code: function Bounce(x, y) {
              var dist = Math.random() * 7;
              var angle = Math.PI + Math.random() * Math.PI;
              this._d = new C2D.Vector(x, y);
              this._u = 0.2 + Math.random() * 0.8;
              this._v = new C2D.Vector(Math.cos(angle) * dist, Math.sin(angle) * dist);
          }
docs: 构造函数。