照相机根据触摸围绕目标旋转

Transform m_Rounder = Camera.main.transform;
Transform m_Center = m_TargetXingqiu.transform;
//当前触摸的坐标与上一个触摸坐标的偏移量
Vector2 offsetPos = m_TouchCurPos - m_TouchPrePos;			//围绕半径
float radius = (m_Rounder.position - m_Center.position).magnitude;
//相机到目标的向量
Vector3 camRelativeToTarget = Vector3.Normalize(m_Rounder.position - m_Center.position);
//触摸为照相机坐标，将此坐标转为世界坐标
Vector3 touchCurWorldPos = m_Rounder.localToWorldMatrix.MultiplyPoint(new Vector3(m_TouchCurPos.x, m_TouchCurPos.y, 0f));
Vector3 touchPreWorldPos = m_Rounder.localToWorldMatrix.MultiplyPoint(new Vector3(m_TouchPrePos.x, m_TouchPrePos.y, 0f));
//触摸方向向量转世界向量
Vector3 touchVec = touchCurWorldPos - touchPreWorldPos;
//围绕的旋转轴
Vector3 roundAxis = Vector3.Cross(touchVec, camRelativeToTarget);
m_Rounder.RotateAround (m_Center.position, Vector3.Normalize(roundAxis), roundAxis.magnitude*0.1f);