| /* |
| * Copyright (C) 2012 Apple Inc. All rights reserved. |
| * |
| * Redistribution and use in source and binary forms, with or without |
| * modification, are permitted provided that the following conditions |
| * are met: |
| * 1. Redistributions of source code must retain the above copyright |
| * notice, this list of conditions and the following disclaimer. |
| * 2. Redistributions in binary form must reproduce the above copyright |
| * notice, this list of conditions and the following disclaimer in the |
| * documentation and/or other materials provided with the distribution. |
| * |
| * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY |
| * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE |
| * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR |
| * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE COMPUTER, INC. OR |
| * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, |
| * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, |
| * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR |
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| * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| */ |
| |
| |
| #ifndef LayoutMultiColumnFlowThread_h |
| #define LayoutMultiColumnFlowThread_h |
| |
| #include "core/CoreExport.h" |
| #include "core/layout/LayoutFlowThread.h" |
| |
| namespace blink { |
| |
| class LayoutMultiColumnSet; |
| class LayoutMultiColumnSpannerPlaceholder; |
| |
| enum BalancedColumnHeightCalculation { GuessFromFlowThreadPortion, StretchBySpaceShortage }; |
| |
| // Flow thread implementation for CSS multicol. This will be inserted as an anonymous child block of |
| // the actual multicol container (i.e. the LayoutBlockFlow whose style computes to non-auto |
| // column-count and/or column-width). LayoutMultiColumnFlowThread is the heart of the multicol |
| // implementation, and there is only one instance per multicol container. Child content of the |
| // multicol container is parented into the flow thread at the time of layoutObject insertion. |
| // |
| // Apart from this flow thread child, the multicol container will also have LayoutMultiColumnSet |
| // children, which are used to position the columns visually. The flow thread is in charge |
| // of layout, and, after having calculated the column width, it lays out content as if everything |
| // were in one tall single column, except that there will typically be some amount of blank space |
| // (also known as pagination struts) at the offsets where the actual column boundaries are. This |
| // way, content that needs to be preceded by a break will appear at the top of the next |
| // column. Content needs to be preceded by a break when there's a forced break or when the content |
| // is unbreakable and cannot fully fit in the same column as the preceding piece of |
| // content. Although a LayoutMultiColumnFlowThread is laid out, it does not take up any space in its |
| // container. It's the LayoutMultiColumnSet objects that take up the necessary amount of space, and |
| // make sure that the columns are painted and hit-tested correctly. |
| // |
| // If there is any column content inside the multicol container, we create a |
| // LayoutMultiColumnSet. We only need to create multiple sets if there are spanners |
| // (column-span:all) in the multicol container. When a spanner is inserted, content preceding it |
| // gets its own set, and content succeeding it will get another set. The spanner itself will also |
| // get its own placeholder between the sets (LayoutMultiColumnSpannerPlaceholder), so that it gets |
| // positioned and sized correctly. The column-span:all element is inside the flow thread, but its |
| // containing block is the multicol container. |
| // |
| // Some invariants for the layout tree structure for multicol: |
| // - A multicol container is always a LayoutBlockFlow |
| // - Every multicol container has one and only one LayoutMultiColumnFlowThread |
| // - All multicol DOM children and pseudo-elements associated with the multicol container are |
| // reparented into the flow thread |
| // - The LayoutMultiColumnFlowThread is the first child of the multicol container |
| // - A multicol container may only have LayoutMultiColumnFlowThread, LayoutMultiColumnSet and |
| // LayoutMultiColumnSpannerPlaceholder children |
| // - A LayoutMultiColumnSet may not be adjacent to another LayoutMultiColumnSet; there are no |
| // use-cases for it, and there are also implementation limitations behind this requirement. |
| // - The flow thread is not in the containing block chain for children that are not to be laid out |
| // in columns. This means column spanners and absolutely positioned children whose containing |
| // block is outside column content |
| // - Each spanner (column-span:all) establishes a LayoutMultiColumnSpannerPlaceholder |
| // |
| // The width of the flow thread is the same as the column width. The width of a column set is the |
| // same as the content box width of the multicol container; in other words exactly enough to hold |
| // the number of columns to be used, stacked horizontally, plus column gaps between them. |
