学步园

成员m_needsPositionedMovementLayout、m_normalChildNeedsLayout、m_posChildNeedsLayout、m_needsLayout等主要用来描述该RenderObject是否确实需要重新布局；
当一个新的RenderObject对象插入到Render树的时候，它会设置其m_needsLayout属性为true，同时会根据该RenderObject对象在祖先RenderObject看来是一个positioned(拥有positiong:absolute或fixed属性)状态的孩子，如是则将相应祖先RenderObject对象的属性m_posChildNeedsLayout设置为true;

如果是一个in-flow(positon:static或relative)状态的孩子，则将相应祖先RenderObject对象的属性m_normalChildNeedsLayout设置为true;

主要方法：
//与是否需要layout相关
bool needsLayout() const { return m_needsLayout || m_normalChildNeedsLayout ||m_posChildNeedsLayout; }
bool selfNeedsLayout() const { return m_needsLayout; }
bool posChildNeedsLayout() const { return m_posChildNeedsLayout; }
bool normalChildNeedsLayout() const { return m_normalChildNeedsLayout; }

//与基本属性相关
bool isFloating() const { return m_floating; }
bool isPositioned() const { return m_positioned; } // absolute or fixed positioning
bool isRelPositioned() const { return m_relPositioned; } // relative positioning
bool isText() const { return m_isText; }
bool isInline() const { return m_inline; } // inline object
bool isCompact() const { return style()->display() == COMPACT; } // compact object
bool isRunIn() const { return style()->display() == RUN_IN; } // run-in object
bool isDragging() const { return m_isDragging; }
bool isReplaced() const { return m_replaced; } // a "replaced" element (see CSS)

//与外部DOM关联相关
RenderView* view() const;
// don't even think about making this method virtual!
Node* element() const { return m_isAnonymous ? 0 : m_node; }
Document* document() const { return m_node->document(); }
void setNode(Node* node) { m_node = node; }
Node* node() const { return m_node; }

// RenderObject tree manipulation
//////////////////////////////////////////
virtual bool canHaveChildren() const;
virtual bool isChildAllowed(RenderObject*, RenderStyle*) const { return true; }
virtual void addChild(RenderObject* newChild, RenderObject* beforeChild = 0);
virtual void removeChild(RenderObject*);
virtual bool createsAnonymousWrapper() const { return false; }

// raw tree manipulation
virtual RenderObject* removeChildNode(RenderObject*, bool fullRemove = true);
virtual void appendChildNode(RenderObject*, bool fullAppend = true);
virtual void insertChildNode(RenderObject* child, RenderObject* before, bool fullInsert = true);
// Designed for speed. Don't waste time doing a bunch of work like layer updating and repainting when we know that our
// change in parentage is not going to affect anything.
virtual void moveChildNode(RenderObject*);

virtual void paint(PaintInfo&, int tx, int ty);

/*
* This function should cause the Element to calculate its
* width and height and the layout of its content
*
* when the Element calls setNeedsLayout(false), layout() is no
* longer called during relayouts, as long as there is no
* style sheet change. When that occurs, m_needsLayout will be
* set to true and the Element receives layout() calls
* again.
*/
virtual void layout() = 0;

其中很多方法如paint()、layout()等是虚拟的，不同的子类可以重载它；

其中方法container() 、containingBlock()、paint()、layout()很值得大家深入研究；

总的说来RenderObject基类定义一些通用属性、方法，以便维护、布局、渲染Render树。