1 /* 2 * Copyright (C) 2012 Apple Inc. All rights reserved. 3 * 4 * Redistribution and use in source and binary forms, with or without 5 * modification, are permitted provided that the following conditions 6 * are met: 7 * 1. Redistributions of source code must retain the above copyright 8 * notice, this list of conditions and the following disclaimer. 9 * 2. Redistributions in binary form must reproduce the above copyright 10 * notice, this list of conditions and the following disclaimer in the 11 * documentation and/or other materials provided with the distribution. 12 * 13 * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY 14 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 16 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE COMPUTER, INC. OR 17 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 18 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, 19 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR 20 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY 21 * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 22 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 23 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 24 */ 25 26 27 #ifndef RenderMultiColumnFlowThread_h 28 #define RenderMultiColumnFlowThread_h 29 30 #include "core/rendering/RenderFlowThread.h" 31 32 namespace blink { 33 34 class RenderMultiColumnSet; 35 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. 41 // 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. 53 // 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. 57 // 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. 65 // 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. 82 // 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 86 // optimized manner. 87 class RenderMultiColumnFlowThread : public RenderFlowThread { 88 public: 89 virtual ~RenderMultiColumnFlowThread(); 90 91 static RenderMultiColumnFlowThread* createAnonymous(Document&, RenderStyle* parentStyle); 92 93 virtual bool isRenderMultiColumnFlowThread() const OVERRIDE FINAL { return true; } 94 95 RenderBlockFlow* multiColumnBlockFlow() const { return toRenderBlockFlow(parent()); } 96 97 RenderMultiColumnSet* firstMultiColumnSet() const; 98 RenderMultiColumnSet* lastMultiColumnSet() const; 99 100 virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0) OVERRIDE; 101 102 // Populate the flow thread with what's currently its siblings. Called when a regular block 103 // becomes a multicol container. 104 void populate(); 105 106 // 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 108 // block. 109 void evacuateAndDestroy(); 110 111 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; } 116 117 virtual LayoutSize columnOffset(const LayoutPoint&) const OVERRIDE FINAL; 118 119 // Do we need to set a new width and lay out? 120 virtual bool needsNewWidth() const; 121 122 void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&); 123 124 bool recalculateColumnHeights(); 125 126 protected: 127 RenderMultiColumnFlowThread(); 128 void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; } 129 130 virtual void layout() OVERRIDE; 131 132 private: 133 void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const; 134 135 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; 145 146 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. 151 }; 152 153 } // namespace blink 154 155 #endif // RenderMultiColumnFlowThread_h 156 157
