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27 #ifndef RenderMultiColumnFlowThread_h
28 #define RenderMultiColumnFlowThread_h
30 #include "core/rendering/RenderFlowThread.h"
34 class RenderMultiColumnSet;
36 // Flow thread implementation for CSS multicol. This will be inserted as an anonymous child block of
37 // the actual multicol container (i.e. the RenderBlockFlow whose style computes to non-auto
38 // column-count and/or column-width). RenderMultiColumnFlowThread is the heart of the multicol
39 // implementation, and there is only one instance per multicol container. Child content of the
40 // multicol container is parented into the flow thread at the time of renderer insertion.
42 // Apart from this flow thread child, the multicol container will also have RenderMultiColumnSet
43 // "region" children, which are used to position the columns visually. The flow thread is in charge
44 // of layout, and, after having calculated the column width, it lays out content as if everything
45 // were in one tall single column, except that there will typically be some amount of blank space
46 // (also known as pagination struts) at the offsets where the actual column boundaries are. This
47 // way, content that needs to be preceded by a break will appear at the top of the next
48 // column. Content needs to be preceded by a break when there's a forced break or when the content
49 // is unbreakable and cannot fully fit in the same column as the preceding piece of
50 // content. Although a RenderMultiColumnFlowThread is laid out, it does not take up any space in its
51 // container. It's the RenderMultiColumnSet objects that take up the necessary amount of space, and
52 // make sure that the columns are painted and hit-tested correctly.
54 // The width of the flow thread is the same as the column width. The width of a column set is the
55 // same as the content box width of the multicol container; in other words exactly enough to hold
56 // the number of columns to be used, stacked horizontally, plus column gaps between them.
58 // Since it's the first child of the multicol container, the flow thread is laid out first, albeit
59 // in a slightly special way, since it's not to take up any space in its ancestors. Afterwards, the
60 // column sets are laid out. They get their height from the columns that they hold. In single
61 // column-row constrained height non-balancing cases this will simply be the same as the content
62 // height of the multicol container itself. In most other cases we'll have to calculate optimal
63 // column heights ourselves, though. This process is referred to as column balancing, and then we
64 // infer the column set height from the flow thread's height.
66 // More on column balancing: the columns' height is unknown in the first layout pass when
67 // balancing. This means that we cannot insert any implicit (soft / unforced) breaks (and pagination
68 // struts) when laying out the contents of the flow thread. We'll just lay out everything in tall
69 // single strip. After the initial flow thread layout pass we can determine a tentative / minimal /
70 // initial column height. This is calculated by simply dividing the flow thread's height by the
71 // number of specified columns. In the layout pass that follows, we can insert breaks (and
72 // pagination struts) at column boundaries, since we now have a column height. It may very easily
73 // turn out that the calculated height wasn't enough, though. We'll notice this at end of layout. If
74 // we end up with too many columns (i.e. columns overflowing the multicol container), it wasn't
75 // enough. In this case we need to increase the column heights. We'll increase them by the lowest
76 // amount of space that could possibly affect where the breaks occur (see
77 // RenderMultiColumnSet::recordSpaceShortage()). We'll relayout (to find new break points and the
78 // new lowest amount of space increase that could affect where they occur, in case we need another
79 // round) until we've reached an acceptable height (where everything fits perfectly in the number of
80 // columns that we have specified). The rule of thumb is that we shouldn't have to perform more of
81 // such iterations than the number of columns that we have.
83 // For each layout iteration done for column balancing, the flow thread will need a deep layout if
84 // column heights changed in the previous pass, since column height changes may affect break points
85 // and pagination struts anywhere in the tree, and currently no way exists to do this in a more
87 class RenderMultiColumnFlowThread : public RenderFlowThread {
89 virtual ~RenderMultiColumnFlowThread();
91 static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle);
93 virtual bool isRenderMultiColumnFlowThread() const OVERRIDE FINAL { return true; }
95 RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); }
97 RenderMultiColumnSet* firstMultiColumnSet() const;
98 RenderMultiColumnSet* lastMultiColumnSet() const;
100 virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0) OVERRIDE;
102 // Populate the flow thread with what's currently its siblings. Called when a regular block
103 // becomes a multicol container.
106 // Empty the flow thread by moving everything to the parent. Remove all multicol specific
107 // renderers. Then destroy the flow thread. Called when a multicol container becomes a regular
109 void evacuateAndDestroy();
111 unsigned columnCount() const { return m_columnCount; }
112 LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; }
113 void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; }
114 virtual bool heightIsAuto() const { return !columnHeightAvailable() || multiColumnBlockFlow()->style()->columnFill() == ColumnFillBalance; }
115 bool progressionIsInline() const { return m_progressionIsInline; }
117 virtual LayoutSize columnOffset(const LayoutPoint&) const OVERRIDE FINAL;
119 // Do we need to set a new width and lay out?
120 virtual bool needsNewWidth() const;
122 void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&);
124 bool recalculateColumnHeights();
127 RenderMultiColumnFlowThread();
128 void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; }
130 virtual void layout() OVERRIDE;
133 void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const;
135 virtual const char* renderName() const OVERRIDE;
136 virtual void addRegionToThread(RenderMultiColumnSet*) OVERRIDE;
137 virtual void willBeRemovedFromTree() OVERRIDE;
138 virtual void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const OVERRIDE;
139 virtual void updateLogicalWidth() OVERRIDE;
140 virtual void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) OVERRIDE;
141 virtual void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) OVERRIDE;
142 virtual RenderMultiColumnSet* columnSetAtBlockOffset(LayoutUnit) const OVERRIDE;
143 virtual bool addForcedRegionBreak(LayoutUnit, RenderObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = 0) OVERRIDE;
144 virtual bool isPageLogicalHeightKnown() const OVERRIDE;
146 unsigned m_columnCount; // The used value of column-count
147 LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto.
148 bool m_inBalancingPass; // Set when relayouting for column balancing.
149 bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout.
150 bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow.
155 #endif // RenderMultiColumnFlowThread_h