Events and callbacks

Posted at — Sep 23, 2020

There was one moment in the default program that still remains unexplained. That’s the button that closed the application.

Using CC_CALLBACK_1 from MenuItemImage

As you remember the program looked like this:

And after user pressed the button in the bottom left corner the following method was called:

void SmokeTestScene::menuCloseCallback(Ref *pSender) {
  // Close the cocos2d-x game scene and quit the application
  Director::getInstance()->end();
}

But how does it work? Why the method was called? The whole button gets created like this:

/////////////////////////////
// 2. add a menu item with "X" image, which is clicked to quit the program
//    you may modify it.

// add a "close" icon to exit the progress. it's an autorelease object
auto closeItem = MenuItemImage::create(
  "CloseNormal.png",
  "CloseSelected.png",
  CC_CALLBACK_1(SmokeTestScene::menuCloseCallback, this));

if ((closeItem == nullptr) ||
    (closeItem->getContentSize().width <= 0) ||
    (closeItem->getContentSize().height <= 0)) {
  problemLoading("'CloseNormal.png' and 'CloseSelected.png'");
}
else {
  float x = origin.x + visibleSize.width - closeItem->getContentSize().width / 2;
  float y = origin.y + closeItem->getContentSize().height / 2;
  closeItem->setPosition(Vec2(x, y));
}


// create menu, it's an autorelease object
auto menu = Menu::create(closeItem, NULL);
menu->setPosition(Vec2::ZERO);
this->addChild(menu, 1);

Most of this code is similar to construction of a joined node we created in previous post. However, there is one more thing: the CC_CALLBACK_1 macro is used to connect the callback processing method and the menu item.

The CC_CALLBACK_* macros id used very often across Cocos2d programs. It’s implemented with a bit of C++11 magic like this:

// 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__)

The MenuItemImage::create was declared like:

static MenuItemImage * 	create (const std::string &normalImage, const std::string &selectedImage, const std::string &disabledImage, const ccMenuCallback &callback)

and ccMenuCallback here is a short form for std::function:

typedef std::function<void(Ref*)> ccMenuCallback;

So, the CC_CALBACK_* macro gets two parameters:

a function (or method) that should be called
an object that owns this method.

The number in CC_CALBACK_* means the number of arguments the callback function will receive.

As a result you’ll get a std::function instance created by std::bind. The Cocos2d-x object will call this instance when the actual event will appear.

Cocos2d-x basics

Some articles about Cocos2d-x framework

Events and callbacks

Using CC_CALLBACK_1 from MenuItemImage

Default application