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 our style info is up-to-date. This can happen in situations where the layout beats any sort of style 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 state for 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 time defer 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 new width/height, since this will take the full page 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 push a 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 letter info 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 be table 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 intrinsic height 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 intrinsic height, since they can slightly affect the intrinsic height as we adjust for clean column breaks.
int singleColumnBottom = layoutColumns();
// Calculate our new height.//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。。
posted on 2012-04-23 17:35
Bluesea 阅读(2184)
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