flex_layout.h

// Copyright 2018 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef UI_VIEWS_LAYOUT_FLEX_LAYOUT_H_
#define UI_VIEWS_LAYOUT_FLEX_LAYOUT_H_

#include <list>
#include <map>
#include <memory>
#include <set>
#include <utility>
#include <vector>

#include "base/memory/raw_ptr.h"
#include "third_party/abseil-cpp/absl/types/optional.h"
#include "ui/base/class_property.h"
#include "ui/gfx/geometry/insets.h"
#include "ui/views/layout/flex_layout_types.h"
#include "ui/views/layout/layout_manager_base.h"
#include "ui/views/view_class_properties.h"
#include "ui/views/views_export.h"

namespace views {

class NormalizedSize;
class NormalizedSizeBounds;
class View;

// Provides CSS-like layout for a one-dimensional (vertical or horizontal)
// arrangement of child views. Independent alignment can be specified for the
// main and cross axes.
//
// Per-View margins (provided by view property kMarginsKey) specify how much
// space to leave around each child view. The |interior_margin| says how much
// empty space to leave at the edges of the parent view. If |collapse_margins|
// is false, these values are additive; if true, the greater of the two values
// is used. The |default_child_margins| provides a fallback for views without
// kMarginsKey set.
//
// collapse_margins = false:
//
// | interior margin>                      <margin [view]...
// |                 <margin [view] margin>
//
// collapse_margins = true:
//
// | interior margin>      <margin [view]
// |         <margin [view] margin>       ...
//
// Views can have their own internal padding, using the kInternalPaddingKey
// property, which is subtracted from the margin space between child views.
//
// Calling SetVisible(false) on a child view outside of the FlexLayout will
// result in the child view being hidden until SetVisible(true) is called. This
// is irrespective of whether the FlexLayout has set the child view to be
// visible or not based on, for example, flex rules.
//
// If you want the host view to maintain control over a child view, you can
// exclude it from the layout. Excluded views are completely ignored during
// layout and do not have their properties modified.
//
// FlexSpecification objects determine how child views are sized. You can set
// individual flex rules for each child view, or a default for any child views
// without individual flex rules set. If you don't set anything, each view will
// take up its preferred size in the layout.
//
// The core function of this class is contained in
// GetPreferredSize(maximum_size) and Layout(). In both cases, a layout will
// be cached and typically not recalculated as long as none of the layout's
// properties or the preferred size or visibility of any of its children has
// changed.
class VIEWS_EXPORT FlexLayout : public LayoutManagerBase {
 public:
  FlexLayout();

  FlexLayout(const FlexLayout&) = delete;
  FlexLayout& operator=(const FlexLayout&) = delete;

  ~FlexLayout() override;

  // Note: setters provide a Builder-style interface, so you can type:
  // layout.SetMainAxisAlignment()
  //       .SetCrossAxisAlignment()
  //       .SetDefaultFlex(...);
  // Note that cross-axis alignment can be overridden per-child using:
  //   child->SetProperty(kCrossAxisAlignmentKey, <value>);
  FlexLayout& SetOrientation(LayoutOrientation orientation);
  FlexLayout& SetMainAxisAlignment(LayoutAlignment main_axis_alignment);
  FlexLayout& SetCrossAxisAlignment(LayoutAlignment cross_axis_alignment);
  FlexLayout& SetInteriorMargin(const gfx::Insets& interior_margin);
  FlexLayout& SetMinimumCrossAxisSize(int size);
  FlexLayout& SetCollapseMargins(bool collapse_margins);
  FlexLayout& SetIncludeHostInsetsInLayout(bool include_host_insets_in_layout);
  FlexLayout& SetIgnoreDefaultMainAxisMargins(
      bool ignore_default_main_axis_margins);
  FlexLayout& SetFlexAllocationOrder(FlexAllocationOrder flex_allocation_order);

