Cocos2d iphone Box2d径向重力极限coco2d

我一直在使用下面的代码创建径向重力

现在它可以正常工作了，但是我想要它，例如，只有当我的当前精灵在精灵的特定距离内时，才会触发此代码

我需要一些向导，我没有给这个世界重力，所以现在我的精灵在漂浮，但我想做如下的事情

一个精灵在X的项目路径上移动，因为我已经关闭了重力，所以没有重力应用于此。如果我们称之为D的物体在这条路径上的某个极限内，那么径向重力代码就会生效，换句话说，它会沿着它的投影路径继续

有人知道如何修改这段代码来做到这一点吗？我已经在这方面做了很多年了，我似乎无法接近创建一个范围并将这段代码应用到其中

      // Import the interfaces  
    #import "HelloWorldLayer.h"

    // Needed to obtain the Navigation Controller
    #import "AppDelegate.h"

    #import "PhysicsSprite.h"

    enum {
kTagParentNode = 1,
    };

    CGPoint location;
    CGPoint nodePosition;

    #pragma mark - HelloWorldLayer

    @interface HelloWorldLayer()
    -(void) initPhysics;
    -(void) addNewSpriteAtPosition:(CGPoint)p;
    -(void) createMenu;
    @end

    @implementation HelloWorldLayer

    +(CCScene *) scene
    {
// 'scene' is an autorelease object.
CCScene *scene = [CCScene node];

// 'layer' is an autorelease object.
HelloWorldLayer *layer = [HelloWorldLayer node];

// add layer as a child to scene
[scene addChild: layer];

// return the scene
return scene;
    }

    -(id) init
    {
if( (self=[super init])) {

    // enable events

    self.isTouchEnabled = YES;
    self.isAccelerometerEnabled = YES;
    CGSize s = [CCDirector sharedDirector].winSize;

    // init physics
    [self initPhysics];

    // create reset button
    [self createMenu];

    //Set up sprite

    #if 1
    // Use batch node. Faster
    CCSpriteBatchNode *parent = [CCSpriteBatchNode batchNodeWithFile:@"blocks.png"                      capacity:100];
    spriteTexture_ = [parent texture];
    #else
    // doesn't use batch node. Slower
    spriteTexture_ = [[CCTextureCache sharedTextureCache] addImage:@"blocks.png"];
    CCNode *parent = [CCNode node];
    #endif
    [self addChild:parent z:0 tag:kTagParentNode];


    [self addNewSpriteAtPosition:ccp(s.width/2 - 150, s.height/2)];


    [self scheduleUpdate];
}
return self;
            }

    -(void) dealloc
    {
delete world;
world = NULL;

delete m_debugDraw;
m_debugDraw = NULL;

[super dealloc];
    }   

    -(void) createMenu
    {
// Default font size will be 22 points.
[CCMenuItemFont setFontSize:22];

// Reset Button
CCMenuItemLabel *reset = [CCMenuItemFont itemWithString:@"Reset" block:^(id sender){
    [[CCDirector sharedDirector] replaceScene: [HelloWorldLayer scene]];
}];



CCMenu *menu = [CCMenu menuWithItems:reset, nil];

[menu alignItemsVertically];

CGSize size = [[CCDirector sharedDirector] winSize];
[menu setPosition:ccp( size.width/2, size.height/2)];


[self addChild: menu z:-1]; 
    }

    -(void) initPhysics
    {

CGSize s = [[CCDirector sharedDirector] winSize];

b2Vec2 gravity;
gravity.Set(0.0f, 0.0f);
world = new b2World(gravity);


// Do we want to let bodies sleep?
world->SetAllowSleeping(true);

world->SetContinuousPhysics(true);

m_debugDraw = new GLESDebugDraw( PTM_RATIO );
world->SetDebugDraw(m_debugDraw);

uint32 flags = 0;
flags += b2Draw::e_shapeBit;
//      flags += b2Draw::e_jointBit;
//      flags += b2Draw::e_aabbBit;
//      flags += b2Draw::e_pairBit;
//      flags += b2Draw::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.
b2EdgeShape groundBox;      

// bottom

groundBox.Set(b2Vec2(0,0), b2Vec2(s.width/PTM_RATIO,0));
groundBody->CreateFixture(&groundBox,0);

// top
groundBox.Set(b2Vec2(0,s.height/PTM_RATIO),            b2Vec2(s.width/PTM_RATIO,s.height/PTM_RATIO));
groundBody->CreateFixture(&groundBox,0);

// left
groundBox.Set(b2Vec2(0,s.height/PTM_RATIO), b2Vec2(0,0));
groundBody->CreateFixture(&groundBox,0);

