Refactor cc scrollbar layers to separate solid-color vs desktop.

cc/layers/scrollbar_layer_impl_base.cc

+// Copyright 2012 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 "cc/layers/scrollbar_layer_impl_base.h"
+#include "ui/gfx/rect_conversions.h"
+
+namespace cc {
+
+void ScrollbarLayerImplBase::PushPropertiesTo(LayerImpl* layer) {
+  LayerImpl::PushPropertiesTo(layer);
+}
+
+ScrollbarLayerImplBase* ScrollbarLayerImplBase::ToScrollbarLayerBase() {
+  return this;
+}
+
+gfx::Rect ScrollbarLayerImplBase::ScrollbarLayerRectToContentRect(
+    gfx::RectF layer_rect) const {
+  // Don't intersect with the bounds as in layerRectToContentRect() because
+  // layer_rect here might be in coordinates of the containing layer.
+  gfx::RectF content_rect = gfx::ScaleRect(layer_rect,
+      contents_scale_x(),
+      contents_scale_y());
+  return gfx::ToEnclosingRect(content_rect);
+}
+
+gfx::Rect ScrollbarLayerImplBase::ComputeThumbQuadRectInternal(
+    int thumb_length,
+    float track_length,
+    int track_start,
+    int thumb_thickness) const {
+  // Thumb extent is the length of the thumb in the scrolling direction, thumb
+  // thickness is in the perpendicular direction. Here's an example of a
+  // horizontal scrollbar - inputs are above the scrollbar, computed values
+  // below:
+  //
+  //    |<------------------- track_length_ ------------------->|
+  //
+  // |--| <-- start_offset
+  //
+  // +--+----------------------------+------------------+-------+--+
+  // |<||                            |##################|       ||>|
+  // +--+----------------------------+------------------+-------+--+
+  //
+  //                                 |<- thumb_length ->|
+  //
+  // |<------- thumb_offset -------->|
+  //
+  // For painted, scrollbars, the length is fixed. For solid color scrollbars we
+  // have to compute it. The ratio of the thumb's length to the track's length
+  // is the same as that of the visible viewport to the total viewport, unless
+  // that would make the thumb's length less than its thickness.
+  //
+  // vertical_adjust_ is used when the layer geometry from the main thread is
+  // not in sync with what the user sees. For instance on Android scrolling the
+  // top bar controls out of view reveals more of the page content. We want the
+  // root layer scrollbars to reflect what the user sees even if we haven't
+  // received new layer geometry from the main thread.  If the user has scrolled
+  // down by 50px and the initial viewport size was 950px the geometry would
+  // look something like this:
+  //
+  // vertical_adjust_ = 50, scroll position 0, visible ratios 99%
+  // Layer geometry:             Desired thumb positions:
+  // +--------------------+-+   +----------------------+   <-- 0px
+  // |                    |v|   |                     #|
+  // |                    |e|   |                     #|
+  // |                    |r|   |                     #|
+  // |                    |t|   |                     #|
+  // |                    |i|   |                     #|
+  // |                    |c|   |                     #|
+  // |                    |a|   |                     #|
+  // |                    |l|   |                     #|
+  // |                    | |   |                     #|
+  // |                    |l|   |                     #|
+  // |                    |a|   |                     #|
+  // |                    |y|   |                     #|
+  // |                    |e|   |                     #|
+  // |                    |r|   |                     #|
+  // +--------------------+-+   |                     #|
+  // | horizontal  layer  | |   |                     #|
+  // +--------------------+-+   |                     #|  <-- 950px
+  // |                      |   |                     #|
+  // |                      |   |##################### |
+  // +----------------------+   +----------------------+  <-- 1000px
+  //
+  // The layer geometry is set up for a 950px tall viewport, but the user can
+  // actually see down to 1000px. Thus we have to move the quad for the
+  // horizontal scrollbar down by the vertical_adjust_ factor and lay the
+  // vertical thumb out on a track lengthed by the vertical_adjust_ factor. This
+  // means the quads may extend outside the layer's bounds.
+
+  // With the length known, we can compute the thumb's position.
+  float ratio = current_pos_ / maximum_;
+  float max_offset = track_length - thumb_length;
+  int thumb_offset = static_cast<int>(ratio * max_offset) + track_start;
+
+  gfx::RectF thumb_rect;
+  if (orientation_ == HORIZONTAL) {
+    thumb_rect = gfx::RectF(thumb_offset, vertical_adjust_,
+                            thumb_length, thumb_thickness);
+  } else {
+    thumb_rect = gfx::RectF(0.f, thumb_offset,
+                            thumb_thickness, thumb_length);
+  }
+
+  return ScrollbarLayerRectToContentRect(thumb_rect);
+}
+
+}  // namespace cc