UGUI内核大探究（十八）Raycaster

射线其实是属于事件系统，它在EventSystem/Raycasters目录下，有BaseRaycaster、PhysicsRaycaster和Physics2DRaycaster三个类，命名空间也是UnityEngine.EventSystems。但是UI/Core目录下还有一个GraphicRaycaster文件，命名空间却是UnityEngine.UI。当我们在编辑器里新建（或间接新建）一个Canvas时，会为Canvas添加GraphicRaycaster组件，而PhysicsRaycaster和Physics2DRaycaster似乎在UGUI里没有被添加过。当然，我们在编辑器里可以添加这两个组件。本文就讨论一下这些射线照射器的原理。

按照惯例，附上UGUI源码下载地址。

BaseRaycaster是其他Raycaster的基类，是一个抽象类。在它OnEnable（调用时机参考Untiy3D组件小贴士（一）OnEnabled与OnDisabled）里将自己注册到RaycasterManager，并在OnDisable的时候从后者移除。

RaycasterManager是一个静态类，维护了一个BaseRaycaster类型的List。EventSystem（参考UGUI内核大探究（一）EventSystem）里也通过这个类来管理所有的射线照射器。

PhysicsRaycaster（物理射线照射器）添加了特性

[RequireComponent(typeof(Camera))]
说明它依赖于Camera组件。它通过eventCamera属性来获取对象上的Camera组件。
Raycast方法重写了BaseRaycaster的同名抽象方法：

public override void Raycast(PointerEventData eventData, List<RaycastResult> resultAppendList)
{
if (eventCamera == null)
return;

var ray = eventCamera.ScreenPointToRay(eventData.position);
float dist = eventCamera.farClipPlane - eventCamera.nearClipPlane;

var hits = Physics.RaycastAll(ray, dist, finalEventMask);

if (hits.Length > 1)
System.Array.Sort(hits, (r1, r2) => r1.distance.CompareTo(r2.distance));

if (hits.Length != 0)
{
for (int b = 0, bmax = hits.Length; b < bmax; ++b)
{
var result = new RaycastResult
{
gameObject = hits[b].collider.gameObject,
module = this,
distance = hits[b].distance,
worldPosition = hits[b].point,
worldNormal = hits[b].normal,
screenPosition = eventData.position,
index = resultAppendList.Count,
sortingLayer = 0,
sortingOrder = 0
};
resultAppendList.Add(result);
}
}
}
通过Physics.RaycastAll来获取所有被照射到的对象（finalEventMask是通过将Camera的cullingMask属性和编辑器设置中的EventMask属性做与运算获得的）。根据距离进行排序，然后包装成RaycastResult结构，加入到resultAppendList里面。EventSystem会将所有的Raycast的照射结果合在一起并排序，然后输入模块（参考UGUI内核大探究（三）输入模块）取到第一个结果的对象（距离最短）作为受输入事件影响的对象。
Physics2DRaycaster继承自PhysicsRaycaster，其他都一样，只重写了Raycast方法：

public override void Raycast(PointerEventData eventData, List<RaycastResult> resultAppendList)
{
if (eventCamera == null)
return;

var ray = eventCamera.ScreenPointToRay(eventData.position);

float dist = eventCamera.farClipPlane - eventCamera.nearClipPlane;

var hits = Physics2D.RaycastAll(ray.origin, ray.direction, dist, finalEventMask);

if (hits.Length != 0)
{
for (int b = 0, bmax = hits.Length; b < bmax; ++b)
{
var sr = hits[b].collider.gameObject.GetComponent<SpriteRenderer>();

var result = new RaycastResult
{
gameObject = hits[b].collider.gameObject,
module = this,
distance = Vector3.Distance(eventCamera.transform.position, hits[b].transform.position),
worldPosition = hits[b].point,
worldNormal = hits[b].normal,
screenPosition = eventData.position,
index = resultAppendList.Count,
sortingLayer = sr != null ? sr.sortingLayerID : 0,
sortingOrder = sr != null ? sr.sortingOrder : 0
};
resultAppendList.Add(result);
}
}
}
改为用Physics2D.RaycastAll来照射对象，并且根据SpriteRenderer组件设置结果变量（在EventSystem里会作为排序依据，毕竟是2D对象）。
GraphicRaycaster继承自BaseRaycaster，它添加了特性：

