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.

????// 進行CSSStyleSelector的更新處理，因為一旦CSS發生變化，布局的結果也可能 發生相關變化，所以在開始布局之前，需要檢查CSS是否發生變化，如果有則需要作相應調整，進而可能影響Render樹等。

????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;
????}

????//布局的處理可能相互嵌套，這與發起布局處理的時機相關。
????//設置滾動條相關
????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 而且設置了frameFlatteningEnabled，則關閉滾動條
????????????????body->renderer()->setChildNeedsLayout(true);
????????????????vMode = ScrollbarAlwaysOff;
????????????????hMode = ScrollbarAlwaysOff;
????????????} else if (body->hasTagName(bodyTag)){
????????????????// 設置滾動條
????????????????applyOverflowToViewport(o, hMode, vMode);
????????????}
????}

????//root往往為RenderView對象
????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.
????//對當前Render樹布局完后，設置RenderLayer樹的布局信息，其中m_doFullRepaint描述是否需要發起渲染處理。

???beginDeferredRepaints();
???layer->updateLayerPositions(...);
???endDeferredRepaints();

????// 設置快速blit
????setCanBlitOnScroll(!useSlowRepaints());

????//因為在布局的過程中，可能進一步獲得網頁數據，則需要繼續布局處理。
????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();
????}
}

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();//類繼承的好處，直接調用父類的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();
}

void RenderBlock::layoutBlock(bool relayoutChildren)
{
??? ...
??? calcWidth();//先計算寬度
??? calcColumnWidth();
???

??? m_overflow.clear();???

??? if(oldWidth != width()|| oldColumnWidth != desiredColumnWidth())
??????? relayoutChildren = true;
??? clearFloats();

??? int previousHeight = height();
??? setHeight(0);

???? //這就是在布局基本概念中提到的Block-level元素的子節點要么是Block-level元素要么為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.//布局完子節點后確定父節點高度
???? int oldHeight = height();
???? calcHeight();

???? if (previousHeight != height())
???????? relayoutChildren = true;

???? //另外布局屬性為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())) {
??????? // 設置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);
}

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);

//遍歷子節點

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);
}