BoxLayout now suports per-view margins.

ui/views/layout/box_layout.h

 // 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.
 
 #ifndef UI_VIEWS_LAYOUT_BOX_LAYOUT_H_
 #define UI_VIEWS_LAYOUT_BOX_LAYOUT_H_
 
 #include <map>
 
 #include "base/compiler_specific.h"
@@ skipping 79 matching lines @@
   // negative.
   void SetFlexForView(const View* view, int flex);
 
   // Clears the flex for the given |view|, causing it to use the default
   // flex.
   void ClearFlexForView(const View* view);
 
   // Sets the flex for views to use when none is specified.
   void SetDefaultFlex(int default_flex);
 
+  // Clears the margins for the given |view|.
+  void ClearMarginsForView(const View* view);
+
+  // Sets the margins for a specific |view|. The margins are the area around
+  // the outside of the given view into which no other view is placed. This is
+  // in addition to any other horizontal or vertical spacing.
+  void SetViewMargins(const View* view, const gfx::Insets& margins);

[sky, 2017/04/25 17:28:58]

Is there a reason you aren't using a UI property for the insets? I think a UI
property is better given we want to support margins on multiple layout managers.
This means you can get rid of both SetViewMargins and ClearMarginsForView.

Please also document the expectations of views placed right next to each other.
e.g. is the margin the total of the two sides? I could also see wanting to have
no margins along the edges, or possibly options as to which edges should have
margins. I started reviewing this, but given you are doing this for Peter I
would rather make sure you are addressing his needs. Peter, what specific
behavior do you need?

+
   // Overridden from views::LayoutManager:
   void Installed(View* host) override;
   void ViewRemoved(View* host, View* view) override;
   void Layout(View* host) override;
   gfx::Size GetPreferredSize(const View* host) const override;
   int GetPreferredHeightForWidth(const View* host, int width) const override;
 
  private:
+  struct ViewWrapper {

[sky, 2017/04/25 17:28:59]

Please add a description of this.

+    ViewWrapper();
+    ViewWrapper(const BoxLayout* layout, const View* view);
+    ViewWrapper(const ViewWrapper& wrapper);
+    ~ViewWrapper();
+    gfx::Size GetPreferredSize() const;
+    void SetBoundsRect(const gfx::Rect& bounds);
+    const View* view() const { return view_; }
+    bool visible() const;
+
+   private:
+    const View* view_;
+    const BoxLayout* layout_;
+    gfx::Insets margins_;
+  };
+  using MarginMap = std::map<const View*, gfx::Insets>;
+  using FlexMap = std::map<const View*, int>;
+  friend struct ViewWrappper;
+
   // Returns the flex for the specified |view|.
   int GetFlexForView(const View* view) const;
 
   // Returns the size and position along the main axis of |rect|.
   int MainAxisSize(const gfx::Rect& rect) const;
   int MainAxisPosition(const gfx::Rect& rect) const;
 
   // Sets the size and position along the main axis of |rect|.
   void SetMainAxisSize(int size, gfx::Rect* rect) const;
   void SetMainAxisPosition(int position, gfx::Rect* rect) const;
@@ skipping 14 matching lines @@
   int CrossAxisSizeForView(const View* view) const;
 
   // The preferred size for the dialog given the width of the child area.
   gfx::Size GetPreferredSizeForChildWidth(const View* host,
                                           int child_area_width) const;
 
   // The amount of space the layout requires in addition to any space for the
   // child views.
   gfx::Size NonChildSize(const View* host) const;
 
+  // Returns the margins for the specified |view|.
+  gfx::Insets GetViewMargins(const View* view) const;
+
   const Orientation orientation_;
 
   // Spacing between child views and host view border.
   gfx::Insets inside_border_insets_;
 
   // Spacing to put in between child views.
   const int between_child_spacing_;
 
   // The alignment of children in the main axis. This is
   // MAIN_AXIS_ALIGNMENT_START by default.
   MainAxisAlignment main_axis_alignment_;
 
   // The alignment of children in the cross axis. This is
   // CROSS_AXIS_ALIGNMENT_STRETCH by default.
   CrossAxisAlignment cross_axis_alignment_;
 
   // A map of views to their flex weights.
-  std::map<const View*, int> flex_map_;
+  FlexMap flex_map_;
 
   // The flex weight for views if none is set. Defaults to 0.
   int default_flex_;
 
   // The minimum cross axis size for the layout.
   int minimum_cross_axis_size_;
 
   // The view that this BoxLayout is managing the layout for.
   views::View* host_;
 
+  // Unique margins around specific views.
+  MarginMap margin_map_;
+
   DISALLOW_IMPLICIT_CONSTRUCTORS(BoxLayout);
 };
 
 } // namespace views
 
 #endif  // UI_VIEWS_LAYOUT_BOX_LAYOUT_H_