WebKit Layout (布局)

 WebKit 在渲染页面之前，需要确定各个元素的位置、大小，而这个过程就是layout(布局)。下面，我们对layout的主要过程进行一番说明。

一、FrameView::layout方法
FrameView作为与View相关的类，其主要涉及与显示相关的内容，而其中对页面元素的布局至关重要，这也是浏览器的核心处理部分。

我们都知道浏览器从Web服务器获得数据后，经解析会构建DOM树、Render树，然后进行布局处理，进而为渲染页面作好准备，其中的布局处理往往由FrameView::layout方法发起，让我们来具体看看其实现，一窥其主要实现过程。

void FrameView::layout(bool allowSubtree) {     if (m_midLayout)       return;     // Always ensure ourstyleinfo is up-to-date. This can happen in situations where the layout beats any sort ofstyle recalc update that needs to occur.     // 进行CSSStyleSelector的更新处理，因为一旦CSS发生变化，布局的结果也可能 发生相关变化，所以在开始布局之前，需要检查CSS是否发生变化，如果有则需要作相应调整，进而可能影响Render树等。     if (m_frame->needsReapplyStyles())        m_frame->reapplyStyles();     else if (document->childNeedsStyleRecalc())        document->recalcStyle();     bool subtree = m_layoutRoot;     RenderObject* root = subtree ? m_layoutRoot : document->renderer();     if (!root){         // FIXME: Do we need to set m_size here?         m_layoutSchedulingEnabled = true;         return;     }     //布局的处理可能相互嵌套，这与发起布局处理的时机相关。     //设置滚动条相关     if (m_canHaveScrollbars){         hMode = ScrollbarAuto;         vMode = ScrollbarAuto;     } else {         hMode = ScrollbarAlwaysOff;         vMode = ScrollbarAlwaysOff;     }     if (!subtree){         // Now set our scrollbar statefor the layout.         if (body->hasTagName(framesetTag)&& !m_frame->settings()->frameFlatteningEnabled()){             //frameset 而且设置了frameFlatteningEnabled，则关闭滚动条                 body->renderer()->setChildNeedsLayout(true);                 vMode = ScrollbarAlwaysOff;                 hMode = ScrollbarAlwaysOff;             } else if (body->hasTagName(bodyTag)){                 // 设置滚动条                 applyOverflowToViewport(o, hMode, vMode);             }     }     //root往往为RenderView对象     RenderLayer* layer = root->enclosingLayer();     m_midLayout = true;     beginDeferredRepaints();     root->layout();     endDeferredRepaints();     m_midLayout = false;     m_layoutSchedulingEnabled = true;     if (!subtree && !static_cast(root)->printing())         adjustViewSize();     // Now update the positions of all layers.     //对当前Render树布局完后，设置RenderLayer树的布局信息，其中m_doFullRepaint描述是否需要发起渲染处理。    beginDeferredRepaints();    layer->updateLayerPositions(...);    endDeferredRepaints();     // 设置快速blit     setCanBlitOnScroll(!useSlowRepaints());     //因为在布局的过程中，可能进一步获得网页数据，则需要继续布局处理。     if (!m_postLayoutTasksTimer.isActive()){         // Calls resumeScheduledEvents()         performPostLayoutTasks();         if (!m_postLayoutTasksTimer.isActive()&& needsLayout()){             // Post-layout widget updates or an event handler made us need layout again. Lay out again, but this timedefer widget updates and event dispatch until after we return.             m_postLayoutTasksTimer.startOneShot(0);             pauseScheduledEvents();             layout();         }     } else {         resumeScheduledEvents();     } }

 

FrameView::layout方法，简单的说来就是发起对Render树中的每一个节点按照从父节点到子节点的方式进行x、y、width、height计算，当每一个树节点的位置及大小确定之后就可以进行后面的渲染。