  LayoutOrientation orientation() const { return orientation_; }
  bool collapse_margins() const { return collapse_margins_; }
  LayoutAlignment main_axis_alignment() const { return main_axis_alignment_; }
  LayoutAlignment cross_axis_alignment() const {
    return *GetDefault(kCrossAxisAlignmentKey);
  }
  const gfx::Insets& interior_margin() const { return interior_margin_; }
  int minimum_cross_axis_size() const { return minimum_cross_axis_size_; }
  bool include_host_insets_in_layout() const {
    return include_host_insets_in_layout_;
  }
  bool ignore_default_main_axis_margins() const {
    return ignore_default_main_axis_margins_;
  }
  FlexAllocationOrder flex_allocation_order() const {
    return flex_allocation_order_;
  }

  // Returns a flex rule that allows flex layouts to be nested with expected
  // behavior.
  FlexRule GetDefaultFlexRule() const;

  // Moves and uses |value| as the default value for layout property |key|.
  template <class T, class U>
  FlexLayout& SetDefault(const ui::ClassProperty<T>* key, U&& value) {
    layout_defaults_.SetProperty(key, std::forward<U>(value));
    return *this;
  }

  // Copies and uses |value| as the default value for layout property |key|.
  template <class T, class U>
  FlexLayout& SetDefault(const ui::ClassProperty<T>* key, const U& value) {
    layout_defaults_.SetProperty(key, value);
    return *this;
  }

 protected:
  // LayoutManagerBase:
  ProposedLayout CalculateProposedLayout(
      const SizeBounds& size_bounds) const override;

 private:
  struct ChildLayoutParams;
  class ChildViewSpacing;
  struct FlexLayoutData;

  class PropertyHandler : public ui::PropertyHandler {
   public:
    explicit PropertyHandler(FlexLayout* layout);

   protected:
    // ui::PropertyHandler:
    void AfterPropertyChange(const void* key, int64_t old_value) override;

   private:
    const raw_ptr<FlexLayout> layout_;
  };

  using ChildIndices = std::list<size_t>;

  // Maps a flex order (lower = allocated first, and therefore higher priority)
  // to the indices of child views within that order that can flex.
  // See FlexSpecification::order().
  using FlexOrderToViewIndexMap = std::map<int, ChildIndices>;

  // Alignment used when the main-axis alignment is not specified.
  static constexpr LayoutAlignment kDefaultMainAxisAlignment =
      LayoutAlignment::kStart;

  // Layout used when the cross-axis alignment is not specified.
  static constexpr LayoutAlignment kDefaultCrossAxisAlignment =
      LayoutAlignment::kStretch;

  // Returns the preferred size for a given |rule| and |child| given unbounded
  // space, with the caveat that for vertical layouts the horizontal axis is
  // bounded to |available_cross| to factor in height-for-width considerations.
  // This corresponds to the FlexSpecification "preferred size".
  NormalizedSize GetPreferredSizeForRule(
      const FlexRule& rule,
      const View* child,
      const SizeBound& available_cross) const;

  // Returns the size for a given |rule| and |child| with |available| space.
  NormalizedSize GetCurrentSizeForRule(
      const FlexRule& rule,
      const View* child,
      const NormalizedSizeBounds& available) const;

  // Returns the combined margins across the cross axis of the host view, for a
  // particular child view.
  Inset1D GetCrossAxisMargins(const FlexLayoutData& layout,
                              size_t child_index) const;

  // Calculates a margin between two child views based on each's margin,
  // inter-child spacing, and any internal padding present in one or both
  // elements. Uses properties of the layout, like whether adjacent margins
  // should be collapsed.
  int CalculateMargin(int margin1, int margin2, int internal_padding) const;

  // Calculates the cross-layout space available to a view based on the
  // available space and margins.
  SizeBound GetAvailableCrossAxisSize(const FlexLayoutData& layout,
                                      size_t child_index,
                                      const NormalizedSizeBounds& bounds) const;

