Arrange compositor scrolling into scroll customization format

cc/input/scroll_state.h

Index: cc/input/scroll_state.h
diff --git a/cc/input/scroll_state.h b/cc/input/scroll_state.h
new file mode 100644
index 0000000000000000000000000000000000000000..16202b13fd5774bd3c6a520e680e3c76f59a45af
--- /dev/null
+++ b/cc/input/scroll_state.h
@@ -0,0 +1,93 @@
+// Copyright 2015 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.
+
+#ifndef CC_INPUT_SCROLL_STATE_H_
+#define CC_INPUT_SCROLL_STATE_H_
+
+#include <list>
+
+#include "cc/base/cc_export.h"
+
+namespace cc {
+
+class LayerImpl;
+
+// ScrollState is based on the proposal for scroll customization in blink, found
+// here: https://goo.gl/1ipTpP.
+class CC_EXPORT ScrollState {
+ public:
+  ScrollState(double delta_x,
+              double delta_y,
+              int start_position_x,
+              int start_position_y,
+              bool in_inertial_phase,
+              bool should_propagate,
+              bool delta_consumed_for_scroll_sequence,
+              bool is_direct_manipulation);
+  ~ScrollState();
+
+  // Reduce deltas by x, y.
+  void ConsumeDelta(double x, double y);
+  // Pops the first layer off of |scroll_chain_| and calls
+  // |DistributeScroll| on it.
+  void DistributeToScrollChainDescendant();
+  // Positive when scrolling left.
+  double delta_x() const { return delta_x_; }
+  // Positive when scrolling up.
+  double delta_y() const { return delta_y_; }
+  // The location the scroll started at. For touch, the starting
+  // position of the finger. For mouse, the location of the cursor.
+  int start_position_x() const { return start_position_x_; }
+  int start_position_y() const { return start_position_y_; }
+
+  // True for events dispatched after the users's gesture has finished.
+  bool in_inertial_phase() const { return in_inertial_phase_; }
+  // True if this scroll is allowed to bubble upwards.
+  bool should_propagate() const { return should_propagate_; }
+  // True if the user interacts directly with the screen, e.g., via touch.
+  bool is_direct_manipulation() const { return is_direct_manipulation_; }
+
+  void set_scroll_chain(const std::list<LayerImpl*>& scroll_chain) {
+    scroll_chain_ = scroll_chain;
+  }
+
+  void set_current_native_scrolling_layer(LayerImpl* layer) {
+    current_native_scrolling_layer_ = layer;
+  }
+
+  LayerImpl* current_native_scrolling_layer() const {
+    return current_native_scrolling_layer_;
+  }
+
+  bool delta_consumed_for_scroll_sequence() const {
+    return delta_consumed_for_scroll_sequence_;
+  }
+
+  bool FullyConsumed() const { return !delta_x_ && !delta_y_; }
+
+ private:
+  ScrollState();
+  double delta_x_;
+  double delta_y_;
+  double start_position_x_;
+  double start_position_y_;
+
+  bool in_inertial_phase_;
+  bool should_propagate_;
+
+  // The last layer to respond to a scroll, or null if none exists.
+  LayerImpl* current_native_scrolling_layer_;
+  // Whether the scroll sequence has had any delta consumed, in the
+  // current frame, or any child frames.
+  bool delta_consumed_for_scroll_sequence_;
+  // True if the user interacts directly with the display, e.g., via
+  // touch.
+  bool is_direct_manipulation_;
+
+  std::list<LayerImpl*> scroll_chain_;

[Ian Vollick, 2015/07/09 14:03:55]

I'm a little nervous about the LayerImpl pointers because we're  eventually like
to unify scrolling and this is certainly compositor-thread specific, and because
we're moving away from layer trees.

How would you feel about introducing a ScrollNode interface (implemented by
LayerImpl) and hanging onto pointers to those?

[tdresser, 2015/07/09 16:07:45]

We'll absolutely want to introduce a layer of abstraction here at some point. In
my experience abstracting things before it's necessary causes more trouble than
it's worth, but I'm fine introducing that abstraction now if you think it's
superior to introducing it later.

[Ian Vollick, 2015/07/09 16:46:51]

Please put a TODO here, then, and link to the scroll unification bug.

[tdresser, 2015/07/16 14:04:49]

Done.

+};
+
+}  // namespace cc
+
+#endif  // CC_INPUT_SCROLL_STATE_H_