Implement workspace scrubbing

ash/wm/workspace/workspace_cycler.cc

Index: ash/wm/workspace/workspace_cycler.cc
diff --git a/ash/wm/workspace/workspace_cycler.cc b/ash/wm/workspace/workspace_cycler.cc
new file mode 100644
index 0000000000000000000000000000000000000000..9bdc388ccf1775e944597aa5163cd60a5e8db6c0
--- /dev/null
+++ b/ash/wm/workspace/workspace_cycler.cc
@@ -0,0 +1,91 @@
+// Copyright (c) 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 "ash/wm/workspace/workspace_cycler.h"
+
+#include "ash/shell.h"
+#include "ash/wm/workspace/workspace_manager.h"
+#include "ui/base/events/event.h"
+#include "ui/base/events/event_utils.h"
+
+namespace ash {
+namespace internal {
+
+namespace {
+
+// The required vertical distance to scrub to the next workspace.
+const int kWorkspaceStepSize = 10;
+
+// Returns true is scrubbing is enabled.
+bool IsScrubbingEnabled() {
+  // Scrubbing is disabled if the screen is locked or a modal dialog is open.
+  return !Shell::GetInstance()->IsScreenLocked() &&
+         !Shell::GetInstance()->IsSystemModalWindowOpen();
+}
+
+}  // namespace
+
+WorkspaceCycler::WorkspaceCycler(WorkspaceManager* workspace_manager)
+    : workspace_manager_(workspace_manager),
+      scrubbing_(false),
+      scroll_x_(0),
+      scroll_y_(0) {
+  ash::Shell::GetInstance()->AddPreTargetHandler(this);
+}
+
+WorkspaceCycler::~WorkspaceCycler() {
+  scrubbing_ = false;
+  ash::Shell::GetInstance()->RemovePreTargetHandler(this);
+}
+
+ui::EventResult WorkspaceCycler::OnScrollEvent(ui::ScrollEvent* event) {
+  if (event->finger_count() != 3 ||
+      event->type() != ui::ET_SCROLL) {
+    scrubbing_ = false;
+    return ui::ER_UNHANDLED;
+  }
+
+  if (!IsScrubbingEnabled()) {
+    scrubbing_ = false;
+    return ui::ER_UNHANDLED;
+  }
+
+  if (!scrubbing_) {
+    scrubbing_ = true;
+    scroll_x_ = 0;
+    scroll_y_ = 0;
+  }
+
+  if (ui::IsNaturalScrollEnabled()) {
+    scroll_x_ += event->x_offset();
+    scroll_y_ += event->y_offset();
+  } else {
+    scroll_x_ -= event->x_offset();
+    scroll_y_ -= event->y_offset();
+  }
+
+  if (std::abs(scroll_y_) > kWorkspaceStepSize) {
+    workspace_manager_->CycleToWorkspace(scroll_y_ > 0 ?
+        WorkspaceManager::CYCLE_NEXT : WorkspaceManager::CYCLE_PREVIOUS);
+
+    scroll_x_ = 0;
+    scroll_y_ = 0;

[sky, 2012/11/29 23:09:51]

Lets say I've scrolled by 100. If you set scroll_y_ to 0 here won't it get reset
to 100+delta on the next scroll event around line 62/64 and you'll try to update
again since the increment is > kWorkspaceStepSize.

I suggest trying for the following:
. Once you cycle the user should have to cycle a certain amount before cycling
again.
. The cycle stops should be relative to the start of the scroll.

I'm thinking something like this. Do away with 60-66 and convert to something
like:

// Only consider moving if the user moves at least kAmountNeededToScroll since
last scroll.
if (std::abs(last_scroll_offset - event->y_offset()) > kAmountNeededToScroll) {
  const int cycle_delta = event->y_offset() / kWorkspaceStepSize -
last_scroll_offset_ / kWorskpaceSteSize;
  if (cycle_delta > 0) {
    // cycle through cycle_delta workspaces.
    last_scroll_offset_ = event->y_offset();
  }
}

I'm sure I'm not handling the directionality (line 60), but you get the idea.

[pkotwicz, 2012/11/30 01:19:45]

As far as I understand it, event->x_offset() is a delta as opposed to being
absolute. So a subsequent event will not include the offset of the previous
event. I have tested this and the test results seem consistent.

+    return ui::ER_HANDLED;
+  }
+
+  if (std::abs(scroll_y_) > kWorkspaceStepSize) {

[sky, 2012/11/29 23:09:51]

You should never get here, right?

[pkotwicz, 2012/11/30 01:19:45]

This should be scroll_x_, fixed!

+    // Update |scroll_x_| and |scroll_y_| such that workspaces are only cycled
+    // through when there recently was a significant amount of vertical movement
+    // as opposed to vertical movement accumulated over a long horizontal three
+    // finger drag.
+    scroll_x_ = 0;
+    scroll_y_ = 0;
+  }
+
+  // The active workspace was not changed, do not consume the event.
+  return ui::ER_UNHANDLED;
+}
+
+}  // namespace internal
+}  // namespace ash