FrameView::layout往往会调用Render树根的layout方法即RenderView::layout。

二、RenderView::layout方法

void RenderView::layout() {      if (printing())          m_minPrefWidth = m_maxPrefWidth = m_width;      // Use calcWidth/Height to get the newwidth/height, since this will take the fullpage zoom factor into account.      bool relayoutChildren = !printing()&&(!m_frameView|| m_width != viewWidth()|| m_height != viewHeight());      if (relayoutChildren) {         setChildNeedsLayout(true, false);         for (RenderObject* child = firstChild(); child; child = child->nextSibling()) {             if (child->style()->height().isPercent() || child->style()->minHeight().isPercent() || child->style()->maxHeight().isPercent())                 child->setChildNeedsLayout(true, false);         }      }      ASSERT(!m_layoutState);      LayoutState state;      // FIXME: May be better to pusha clip and avoid issuing offscreen repaints.      state.m_clipped = false;      m_layoutState = &state;      if (needsLayout())          RenderBlock::layout();//类继承的好处，直接调用父类的layout     // Reset overflow and then replace it with docWidth and docHeight.     m_overflow.clear();     addLayoutOverflow(IntRect(0, 0, docWidth(), docHeight()));      ASSERT(m_layoutStateDisableCount == 0);      ASSERT(m_layoutState == &state);      m_layoutState = 0;      setNeedsLayout(false); } void RenderBlock::layout() {      // Update our first letterinfo now.      updateFirstLetter();      // Table cells call layoutBlock directly, so don't add any logic here. Put code into layoutBlock().      layoutBlock(false);      // It's safe to check for control clip here, since controls can never betable cells. If we have a lightweight clip, there can never be any overflow from children.     if (hasControlClip() && m_overflow)         clearLayoutOverflow(); }

三、RenderBlock::layoutBlock方法

layoutBlock 会分成两个分支：layoutInlineChildren 和layoutBlockChildren



void RenderBlock::layoutBlock(bool relayoutChildren) {     ...     calcWidth();//先计算宽度     calcColumnWidth();         m_overflow.clear();        if(oldWidth != width()|| oldColumnWidth != desiredColumnWidth())         relayoutChildren = true;     clearFloats();     int previousHeight = height();     setHeight(0);      //这就是在布局基本概念中提到的Block-level元素的子节点要么是Block-level元素要么为Inline-level元素。      if (childrenInline())          layoutInlineChildren(relayoutChildren, repaintTop, repaintBottom);      else          layoutBlockChildren(relayoutChildren, maxFloatBottom);      // Expand our intrinsicheight to encompass floats.      int toAdd = borderBottom()+ paddingBottom()+ horizontalScrollbarHeight();      if (floatBottom()>(m_height - toAdd)&&(isInlineBlockOrInlineTable()|| isFloatingOrPositioned()|| hasOverflowClip()|| (parent()&& parent()->isFlexibleBox()|| m_hasColumns)))      setHeight(floatBottom()+ toAdd);      // Now lay out our columns within this intrinsicheight, since they can slightly affect the intrinsicheight as we adjust for clean column breaks.      int singleColumnBottom = layoutColumns();      // Calculate our newheight.//布局完子节点后确定父节点高度      int oldHeight = height();      calcHeight();      if (previousHeight != height())          relayoutChildren = true;      //另外布局属性为Fixed和absolute的元素      layoutPositionedObjects(relayoutChildren || isRoot());      // Update our scroll information if we're overflow:auto/scroll/hidden now that we know if we overflow or not.     updateScrollInfoAfterLayout();     // Repaint with our new bounds if they are different from our old bounds.     bool didFullRepaint = repainter.repaintAfterLayout();     if (!didFullRepaint && repaintTop != repaintBottom && (style()->visibility() == VISIBLE || enclosingLayer()->hasVisibleContent())) {         // 设置repaintRect        // Make sure the rect is still non-empty after intersecting for overflow above         if (!repaintRect.isEmpty()) {             repaintRectangle(repaintRect); // We need to do a partial repaint of our content.             if (hasReflection())                 repaintRectangle(reflectedRect(repaintRect));         }     }     setNeedsLayout(false); }

