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Cocos2d之Node类详解之节点树（二）

2024-08-01 10:19:12   216人阅读

一、声明

本文属于笔者原创，允许读者转载和分享，只要注明文章来源即可。

笔者使用cocos2d框架的cocos2d-x-3.3rc0版本的源代码做分析。这篇文章承接上篇《Cocos2d之Node类详解之节点树（一）》。

二、简介

节点

一个Node对象。

节点树

上篇文章介绍到，Node类有一个成员变量 Vector<Node*> _children，这是一个保存所有子节点的数组，因为Node类采用遍历树的方式获取子节点进行渲染，所以我管这两个东西的结合叫节点树。

三、源码详解

《Cocos2d之Node类详解之节点树（一）》一文中已经介绍了Node对象如何往节点树中添加子节点，现在介绍从节点树中获取节点和删除节点的实现过程。

获取子节点

相关函数的声明：

/**     * Gets a child from the container with its tag     *     * @param tag   An identifier to find the child node.     *     * @return a Node object whose tag equals to the input parameter     *     * Please use `getChildByName()` instead     */     virtual Node * getChildByTag(int tag) const;    /**     * Gets a child from the container with its name     *     * @param name   An identifier to find the child node.     *     * @return a Node object whose name equals to the input parameter     *     * @since v3.2     */    virtual Node* getChildByName(const std::string& name) const;

相关函数实现：

Node* Node::getChildByTag(int tag) const{    CCASSERT( tag != Node::INVALID_TAG, "Invalid tag");    for (auto& child : _children)    {        if(child && child->_tag == tag)            return child;    }    return nullptr;}Node* Node::getChildByName(const std::string& name) const{    CCASSERT(name.length() != 0, "Invalid name");        std::hash<std::string> h;    size_t hash = h(name);        for (const auto& child : _children)    {        // Different strings may have the same hash code, but can use it to compare first for speed        if(child->_hashOfName == hash && child->_name.compare(name) == 0)            return child;    }    return nullptr;}

从源码可以看出，Node类提供分别以name、tag为关键字查询子节点的方式。每种方式的实现都是对 _children 子节点数组进行遍历匹配。值得注意的是，在 getChildByName 函数中，为了提高字符串匹配的效率，先进行哈希匹配再进行字符串内容对比。

 

删除子节点

相关函数声明。

/**     * Removes a child from the container. It will also cleanup all running actions depending on the cleanup parameter.     *     * @param child     The child node which will be removed.     * @param cleanup   true if all running actions and callbacks on the child node will be cleanup, false otherwise.     */    virtual void removeChild(Node* child, bool cleanup = true);    /**     * Removes a child from the container by tag value. It will also cleanup all running actions depending on the cleanup parameter     *     * @param tag       An interger number that identifies a child node     * @param cleanup   true if all running actions and callbacks on the child node will be cleanup, false otherwise.     *     * Please use `removeChildByName` instead.     */     virtual void removeChildByTag(int tag, bool cleanup = true);    /**     * Removes a child from the container by tag value. It will also cleanup all running actions depending on the cleanup parameter     *     * @param name       A string that identifies a child node     * @param cleanup   true if all running actions and callbacks on the child node will be cleanup, false otherwise.     */    virtual void removeChildByName(const std::string &name, bool cleanup = true);    /**     * Removes all children from the container with a cleanup.     *     * @see `removeAllChildrenWithCleanup(bool)`     */    virtual void removeAllChildren();

删除子节点函数实现。

void Node::removeChild(Node* child, bool cleanup /* = true */){    // explicit nil handling    if (_children.empty())    {        return;    }    ssize_t index = _children.getIndex(child);    if( index != CC_INVALID_INDEX )        this->detachChild( child, index, cleanup );}void Node::removeChildByTag(int tag, bool cleanup/* = true */){    CCASSERT( tag != Node::INVALID_TAG, "Invalid tag");    Node *child = this->getChildByTag(tag);    if (child == nullptr)    {        CCLOG("cocos2d: removeChildByTag(tag = %d): child not found!", tag);    }    else    {        this->removeChild(child, cleanup);    }}void Node::removeChildByName(const std::string &name, bool cleanup){    CCASSERT(name.length() != 0, "Invalid name");        Node *child = this->getChildByName(name);        if (child == nullptr)    {        CCLOG("cocos2d: removeChildByName(name = %s): child not found!", name.c_str());    }    else    {        this->removeChild(child, cleanup);    }}void Node::removeAllChildren(){    this->removeAllChildrenWithCleanup(true);}

