Box2D自定義重力

需要給剛體添加一個自定義的屬性：m_customGravity，這樣就可以動態的修改每一個剛體自定義的重力，查找box2d源碼大約在5486行，加上紅色的一句代碼

b2Island.prototype.Solve = function (step, gravity, allowSleep) {
      var i = 0;
      var j = 0;
      var b;
      var joint;
      for (i = 0;
      i < this.m_bodyCount; ++i) {
         b = this.m_bodies[i];
         if (b.GetType() != b2Body.b2_dynamicBody) continue;

         gravity = b.m_customGravity || gravity;         

         b.m_linearVelocity.x += step.dt * (gravity.x + b.m_invMass * b.m_force.x);
         b.m_linearVelocity.y += step.dt * (gravity.y + b.m_invMass * b.m_force.y);
         b.m_angularVelocity += step.dt * b.m_invI * b.m_torque;
         b.m_linearVelocity.Multiply(b2Math.Clamp(1.0 - step.dt * b.m_linearDamping, 0.0, 1.0));
         b.m_angularVelocity *= b2Math.Clamp(1.0 - step.dt * b.m_angularDamping, 0.0, 1.0);
      }
..............代碼略........................
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};

使用的方法：

//創建一個圓形剛體
var circle = new Circle({x:mousePoint.x,y:mousePoint.y,r:20,type:sc.dynamicBody});
//給該剛體加上m_customGravity屬性，並賦值垂直重力為1
circle.b2Obj.body.m_customGravity = new xengine.EasyB2D.b2Vec2(0,1);