你可以尝试改变

camera.AutoFocus(OnAutoFocus);

到

camera.AutoFocus(this);

并且它将使用OnAutoFocus，因为它是从IAutoFocusCallback实现的。
关于订阅事件的问题，您可以尝试在OnElementChanged中订阅事件，如下所示

protected override void OnElementChanged(ElementChangedEventArgs<Xamarin.Forms.Page> e)
        {
            base.OnElementChanged(e);
            if (e.OldElement != null || Element == null)
            {
                return;
            }
            try
            {
                this.SetOnTouchListener(this);
            }
            catch (Exception e)
            {

            }

        }

顺便说一句，我没有看到在这个代码中使用TextureView.ISurfaceTextureListener。

在链接的Java答案中所发生的一切就是他们提供了在操作系统调用回调时运行的代码：

camera.autoFocus(new Camera.AutoFocusCallback() {                   
    @Override
      public void onAutoFocus(boolean success, Camera camera) {
           camera.cancelAutoFocus();
           Parameters params = camera.getParameters();
           if(params.getFocusMode() != Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE){
                params.setFocusMode(Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE);
                camera.setParameters(params);
        }
    }
});

上面没有“调用”回调，只是提供了回调代码来运行。操作系统调用回调。所以在Xamarin中，你需要传入实现IAutoFocusCallback接口的类型，所以我认为你应该能够做到这一点，因为CameraPage实现了IAutoFocusCallback接口：

camera.AutoFocus(this); // "this" refers to your current CameraPage which implements the interface.

这里的线索是，当你在camera.AutoFocus后面输入左括号时，弹出窗口显示你需要传入一个类型IAutoFocusCallback，这意味着任何实现该接口的类型，所以在本例中是“this”CameraPage。：-）

因为这里没有完整的例子，这是我的例子。
这个解决方案很好用，至少对我来说是这样。相机会持续对焦，直到一个对焦点被点击。然后它会对焦在点击点上，直到你把相机移开。然后它会回到持续对焦模式。

public class CameraPageRenderer : PageRenderer, TextureView.ISurfaceTextureListener, Android.Hardware.Camera.IPictureCallback, Android.Hardware.Camera.IShutterCallback, IAutoFocusCallback 
{

    // ... code removed for brevity

    /// <summary>
    /// Occurs whenever the user touches the screen. Here we set the focus mode to FocusModeAuto and set a focus area based on the tapped coordinates.
    /// </summary>
    public override bool OnTouchEvent(MotionEvent e)
    {
        var parameters = camera.GetParameters();

        parameters.FocusMode = Camera.Parameters.FocusModeAuto;

        if (parameters.MaxNumFocusAreas > 0)
        {
            var focusRect = CalculateTapArea(e.GetX(), e.GetY(), textureView.Width, textureView.Height, 50f);

            parameters.FocusAreas = new List<Area>()
            {
                new Area(focusRect, 1000)
            };
        }

        try
        {
            camera.CancelAutoFocus();
            camera.SetParameters(parameters);
            camera.AutoFocus(this);
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex);
        }

        return true;
    }

    /// <summary>
    /// Auto focus callback. Here we reset the focus mode to FocusModeContinuousPicture and remove any focus areas
    /// </summary>
    public void OnAutoFocus(bool success, Camera camera)
    {
        var parameters = camera.GetParameters();

        parameters.FocusMode = Parameters.FocusModeContinuousPicture;

        if (parameters.MaxNumFocusAreas > 0)
        {
            parameters.FocusAreas = null;
        }

        camera.SetParameters(parameters);
    }

    /// <summary>
    /// Calculates a tap area using the focus coordinates mentioned in <see href="https://developer.android.com/reference/android/hardware/Camera.Parameters.html#getFocusAreas()"/>
    /// <para>
    /// Coordinates of the rectangle range from -1000 to 1000. (-1000, -1000) is the upper left point. (1000, 1000) is the lower right point. The width and height of focus areas cannot be 0 or negative.</para>
    /// </summary>
    /// <param name="x">The X coordinate of the tapped area</param>
    /// <param name="y">The Y coordinate of the tapped area</param>
    /// <param name="width">The total width of the tappable area</param>
    /// <param name="height">The total height of the tappable area</param>
    /// <param name="focusAreaSize">The desired size (widht, height) of the created rectangle</param>
    /// <returns></returns>
    private Rect CalculateTapArea(float x, float y, float width, float height, float focusAreaSize)
    {
        var leftFloat = x * 2000 / width - 1000;
        var topFloat = y * 2000 / height - 1000;

        var left = RoundFocusCoordinate(leftFloat);
        var top = RoundFocusCoordinate(topFloat);
        var right = RoundFocusCoordinate(leftFloat + focusAreaSize);
        var bottom = RoundFocusCoordinate(topFloat + focusAreaSize);

        return new Rect(left, top, right, bottom);
    }

    /// <summary>
    /// Round, convert to int, and clamp between -1000 and 1000
    /// </summary>
    private int RoundFocusCoordinate(float value)
    {
        var intValue = (int)Math.Round(value, 0, MidpointRounding.AwayFromZero);
        return Math.Clamp(intValue, -1000, 1000);
    }

    // ... code removed for brevity

}