Moved InitialCurrentTime() to AnimationTimeline classes

This is part of an effort to remove any knowledge of
what kind of timeline has been supplied to Animation
and WorkletAnimation.

This change eliminates a call to IsScrollTimeline() in
WorkletAnimation.

Bug: 1012073
Change-Id: Ia8d41cb22bea377230bb3c10e1d5a4d9fc19d30c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1846247
Commit-Queue: Jordan Taylor <jortaylo@microsoft.com>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708091}

third_party/blink/renderer/core/animation/animation_timeline.h

@@ -37,6 +37,15 @@ class CORE_EXPORT AnimationTimeline : public ScriptWrappable {
   virtual bool IsDocumentTimeline() const { return false; }
   virtual bool IsScrollTimeline() const { return false; }
   virtual bool IsActive() const = 0;
+  // Returns the initial start time for animations that are linked to this
+  // timeline. This method gets invoked when initializing the start time of an
+  // animation on this timeline for the first time. It exists because the
+  // initial start time for scroll-linked and time-linked animations are
+  // different.
+  //
+  // Changing scroll-linked animation start_time initialization is under
+  // consideration here: https://github.com/w3c/csswg-drafts/issues/2075.
+  virtual base::Optional<base::TimeDelta> InitialStartTimeForAnimations() = 0;
   virtual Document* GetDocument() = 0;
   virtual void AnimationAttached(Animation*) = 0;
   virtual void AnimationDetached(Animation*) = 0;

third_party/blink/renderer/core/animation/document_timeline.cc

@@ -114,6 +114,17 @@ bool DocumentTimeline::IsActive() const {
   return document_->GetPage();
 }
 
+// Document-linked animations are initialized with start time of the document
+// timeline current time.
+base::Optional<base::TimeDelta>
+DocumentTimeline::InitialStartTimeForAnimations() {
+  base::Optional<double> current_time_ms = CurrentTime();
+  if (current_time_ms.has_value()) {
+    return base::TimeDelta::FromMillisecondsD(current_time_ms.value());
+  }
+  return base::nullopt;
+}
+
 void DocumentTimeline::AnimationAttached(Animation* animation) {
   DCHECK_EQ(&animation->GetDocument()->Timeline(), this);
   DCHECK(!animations_.Contains(animation));

third_party/blink/renderer/core/animation/document_timeline.h

@@ -94,6 +94,7 @@ class CORE_EXPORT DocumentTimeline : public AnimationTimeline {
   void AnimationDetached(Animation*) override {}
 
   bool IsActive() const override;
+  base::Optional<base::TimeDelta> InitialStartTimeForAnimations() override;
   bool HasPendingUpdates() const {
     return !animations_needing_update_.IsEmpty();
   }

third_party/blink/renderer/core/animation/scroll_timeline.cc

@@ -136,6 +136,12 @@ bool ScrollTimeline::IsActive() const {
   return layout_box && layout_box->HasOverflowClip();
 }
 
+// Scroll-linked animations are initialized with the start time of zero.
+base::Optional<base::TimeDelta>
+ScrollTimeline::InitialStartTimeForAnimations() {
+  return base::TimeDelta();
+}
+
 double ScrollTimeline::currentTime(bool& is_null) {
   is_null = true;
 

third_party/blink/renderer/core/animation/scroll_timeline.h

@@ -56,6 +56,7 @@ class CORE_EXPORT ScrollTimeline final : public AnimationTimeline {
   // have a CSS layout box, or if its layout box is not a scroll container.
   // https://github.com/WICG/scroll-animations/issues/31
   bool IsActive() const override;
+  base::Optional<base::TimeDelta> InitialStartTimeForAnimations() override;
 
   // IDL API implementation.
   Element* scrollSource();

third_party/blink/renderer/modules/animationworklet/worklet_animation.cc

@@ -161,12 +161,9 @@ base::Optional<base::TimeDelta> CalculateStartTime(
     base::TimeDelta current_time,
     double playback_rate,
     AnimationTimeline& timeline) {
-  bool is_null;
-  double time_ms = timeline.currentTime(is_null);
-  DCHECK(!is_null);
-
-  auto timeline_time = base::TimeDelta::FromMillisecondsD(time_ms);
-  return timeline_time - (current_time / playback_rate);
+  base::Optional<double> timeline_current_time_ms = timeline.CurrentTime();
+  return base::TimeDelta::FromMillisecondsD(timeline_current_time_ms.value()) -
+         (current_time / playback_rate);
 }
 }  // namespace
 
@@ -711,8 +708,7 @@ bool WorkletAnimation::IsCurrentTimeInitialized() const {
 // zero (i.e., scroll origin) and the current time corresponding to the current
 // scroll position adjusted by the playback rate.
 //
-// Changing scroll-linked animation start_time initialization is under
-// consideration here: https://github.com/w3c/csswg-drafts/issues/2075.
+// More information at AnimationTimeline::InitialStartTimeForAnimations
 //
 // TODO(https://crbug.com/986925): The playback rate should be taken into
 // consideration when calculating the initial current time.
@@ -722,16 +718,17 @@ base::Optional<base::TimeDelta> WorkletAnimation::InitialCurrentTime() const {
       !IsTimelineActive())
     return base::nullopt;
 
-  if (timeline_->IsScrollTimeline()) {
-    bool is_null;
-    double timeline_time_ms = timeline_->currentTime(is_null);
-    if (is_null)
-      return base::nullopt;
+  base::Optional<base::TimeDelta> starting_time =
+      timeline_->InitialStartTimeForAnimations();
+  base::Optional<double> current_time = timeline_->CurrentTime();
 
-    return base::TimeDelta::FromMillisecondsD(timeline_time_ms) *
-           playback_rate_;
+  if (!starting_time || !current_time) {
+    return base::nullopt;
   }
-  return base::TimeDelta();
+
+  return (base::TimeDelta::FromMillisecondsD(current_time.value()) -
+          starting_time.value()) *
+         playback_rate_;
 }
 
 void WorkletAnimation::UpdateCurrentTimeIfNeeded() {