片段surfaceview中的相机预览

Question

最近我正在尝试在Xamarin制作Android应用程序，它将使用相机预览识别人物并测量摄影师与目标之间的距离。

要实现这一目标，我需要一个简单的相机预览，不需要捕捉任何图片，只需简单预览。

我能够打开相机并将其预览为全屏，但这不是我需要的。我的代码没有正确报告任何错误，应用程序部署和启动，但我的片段中的surfaceView是黑色的。 Logcat从Camera抛出一条消息，表明app传递了null surface。

我的MainActivity.cs：

using System;
using Android.App;
using Android.Graphics;
using Android.OS;
using Android.Views;
using Android.Widget;

namespace App1
{
    [Activity(Label = "App1", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        bool color = false;
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            SetContentView (Resource.Layout.Main);
        }
    }
}

FirstFragment.cs - 它是带有surfaceView的片段（第二个片段只包含一个简单的textview，与案例无关）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Android.App;
using Android.Content;
using Android.OS;
using Android.Runtime;
using Android.Util;
using Android.Views;
using Android.Widget;
using Android.Hardware;
using Java.Lang;

namespace App1
{
    public class FirstFragment : Fragment, ISurfaceHolderCallback
    {
    static Camera camera = null;
    SurfaceView surfaceView;
    ISurfaceHolder surfaceHolder;

    private string tag = "FirstFragment";

    public override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        start_camera();
        // Create your fragment here
    }

    private void start_camera()
    {
        try
        {
            camera = Camera.Open();
        }
        catch (RuntimeException e)
        {
            Log.Error(tag, "init_camera: " + e);
            return;
        }
        Camera.Parameters param;
        param = camera.GetParameters();
        //modify parameter
        param.PreviewFrameRate = 20;
        param.SetPreviewSize(176, 144);
        camera.SetParameters(param);
        try
        {
            //camera.SetPreviewDisplay(surfaceHolder);
            //camera.StartPreview();
        }
        catch (Java.Lang.Exception e)
        {
            Log.Error(tag, "init_camera: " + e);
            return;
        }
    }

    public void SurfaceChanged(ISurfaceHolder arg0, Android.Graphics.Format arg1, int arg2, int arg3)
    {
        // TODO Auto-generated method stub
    }

    public void SurfaceCreated(ISurfaceHolder holder)
    {
        // TODO Auto-generated method stub
    }

    public void SurfaceDestroyed(ISurfaceHolder holder)
    {

        Log.Error(tag, "surfaceDestroyed() is called");
            // Stop preview and release camera 
            if (holder.Surface != null)
        {
            camera.StopPreview();
            //mCamera.release();  //should we release camera here?
            //mCamera = null;
        }
        else
        {
            Log.Error(tag, "surfaceDestroyed(): preview surface does not exist");
        }
    }

        public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
        {
            View view = inflater.Inflate(Resource.Layout.FirstFragment, container, false);
            surfaceView = view.FindViewById<SurfaceView>(Resource.Id.surfaceView1);
            surfaceHolder = surfaceView.Holder;
            surfaceHolder.AddCallback(this);
            camera.SetPreviewDisplay(surfaceHolder);
            camera.StartPreview();
            surfaceHolder.SetType(SurfaceType.PushBuffers);
            return view;
        }
    }
}

这是布局：

主：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/linearLayout1">
    <fragment
        android:name="App1.FirstFragment"
        android:layout_width="match_parent"
        android:layout_height="279.5dp"
        android:id="@+id/fragment1" />
    <fragment
        android:name="App1.SecondFragment"
        android:layout_width="match_parent"
        android:layout_height="125.0dp"
        android:id="@+id/fragment2" />
</LinearLayout>

和FirstFragment：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:minWidth="25px"
    android:minHeight="25px">
    <SurfaceView
        android:layout_width="match_parent"
        android:layout_height="200dp"
        android:id="@+id/surfaceView1"
        android:layout_marginTop="0.0dp" />
</LinearLayout>

我知道原因可能很简单，但我做了很多研究，如何修复它却没有成功。