h1. 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 to let him 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,

| <pre><code class="cplusplus">

// cpp with cocos2d-x

void HelloWorld::addTarget()

{

	CCSprite *target = CCSprite::spriteWithFile("Target.png", 

		CCRectMake(0,0,27,40) );

	// Determine where to spawn the target along the Y axis

	CCSize winSize = CCDirector::sharedDirector()->getWinSize();

	int minY = target->getContentSize().height/2;

	int maxY = winSize.height

                          -  target->getContentSize().height/2;

	int rangeY = maxY - minY;

	// srand( TimGetTicks() );

	int actualY = ( rand() % rangeY ) + minY;

	// Create the target slightly off-screen along the right edge,

	// and along a random position along the Y axis as calculated

	target->setPosition( 

		ccp(winSize.width + (target->getContentSize().width/2), 

		actualY) );

	this->addChild(target);

	// Determine speed of the target

	int minDuration = (int)2.0;

	int maxDuration = (int)4.0;

	int rangeDuration = maxDuration - minDuration;

	// srand( TimGetTicks() );

	int actualDuration = ( rand() % rangeDuration )

                                        + minDuration;

	// Create the actions

	CCFiniteTimeAction* actionMove = 

		CCMoveTo::actionWithDuration( (ccTime)actualDuration, 

		ccp(0 - target->getContentSize().width/2, actualY) );

	CCFiniteTimeAction* actionMoveDone = 

		CCCallFuncN::actionWithTarget( this, 

		callfuncN_selector(HelloWorld::spriteMoveFinished));

	target->runAction( CCSequence::actions(actionMove, 

		actionMoveDone, NULL) );

}

</code></pre> | <pre><code class="cplusplus">

// objc with cocos2d-iphone

-(void)addTarget 

{

  CCSprite *target = [CCSprite spriteWithFile:@"Target.png"

                                rect:CGRectMake(0, 0, 27, 40)];

  // Determine where to spawn the target along the Y axis

  CGSize winSize = [[CCDirector sharedDirector] winSize];

  int minY = target.contentSize.height/2;

  int maxY = winSize.height - target.contentSize.height/2;

  int rangeY = maxY - minY;

  int actualY = (arc4random() % rangeY) + minY;

  // Create the target slightly off-screen along the right edge,

  // and along a random position along the Y axis as calculated

  target.position = 

    ccp(winSize.width + (target.contentSize.width/2), 

    actualY);

  [self addChild:target];

  // Determine speed of the target

  int minDuration = 2.0;

  int maxDuration = 4.0;

  int rangeDuration = maxDuration - minDuration;

  int actualDuration = (arc4random() % rangeDuration)

                                  + minDuration;

  // Create the actions

  id actionMove = 

    [CCMoveTo actionWithDuration:actualDuration

          position:ccp(-target.contentSize.width/2, actualY)];

  id actionMoveDone = 

    [CCCallFuncN actionWithTarget:self

                selector:@selector(spriteMoveFinished:)];

  [target runAction:[CCSequence actions:actionMove, 

                    actionMoveDone, nil]]; 

} 

</code></pre> |

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

|<pre><code class="cplusplus">

// cpp with cocos2d-x

void HelloWorld::spriteMoveFinished(CCNode* sender)

{

  CCSprite *sprite = (CCSprite *)sender;

  this->removeChild(sprite, true);

}

</code></pre> |<pre><code class="cplusplus">

// objc with cocos2d-iphone

-(void)spriteMoveFinished:(id)sender 

{

  CCSprite *sprite = (CCSprite *)sender;

  [self removeChild:sprite cleanup:YES]; 

} 

</code></pre> |

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 WoPhone, the function is TimGetTickes(), and 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 realize 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(ccTime), 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.

|<pre><code class="cplusplus">

// cpp with cocos2d-x

// Call game logic about every second

this->schedule( schedule_selector(HelloWorld::gameLogic), 1.0 );

</code></pre> |<pre><code class="cplusplus">

// objc with cocos2d-iphone

// Call game logic about every second

[self schedule:@selector(gameLogic:) interval:1.0];

</code></pre> |

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

|<pre><code class="cplusplus">

// cpp with cocos2d-x

void HelloWorld::gameLogic(ccTime dt)

{

    this->addTarget();

}

</code></pre>|<pre><code class="cplusplus">

// objc with cocos2d-iphone

-(void)gameLogic:(ccTime)dt

{

    [self addTarget];

}

</code></pre>|

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

IPhone !IOSSpriteAndEnemy.png!

Android !androidSpriteAndEnemy.png!

WoPhone !WoPhoneSpriteAndEmemy.png!

Win32   !Win32SpriteAndEmemies.png!