how_to_detect_the_Collisions.html

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<h1>How to Detect the Collisions</h1>
<hr />
<p>Our hero could fire bullets now, but the bullets are only visually there, how could they kill the enemies?</p>
<p>In this chapter, we will introduce Collision Detection to implement it.</p>
<p>Firstly, it's necessary to track the enemies and the bullets.</p>
<p>In the game, we add a tag for these two kinds of sprites to identify them. tag = 1 stands for a enemy, and tag = 2 means a bullet. Because there is already a member named m_nTag of CCNode, and the methods setTag() and getTag() also exist, CCSprite has inherited them, which we could utilize.</p>
<p>Add the two member variabls below to HelloWorld in HelloWorldScene.h, these two member variables are used to store the existing enemies and bullets.</p>
<pre><code>     // cpp with cocos2d-x
     protected:
       cocos2d::CCMutableArray&lt;cocos2d::CCSprite*&gt; *_targets;
       cocos2d::CCMutableArray&lt;cocos2d::CCSprite*&gt; *_projectiles;
</code></pre>
<p>Then initialize the two variables in the construct function, new them in init(), and release them in the destruct function.</p>
<pre><code>     // cpp with cocos2d-x

     // in init()
     // Initialize arrays
     _targets = new CCMutableArray&lt;CCSprite*&gt;;
     _projectiles = new CCMutableArray&lt;CCSprite*&gt;;

     HelloWorld::~HelloWorld()
     {
       if (_targets)
       {
         _targets-&gt;release();
         _targets = NULL;
       }

       if (_projectiles)
       {
         _projectiles-&gt;release();
         _projectiles = NULL;
       }

       // cpp don't need to call super dealloc
       // virtual destructor will do this
     }

     HelloWorld::HelloWorld()
     :_targets(NULL)
     ,_projectiles(NULL)
     {
     }
</code></pre>
<p>Now modify addTarget() to add a new target to targets array, and set its tag to be 1.</p>
<pre><code>     // cpp with cocos2d-x
     // Add to targets array
     target-&gt;setTag(1);
     _targets-&gt;addObject(target);
</code></pre>
<p>Modify ccTouchesEnded() to add a new bullet to bullets array, and set its tag to be 2.</p>
<pre><code>     // cpp with cocos2d-x
     // Add to projectiles array
     projectile-&gt;setTag(2);
     _projectiles-&gt;addObject(projectile);
</code></pre>
<p>Then, modify spriteMoveFinished() as follows. Here we remove the sprites from their corresponding arrays. <br />
</p>
<pre><code>     // cpp with cocos2d-x
     void HelloWorld::spriteMoveFinished(CCNode* sender)
     {
       CCSprite *sprite = (CCSprite *)sender;
       this-&gt;removeChild(sprite, true);

       if (sprite-&gt;getTag() == 1)  // target
       {
         _targets-&gt;removeObject(sprite);
       }
       else if (sprite-&gt;getTag() == 2) // projectile
       {
         _projectiles-&gt;removeObject(sprite);
       }
     }
</code></pre>
<p>The function update() below is used to detect collisions every frame, remove the collided bullet and enemy from the scene. 
Declare it in HelloWorldScene.h and Define it in HelloWorldScene.cpp.</p>
<pre><code>     // cpp with cocos2d-x
     void HelloWorld::update(ccTime dt)
     {
       CCMutableArray&lt;CCSprite*&gt; *projectilesToDelete =        
                                      new CCMutableArray&lt;CCSprite*&gt;;
       CCMutableArray&lt;CCSprite*&gt;::CCMutableArrayIterator it, jt;

       for (it = _projectiles-&gt;begin(); it != _projectiles-&gt;end(); it++)
       {
         CCSprite *projectile =*it;
         CCRect projectileRect = CCRectMake(
             projectile-&gt;getPosition().x 
                               - (projectile-&gt;getContentSize().width/2),
             projectile-&gt;getPosition().y 
                               - (projectile-&gt;getContentSize().height/2),
             projectile-&gt;getContentSize().width,
             projectile-&gt;getContentSize().height);

         CCMutableArray&lt;CCSprite*&gt;*targetsToDelete 
                               = new CCMutableArray&lt;CCSprite*&gt;;

         for (jt = _targets-&gt;begin(); jt != _targets-&gt;end(); jt++)
         {
           CCSprite *target =*jt;
           CCRect targetRect = CCRectMake(
             target-&gt;getPosition().x - (target-&gt;getContentSize().width/2),
             target-&gt;getPosition().y - (target-&gt;getContentSize().height/2),
             target-&gt;getContentSize().width,
             target-&gt;getContentSize().height);

           if (CCRect::CCRectIntersectsRect(projectileRect, targetRect))
           {
             targetsToDelete-&gt;addObject(target);
           }
         }

         for (jt = targetsToDelete-&gt;begin(); 
               jt != targetsToDelete-&gt;end(); 
               jt++)
         {
           CCSprite *target =*jt;
           _targets-&gt;removeObject(target);
           this-&gt;removeChild(target, true);
         }

         if (targetsToDelete-&gt;count() &gt;0)
         {
           projectilesToDelete-&gt;addObject(projectile);
         }
         targetsToDelete-&gt;release();
       }

       for (it = projectilesToDelete-&gt;begin(); 
              it != projectilesToDelete-&gt;end(); 
              it++)
       {
         CCSprite* projectile =*it;
         _projectiles-&gt;removeObject(projectile);
         this-&gt;removeChild(projectile, true);
       }
       projectilesToDelete-&gt;release();
     }
</code></pre>
<p>Ok, the last thing we should do is adding update() to the schedule to let it be called every frame.</p>
<pre><code>     // cpp with cocos2d-x
     this-&gt;schedule( schedule_selector(HelloWorld::update) );
</code></pre>
<p>Compile and run the project, fire the bullets as you like, then: AH..AH.., the enemies are killed one by one.</p>