Cocos2dx 3.0 过渡篇（十七） std::bind与CC_CALLBACK不得不说的故事

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本篇的主题就是揭露CC_CALLBACK 与 std::bind之间不可告人的秘密......

首先看一段代码：

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1. //先是创建3个精灵 
2. boy = Sprite::create("boy.png");//创建boy 
3. boy->setPosition(Point(visibleSize.width/2,visibleSize.height/2)); 
4. this->addChild(boy,1); 
5.  
6. girl_1 = Sprite::create("girl_1.png");//创建girl1 
7. girl_1->setPosition(Point(visibleSize.width/3,visibleSize.height/2)); 
8. girl_1->setTag(10); 
9. this->addChild(girl_1,1); 
10.  
11. girl_2 = Sprite::create("girl_3.png");//创建girl2 
12. girl_2->setPosition(Point(2*visibleSize.width/3,visibleSize.height/2)); 
13. girl_2->setTag(20); 
14. this->addChild(girl_2,1); 
15.  
16. //让boy运动，通过Callfunc回调到callback1 
17. boy->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,100)), 
18.                                     CallFunc::create(CC_CALLBACK_0(HelloWorld::callback1,this)), 
19.                                     NULL)); 

//先是创建3个精灵boy = Sprite::create("boy.png");//创建boyboy->setPosition(Point(visibleSize.width/2,visibleSize.height/2));this->addChild(boy,1);girl_1 = Sprite::create("girl_1.png");//创建girl1girl_1->setPosition(Point(visibleSize.width/3,visibleSize.height/2));girl_1->setTag(10);this->addChild(girl_1,1);girl_2 = Sprite::create("girl_3.png");//创建girl2girl_2->setPosition(Point(2*visibleSize.width/3,visibleSize.height/2));girl_2->setTag(20);this->addChild(girl_2,1);//让boy运动，通过Callfunc回调到callback1boy->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,100)),CallFunc::create(CC_CALLBACK_0(HelloWorld::callback1,this)),NULL));

三个回调函数的实现：

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1. void HelloWorld::callback1() 
2. { 
3.     CCLOG("in callback1"); 
4.     //girl1运动，最后回调到callback2 
5.     girl_1->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,150)), 
6.                                             CallFunc::create(CC_CALLBACK_0(HelloWorld::callback2,this,girl_1)), 
7.                                             NULL)); 
8. } 
9. void HelloWorld::callback2(Node* sender) 
10. { 
11.     //girl2运动，最后回调到callback3 
12.     girl_2->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,200)), 
13.         CallFunc::create(CC_CALLBACK_0(HelloWorld::callback3,this,girl_2,99)), 
14.         NULL)); 
15.  
16.     CCLOG("in callback2,sender tag is:%d",(Sprite*)sender->getTag()); 
17. } 
18. void HelloWorld::callback3(Node* sender,long data) 
19. { 
20.     //最终输出 
21.     CCLOG("in callback3,everything is OK,sender tag is:%d,date is:%ld",(Sprite*)sender->getTag(),data); 
22.     CCLOG("girl2 dandan ask:what fake the CC_CALLBACK is?"); 
23. } 

void HelloWorld::callback1(){CCLOG("in callback1");//girl1运动，最后回调到callback2girl_1->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,150)),CallFunc::create(CC_CALLBACK_0(HelloWorld::callback2,this,girl_1)),NULL));}void HelloWorld::callback2(Node* sender){//girl2运动，最后回调到callback3girl_2->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,200)),CallFunc::create(CC_CALLBACK_0(HelloWorld::callback3,this,girl_2,99)),NULL));CCLOG("in callback2,sender tag is:%d",(Sprite*)sender->getTag());}void HelloWorld::callback3(Node* sender, long data){//最终输出CCLOG("in callback3,everything is OK,sender tag is:%d,date is:%ld",(Sprite*)sender->getTag(),data);CCLOG("girl2 dandan ask:what fake the CC_CALLBACK is?");}

整个过程就是boy“勾引”girl1，但girl1显然对异性兴趣不大，于是她也勾引girl2......可是，girl2对同性异性都没兴趣，她只是淡淡的说了句：CC_CALLBACK到底是什么？，调试如图：

好吧，先让我回口血，然后再来回答girl2的问题：CC_CALLBACK到底是什么碗糕（东东）？
我们先进CC_CALLBACK源码里看看：

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1. // new callbacks based on C++11 
2. #define CC_CALLBACK_0(__selector__,__target__, ...) std::bind(&__selector__,__target__, ##__VA_ARGS__) 
3. #define CC_CALLBACK_1(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, ##__VA_ARGS__) 
4. #define CC_CALLBACK_2(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, std::placeholders::_2, ##__VA_ARGS__) 
5. #define CC_CALLBACK_3(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3 ##__VA_ARGS__) 

// new callbacks based on C++11#define CC_CALLBACK_0(__selector__,__target__, ...) std::bind(&__selector__,__target__, ##__VA_ARGS__)#define CC_CALLBACK_1(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, ##__VA_ARGS__)#define CC_CALLBACK_2(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, std::placeholders::_2, ##__VA_ARGS__)#define CC_CALLBACK_3(__selector__,__target__, ...) std::bind(&__selector__,__target__, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3 ##__VA_ARGS__)

