Added common layout framework for system menu rows.

ash/common/system/tray/tray_popup_layout_factory.h

Index: ash/common/system/tray/tray_popup_layout_factory.h
diff --git a/ash/common/system/tray/tray_popup_layout_factory.h b/ash/common/system/tray/tray_popup_layout_factory.h
new file mode 100644
index 0000000000000000000000000000000000000000..5e5669eb93d48fe182ab829f34503c081505947d
--- /dev/null
+++ b/ash/common/system/tray/tray_popup_layout_factory.h
@@ -0,0 +1,55 @@
+// Copyright 2016 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 ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LAYOUT_FACTORY_H_
+#define ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LAYOUT_FACTORY_H_
+
+#include <memory>
+
+#include "ash/common/system/tray/three_view_layout.h"
+#include "base/macros.h"
+
+namespace views {
+class View;
+}  // namespace views
+
+namespace ash {
+
+// Factory functions for system menu row layout managers.
+class TrayPopupLayoutFactory {
+ public:
+  // Sets the layout manager of |host| to the default layout manager used by
+  // most system menu rows. The newly created layout manager is owned by the
+  // |host| but is returned so child Views can be added.
+  //
+  // The returned layout manager consists of 3 regions: START, CENTER, and END.
+  // Any child Views added to the START and END containers will be added
+  // horizonatlly and any Views added to the CENTER container will be added
+  // vertically.
+  //
+  // The START and END containers have a fixed width but can grow into the
+  // CENTER container if space is required and available.
+  //
+  // The CENTER container has a flexible width.
+  static ThreeViewLayout* InstallDefaultLayout(views::View* host);
+
+  // Configures the different |container| Views with the default layout
+  // managers.
+  //
+  // This can be used by clients who need to embed portions of the default
+  // layout manager into a different container view, e.g. just the END layout
+  // into a clickable View.
+  static void ConfigureDefaultLayout(ThreeViewLayout* layout,

[tdanderson, 2016/10/19 18:31:00]

Maybe it's the function name or the description, but I'm having a hard time
understanding when and how this would be used by something other than as a
helper function for InstallDefaultLayout(). Can you please clarify?

[bruthig, 2016/10/20 04:40:13]

I envision this being used by menu rows that have more than one targetable
space.  It will allow these rows to set the layout for each targetable container
(e.g. the audio row)

Example:

VolumeView::VolumeView() {
...
  mute_button_ = new CustomButton();
  ThreeViewLayout* start_layout = new ThreeViewLayout();
  TrayPopupLayoutFactory::ConfigureDefaultLayout(start_layout, START);
  mute_button_->SetLayoutManager(end_layout);
  start_layout.AddView(START, head_phone_image);
  
  center_view_ = new views::View();
  ThreeViewLayout* center_layout = new ThreeViewLayout();
  TrayPopupLayoutFactory::ConfigureDefaultLayout(center_layout, CENTER);
  center_view_->SetLayoutManager(center_layout);
  center_layout->AddView(CENTER, slider);

  more_region_ = views::View();
  ThreeViewLayout* end_layout = new ThreeViewLayout();
  TrayPopupLayoutFactory::ConfigureDefaultLayout(end_layout, END);
  more_region_.SetLayoutManager(end_layout);
  end_layout->AddView(END, detail_view_button_);
...
}

Do you think adding an example like this to the docs would help?

[tdanderson, 2016/10/20 18:16:49]

Following up on our discussion in person, let's defer the decision of whether to
make this method public or private until you start implementing the new audio
row.

[bruthig, 2016/10/20 19:01:34]

Acknowledged.

+                                     ThreeViewLayout::Container container);
+
+ private:
+  TrayPopupLayoutFactory();
+  ~TrayPopupLayoutFactory();
+
+  DISALLOW_COPY_AND_ASSIGN(TrayPopupLayoutFactory);
+};
+
+}  // namespace ash
+
+#endif  // ASH_COMMON_SYSTEM_TRAY_TRAY_POPUP_LAYOUT_FACTORY_H_