6、实现圆形组件

圆形在很多地方都会有应用，圆形小地图，圆形头像框，圆形技能框，圆形瞄准镜框等。

在Unity中UGUI的IMage是方形的，没有直接的圆形的组件，要实现圆形可以先挂载一个mask，然后在mask下再放一张想要显示的圆形图片即可。
这种办法不好的地方是:
1、增加DrawCall，影响性能。
2、需要一个mask作为父物体，不直观。
3、圆形的外框比较粗糙，不美观。

所以，有必要自己实现一个圆形组件。

using System.Collections.Generic;
using UnityEngine;
using UnityEngine.Sprites;
using UnityEngine.UI;

public class CircleImage : Image
{
    /// <summary>
    /// 圆形由多少块三角形拼成
    /// </summary>
    [SerializeField]
    private int segements = 100;
    //显示部分占圆形的百分比.
    [SerializeField]
    private float showPercent = 1;
    private readonly Color32 GRAY_COLOR = new Color32(60, 60, 60, 255);
    private List<Vector3> _vertexList; 

    protected override void OnPopulateMesh(VertexHelper vh)
    {
        vh.Clear();

        _vertexList = new List<Vector3>();

        AddVertex(vh, segements);

        AddTriangle(vh, segements);
    }

    private void AddVertex(VertexHelper vh, int segements)
    {
        float width = rectTransform.rect.width;
        float heigth = rectTransform.rect.height;
        int realSegments = (int)(segements * showPercent);

        Vector4 uv = overrideSprite != null ? DataUtility.GetOuterUV(overrideSprite) : Vector4.zero;
        float uvWidth = uv.z - uv.x;
        float uvHeight = uv.w - uv.y;
        Vector2 uvCenter = new Vector2(uvWidth * 0.5f, uvHeight * 0.5f);
        Vector2 convertRatio = new Vector2(uvWidth / width, uvHeight / heigth);

        float radian = (2 * Mathf.PI) / segements;
        float radius = width * 0.5f;

        Vector2 originPos = new Vector2((0.5f - rectTransform.pivot.x) * width, (0.5f - rectTransform.pivot.y) * heigth);
        Vector2 vertPos = Vector2.zero;

        Color32 colorTemp = GetOriginColor();
        UIVertex origin = GetUIVertex(colorTemp, originPos, vertPos, uvCenter, convertRatio);
        vh.AddVert(origin);

        int vertexCount = realSegments + 1;
        float curRadian = 0;
        Vector2 posTermp = Vector2.zero;
        for (int i = 0; i < segements + 1; i++)
        {
            float x = Mathf.Cos(curRadian) * radius;
            float y = Mathf.Sin(curRadian) * radius;
            curRadian += radian;

            if (i < vertexCount)
            {
                colorTemp = color;
            }
            else
            {
                colorTemp = GRAY_COLOR;
            }
            posTermp = new Vector2(x, y);
            UIVertex vertexTemp = GetUIVertex(colorTemp, posTermp + originPos, posTermp, uvCenter, convertRatio);
            vh.AddVert(vertexTemp);
            _vertexList.Add(posTermp + originPos);
        }
    }

    private Color32 GetOriginColor()
    {
        Color32 colorTemp = (Color.white - GRAY_COLOR) * showPercent;
        return new Color32(
            (byte) (GRAY_COLOR.r + colorTemp.r), 
            (byte) (GRAY_COLOR.g + colorTemp.g),
            (byte) (GRAY_COLOR.b + colorTemp.b),
            255);
    }

    private void AddTriangle(VertexHelper vh, int realSegements)
    {
        int id = 1;
        for (int i = 0; i < realSegements; i++)
        {
            vh.AddTriangle(id, 0, id + 1);
            id++;
        }
    }

    private UIVertex GetUIVertex(Color32 col, Vector3 pos, Vector2 uvPos, Vector2 uvCenter, Vector2 uvScale)
    {
        UIVertex vertexTemp = new UIVertex();
        vertexTemp.color = col;
        vertexTemp.position = pos;
        vertexTemp.uv0 = new Vector2(uvPos.x * uvScale.x + uvCenter.x, uvPos.y * uvScale.y + uvCenter.y);
        return vertexTemp;
    }

    public override bool IsRaycastLocationValid(Vector2 screenPoint, Camera eventCamera)
    {
        Vector2 localPoint;
        RectTransformUtility.ScreenPointToLocalPointInRectangle(rectTransform, screenPoint, eventCamera, out localPoint);
        return IsValid(localPoint);
    }

    private bool IsValid(Vector2 localPoint)
    {
        return GetCrossPointNum(localPoint, _vertexList) %2 == 1;
    }

    private int GetCrossPointNum(Vector2 localPoint, List<Vector3> vertexList)
    {
        int count = 0;
        Vector3 vert1 = Vector3.zero;
        Vector3 vert2 = Vector3.zero;
        int vertCount = vertexList.Count;

        for (int i = 0; i < vertCount; i++)
        {
            vert1 = vertexList[i];
            vert2 = vertexList[(i + 1)% vertCount];

            if (IsYInRang(localPoint, vert1, vert2))
            {
                if (localPoint.x < GetX(vert1, vert2, localPoint.y))
                {
                    count++;
                }
            }
        }

        return count;
    }

    private bool IsYInRang(Vector2 localPoint, Vector3 vert1, Vector3 vert2)
    {
        if (vert1.y > vert2.y)
        {
            return localPoint.y < vert1.y && localPoint.y > vert2.y;
        }
        else
        {
            return localPoint.y < vert2.y && localPoint.y > vert1.y;
        }
    }

    private float GetX(Vector3 vert1, Vector3 vert2, float y)
    {
        float k = (vert1.y - vert2.y)/(vert1.x - vert2.x);
        return vert1.x + (y - vert1.y)/k;
    }
}