实现代码

private IEnumerator OnlyScreenShot(RectTransform target, string name, ScreenShotCallBack callBack = null, Canvas canvas = null)

{    yield return new WaitForEndOfFrame();

    string filePath = GetScreenShotPath(name);

    //Canvas的sizeDelta

    canvas = canvas ??UGUIRoot.Instance.MainCanvas;

    RectTransform canvasTransform = canvas.GetComponent<RectTransform>();

    Vector3 canvasSize = canvasTransform.sizeDelta;

    RenderTexture screenShotRT = RenderTexture.GetTemporary(Screen.width, Screen.height, 32 );

    screenShotRT.name = "ScreenShotTool_ScreenShot_Texture";

    screenShotRT.Create();

    GrabCameraTexture.GrabCameraTarget(ref screenShotRT, GrabCameraTexture.FinalGrabCameraMask.All, 1);

    RenderTexture.active = screenShotRT;

    float targetWidth = target.rect.width * target.localScale.x;

    float targetHeight = target.rect.height * target.localScale.y;

    float WidthRatio = Screen.width / canvasSize.x;

    float HeightRatio = Screen.height / canvasSize.y;

    Vector3 targetPos = target.transform.position;

    var screenPoint = RectTransformUtility.WorldToScreenPoint(canvas.worldCamera, targetPos);

    var localPosition = new Vector2();

    RectTransformUtility.ScreenPointToLocalPointInRectangle(canvasTransform, screenPoint, canvas.worldCamera, out localPosition);

#if UNITY_EDITOR

//截取目标

    Vector2 origin = new Vector2(((localPosition.x + (canvasSize.x - targetWidth) / 2f) * WidthRatio),

((canvasSize.y - targetHeight) / 2f - target.localPosition.y) * HeightRatio);

#else

    //截取目标

    Vector2 origin = new Vector2(((localPosition.x + (canvasSize.x - targetWidth) / 2f) * WidthRatio),

        ((canvasSize.y - targetHeight) / 2f + localPosition.y) * HeightRatio);

#endif

    //截取目标的大小

    Vector2 size = new Vector2((targetWidth) * WidthRatio, (targetHeight) * HeightRatio);

    //截取

    Texture2D screenShot = new Texture2D((int)Math.Ceiling(size.x), (int)Math.Ceiling(size.y), TextureFormat.ARGB32, false);

    Rect position = new Rect(origin.x, origin.y, size.x, size.y);

    //TODO 分析日志

    LogInfo($"[OnlyScreenShot] 截取了一张图片: Screen.width:{Screen.width} Screen.height:{Screen.height} canvasSize.x:{canvasSize.x} canvasSize.y:{canvasSize.y}");

    LogInfo($"[OnlyScreenShot] 截取了一张图片: WidthRatio:{WidthRatio} HeightRatio:{HeightRatio} screenShotRT.width:{screenShotRT.width} screenShotRT.height:{screenShotRT.height}");

    LogInfo($"[OnlyScreenShot] 截取了一张图片: target.rect.width:{target.rect.width} target.rect.height:{target.rect.height} target.localPosition.x:{target.localPosition.x} target.localPosition.y:{target.localPosition.y}");

    LogInfo($"[OnlyScreenShot] 截取了一张图片: targetWidth:{targetWidth} target.rect.height:{targetHeight}");

    LogInfo($"[OnlyScreenShot] 截取了一张图片: size.x:{size.x} size.y:{size.y} (int)Math.Ceiling(size.x):{(int)Math.Ceiling(size.x)} (int)Math.Ceiling(size.y):{(int)Math.Ceiling(size.y)} ");

    LogInfo($"[OnlyScreenShot] 截取了一张图片: Screen.dpi:{Screen.dpi} origin.x:{origin.x} origin.y:{origin.y}");

    LogInfo(string.Format("[OnlyScreenShot] 截取了一张图片1: {0}", filePath));

    screenShot.ReadPixels(position, 0, 0); //按照设定区域读取像素

    screenShot.Apply();

    yield return screenShot;

    RenderTexture.active = null;

    Logger.LogInfo(string.Format("[OnlyScreenShot] 截取了一张图片2: {0}", filePath));

    //保存

    byte[] byt = screenShot.EncodeToPNG();

    File.WriteAllBytes(filePath, byt);

    GrabCameraTexture.ReleaseCameraTarget(ref screenShotRT);

    DestroyImmediate(screenShot);

    screenShot = null;

    screenShotRT = null;

    Logger.LogInfo(string.Format("[OnlyScreenShot] 截取了一张图片3: {0}", filePath));

    callBack?.Invoke(filePath);

    yield break;

}

区域截取时编辑器和移动端看到的效果不一致问题，是因为移动端和编辑器下，Rect的坑，计算position是注意一下即可

//编辑器下，rect起点(0,0)在左上角！！！

//移动端下，rect起点(0,0)在左下角！！！