[RequireComponent(typeof(Canvas))]
表示它依赖于Canvas组件（通过canvas属性来获取）。
它重写了三个属性sortOrderPriority、renderOrderPriority（获取Canvas的sortingOrder和renderOrder，这在EventSystem里会作为排序依据，呃……毕竟是UI）和eventCamera（获取canvas.worldCamera，为null则返回Camera.main）。

Raycast方法：

[NonSerialized] private List<Graphic> m_RaycastResults = new List<Graphic>();
public override void Raycast(PointerEventData eventData, List<RaycastResult> resultAppendList)
{
if (canvas == null)
return;

// Convert to view space
Vector2 pos;
if (eventCamera == null)
pos = new Vector2(eventData.position.x / Screen.width, eventData.position.y / Screen.height);
else
pos = eventCamera.ScreenToViewportPoint(eventData.position);

// If it's outside the camera's viewport, do nothing
if (pos.x < 0f || pos.x > 1f || pos.y < 0f || pos.y > 1f)
return;

float hitDistance = float.MaxValue;

Ray ray = new Ray();

if (eventCamera != null)
ray = eventCamera.ScreenPointToRay(eventData.position);

if (canvas.renderMode != RenderMode.ScreenSpaceOverlay && blockingObjects != BlockingObjects.None)
{
float dist = 100.0f;

if (eventCamera != null)
dist = eventCamera.farClipPlane - eventCamera.nearClipPlane;

if (blockingObjects == BlockingObjects.ThreeD || blockingObjects == BlockingObjects.All)
{
RaycastHit hit;
if (Physics.Raycast(ray, out hit, dist, m_BlockingMask))
{
hitDistance = hit.distance;
}
}

if (blockingObjects == BlockingObjects.TwoD || blockingObjects == BlockingObjects.All)
{
RaycastHit2D hit = Physics2D.Raycast(ray.origin, ray.direction, dist, m_BlockingMask);

if (hit.collider != null)
{
hitDistance = hit.fraction * dist;
}
}
}

m_RaycastResults.Clear();
Raycast(canvas, eventCamera, eventData.position, m_RaycastResults);

for (var index = 0; index < m_RaycastResults.Count; index++)
{
var go = m_RaycastResults[index].gameObject;
bool appendGraphic = true;

if (ignoreReversedGraphics)
{
if (eventCamera == null)
{
// If we dont have a camera we know that we should always be facing forward
var dir = go.transform.rotation * Vector3.forward;
appendGraphic = Vector3.Dot(Vector3.forward, dir) > 0;
}
else
{
// If we have a camera compare the direction against the cameras forward.
var cameraFoward = eventCamera.transform.rotation * Vector3.forward;
var dir = go.transform.rotation * Vector3.forward;
appendGraphic = Vector3.Dot(cameraFoward, dir) > 0;
}
}

if (appendGraphic)
{
float distance = 0;

if (eventCamera == null || canvas.renderMode == RenderMode.ScreenSpaceOverlay)
distance = 0;
else
{
Transform trans = go.transform;
Vector3 transForward = trans.forward;
// http://geomalgorithms.com/a06-_intersect-2.html distance = (Vector3.Dot(transForward, trans.position - ray.origin) / Vector3.Dot(transForward, ray.direction));

// Check to see if the go is behind the camera.
if (distance < 0)
continue;
}

if (distance >= hitDistance)
continue;

var castResult = new RaycastResult
{
gameObject = go,
module = this,
distance = distance,
screenPosition = eventData.position,
index = resultAppendList.Count,
depth = m_RaycastResults[index].depth,
sortingLayer = canvas.sortingLayerID,
sortingOrder = canvas.sortingOrder
};
resultAppendList.Add(castResult);
}
}
}首先将屏幕点转换为Camera的视窗坐标，用于判断是否在视窗外。然后根据blockingObjects来选择Physics或RaycastHit2D，不过这里只是用来计算距离hitDistance。然后调用静态方法Raycast获取屏幕点在其区域内的Graphic的列表m_RaycastResults（会调用Graphic的Raycast方法，参考UGUI内核大探究（七）Graphic）。接着遍历m_RaycastResults，判断Graphic的方向向量和Camera的方向向量是否相交，然后判断Graphic是否在Camera的前面，并且距离小于等于hitDistance，满足了这些条件，才会把它打包成RaycastResult添加到resultAppendList里。
由此可见GraphicRaycaster与其他射线照射器的区别就在于，它把照射对象限定为了Graphic，这也是UGUI里的常规用法。