从源码中可以看出，addChild函数并没有直接将子节点从 _children 数组中删除，而是获取子节点在 _children 数组中的位置，然后调用 detachChild( child, index, cleanup )函数。detachChild函数的声明如下：

/// Removes a child, call child->onExit(), do cleanup, remove it from children array.void detachChild(Node *child, ssize_t index, bool doCleanup);

该函数的实现如下：

void Node::detachChild(Node *child, ssize_t childIndex, bool doCleanup){    // IMPORTANT:    //  -1st do onExit    //  -2nd cleanup    if (_running)    {        child->onExitTransitionDidStart();        child->onExit();    }    #if CC_USE_PHYSICS    child->removeFromPhysicsWorld();#endif    // If you don‘t do cleanup, the child‘s actions will not get removed and the    // its scheduledSelectors_ dict will not get released!    if (doCleanup)    {        child->cleanup();    }    // set parent nil at the end    child->setParent(nullptr);    _children.erase(childIndex);}

detachChild 函数其实就是去释放子节点拥有的资源，这种设置是很合理的。

节点树重排序

相关函数声明：

/**     * Reorders a child according to a new z value.     *     * @param child     An already added child node. It MUST be already added.     * @param localZOrder Z order for drawing priority. Please refer to setLocalZOrder(int)     */    virtual void reorderChild(Node * child, int localZOrder);    /**     * Sorts the children array once before drawing, instead of every time when a child is added or reordered.     * This appraoch can improves the performance massively.     * @note Don‘t call this manually unless a child added needs to be removed in the same frame     */    virtual void sortAllChildren();

函数实现：

void Node::reorderChild(Node *child, int zOrder){    CCASSERT( child != nullptr, "Child must be non-nil");    _reorderChildDirty = true;    child->setOrderOfArrival(s_globalOrderOfArrival++);    child->_localZOrder = zOrder;}void Node::sortAllChildren(){    if( _reorderChildDirty ) {        std::sort( std::begin(_children), std::end(_children), nodeComparisonLess );        _reorderChildDirty = false;    }}

值得注意的是， reorderChild 函数只是改变子节点 _localZOrder 的值，还有将父节点的 _reorderChildDirty 标志位置true。_reorderChildDirty 标志位为true说明子节点的 _localZOrder 发生了改变，因此需要调用 sortAllChildren 函数对所有子节点进行排序。还有一点要注意的是，改变一次子节点的_localZOrder时，s_globalOrderOfArrival属性加1（这个属性的介绍在《Cocos2d之Node类详解之节点树（一）》添加子节点部分由介绍）。

sortAllChildren函数使用了C++标准库提供的 sort函数对所有子节点进行排序。笔者通过下面这个例子详细为读者解释这个函数的用法。

#include <iostream>     // std::cout#include <algorithm>    // std::sort#include <vector>       // std::vectorbool myfunction (int i,int j) { return (i<j); }struct myclass {  bool operator() (int i,int j) { return (i<j);}} myobject;int main () {  int myints[] = {32,71,12,45,26,80,53,33};  std::vector<int> myvector (myints, myints+8);               // 32 71 12 45 26 80 53 33  // using default comparison (operator <):  std::sort (myvector.begin(), myvector.begin()+4);           //(12 32 45 71)26 80 53 33  // using function as comp  std::sort (myvector.begin()+4, myvector.end(), myfunction); // 12 32 45 71(26 33 53 80)  // using object as comp  std::sort (myvector.begin(), myvector.end(), myobject);     //(12 26 32 33 45 53 71 80)  return 0;}

了解了 sort 函数的用法之后，我们看Node类是怎么具体使用sort函数的吧。

void Node::sortAllChildren(){    if( _reorderChildDirty ) {        std::sort( std::begin(_children), std::end(_children), nodeComparisonLess );        _reorderChildDirty = false;    }}bool nodeComparisonLess(Node* n1, Node* n2){    return( n1->getLocalZOrder() < n2->getLocalZOrder() ||           ( n1->getLocalZOrder() == n2->getLocalZOrder() && n1->getOrderOfArrival() < n2->getOrderOfArrival() )           );}

 

四、结束

介绍Node类实现节点树的添加、获取、删除子节点等功能的内容就到此结束咯。

Cocos2d之Node类详解之节点树（二）


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