Android 为什么相机预览不起作用

我尝试在曲面上显示相机预览。但它只是显示黑色的表面。我该怎么办？我把代码放在下面 谢谢你的帮助

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将以下代码放入

表面处理（）

使用前需要先设置曲面

import android.graphics.PixelFormat;
import android.hardware.Camera;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.SurfaceHolder;
import android.view.SurfaceView;

import java.io.IOException;


public class MainActivity extends ActionBarActivity implements SurfaceHolder.Callback {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        getWindow().setFormat(PixelFormat.UNKNOWN);
        SurfaceView surfaceView = (SurfaceView) findViewById(R.id.surfaceView);
        SurfaceHolder surfaceHolder = surfaceView.getHolder();
        surfaceHolder.addCallback(this);
        surfaceHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
        Camera camera = Camera.open();
        try {
            camera.setPreviewDisplay(surfaceHolder);
            camera.startPreview();

        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {

    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {

    }
}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">
    <SurfaceView
        android:id="@+id/surfaceView"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</RelativeLayout>

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="test.frontcamerateststartup" >
    <uses-permission android:name="android.permission.CAMERA"/>
    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

    try {
        camera.setPreviewDisplay(surfaceHolder);
        camera.startPreview();

    } catch (IOException e) {
        e.printStackTrace();
    }