Chapter 3 - How to Move a sprite

http://www.eoeandroid.com/forum.php?mod=viewthread&tid=250529

http://www.cocos2d-x.org/boards/6/topics/10055

Chapter 3 - How to Move a sprite

We have added a hero to the scene in the last chapter Chapter 2 - How to Add a sprite. But the hero is so lonely that we should add some enemies for him to beat down.
The function void addTarget() will complete the work, the enemies will be added into the scene from right to left at random speed.

Declare void addTarget() in HelloWorldScene.h and add the following code to HelloWorldScene.cpp, (and don't forget to add using namespace cocos2d; at the start of HelloWorldScene.cpp)

1// cpp with cocos2d-x
 2void HelloWorld::addTarget()
 3{
 4    CCSprite *target = CCSprite::create("Target.png",
 5        CCRectMake(0,0,27,40) );
 6
 7    // Determine where to spawn the target along the Y axis
 8    CCSize winSize = CCDirector::sharedDirector()->getWinSize();
 9    int minY = target->getContentSize().height/2;
10    int maxY = winSize.height
11                          -  target->getContentSize().height/2;
12    int rangeY = maxY - minY;
13    // srand( TimGetTicks() );
14    int actualY = ( rand() % rangeY ) + minY;
15
16    // Create the target slightly off-screen along the right edge,
17    // and along a random position along the Y axis as calculated
18    target->setPosition(
19        ccp(winSize.width + (target->getContentSize().width/2),
20        actualY) );
21    this->addChild(target);
22
23    // Determine speed of the target
24    int minDuration = (int)2.0;
25    int maxDuration = (int)4.0;
26    int rangeDuration = maxDuration - minDuration;
27    // srand( TimGetTicks() );
28    int actualDuration = ( rand() % rangeDuration )
29                                        + minDuration;
30
31    // Create the actions
32    CCFiniteTimeAction* actionMove =
33        CCMoveTo::create( (float)actualDuration,
34        ccp(0 - target->getContentSize().width/2, actualY) );
35    CCFiniteTimeAction* actionMoveDone =
36        CCCallFuncN::create( this,
37        callfuncN_selector(HelloWorld::spriteMoveFinished));
38    target->runAction( CCSequence::create(actionMove,
39        actionMoveDone, NULL) );
40}

Here, callfuncN_selector(HelloWorld::spriteMoveFinished) backcalls the function spriteMoveFinished(), we need to declare it in the HelloWorldScene.h and define it as follows,

1// cpp with cocos2d-x
2void HelloWorld::spriteMoveFinished(CCNode* sender)
3{
4  CCSprite *sprite = (CCSprite *)sender;
5  this->removeChild(sprite, true);
6}

TIPs:
1. About rand function. srand and rand are c std function. For each platform, you could get the mili-second system time as sand to get a random number. On IPhone, you could get the random number by arc4random() directly

2. The YES and NO in objc are true and false in cpp

3. The callback function is selector:@selector(spriteMoveFinished) in objc, but it is a little complicated to implement in cpp, you could refer to the declarations in cocos2dx\include\selector_protocol.h. There are five different callback types:

• schedule_selector
• callfunc_selector
• callfuncN_selector
• callfuncND_selector
• menu_selector

How to use them is according to the callback function definition. For example, when use the function CCTimer::initWithTarget whose second parameter is a type of SEL_SCHEDULE, we could find its macro-definition schedule_selector(_SELECTOR) in selector_protocol.h, then we declare a callback function void MyClass::MyCallbackFuncName(float), and transform it as the second parameter of CCTimer::initWithTarget.

Then, we should put the enemies into the scene at intervals, add the codes before init() function returns.

1// cpp with cocos2d-x
2// Call game logic about every second
3this->schedule( schedule_selector(HelloWorld::gameLogic), 1.0 );

and implement gameLogic() in HelloWorldScene.cpp. Notice that gameLogic() should be declared as public, otherwise it won't be backcalled.

1// cpp with cocos2d-x
2void HelloWorld::gameLogic(float dt)
3{
4    this->addTarget();
5}

Ok, everything is done, build and run, and enjoy your fruit.
Win32