Basic ScrollTimeline implementation for Animation Worklet

third_party/WebKit/Source/core/animation/ScrollTimeline.cpp

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/animation/ScrollTimeline.h"
+
+#include "core/dom/ExceptionCode.h"
+#include "core/layout/LayoutBox.h"
+#include "core/paint/PaintLayerScrollableArea.h"
+
+namespace blink {
+
+namespace {
+bool StringToScrollDirection(String scroll_direction,
+                             ScrollTimeline::ScrollDirection* result) {
+  if (scroll_direction == "auto") {
+    *result = ScrollTimeline::Auto;
+    return true;
+  }
+  if (scroll_direction == "block") {
+    *result = ScrollTimeline::Block;
+    return true;
+  }
+  if (scroll_direction == "inline") {
+    *result = ScrollTimeline::Inline;
+    return true;
+  }
+  return false;
+}
+}  // namespace
+
+ScrollTimeline* ScrollTimeline::Create(Document& document,
+                                       ScrollTimelineOptions options,
+                                       ExceptionState& exception_state) {
+  Element* scroll_source = options.scrollSource() ? options.scrollSource()
+                                                  : document.scrollingElement();
+
+  ScrollDirection orientation;
+  if (!StringToScrollDirection(options.orientation(), &orientation)) {
+    exception_state.ThrowDOMException(kNotSupportedError,
+                                      "Invalid orientation");
+    return nullptr;
+  }
+
+  // TODO(smcgruer): Support 'auto' value.
+  if (options.timeRange().isScrollTimelineAutoKeyword()) {
+    exception_state.ThrowDOMException(
+        kNotSupportedError, "'auto' value for timeRange not yet supported");
+    return nullptr;
+  }
+
+  return new ScrollTimeline(document, scroll_source, orientation,
+                            options.timeRange().getAsDouble());
+}
+
+ScrollTimeline::ScrollTimeline(const Document& document,
+                               Element* scroll_source,
+                               ScrollDirection orientation,
+                               double time_range)
+    : AnimationTimeline(const_cast<Document*>(&document), nullptr),
+      scroll_source_(scroll_source),
+      orientation_(orientation),
+      time_range_(time_range) {}
+
+double ScrollTimeline::currentTime(bool& is_null) {
+  // 1. If scrollSource does not currently have a CSS layout box, or if its
+  // layout box is not a scroll container, return an unresolved time value.
+  LayoutBox* layout_box = scroll_source_->GetLayoutBox();
+  if (!layout_box || !layout_box->HasOverflowClip()) {
+    is_null = true;
+    return std::numeric_limits<double>::quiet_NaN();
+  }
+
+  // 2. Otherwise, let current scroll offset be the current scroll offset of
+  // scrollSource in the direction specified by orientation.
+
+  // 'auto' ScrollDirection is specified as: If only one direction is
+  // scrollable, selects that direction. Otherwise selects the direction along
+  // the block axis.
+  ScrollDirection local_orientation = orientation_;
+  bool is_horizontal = layout_box->IsHorizontalWritingMode();
+  if (local_orientation == Auto) {
+    if (layout_box->HasScrollableOverflowX() &&
+        !layout_box->HasScrollableOverflowY()) {
+      local_orientation = is_horizontal ? Inline : Block;
+    } else if (!layout_box->HasScrollableOverflowX() &&
+               layout_box->HasScrollableOverflowY()) {
+      local_orientation = is_horizontal ? Block : Inline;
+    } else {
+      // Either neither or both axes are scrollable, so select the block
+      // direction.
+      local_orientation = Block;
+    }
+  }
+
+  // Depending on the writing-mode and direction, the scroll origin shifts and
+  // the scroll offset may be negative. The easiest way to deal with this is to
+  // use only the magnitude of the scroll offset, and compare it to (max-offset
+  // - min_offset).

[flackr, 2017/05/18 15:11:50]

There is always the risk that the max or min may not be based around 0 and so
the absolute magnitude loses information, I don't think this ever happens
though. Probably worth a comment.

[smcgruer, 2017/05/18 15:42:57]

Added a DCHECK, since this is a pre-requisite of the code operating correctly.

+  PaintLayerScrollableArea* scrollable_area = layout_box->GetScrollableArea();
+  ScrollOffset scroll_offset = scrollable_area->GetScrollOffset();
+  ScrollOffset scroll_dimensions = scrollable_area->MaximumScrollOffset() -
+                                   scrollable_area->MinimumScrollOffset();
+
+  double current_offset;
+  double max_offset;
+  DCHECK(local_orientation == Block || local_orientation == Inline);
+  if (local_orientation == Block) {
+    current_offset =
+        is_horizontal ? scroll_offset.Height() : scroll_offset.Width();
+    max_offset =
+        is_horizontal ? scroll_dimensions.Height() : scroll_dimensions.Width();
+  } else {
+    current_offset =
+        is_horizontal ? scroll_offset.Width() : scroll_offset.Height();
+    max_offset =
+        is_horizontal ? scroll_dimensions.Width() : scroll_dimensions.Height();
+  }
+
+  // 3. If current scroll offset is less than startScrollOffset, return an
+  // unresolved time value if fill is none or forwards, or 0 otherwise.
+  // TODO(smcgruer): Implement |startScrollOffset| and |fill|.
+
+  // 4. If current scroll offset is greater than or equal to endScrollOffset,
+  // return an unresolved time value if fill is none or backwards, or the
+  // effective time range otherwise.
+  // TODO(smcgruer): Implement |endScrollOffset| and |fill|.
+
+  // 5. Return the result of evaluating the following expression:
+  //   ((current scroll offset - startScrollOffset) /
+  //      (endScrollOffset - startScrollOffset)) * effective time range
+
+  is_null = false;
+  return (std::abs(current_offset) / max_offset) * time_range_;
+}
+
+double ScrollTimeline::currentTime() {
+  bool ignored;
+  return currentTime(ignored);
+}
+
+double ScrollTimeline::CurrentTimeInternal(bool& is_null) {
+  return currentTime(is_null);
+}
+
+double ScrollTimeline::CurrentTimeInternal() {
+  bool ignored;
+  return currentTime(ignored);
+}
+
+Element* ScrollTimeline::scrollSource() {
+  return scroll_source_.Get();
+}
+
+String ScrollTimeline::orientation() {
+  switch (orientation_) {
+    case Auto:
+      return "auto";
+    case Block:
+      return "block";
+    case Inline:
+      return "inline";
+  }
+}
+
+void ScrollTimeline::timeRange(DoubleOrScrollTimelineAutoKeyword& result) {
+  result.setDouble(time_range_);
+}
+
+DEFINE_TRACE(ScrollTimeline) {
+  visitor->Trace(scroll_source_);
+  AnimationTimeline::Trace(visitor);
+}
+
+}  // namespace blink