四、RenderBlock::layoutBlockChildren方法

void RenderBlock::layoutBlockChildren(bool relayoutChildren, int& maxFloatBottom) { int top = borderTop()+ paddingTop(); int bottom = borderBottom()+ paddingBottom()+ horizontalScrollbarHeight();     // Fieldsets need to find their legend and position it inside the border of the object.     // The legend then gets skipped during normal layout.     RenderObject* legend = layoutLegend(relayoutChildren); //遍历子节点 RenderBox* next = firstChildBox();     while (next) {         RenderBox* child = next;         next = child->nextSiblingBox();         if (legend == child)             continue; // Skip the legend, since it has already been positioned up in the fieldset's border.        // Make sure we layout children if they need it.         // FIXME: Technically percentage height objects only need a relayout if their percentage isn't going to be turned into         // an auto value.  Add a method to determine this, so that we can avoid the relayout.         if (relayoutChildren || ((child->style()->height().isPercent() || child->style()->minHeight().isPercent() || child->style()->maxHeight().isPercent()) && !isRenderView()))             child->setChildNeedsLayout(true, false);         // If relayoutChildren is set and we have percentage padding, we also need to invalidate the child's pref widths.         if (relayoutChildren && (child->style()->paddingLeft().isPercent() || child->style()->paddingRight().isPercent()))             child->setPrefWidthsDirty(true, false);         // Handle the four types of special elements first.  These include positioned content, floating content, compacts and         // run-ins.  When we encounter these four types of objects, we don't actually lay them out as normal flow blocks.         if (handleSpecialChild(child, marginInfo))             continue;         // Lay out the child.         layoutBlockChild(child, marginInfo, previousFloatBottom, maxFloatBottom);     }     // Now do the handling of the bottom of the block, adding in our bottom border/padding and     // determining the correct collapsed bottom margin information.     handleBottomOfBlock(top, bottom, marginInfo); }

layoutBlockChild 方法就比较复杂了，具体可以自己看代码。

五、RenderBlock::layoutInlineChildren方法
这个方法相当复杂，其作用就是布局文字、图像等，对文字行高确定、断行等处理，同时还包括 文字从左到右或从右到左的布局处理。具体可以参考bidi.cpp中的源码实现。

六、调用FrameView::layout方法的时机
由于从Web服务器获取的网页数据不可能一次性完成，往往需要边获取数据，边布局，然后渲染，这样才可能获得良好的用户感受。

所以一旦获得主要数据如css数据及body等标签后，就可以开始布局，布局完后会根据当前条件决定是否将布局的数据渲染出来，或者继续布局处理后来获取的数据，这样也增加了布局处理过程的复杂度。

而调用layout方法的时机也至关重要，因为layout本身就可能需要花费大量的时间如layoutBlockChildren、layoutInlineChildren等处理，其往往与网页的内容有关，而网页的内容却由网页开发者来确定，对浏览器来讲是千变万化的，这就对layout方法的实现及调用时机提出更高的要求，同时确定了其复杂性。

调用layout的时机主要有获得一定DOM文档数据后调用Document::updateLayout()、需要重新使用CSS数据时调用Document::recalcStyle()、改变窗口大小后调用Frame::forceLayout()等来实现。。。

七、总结
其实WebKit涉及网页布局方面的layout方法蛮复杂的，如其他RenderObject子类也会根据自身情况重载实现layout，还有对float、fixed、absolute、inline元素等的处理，但其主要逻辑就象上面所提，这里只是汇总一下主要流程及概念，针对每一个具体标签或RenderObject的布局实现则需要更深一步的了解，希望大家能对了解WebKit的网页布局过程有一个清晰而准确的认识。。