| // |
| // Since it's the first child of the multicol container, the flow thread is laid out first, albeit |
| // in a slightly special way, since it's not to take up any space in its ancestors. Afterwards, the |
| // column sets are laid out. Column sets get their height from the columns that they hold. In single |
| // column-row constrained height non-balancing cases without spanners this will simply be the same |
| // as the content height of the multicol container itself. In most other cases we'll have to |
| // calculate optimal column heights ourselves, though. This process is referred to as column |
| // balancing, and then we infer the column set height from the height of the flow thread portion |
| // occupied by each set. |
| // |
| // More on column balancing: the columns' height is unknown in the first layout pass when |
| // balancing. This means that we cannot insert any implicit (soft / unforced) breaks (and pagination |
| // struts) when laying out the contents of the flow thread. We'll just lay out everything in tall |
| // single strip. After the initial flow thread layout pass we can determine a tentative / minimal / |
| // initial column height. This is calculated by simply dividing the flow thread's height by the |
| // number of specified columns. In the layout pass that follows, we can insert breaks (and |
| // pagination struts) at column boundaries, since we now have a column height. It may very easily |
| // turn out that the calculated height wasn't enough, though. We'll notice this at end of layout. If |
| // we end up with too many columns (i.e. columns overflowing the multicol container), it wasn't |
| // enough. In this case we need to increase the column heights. We'll increase them by the lowest |
| // amount of space that could possibly affect where the breaks occur (see |
| // LayoutMultiColumnSet::recordSpaceShortage()). We'll relayout (to find new break points and the |
| // new lowest amount of space increase that could affect where they occur, in case we need another |
| // round) until we've reached an acceptable height (where everything fits perfectly in the number of |
| // columns that we have specified). The rule of thumb is that we shouldn't have to perform more of |
| // such iterations than the number of columns that we have. |
| // |
| // For each layout iteration done for column balancing, the flow thread will need a deep layout if |
| // column heights changed in the previous pass, since column height changes may affect break points |
| // and pagination struts anywhere in the tree, and currently no way exists to do this in a more |
| // optimized manner. |
| // |
| // There's also some documentation online: |
| // https://sites.google.com/a/chromium.org/dev/developers/design-documents/multi-column-layout |
| class CORE_EXPORT LayoutMultiColumnFlowThread : public LayoutFlowThread { |
| public: |
| ~LayoutMultiColumnFlowThread() override; |
| |
| static LayoutMultiColumnFlowThread* createAnonymous(Document&, const ComputedStyle& parentStyle); |
| |
| bool isLayoutMultiColumnFlowThread() const final { return true; } |
| |
| LayoutBlockFlow* multiColumnBlockFlow() const { return toLayoutBlockFlow(parent()); } |
| |
| LayoutMultiColumnSet* firstMultiColumnSet() const; |
| LayoutMultiColumnSet* lastMultiColumnSet() const; |
| |
| // Return the first column set or spanner placeholder. |
| LayoutBox* firstMultiColumnBox() const |
| { |
| return nextSiblingBox(); |
| } |
| // Return the last column set or spanner placeholder. |
| LayoutBox* lastMultiColumnBox() const |
| { |
| LayoutBox* lastSiblingBox = multiColumnBlockFlow()->lastChildBox(); |
| // The flow thread is the first child of the multicol container. If the flow thread is also |
| // the last child, it means that there are no siblings; i.e. we have no column boxes. |
| return lastSiblingBox != this ? lastSiblingBox : 0; |
| } |
| |
| // Find the first set inside which the specified layoutObject (which is a flowthread descendant) would be rendered. |
| LayoutMultiColumnSet* mapDescendantToColumnSet(LayoutObject*) const; |
| |
| // Return the spanner placeholder that belongs to the spanner in the containing block chain, if |
| // any. This includes the layoutObject for the element that actually establishes the spanner too. |
| LayoutMultiColumnSpannerPlaceholder* containingColumnSpannerPlaceholder(const LayoutObject* descendant) const; |
| |
| // Populate the flow thread with what's currently its siblings. Called when a regular block |
| // becomes a multicol container. |
| void populate(); |
| |
| // Empty the flow thread by moving everything to the parent. Remove all multicol specific |
| // layoutObjects. Then destroy the flow thread. Called when a multicol container becomes a regular |
| // block. |
| void evacuateAndDestroy(); |
| |
| unsigned columnCount() const { return m_columnCount; } |
| LayoutUnit columnHeightAvailable() const { return m_columnHeightAvailable; } |
| void setColumnHeightAvailable(LayoutUnit available) { m_columnHeightAvailable = available; } |
