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..cacec94b3e296f298f5d3b97ac30a04b839c4b07 100644
--- a/ui/views/layout/box_layout.h
+++ b/ui/views/layout/box_layout.h
@@ -58,11 +58,14 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // Use |inside_border_horizontal_spacing| and
   // |inside_border_vertical_spacing| to add additional space between the child
   // view area and the host view border. |between_child_spacing| controls the
-  // space in between child views.
+  // space in between child views. |collapse_margins_spacing| paramter controls

[sky, 2017/05/15 20:29:09]

parameter

+  // whether or not spacing adjacent to view margins are collapsed base on the

[sky, 2017/05/15 20:29:09]

How about: 'spacing between views along the main axis are collapsed to the max
of the margins of the two adjacent views.' Please also add an example to help
drive it home.

[kylix_rd, 2017/05/16 18:45:47]

It's more than merely "adjacent views". The collapsing also happens with the
inside border spacing (aka. the internal padding). So it is the adjacent
"spacing", regardless of where that spacing is specified, that is collapsed.

I can certainly add an example... based on the ASCII art you and pkasting@ are
bantering about :).

+  // max of the two values
   BoxLayout(Orientation orientation,
             int inside_border_horizontal_spacing,
             int inside_border_vertical_spacing,
-            int between_child_spacing);
+            int between_child_spacing,
+            bool collapse_margins_spacing = false);
   ~BoxLayout() override;
 
   void set_main_axis_alignment(MainAxisAlignment main_axis_alignment) {
@@ -105,6 +108,38 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   int GetPreferredHeightForWidth(const View* host, int width) const override;
 
  private:
+  // This struct is used internally to "wrap" a child view in order to obviate
+  // the need for the main layout logic to be fully aware of the per-view
+  // margins when |collapse_margin_spacing_| is false. Since each view is a
+  // rectangle of a certain size, this wrapper, coupled with any margins set
+  // will increase the apparent size of the view. All necessary view size/
+  // position methods required for the layout logic add or subtract the margins
+  // where appropriate to ensure the actual visible size of the view doesn't
+  // include the margins.
+  // When |collapse_margin_spacing_| is true, this wrapper provides quick access
+  // to the view's margins for use by the layout to collapse adjacent spacing
+  // to the largest of the several values.
+  class ViewWrapper {
+   public:
+    ViewWrapper();
+    ViewWrapper(const BoxLayout* layout, View* view);
+    ViewWrapper(const ViewWrapper& wrapper) = delete;

[sky, 2017/05/15 20:29:09]

Is there a reason to delete this constructor and not use DISALLOW that deletes
more?

[kylix_rd, 2017/05/16 18:45:47]

No, not really.

Done.

+    ~ViewWrapper();
+    int GetHeightForWidth(int width) const;
+    const gfx::Insets& margins() const { return margins_; }
+    gfx::Size GetPreferredSize() const;
+    void SetBoundsRect(const gfx::Rect& bounds);
+    View* view() const { return view_; }
+    bool visible() const;
+
+   private:
+    View* view_;
+    const BoxLayout* layout_;
+    gfx::Insets margins_;
+  };
+
+  using FlexMap = std::map<const View*, int>;
+
   // Returns the flex for the specified |view|.
   int GetFlexForView(const View* view) const;
 
@@ -126,10 +161,33 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
 
   // Returns the main axis size for the given view. |child_area_width| is needed
   // to calculate the height of the view when the orientation is vertical.
-  int MainAxisSizeForView(const View* view, int child_area_width) const;
+  int MainAxisSizeForView(const ViewWrapper& view, int child_area_width) const;
+
+  // Returns the main axis margin spacing between the two views which is the max
+  // of the right margin from the |left| view, the left margin of the |right|
+  // view and |between_child_spacing_|.
+  int MainAxisMarginBetweenViews(const ViewWrapper& left,
+                                 const ViewWrapper& right) const;
+
+  // Return the outer margin along the main axis as insets.
+  gfx::Insets MainAxisOuterMargin() const;
+
+  // Return the outer margin along the cross axis as insets.
+  gfx::Insets CrossAxisOuterMargin() const;
+
+  // Adjusts the main axis for |rect| by collapsing the left or top margin of
+  // the first view with corresponding side of |inside_border_insets_| and the
+  // right or bottom margin of the last view with the corresponding side of
+  // |inside_border_insets_|.
+  void AdjustMainAxisForMargin(gfx::Rect* rect) const;
+
+  // Adjust the cross axis of |rect| by collapsing the max of the cross axis
+  // margins of all children and the corresponding side of
+  // |inside_border_insets_|
+  void AdjustCrossAxisForMargin(gfx::Rect* rect) const;
 
   // Returns the cross axis size for the given view.
-  int CrossAxisSizeForView(const View* view) const;
+  int CrossAxisSizeForView(const ViewWrapper& view) const;
 
   // The preferred size for the dialog given the width of the child area.
   gfx::Size GetPreferredSizeForChildWidth(const View* host,
@@ -139,6 +197,16 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // child views.
   gfx::Size NonChildSize(const View* host) const;
 
+  // The next visible view at > index. If no other views are visible, return
+  // nullptr.
+  View* NextVisibleView(int index) const;
+
+  // Return the first visible view in the host or nullptr if none are visible.
+  View* FirstVisibleView() const;
+
+  // Return the last visible view in the host or nullptr if none are visible.
+  View* LastVisibleView() const;
+
   const Orientation orientation_;
 
   // Spacing between child views and host view border.
@@ -156,7 +224,7 @@ 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_;
@@ -164,6 +232,9 @@ class VIEWS_EXPORT BoxLayout : public LayoutManager {
   // The minimum cross axis size for the layout.
   int minimum_cross_axis_size_;
 
+  // Adjacent view margins and spacing should be collapsed.
+  const bool collapse_margins_spacing_;
+
   // The view that this BoxLayout is managing the layout for.
   views::View* host_;