocSizeOf);
}

/* virtual */ size_t
nsTransitionManager::SizeOfIncludingThis(nsMallocSizeOfFun aMallocSizeOf) const
{
  return aMallocSizeOf(this) + SizeOfExcludingThis(aMallocSizeOf);
}

struct TransitionEventInfo {
  nsCOMPtr<nsIContent> mElement;
  nsTransitionEvent mEvent;

  TransitionEventInfo(nsIContent *aElement, nsCSSProperty aProperty,
                      TimeDuration aDuration, const nsAString& aPseudoElement)
    : mElement(aElement),
      mEvent(true, NS_TRANSITION_END,
             NS_ConvertUTF8toUTF16(nsCSSProps::GetStringValue(aProperty)),
             aDuration.ToSeconds(), aPseudoElement)
  {
  }

  // nsTransitionEvent doesn't support copy-construction, so we need
  // to ourselves in order to work with nsTArray
  TransitionEventInfo(const TransitionEventInfo &aOther)
    : mElement(aOther.mElement),
      mEvent(true, NS_TRANSITION_END,
             aOther.mEvent.propertyName, aOther.mEvent.elapsedTime,
             aOther.mEvent.pseudoElement)
  {
  }
};

/* virtual */ void
nsTransitionManager::WillRefresh(mozilla::TimeStamp aTime)
{
  NS_ABORT_IF_FALSE(mPresContext,
                    "refresh driver should not notify additional observers "
                    "after pres context has been destroyed");
  if (!mPresContext->GetPresShell()) {
    // Someone might be keeping mPresContext alive past the point
    // where it has been torn down; don't bother doing anything in
    // this case.  But do get rid of all our transitions so we stop
    // triggering refreshes.
    RemoveAllElementData();
    return;
  }

  FlushTransitions(Can_Throttle);
}

void
nsTransitionManager::FlushTransitions(FlushFlags aFlags)
{ 
  if (PR_CLIST_IS_EMPTY(&mElementData)) {
    // no transitions, leave early
    return;
  }

  nsTArray<TransitionEventInfo> events;
  TimeStamp now = mPresContext->RefreshDriver()->MostRecentRefresh();
  bool didThrottle = false;
  // Trim transitions that have completed, post restyle events for frames that
  // are still transitioning, and start transitions with delays.
  {
    PRCList *next = PR_LIST_HEAD(&mElementData);
    while (next != &mElementData) {
      ElementTransitions *et = static_cast<ElementTransitions*>(next);
      next = PR_NEXT_LINK(next);

      bool canThrottleTick = aFlags == Can_Throttle &&
        et->CanPerformOnCompositorThread(
          CommonElementAnimationData::CanAnimateFlags(0)) &&
        et->CanThrottleAnimation(now);

      NS_ABORT_IF_FALSE(et->mElement->GetCurrentDoc() ==
                          mPresContext->Document(),
                        "Element::UnbindFromTree should have "
                        "destroyed the element transitions object");

      uint32_t i = et->mPropertyTransitions.Length();
      NS_ABORT_IF_FALSE(i != 0, "empty transitions list?");
      bool transitionStartedOrEnded = false;
      do {
        --i;
        ElementPropertyTransition &pt = et->mPropertyTransitions[i];
        if (pt.IsRemovedSentinel()) {
          // Actually remove transitions one throttle-able cycle after their
          // completion. We only clear on a throttle-able cycle because that
          // means it is a regular restyle tick and thus it is safe to discard
          // the transition. If the flush is not throttle-able, we might still
          // have new transitions left to process. See comment below.
          if (aFlags == Can_Throttle) {
            et->mPropertyTransitions.RemoveElementAt(i);
          }
        } else if (pt.mStartTime + pt.mDuration <= now) {
          nsCSSProperty prop = pt.mProperty;
          if (nsCSSProps::PropHasFlags(prop, CSS_PROPERTY_REPORT_OTHER_NAME))
          {
            prop = nsCSSProps::OtherNameFor(prop);
          }
          nsIAtom* ep = et->mElementProperty;
          NS_NAMED_LITERAL_STRING(before, "::before");
          NS_NAMED_LITERAL_STRING(after, "::after");
          events.AppendElement(
            TransitionEventInfo(et->mElement, prop, pt.mDuration,
                                ep == nsGkAtoms::transitionsProperty ?
                                  EmptyString() :
                                  ep == nsGkAtoms::transitionsOfBeforeProperty ?
                                    before :
                                    after));

          // Leave this transition in the list for one more refresh
          // cycle, since we haven't yet processed its style change, and
          // if we also have (already, or will have from processing
          // transitionend events or other refresh driver notifications)
          // a non-animation style change that would affect it, we need
          // to know not to start a new transition for the transition
          // from the almost-completed value to the final value.
          pt.SetRemovedSentinel();
          et->UpdateAnimationGeneration(mPresContext);
          transitionStartedOrEnded = true;
        } else if (pt.mStartTime <= now && canThrottleTick &&
                   !pt.mIsRunningOnCompositor) {
          // Start a transition with a delay where we should start the
          // transition proper.
          et->UpdateAnimationGeneration(mPresContext);
          transitionStartedOrEnded = true;
        }
      } while (i != 0);

      // We need to restyle even if the transition rule no longer
      // applies (in which case we just made it not apply).
      NS_ASSERTION(et->mElementProperty == nsGkAtoms::transitionsProperty ||
                   et->mElementProperty == nsGkAtoms::transitionsOfBeforeProperty ||
                   et->mElementProperty == nsGkAtoms::transitionsOfAfterProperty,
                   "Unexpected element property; might restyle too much");
      if (!canThrottleTick || transitionStartedOrEnded) {
        nsRestyleHint hint = et->mElementProperty == nsGkAtoms::transitionsProperty ?
          eRestyle_Self : eRestyle_Subtree;
        mPresContext->PresShell()->RestyleForAnimation(et->mElement, hint);
      } else {
        didThrottle = true;
      }

      if (et->mPropertyTransitions.IsEmpty()) {
        et->Destroy();
        // |et| is now a dangling pointer!
        et = nullptr;
      }
    }
  }

  // We might have removed transitions above.
  ElementDataRemoved();

  if (didThrottle) {
    mPresContext->Document()->SetNeedStyleFlush();
  }

  for (uint32_t i = 0, i_end = events.Length(); i < i_end; ++i) {
    TransitionEventInfo &info = events[i];
    nsEventDispatcher::Dispatch(info.mElement, mPresContext, &info.mEvent);

    if (!mPresContext) {
      break;
    }
  }
}