cocos2d学习笔记（七）物理引擎box2d之一

一、准备工作

引入box2d包，在需要使用box2d的文件中加入box2d的头文件；由于box2d是c++编写的，所以要把引入box2d的所有文件后缀名都改为.mm

二、box2d中的一些重要参数

1、gravity，重力加速度，同现实世界中的g，向量

2、shape，形状，形状是有大小的

3、density，密度

4、friction，摩擦力

5、restitution，恢复，此参数用于碰撞，如果两个物体有不同的restitution，box2d总是选择比较大的restitution进行计算

6、meter，距离单位，灵活定义你的meter，当对象为0.1至10meters的时候，box2d可以很好的处理它们，

三、box2d之hello world

让我们先创建一个box2d项目。创建好之后运行：

每当我们点击屏幕时，会落下一个小方块，ok，然我们来详细看下生成的代码。

-(id) init
{
	// always call "super" init
	// Apple recommends to re-assign "self" with the "super" return value
	if( (self=[super init])) {
		
		// enable touches
		self.isTouchEnabled = YES;
		
		// enable accelerometer
		self.isAccelerometerEnabled = YES;
		
		CGSize screenSize = [CCDirector sharedDirector].winSize;
		CCLOG(@"Screen width %0.2f screen height %0.2f",screenSize.width,screenSize.height);
		
		// Define the gravity vector.
		b2Vec2 gravity;
		gravity.Set(0.0f, -10.0f);
		
		// Do we want to let bodies sleep?
		// This will speed up the physics simulation
		bool doSleep = true;
		
		// Construct a world object, which will hold and simulate the rigid bodies.
		world = new b2World(gravity, doSleep);
		
		world->SetContinuousPhysics(true);
		
		// Debug Draw functions
		m_debugDraw = new GLESDebugDraw( PTM_RATIO );
		world->SetDebugDraw(m_debugDraw);
		
		uint32 flags = 0;
		flags += b2DebugDraw::e_shapeBit;
//		flags += b2DebugDraw::e_jointBit;
//		flags += b2DebugDraw::e_aabbBit;
//		flags += b2DebugDraw::e_pairBit;
//		flags += b2DebugDraw::e_centerOfMassBit;
		m_debugDraw->SetFlags(flags);		
		
		
		// Define the ground body.
		b2BodyDef groundBodyDef;
		groundBodyDef.position.Set(0, 0); // bottom-left corner
		
		// Call the body factory which allocates memory for the ground body
		// from a pool and creates the ground box shape (also from a pool).
		// The body is also added to the world.
		b2Body* groundBody = world->CreateBody(&groundBodyDef);
		
		// Define the ground box shape.
		b2PolygonShape groundBox;		
		
		// bottom
		groundBox.SetAsEdge(b2Vec2(0,0), b2Vec2(screenSize.width/PTM_RATIO,0));
		groundBody->CreateFixture(&groundBox,0);
		
		// top
		groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO));
		groundBody->CreateFixture(&groundBox,0);
		
		// left
		groundBox.SetAsEdge(b2Vec2(0,screenSize.height/PTM_RATIO), b2Vec2(0,0));
		groundBody->CreateFixture(&groundBox,0);
		
		// right
		groundBox.SetAsEdge(b2Vec2(screenSize.width/PTM_RATIO,screenSize.height/PTM_RATIO), b2Vec2(screenSize.width/PTM_RATIO,0));
		groundBody->CreateFixture(&groundBox,0);
		
		
		//Set up sprite
		
		CCSpriteBatchNode *batch = [CCSpriteBatchNode batchNodeWithFile:@"blocks.png" capacity:150];
		[self addChild:batch z:0 tag:kTagBatchNode];
		
		[self addNewSpriteWithCoords:ccp(screenSize.width/2, screenSize.height/2)];
		
		CCLabelTTF *label = [CCLabelTTF labelWithString:@"Tap screen" fontName:@"Marker Felt" fontSize:32];
		[self addChild:label z:0];
		[label setColor:ccc3(0,0,255)];
		label.position = ccp( screenSize.width/2, screenSize.height-50);
		
		[self schedule: @selector(tick:)];
	}
	return self;
}

init方法首先创建了重力加速度，加速度是一个向量，-10是因为加速度朝向y轴负方向。之后创建world以及word的四个边缘，防止物体跑出屏幕。再然后是创建精灵（这里用到了之前一篇博客的创建精灵方法，如果有疑问，传送门：http://blog.csdn.net/volcan1987/article/details/7727593）和一个label，然后schedule tick方法