| bool progressionIsInline() const { return m_progressionIsInline; } |
| |
| LayoutSize columnOffset(const LayoutPoint&) const final; |
| |
| // Do we need to set a new width and lay out? |
| virtual bool needsNewWidth() const; |
| |
| bool isPageLogicalHeightKnown() const final; |
| |
| LayoutSize flowThreadTranslationAtOffset(LayoutUnit) const; |
| |
| LayoutPoint visualPointToFlowThreadPoint(const LayoutPoint& visualPoint) const override; |
| |
| LayoutMultiColumnSet* columnSetAtBlockOffset(LayoutUnit) const final; |
| |
| void layoutColumns(bool relayoutChildren, SubtreeLayoutScope&); |
| |
| bool isInInitialLayoutPass() const { return !m_inBalancingPass; } |
| |
| // Skip past a column spanner during flow thread layout. Spanners are not laid out inside the |
| // flow thread, since the flow thread is not in a spanner's containing block chain (since the |
| // containing block is the multicol container). |
| void skipColumnSpanner(LayoutBox*, LayoutUnit logicalTopInFlowThread); |
| |
| bool recalculateColumnHeights(); |
| |
| void columnRuleStyleDidChange(); |
| |
| // Remove the spanner placeholder and return true if the specified object is no longer a valid spanner. |
| bool removeSpannerPlaceholderIfNoLongerValid(LayoutBox* spannerObjectInFlowThread); |
| |
| LayoutMultiColumnFlowThread* enclosingFlowThread() const; |
| LayoutUnit blockOffsetInEnclosingFlowThread() const { ASSERT(enclosingFlowThread()); return m_blockOffsetInEnclosingFlowThread; } |
| |
| // Return true if we have a fragmentainer group that can hold a column at the specified flow thread block offset. |
| bool hasFragmentainerGroupForColumnAt(LayoutUnit offsetInFlowThread) const; |
| |
| // If we've run out of columns in the last fragmentainer group (column row), we have to insert |
| // another fragmentainer group in order to hold more columns. This means that we're moving to |
| // the next outer column (in the enclosing fragmentation context). |
| void appendNewFragmentainerGroupIfNeeded(LayoutUnit offsetInFlowThread); |
| |
| const char* name() const override { return "LayoutMultiColumnFlowThread"; } |
| |
| protected: |
| LayoutMultiColumnFlowThread(); |
| void setProgressionIsInline(bool isInline) { m_progressionIsInline = isInline; } |
| |
| void layout() override; |
| |
| private: |
| void calculateColumnCountAndWidth(LayoutUnit& width, unsigned& count) const; |
| void createAndInsertMultiColumnSet(LayoutBox* insertBefore = nullptr); |
| void createAndInsertSpannerPlaceholder(LayoutBox* spannerObjectInFlowThread, LayoutObject* insertedBeforeInFlowThread); |
| void destroySpannerPlaceholder(LayoutMultiColumnSpannerPlaceholder*); |
| virtual bool descendantIsValidColumnSpanner(LayoutObject* descendant) const; |
| |
| void addColumnSetToThread(LayoutMultiColumnSet*) override; |
| void willBeRemovedFromTree() override; |
| void flowThreadDescendantWasInserted(LayoutObject*) final; |
| void flowThreadDescendantWillBeRemoved(LayoutObject*) final; |
| void flowThreadDescendantStyleWillChange(LayoutBox*, StyleDifference, const ComputedStyle& newStyle) override; |
| void flowThreadDescendantStyleDidChange(LayoutBox*, StyleDifference, const ComputedStyle& oldStyle) override; |
| void computePreferredLogicalWidths() override; |
| void computeLogicalHeight(LayoutUnit logicalHeight, LayoutUnit logicalTop, LogicalExtentComputedValues&) const override; |
| void updateLogicalWidth() override; |
| void setPageBreak(LayoutUnit offset, LayoutUnit spaceShortage) override; |
| void updateMinimumPageHeight(LayoutUnit offset, LayoutUnit minHeight) override; |
| bool addForcedColumnBreak(LayoutUnit, LayoutObject* breakChild, bool isBefore, LayoutUnit* offsetBreakAdjustment = nullptr) override; |
| |
| // The last set we worked on. It's not to be used as the "current set". The concept of a |
| // "current set" is difficult, since layout may jump back and forth in the tree, due to wrong |
| // top location estimates (due to e.g. margin collapsing), and possibly for other reasons. |
| LayoutMultiColumnSet* m_lastSetWorkedOn; |
| |
| unsigned m_columnCount; // The used value of column-count |
| LayoutUnit m_columnHeightAvailable; // Total height available to columns, or 0 if auto. |
| |
| // Cached block offset from this flow thread to the enclosing flow thread, if any. In the |
| // coordinate space of the enclosing flow thread. |
| LayoutUnit m_blockOffsetInEnclosingFlowThread; |
| |
| bool m_inBalancingPass; // Set when relayouting for column balancing. |
| bool m_needsColumnHeightsRecalculation; // Set when we need to recalculate the column set heights after layout. |
| bool m_progressionIsInline; // Always true for regular multicol. False for paged-y overflow. |
| bool m_isBeingEvacuated; |
| }; |
| |
| // Cannot use DEFINE_LAYOUT_OBJECT_TYPE_CASTS here, because isMultiColumnFlowThread() is defined in |
| // LayoutFlowThread, not in LayoutObject. |
| DEFINE_TYPE_CASTS(LayoutMultiColumnFlowThread, LayoutFlowThread, object, object->isLayoutMultiColumnFlowThread(), object.isLayoutMultiColumnFlowThread()); |
| |
| } // namespace blink |
| |
| #endif // LayoutMultiColumnFlowThread_h |