  // Calculates the preferred spacing between two child views, or between a
  // view edge and the first or last visible child views.
  int CalculateChildSpacing(const FlexLayoutData& layout,
                            absl::optional<size_t> child1_index,
                            absl::optional<size_t> child2_index) const;

  // Calculates the position of each child view and the size of the overall
  // layout based on tentative visibilities and sizes for each child.
  void UpdateLayoutFromChildren(const NormalizedSizeBounds& bounds,
                                FlexLayoutData& data,
                                ChildViewSpacing& child_spacing) const;

  // Fills out the child entries for |data| and generates some initial size
  // and visibility data, and stores off information about which views can
  // expand in |flex_order_to_index|.
  void InitializeChildData(const NormalizedSizeBounds& bounds,
                           FlexLayoutData& data,
                           FlexOrderToViewIndexMap& flex_order_to_index) const;

  // Caclulates the child bounds (in screen coordinates) for each visible child
  // in the layout.
  void CalculateChildBounds(const SizeBounds& size_bounds,
                            FlexLayoutData& data) const;

  // Calculates available space along the main axis for non-flex views and
  // the values in |data.child_data|.
  void CalculateNonFlexAvailableSpace(const SizeBound& available_space,
                                      const FlexOrderToViewIndexMap& flex_views,
                                      const ChildViewSpacing& child_spacing,
                                      FlexLayoutData& data) const;

  // Allocates space shortage (when the available space is less than the
  // preferred size of the layout) across child views that can flex.
  //
  // Updates are made to |data| and |child_spacing|, and views that can still
  // expand above their preferred size are added to |expandable_views| for later
  // processing by AllocateFlexExcess().
  void AllocateFlexShortage(const NormalizedSizeBounds& bounds,
                            const FlexOrderToViewIndexMap& order_to_index,
                            FlexLayoutData& data,
                            ChildViewSpacing& child_spacing,
                            FlexOrderToViewIndexMap& expandable_views) const;

  // Allocates space above each child view's preferred size, based on remaining/
  // excess space in the layout.
  void AllocateFlexExcess(const NormalizedSizeBounds& bounds,
                          const FlexOrderToViewIndexMap& order_to_index,
                          FlexLayoutData& data,
                          ChildViewSpacing& child_spacing) const;

  // Updates the available space for each flex child in |child_indices| in
  // |data.child_data| based on |data.total_size|, |bounds|, and the margin data
  // in |child_spacing|.
  void CalculateFlexAvailableSpace(const NormalizedSizeBounds& bounds,
                                   const ChildIndices& child_indices,
                                   const ChildViewSpacing& child_spacing,
                                   FlexLayoutData& data) const;

  // Pre-allocates space associated with zero-weight views at a particular flex
  // priority |flex_order|. Zero-weight child views are removed from
  // |child_list| and their entries are updated in |data|. If |expandable_views|
  // is specified, this is treated as the first pass, and space allocated to
  // each view is capped at its preferred size; if the view would claim more
  // space it is added to |expandable_views| (if specified).
  void AllocateZeroWeightFlex(const NormalizedSizeBounds& bounds,
                              int flex_order,
                              ChildIndices& child_list,
                              FlexLayoutData& data,
                              ChildViewSpacing& child_spacing,
                              FlexOrderToViewIndexMap* expandable_views) const;

  // Tries to allocate all the views in |child_list| in the available |bounds|.
  // If successful, updates |data| and |expandable_views|. Returns the
  // difference between the space needed by all of the views in |child_list| and
  // the space provided by |bounds|.
  SizeBound TryAllocateAll(const NormalizedSizeBounds& bounds,
                           int flex_order,
                           const ChildIndices& child_list,
                           FlexLayoutData& data,
                           ChildViewSpacing& child_spacing,
                           FlexOrderToViewIndexMap& expandable_views) const;

