{stnguyen} Game dev and random stuffs

Callback in cocos2d-x

09 Feb 2014

Callback, the ability to declare a piece of code to be run at some time later, is essential in game development. cocos2d provides an easy-to-use mechanism to run callback via CCNode.runAction() through CCCallFunc family. We can extend its mechanism to run callback any time we want, regardless of CCNode or CCCallFunc.

Callback in CCNode.runAction()

cocos2d-x (and cocos2d-iphone) provides a bunch of CCAction subclasses which allow you to do callback in CCNode.runAction(). They are:

  • CCCallFunc: calls a function with no arguments
  • CCCallFuncN: calls a function with the node as the first argument. N stands for node
  • CCCallFuncND: calls a function with the node as the first argument and the 2nd argument is params. ND means: Node and Data. Data is void *, so it could be anything.
  • CCCallFuncO: calls a function with an CCObject * as the first argument. O stands for object.

Some examples with detailed explanation:

class MyBeautifulLayer
{
    void demoCallbackWithNoArgument()
    {
        // Use CCCallFunc when you don't need to pass any parameter along
        myNode->runAction(CCSequence::create(
            CCFadeOut::create(2),
            CCCallFunc::create(
                this,
                callfunc_selector(MyBeautifulLayer::sayHello1)),
        NULL));

        // If you need to access the object being called callback on, use
        // CCCallFuncN, CCCallFuncND or CCCallFuncO.
        // In this case we don't need to pass any parameter along, so let's
        // use CCCallFuncN
        myNode->runAction(CCCallFuncN::create(
            this, 
            callfuncN_selector(MyBeautifulLayer::sayHelloN)));
    }

    void demoCallbackWithArguments()
    {
        // Use CCCallFuncND or CCCallFuncO when you need to
        // pass some parameters

        // Any object pointer (or CCObject* in case of CCCallFuncO)
        // will work.
        // Here we use CCArray to pass an arbitary number of CCObject*
        // as parameters.
        CCArray* params = CCArray::create(myObject1, myObject2, NULL);

        params->retain();
        // IMPORTANT: Remember to call params->release() on
        // callback function, or else you have a memory leak

        // Use CCCallFuncND
        myNode->runAction(CCCallFuncND::create(
            this, 
            callfuncND_selector(MyBeautifulLayer::sayHelloND),
            (void*) params));

        // Or use CCCallFuncO
        myNode->runAction(CCCallFuncO::create(
            this, 
            callfuncO_selector(
                MyBeautifulLayer::sayHelloO),
                params));
    }

    void sayHello(){}
    void sayHelloN(CCNode* sender){}
    void sayHelloND(CCNode* sender, void* data){}
    void sayHelloO(CCNode* sender, CCObject* data){}
}

Callback outside of CCNode.runAction()

There’re times when you need to do callback outside of runAction(), for example: callback on network response, callback when a long calculation finishes…

In these cases, you need to store 2 things:

  • Pointer to the object being called function on, or target object
  • Selector to the function being called
// Pointers we need
cocos2d::CCObject* mTarget;
cocos2d::SEL_CallFunc mFunctionSelector;

And when you need to run callback:

(mTarget->*mFunctionSelector)();

You can change SEL_CallFunc to SEL_CallFuncN, SEL_CallFuncND or SEL_CallFuncO to pass some parameters along.

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