创建body的步骤

创建fixture步骤

知道了这些，让我们来看看如何创建一个box2d世界中的物体

-(void) addNewSpriteWithCoords:(CGPoint)p
{
	CCLOG(@"Add sprite %0.2f x %02.f",p.x,p.y);
	CCSpriteBatchNode *batch = (CCSpriteBatchNode*) [self getChildByTag:kTagBatchNode];
	
	//We have a 64x64 sprite sheet with 4 different 32x32 images.  The following code is
	//just randomly picking one of the images
	int idx = (CCRANDOM_0_1() > .5 ? 0:1);
	int idy = (CCRANDOM_0_1() > .5 ? 0:1);
	CCSprite *sprite = [CCSprite spriteWithBatchNode:batch rect:CGRectMake(32 * idx,32 * idy,32,32)];
	[batch addChild:sprite];
	
	sprite.position = ccp( p.x, p.y);
	
	// Define the dynamic body.
	//Set up a 1m squared box in the physics world
	b2BodyDef bodyDef;
	bodyDef.type = b2_dynamicBody;

	bodyDef.position.Set(p.x/PTM_RATIO, p.y/PTM_RATIO);
	bodyDef.userData = sprite;
	b2Body *body = world->CreateBody(&bodyDef);
	
	// Define another box shape for our dynamic body.
	b2PolygonShape dynamicBox;
	dynamicBox.SetAsBox(.5f, .5f);//These are mid points for our 1m box
	
	// Define the dynamic body fixture.
	b2FixtureDef fixtureDef;
	fixtureDef.shape = &dynamicBox;	
	fixtureDef.density = 1.0f;
	fixtureDef.friction = 0.3f;
	body->CreateFixture(&fixtureDef);
}

可以看到，代码中的步骤跟我们上面的步骤一样，唯一不同的就是多了一个userData，这个属性用来绑定精灵否则你会看到精灵在你点击的地方不动，而一个粉色的方块掉了下去。fixture是相对于body的位置来的

下面这段代码相对固定，可以先不管

-(void) draw
{
	// Default GL states: GL_TEXTURE_2D, GL_VERTEX_ARRAY, GL_COLOR_ARRAY, GL_TEXTURE_COORD_ARRAY
	// Needed states:  GL_VERTEX_ARRAY, 
	// Unneeded states: GL_TEXTURE_2D, GL_COLOR_ARRAY, GL_TEXTURE_COORD_ARRAY
	glDisable(GL_TEXTURE_2D);
	glDisableClientState(GL_COLOR_ARRAY);
	glDisableClientState(GL_TEXTURE_COORD_ARRAY);
	
	world->DrawDebugData();
	
	// restore default GL states
	glEnable(GL_TEXTURE_2D);
	glEnableClientState(GL_COLOR_ARRAY);
	glEnableClientState(GL_TEXTURE_COORD_ARRAY);

}

接下来是tick方法

-(void) tick: (ccTime) dt
{
	//It is recommended that a fixed time step is used with Box2D for stability
	//of the simulation, however, we are using a variable time step here.
	//You need to make an informed choice, the following URL is useful
	//http://gafferongames.com/game-physics/fix-your-timestep/
	
	int32 velocityIterations = 8;
	int32 positionIterations = 1;
	
	// Instruct the world to perform a single step of simulation. It is
	// generally best to keep the time step and iterations fixed.
	world->Step(dt, velocityIterations, positionIterations);

	
	//Iterate over the bodies in the physics world
	for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
	{
		if (b->GetUserData() != NULL) {
			//Synchronize the AtlasSprites position and rotation with the corresponding body
			CCSprite *myActor = (CCSprite*)b->GetUserData();
			myActor.position = CGPointMake( b->GetPosition().x * PTM_RATIO, b->GetPosition().y * PTM_RATIO);
			myActor.rotation = -1 * CC_RADIANS_TO_DEGREES(b->GetAngle());
		}	
	}
}

先说下step方法，velocityIterations和positionIterations这两个参数越大，box2d就能进行更好的模拟，但是性能就会下降，这两个参数你应该自己把握以适合你的游戏。

下面的循环是为了让你的sprite与box2d中的对象同步，你可以注释掉这段代码看下效果，你会发现粉方块掉下去了，你的sprite没掉下去。

ok，今天先这么多，没想到想着挺简单，却写了这么久，有时间继续！