Unity Tutorial - Tanks! 总结（下）

Unity Tutorial - Tanks! 总结（下）

紧接着上一篇Unity Tutorial - Tanks! 总结（上），我们来到第五章

05 - SHELLS

现在有坦克有血条，自然而然就是要造炮弹了，对于炮弹首先要思考个问题，我们希望炮弹做什么，我们希望炮弹在碰撞其他的东西时，显示一个爆炸特效，除此之外，我们并不想要它会像一个实体一样爆炸后依然留在地上，为此我们需要为炮弹的 collider 中 Is Trigger 打上勾。这里解释一下 Is Trigger，勾选它之后，炮弹将不会再被当作一个实体，它是一个只有表面，没有内容的模型，但是再加上 rigidbody 之后它的运动会符合物理规律，只是不会和任何其他的 collider 真实地发生碰撞，而是触发 OnTriggerEnter()，OnTriggerExit()，OnTriggerStay() 等方法。于是乎，这里的炮弹发生碰撞的行为我们可以理解为，在炮弹触发到 OnTriggerEnter() 方法时，显示爆炸效果，同时销毁炮弹实例。

知道炮弹是如何爆炸之后，接着来想象炮弹爆炸会引发哪些事情，可以想象得到，爆炸是一种范围性的影响，我们希望炮弹在爆炸的时候会对一定范围里面的敌人造成伤害，以及给它一个爆炸所产生的作用力，而对于其他的物体，我们不想对它们产生影响。这里使用的方法，是将坦克的 LayerMask 标记为 player 从而在爆炸范围监测时忽略掉其他的 collider ，结合源码感受一下具体实现

public class ShellExplosion : MonoBehaviour
{
    public LayerMask m_TankMask;                        // Used to filter what the explosion affects, this should be set to "Players".
    public ParticleSystem m_ExplosionParticles;         // Reference to the particles that will play on explosion.
    public AudioSource m_ExplosionAudio;                // Reference to the audio that will play on explosion.
    public float m_MaxDamage = 100f;                    // The amount of damage done if the explosion is centred on a tank.
    public float m_ExplosionForce = 1000f;              // The amount of force added to a tank at the centre of the explosion.
    public float m_MaxLifeTime = 2f;                    // The time in seconds before the shell is removed.
    public float m_ExplosionRadius = 5f;                // The maximum distance away from the explosion tanks can be and are still affected.


    private void Start ()
    {
        // If it isn't destroyed by then, destroy the shell after it's lifetime.
        Destroy (gameObject, m_MaxLifeTime);
    }


    private void OnTriggerEnter (Collider other)
    {
  // Collect all the colliders in a sphere from the shell's current position to a radius of the explosion radius.
        Collider[] colliders = Physics.OverlapSphere (transform.position, m_ExplosionRadius, m_TankMask);

        // Go through all the colliders...
        for (int i = 0; i < colliders.Length; i++)
        {
            // ... and find their rigidbody.
            Rigidbody targetRigidbody = colliders[i].GetComponent<Rigidbody> ();

            // If they don't have a rigidbody, go on to the next collider.
            if (!targetRigidbody)
                continue;

            // Add an explosion force.
            targetRigidbody.AddExplosionForce (m_ExplosionForce, transform.position, m_ExplosionRadius);

            // Find the TankHealth script associated with the rigidbody.
            TankHealth targetHealth = targetRigidbody.GetComponent<TankHealth> ();

            // If there is no TankHealth script attached to the gameobject, go on to the next collider.
            if (!targetHealth)
                continue;

            // Calculate the amount of damage the target should take based on it's distance from the shell.
            float damage = CalculateDamage (targetRigidbody.position);

            // Deal this damage to the tank.
            targetHealth.TakeDamage (damage);
        }

        // Unparent the particles from the shell.
        m_ExplosionParticles.transform.parent = null;

        // Play the particle system.
        m_ExplosionParticles.Play();

        // Play the explosion sound effect.
        m_ExplosionAudio.Play();

        // Once the particles have finished, destroy the gameobject they are on.
        ParticleSystem.MainModule mainModule = m_ExplosionParticles.main;
        Destroy (m_ExplosionParticles.gameObject, mainModule.duration);

        // Destroy the shell.
        Destroy (gameObject);
    }


    private float CalculateDamage (Vector3 targetPosition)
    {
        // Create a vector from the shell to the target.
        Vector3 explosionToTarget = targetPosition - transform.position;

        // Calculate the distance from the shell to the target.
        float explosionDistance = explosionToTarget.magnitude;

        // Calculate the proportion of the maximum distance (the explosionRadius) the target is away.
        float relativeDistance = (m_ExplosionRadius - explosionDistance) / m_ExplosionRadius;

        // Calculate damage as this proportion of the maximum possible damage.
        float damage = relativeDistance * m_MaxDamage;

        // Make sure that the minimum damage is always 0.
        damage = Mathf.Max (0f, damage);

        return damage;
    }
}

整个脚本的主要逻辑是遍历 Physice.OverlapSphere() 返回的一组 collider ，通过 GetComponent<Rigidbody>() 获取 targetRigidbody 来施加爆炸作用力，通过 GetComponent<TankHealth> 获取 targetHealth 来造成伤害。随后在销毁炮弹实例前，将原本作为炮弹子对象的爆炸特效抽离出来，播放完成后再进行销毁。

• 这里有一个问题我个人思考了很久，为什么将爆炸特效抽离出来不是用 m_ExplosionParticles.gameObject.parent = null 而使用 m_ExplosionParticles.transform.parent = null 经过一番搜索后依然得不到答案，索性就自己改一下，发现 m_ExplosionParticles.gameObject.parent = null 是不合法的，无法通过编译的，最后我强行猜想原因： 原本在 hierarchy 中，通过拖拽让一个对象变成另一个对象的子对象，他们之间的联系也就仅仅是通过在 transform 层面上建立起来的，所以通过将 transform.parent 置 null 相当于将他们建立联系的桥梁砍断，也意味着此时炮弹的销毁不再会影响到爆炸特效

• 第二个需要注意的点是，脚本中的 CalculateDamage(Vector3 targetPosition) 方法之所以传入参数是一个 Vector3 是因为需要根据坦克和爆炸中心的距离比例来计算伤害。另外之所以要用到 damage = Mathf.Max (0f, damage); 是因为 damage 有可能出现负数，出现负数的原因是坦克中心到爆炸范围的中心可能会大于爆炸半径，而同时又刚好能触发 OnTriggerE