看完后恍然大悟！不看不知道，一看...和没看一样...
这里主要注意两点：一是 std::bind，二是##_VA_ARGS_; ##_VA_ARGS_是可变参数宏，我就不多说了。重点讲的是std::bind。
std::bind是在C++ 11里新加入的成员。可以将bind函数看作一个通用的函数适配器，它接受一个可调用对象，生成一个新的可调用对象来“适应”原对象的参数列表.
调用bind的一般形式为：
auto newCallback = bind(callback,arg_list);
其中，newCallback是一个可调用对象，arg_list是可以用逗号分隔的参数列表，至于是啥参数，那就看callback函数里有啥参数啦。也就是说，当我们调用newCallback时，newCallback会调用函数callback,并传递参数arg_list给callback.
看完上面的内容你的理解可能还比较模糊，那直接来个例子：有一个函数callback，如下，

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1. int callback(int one,char two,double three); 

int callback(int one,char two,double three);

下面我们用bind来调用callback

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1. auto newCallback = bind(callback,_1,_2,1.5); 
2. int x = newCallback(10,'h'); //这句相当于:int x = callback(10,'h',1.5); 

auto newCallback = bind(callback,_1,_2,1.5);int x = newCallback(10,'h');  //这句相当于:int x = callback(10,'h',1.5);

“_1″是一个占位符对象，用于表示当函数callback通过函数newCallback进行调用时，函数newCallback的第一个参数在函数callback的参数列表中的位置。第一个参数称为”_1″, 第二个参数为”_2″，依此类推，有意思吧。至于‘1.5’是指默认参数,它处于_1和_2的后面，所以它就是double类型的参数了.
在强调一点就是：_1这类占位符都定义在一个名为placeholders的命名空间中，而这个命名空间本身定义在std的命名空间中。为了使用这些名字，两个命名空间都要写上，如：

std::placeholders::_1;

这样编写贼麻烦，所以在要使用_1时，可以加上这么一句：
using namespace namespace_name; 恩，ok

恩，bind就介绍到这，讲的比较浅，不理解的可以百度研究下。最后再回过头来看下CC_CALLBACK的定义，是不是清晰多了？
最后在举个例子吧，还是之前的boy,girl1,girl2，只是他们之间传递“爱意”的方式要换下了。不用CC_CALLBACK,改用std::bind。代码如下 ：

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1. //让boy运动，通过Callfunc回调到callback1 
2. boy->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,100)), 
3.                                     CallFunc::create(std::bind(&HelloWorld::callback1,this)), 
4.                                     NULL)); 

//让boy运动，通过Callfunc回调到callback1boy->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,100)),CallFunc::create(std::bind(&HelloWorld::callback1,this)),NULL));

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1. void HelloWorld::callback1() 
2. { 
3.     CCLOG("in callback1"); 
4.     //girl1运动，最后回调到callback2 
5.     girl_1->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,150)), 
6.                                             CallFunc::create(std::bind(&HelloWorld::callback2,this,girl_1)), 
7.                                             NULL)); 
8.     CCLOG("boy ask girl_1:can you do my girlFriends?"); 
9. } 
10. void HelloWorld::callback2(Node* sender) 
11. { 
12.     //girl2运动，最后回调到callback3 
13.     girl_2->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,200)), 
14.         CallFunc::create(std::bind(&HelloWorld::callback3,this,girl_1,99)), 
15.         NULL)); 
16.  
17.     CCLOG("in callback2,sender tag is:%d",(Sprite*)sender->getTag()); 
18.     CCLOG("girl_1 ask girl_2:I love girl_2"); 
19. } 
20. void HelloWorld::callback3(Node* sender,long data) 
21. { 
22.     //最终输出 
23.     CCLOG("in callback3,everything is OK,sender tag is:%d,date is:%ld",(Sprite*)sender->getTag(),data); 
24.     CCLOG("girl2 dandan say:I know how to use CC_CALLBACK!"); 
25. }    

void HelloWorld::callback1(){CCLOG("in callback1");//girl1运动，最后回调到callback2girl_1->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,150)),CallFunc::create(std::bind(&HelloWorld::callback2,this,girl_1)),NULL));CCLOG("boy ask girl_1:can you do my girlFriends?");}void HelloWorld::callback2(Node* sender){//girl2运动，最后回调到callback3girl_2->runAction(CCSequence::create(MoveBy::create(1.0f,Point(0,200)),CallFunc::create(std::bind(&HelloWorld::callback3,this,girl_1,99)),NULL));CCLOG("in callback2,sender tag is:%d",(Sprite*)sender->getTag());CCLOG("girl_1 ask girl_2:I love girl_2");}void HelloWorld::callback3(Node* sender, long data){//最终输出CCLOG("in callback3,everything is OK,sender tag is:%d,date is:%ld",(Sprite*)sender->getTag(),data);CCLOG("girl2 dandan say:I know how to use CC_CALLBACK!");}

恩，就是这样子了。bind与CC_CALLBACK之间的关系就是这么的...简单。

尊重原创，转载请注定来自star特530：http://blog.csdn.net/start530/article/details/21245565