BoxLayout now suports per-view margins.

ui/views/layout/box_layout.h

Index: ui/views/layout/box_layout.h
diff --git a/ui/views/layout/box_layout.h b/ui/views/layout/box_layout.h
index 9a87984d51f77cec0caa488860fa6dc1986f9e28..2705646140c0e4b6dc9d15c2b253291d1ea0697f 100644
--- a/ui/views/layout/box_layout.h
+++ b/ui/views/layout/box_layout.h
@@ -97,6 +97,14 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // 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;
@@ -105,6 +113,25 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   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;
 
@@ -139,6 +166,9 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // 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.
@@ -156,7 +186,7 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   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_;
@@ -167,6 +197,9 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // 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);
 };