cocos2d学习笔记（八）物理引擎box2d之二

今天我们来看看如何通过拖拽移动一个物体

拖移物体需要用到box2d中的b2MouseJoint

首先我们在touchbegan方法中为我们点击到的物体创建b2MouseJoint对象，那么问题来了，我们如何获取点击到的物体呢？

box2d为我们提供了相关方法，即AABB（axis-aligned bounding box    ），原理为：首先，我们点击位置的四边加上了1-point的边，这样我们点击的位置变成了一个小方块；之后，以某个形状的物体做个长方形，看看我们点击的位置是否在这个长方形里面。如图：

如果在，那么调用callback对象，这个callback对象是我们自己写的，可以通过TestPoint看看点击的位置是否的确在AABB找出的物体上，如果在，那么我们就获取到了点击的物体。上图可以发现其实我们并没有点到那个黑色的物体上，所以TestPoint会返回false。如果同时点到了多个物体，只返回第一个被发现的。

之后是touch began代码，我们可以看到b2MouseJoint如何被创建的：

- (void)ccTouchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{    for( UITouch *touch in touches ) {        CGPoint location = [touch locationInView:touch.view];        location = [[CCDirector sharedDirector] convertToGL:location];        location = [self convertToNodeSpace:location];        b2Vec2 b2Location = b2Vec2(location.x / PTM_RATIO, location.y / PTM_RATIO);                b2AABB aabb;        b2Vec2 delta = b2Vec2(1.0 / PTM_RATIO, 1.0 / PTM_RATIO);        aabb.upperBound = b2Location + delta;        aabb.lowerBound = b2Location - delta;                SimpleQueryCallback callback(b2Location);        world->QueryAABB(&callback, aabb);                if (callback.fixtureFound) {            b2Body *body = callback.fixtureFound->GetBody();            CCSprite *sprite = (CCSprite *)body->GetUserData();            if (sprite == nil) {                return;            }            b2MouseJointDef mouseJointDef;            mouseJointDef.bodyA = groundBody;            mouseJointDef.bodyB = body;            mouseJointDef.target = b2Location;            mouseJointDef.maxForce = 1000 * body->GetMass();            mouseJointDef.collideConnected = true;                        mouseJoint = (b2MouseJoint *) world->CreateJoint(&mouseJointDef);            body->SetAwake(true);        } else {            [self addNewSpriteWithCoords: location];        }    }}

其中bodyA一般会指定固定的body，bodyB指定你想移动的body，target为你要移动到哪里，maxForce越大，拖动速度越快，collideConnected为true时bodyA和bodyB会相撞。

SimpleQueryCallback是我们自己写的一个类：

class SimpleQueryCallback : public b2QueryCallback{public:    b2Vec2 pointToTest;    b2Fixture * fixtureFound;        SimpleQueryCallback(const b2Vec2& point) {        pointToTest = point;        fixtureFound = NULL;    }        bool ReportFixture(b2Fixture* fixture) {        b2Body* body = fixture->GetBody();        if (body->GetType() == b2_dynamicBody) {            if (fixture->TestPoint(pointToTest)) {                fixtureFound = fixture;                return false;            }        }                return true;    }        };

然后是move和touch ended的代码，move中setTarget即会使物体移到新位置，别忘了在touch ended中销毁这个mouseJoint

- (void)ccTouchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{    for( UITouch *touch in touches ) {        CGPoint location = [touch locationInView:touch.view];        location = [[CCDirector sharedDirector] convertToGL:location];        location = [self convertToNodeSpace:location];        b2Vec2 b2Location = b2Vec2(location.x / PTM_RATIO, location.y / PTM_RATIO);                if (mouseJoint) {            mouseJoint->SetTarget(b2Location);        }    }}- (void)ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{//Add a new body/atlas sprite at the touched locationfor( UITouch *touch in touches ) {CGPoint location = [touch locationInView: [touch view]];location = [[CCDirector sharedDirector] convertToGL: location];//[self addNewSpriteWithCoords: location];        if (mouseJoint) {            world->DestroyJoint(mouseJoint);            mouseJoint = NULL;        }}}