  // Allocates flex excess |to_allocate| for a list of child views at the same
  // priority order.
  //
  // It will attempt to do the entire allocation in one pass, removing all
  // elements from |child_list| that it successfully allocates space for, but in
  // the event a member of |child_list| does not take its full allocation, it
  // will remove just that child and set aside its smaller size. At least one
  // child will always be removed and |to_allocate| will be updated with the
  // remaining space in the layout.
  //
  // This method should be called repeatedly until |child_list| is empty.
  void AllocateFlexExcessAtOrder(const NormalizedSizeBounds& bounds,
                                 SizeBound& to_allocate,
                                 ChildIndices& child_list,
                                 FlexLayoutData& data,
                                 ChildViewSpacing& child_spacing) const;

  // Allocates flex shortage for a list of child views at priority |order|.
  //
  // It will attempt to allocate the entire |child_list| in one pass, removing
  // all elements that it successfully allocates space for, but in the event one
  // or more members of |child_list| do not take their full allocation, those
  // views will be allocated and removed from the list, and this method should
  // be called again on the new, smaller list. At least one child is guaranteed
  // to be allocated and removed each invocation.
  //
  // This method should be called repeatedly until |child_list| is empty.
  void AllocateFlexShortageAtOrder(const NormalizedSizeBounds& bounds,
                                   SizeBound deficit,
                                   ChildIndices& child_list,
                                   FlexLayoutData& data,
                                   ChildViewSpacing& child_spacing) const;

  // Returns the total weight for all children listed in |child_indices|.
  static int CalculateFlexTotal(const FlexLayoutData& data,
                                const ChildIndices& child_indices);

  // Gets the default value for a particular layout property, which will be used
  // if the property is not set on a child view being laid out (e.g.
  // kMarginsKey).
  template <class T>
  T* GetDefault(const ui::ClassProperty<T*>* key) const {
    return layout_defaults_.GetProperty(key);
  }

  static gfx::Size DefaultFlexRuleImpl(const FlexLayout* flex_layout,
                                       const View* view,
                                       const SizeBounds& size_bounds);

  LayoutOrientation orientation_ = LayoutOrientation::kHorizontal;

  // Adjacent view margins should be collapsed.
  bool collapse_margins_ = false;

  // Spacing between child views and host view border.
  gfx::Insets interior_margin_;

  // The alignment of children in the main axis. This is start by default.
  LayoutAlignment main_axis_alignment_ = kDefaultMainAxisAlignment;

  // The minimum cross axis size for the layout.
  int minimum_cross_axis_size_ = 0;

  // Whether to include host insets in the layout. Use when e.g. the host has an
  // empty border and you want to treat that empty space as part of the interior
  // margin of the host view.
  //
  // Most useful in conjunction with |collapse_margins| so child margins can
  // overlap with the host's insets.
  //
  // In the future, we might consider putting this as metadata on the host's
  // border - e.g. an EmptyBorder would be included in host insets but a thick
  // frame would not be.
  bool include_host_insets_in_layout_ = false;

  // Whether host |interior_margin| overrides default child margins at the
  // leading and trailing edge of the host view.
  //
  // Example:
  // layout->SetIgnoreDefaultMainAxisMargins(true)
  //        .SetCollapseMargins(true)
  //        .SetDefault(kMarginsKey, {5, 10})
  //        .SetInteriorMargin({5, 5});
  //
  // This produces a margin of 5 DIP on all edges of the host view, with 10 DIP
  // between child views. If SetIgnoreDefaultMainAxisMargins(true) was not
  // called, the default child margin of 10 would also apply on the leading and
  // trailing edge of the host view.
  bool ignore_default_main_axis_margins_ = false;

  // Order in which the host's child views receive their flex allocation.
  // Setting to reverse is useful when, for example, you want views to drop out
  // left-to-right when there's insufficient space to display them all instead
  // of right-to-left.
  FlexAllocationOrder flex_allocation_order_ = FlexAllocationOrder::kNormal;

  // Default properties for any views that don't have them explicitly set for
  // this layout.
  PropertyHandler layout_defaults_{this};
};

}  // namespace views

#endif  // UI_VIEWS_LAYOUT_FLEX_LAYOUT_H_