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27 #ifndef RenderMultiColumnFlowThread_h
28 #define RenderMultiColumnFlowThread_h
30 #include "core/rendering/RenderFlowThread.h"
31 #include "wtf/HashMap.h"
35 class RenderMultiColumnSet;
36 class RenderMultiColumnSpannerSet;
38 // Flow thread implementation for CSS multicol. This will be inserted as an anonymous child block of
39 // the actual multicol container (i.e. the RenderBlockFlow whose style computes to non-auto
40 // column-count and/or column-width). RenderMultiColumnFlowThread is the heart of the multicol
41 // implementation, and there is only one instance per multicol container. Child content of the
42 // multicol container is parented into the flow thread at the time of renderer insertion.
44 // Apart from this flow thread child, the multicol container will also have RenderMultiColumnSet
45 // "region" children, which are used to position the columns visually. The flow thread is in charge
46 // of layout, and, after having calculated the column width, it lays out content as if everything
47 // were in one tall single column, except that there will typically be some amount of blank space
48 // (also known as pagination struts) at the offsets where the actual column boundaries are. This
49 // way, content that needs to be preceded by a break will appear at the top of the next
50 // column. Content needs to be preceded by a break when there's a forced break or when the content
51 // is unbreakable and cannot fully fit in the same column as the preceding piece of
52 // content. Although a RenderMultiColumnFlowThread is laid out, it does not take up any space in its
53 // container. It's the RenderMultiColumnSet objects that take up the necessary amount of space, and
54 // make sure that the columns are painted and hit-tested correctly.
56 // If there is any column content inside the multicol container, we create a
57 // RenderMultiColumnSet. We only need to create multiple sets if there are spanners
58 // (column-span:all) in the multicol container. When a spanner is inserted, content preceding it
59 // gets its own set, and content succeeding it will get another set. The spanner itself will also
60 // get its own set (RenderMultiColumnSpannerSet).
62 // The width of the flow thread is the same as the column width. The width of a column set is the
63 // same as the content box width of the multicol container; in other words exactly enough to hold
64 // the number of columns to be used, stacked horizontally, plus column gaps between them.
66 // Since it's the first child of the multicol container, the flow thread is laid out first, albeit
67 // in a slightly special way, since it's not to take up any space in its ancestors. Afterwards, the
68 // column sets are laid out. Column sets get their height from the columns that they hold. In single
69 // column-row constrained height non-balancing cases without spanners this will simply be the same
70 // as the content height of the multicol container itself. In most other cases we'll have to
71 // calculate optimal column heights ourselves, though. This process is referred to as column
72 // balancing, and then we infer the column set height from the height of the flow thread portion
73 // occupied by each set.
75 // More on column balancing: the columns' height is unknown in the first layout pass when
76 // balancing. This means that we cannot insert any implicit (soft / unforced) breaks (and pagination
77 // struts) when laying out the contents of the flow thread. We'll just lay out everything in tall
78 // single strip. After the initial flow thread layout pass we can determine a tentative / minimal /
79 // initial column height. This is calculated by simply dividing the flow thread's height by the
80 // number of specified columns. In the layout pass that follows, we can insert breaks (and
81 // pagination struts) at column boundaries, since we now have a column height. It may very easily
82 // turn out that the calculated height wasn't enough, though. We'll notice this at end of layout. If
83 // we end up with too many columns (i.e. columns overflowing the multicol container), it wasn't
84 // enough. In this case we need to increase the column heights. We'll increase them by the lowest
85 // amount of space that could possibly affect where the breaks occur (see
86 // RenderMultiColumnSet::recordSpaceShortage()). We'll relayout (to find new break points and the
87 // new lowest amount of space increase that could affect where they occur, in case we need another
88 // round) until we've reached an acceptable height (where everything fits perfectly in the number of
89 // columns that we have specified). The rule of thumb is that we shouldn't have to perform more of
90 // such iterations than the number of columns that we have.
92 // For each layout iteration done for column balancing, the flow thread will need a deep layout if
93 // column heights changed in the previous pass, since column height changes may affect break points
94 // and pagination struts anywhere in the tree, and currently no way exists to do this in a more
96 class RenderMultiColumnFlowThread : public RenderFlowThread {
98 virtual ~RenderMultiColumnFlowThread();
100 static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle);
102 virtual bool isRenderMultiColumnFlowThread() const override final { return true; }
104 RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
106 RenderMultiColumnSet* firstMultiColumnSet() const;
107 RenderMultiColumnSet* lastMultiColumnSet() const;
109 // Return the spanner set (if any) that contains the specified renderer. This includes the
110 // renderer for the element that actually establishes the spanner too.
111 RenderMultiColumnSpannerSet* containingColumnSpannerSet(const RenderObject* descendant) const;
113 // Populate the flow thread with what's currently its siblings. Called when a regular block
114 // becomes a multicol container.
117 // Empty the flow thread by moving everything to the parent. Remove all multicol specific
118 // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
120 void evacuateAndDestroy();
122 unsigned columnCount() const { return m_columnCount; }
123 LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
124 void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
125 virtual bool heightIsAuto() const { return !columnHeightAvailable() || multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance; }
126 bool progressionIsInline() const { return m_progressionIsInline; }
128 virtual LayoutSize columnOffset(const LayoutPoint&) const override final;
130 // Do we need to set a new width and lay out?
131 virtual bool needsNewWidth() const;
133 void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
135 bool recalculateColumnHeights();
138 RenderMultiColumnFlowThread();
139 void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
141 virtual void layout() override;
144 void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
145 void createAndInsertMultiColumnSet();
146 void createAndInsertSpannerSet(RenderBox* spanner);
147 virtual bool descendantIsValidColumnSpanner(RenderObject* descendant) const;
149 virtual const char* renderName() const override;
150 virtual void addRegionToThread(RenderMultiColumnSet*) override;
151 virtual void willBeRemovedFromTree() override;
152 virtual void flowThreadDescendantWasInserted(RenderObject*) override;
153 virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override;
154 virtual void updateLogicalWidth() override;
155 virtual void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) override;
156 virtual void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) override;
157 virtual RenderMultiColumnSet* columnSetAtBlockOffset(LayoutUnit) const override;
158 virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) override;
159 virtual bool isPageLogicalHeightKnown() const override;
161 typedef HashMap<const RenderObject*, RenderMultiColumnSpannerSet*> SpannerMap;
162 SpannerMap m_spannerMap;
164 unsigned m_columnCount; // The used value of column-count
165 LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
166 bool m_inBalancingPass; // Set when relayouting for column balancing.
167 bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
168 bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
173 #endif // RenderMultiColumnFlowThread_h