// right
groundBox.Set(b2Vec2(s.width/PTM_RATIO,s.height/PTM_RATIO), b2Vec2(s.width/PTM_RATIO,0));
groundBody->CreateFixture(&groundBox,0);

b2CircleShape shape;
shape.m_radius = 0.5f;
shape.m_p.Set(8.0f, 5.0f);
b2FixtureDef fd;
fd.shape = &shape;
planet = groundBody->CreateFixture(&fd);

    }

    -(void) draw
    {
//
// IMPORTANT:
// This is only for debug purposes
// It is recommend to disable it
//
[super draw];

ccGLEnableVertexAttribs( kCCVertexAttribFlag_Position );

kmGLPushMatrix();

world->DrawDebugData(); 

kmGLPopMatrix();
    }

    -(void) addNewSpriteAtPosition:(CGPoint)p
    {
CCLOG(@"Add sprite %0.2f x %02.f",p.x,p.y);
CCNode *parent = [self getChildByTag:kTagParentNode];

//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);
PhysicsSprite *sprite = [PhysicsSprite spriteWithTexture:spriteTexture_ rect:CGRectMake(16 * idx,16 * idy,16,16)];
[parent 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);
b2Body *body = world->CreateBody(&bodyDef);

// Define another box shape for our dynamic body.
b2PolygonShape dynamicBox;
dynamicBox.SetAsBox(.3f, .3f);//These are mid points for our 1m box

// Define the dynamic body fixture.
b2FixtureDef fixtureDef;
fixtureDef.shape = &dynamicBox; 
fixtureDef.density = 2.0f;
fixtureDef.friction = 0.3f;
body->CreateFixture(&fixtureDef);

[sprite setPhysicsBody:body];

        b2Vec2 currentPosition = body->GetPosition();
        b2Vec2 desiredPosition;
        desiredPosition.Set(467.0f/PTM_RATIO, 169.0f/PTM_RATIO);

        b2Vec2 necessaryMovement = desiredPosition - currentPosition;
        float necessaryDistance = necessaryMovement.Length();
        necessaryMovement.Normalize();
        float forceMagnitude = b2Min(150.0f, necessaryDistance *PTM_RATIO /2);
        b2Vec2 force = forceMagnitude * necessaryMovement;
        body->ApplyForce( force, body->GetWorldCenter() );

    }

    -(void) update: (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);

for (b2Body* b = world->GetBodyList(); b; b = b->GetNext())
{
    b2Body* ground = planet->GetBody();
    b2CircleShape* circle = (b2CircleShape*)planet->GetShape();
    // Get position of our "Planet"
    b2Vec2 center = ground->GetWorldPoint(circle->m_p);
    // Get position of our current body in the iteration
    b2Vec2 position = b->GetPosition();
    // Get the distance between the two objects.
    b2Vec2 d = center - position;
    // The further away the objects are, the weaker the gravitational force is
    float force = 0.0f / d.LengthSquared(); // 150 can be changed to adjust the amount of force
    d.Normalize();
    b2Vec2 F = force * d;
    // Finally apply a force on the body in the direction of the "Planet"
    b->ApplyForce(F, position);

    if (b->GetUserData() != NULL) {
        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());
    }   
}

    }

    - (void)ccTouchesEnded:(NSSet *)touches withEvent:(UIEvent *)event
    {
//Add a new body/atlas sprite at the touched location
for( UITouch *touch in touches ) {
    location = [touch locationInView: [touch view]];

    location = [[CCDirector sharedDirector] convertToGL: location];

    nodePosition = [self convertToNodeSpace: location];

    [self addNewSpriteAtPosition: location];
}
    }



    @end

---

你没有对你的身体施加任何力量

float force = 0.0f / d.LengthSquared();

不管d的值是多少，它总是为零

这是一个线性重力井的简单公式。这当然不是重力真正的工作原理，但它可能足以满足你的需要

如果要在某个最大距离maxDistance处应用零力，在某个最大距离maxDistance处应用最大力maxForce，则力的大小仅为：

Force = maxForce * (maxDistance - d)/maxDistance;

因此，当d=maxDistance时，力为0，当d=0时，力为maxForce

---

就像我说的，这不是真正的重力是如何工作的，因为实际上重力与距离的平方成正比，所以如果这对你来说不起作用，你可能需要进行实验。

感谢Sylvan关于这个问题，我如何确定最大距离？这让我大吃一惊。比如，我怎么能给它一个变量，然后全部检查a体是否在范围内？我想这取决于你，基于反复试验和什么“感觉正确”。你可以任意选择一个你认为重力应该影响的距离，然后细化它直到你喜欢它的行为方式。你所要做的就是选择一个最大范围开始，并将其与每帧计算的距离进行比较。我正在这样做，但它给了我一个b2vec2无效操作数到二进制表达式<代码>b2Vec2 e=中心位置；浮动最大距离=20.0f；如果（e知道我做错了什么吗？你不能直接将浮点与b2Vec2进行比较。必须得到b2Vec2的长度并进行比较