android - 如何在 galaxy nexus 中使用手电筒模式？

Question

我按照本指南为 galaxy nexus 创建了一个手电筒模组: Opening Flashlight of Galaxy Nexus

当我启动它时，我有一个空指针异常，我不知道为什么？!

import java.io.IOException;

import android.hardware.Camera;
import android.hardware.Camera.Parameters;
import android.os.Bundle;
import android.app.Activity;
import android.util.Log;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.Toast;

public class MainActivity extends Activity implements SurfaceHolder.Callback{


    Camera mCamera;
    public static SurfaceView preview;
    public static SurfaceHolder mHolder;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Log.v("test", "1");
        preview = (SurfaceView) findViewById(R.id.surfaceView);
        Log.v("test", "2");
        mHolder.addCallback(this);
        Log.v("test", "3");
        mHolder = preview.getHolder();
        Log.v("test", "4");
        mCamera = Camera.open();
        Log.v("test", "5");
        try {
            mCamera.setPreviewDisplay(mHolder);
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        Button onLEDbtn = (Button) findViewById(R.id.onLED_btn);
        onLEDbtn.setOnClickListener(new OnClickListener(){

            public void onClick(View v) {
                // TODO Auto-generated method stub
                Toast.makeText(getApplicationContext(), "test", Toast.LENGTH_SHORT).show();
                Parameters params = mCamera.getParameters();
                params.setFlashMode(Parameters.FLASH_MODE_TORCH);
                mCamera.setParameters(params);      
                mCamera.startPreview();
            }

        });
    }

        public void surfaceChanged(SurfaceHolder holder, int format, int width,
                int height) {
            // TODO Auto-generated method stub

        }


        public void surfaceCreated(SurfaceHolder holder) {
            // TODO Auto-generated method stub
            mHolder = holder;
            try {
                mCamera.setPreviewDisplay(mHolder);
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
        }

        public void surfaceDestroyed(SurfaceHolder holder) {
            // TODO Auto-generated method stub
             mCamera.stopPreview();
                mHolder = null;
        }

}

LogCat

11-06 17:13:02.675: V/test(17050): 1
11-06 17:13:02.675: V/test(17050): 2
11-06 17:13:02.679: D/AndroidRuntime(17050): Shutting down VM
11-06 17:13:02.679: W/dalvikvm(17050): threadid=1: thread exiting with uncaught exception (group=0x40d34300)
11-06 17:13:02.695: E/AndroidRuntime(17050): FATAL EXCEPTION: main
11-06 17:13:02.695: E/AndroidRuntime(17050): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.torchnexus/com.example.torchnexus.MainActivity}: java.lang.NullPointerException
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread.access$600(ActivityThread.java:130)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.os.Looper.loop(Looper.java:137)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread.main(ActivityThread.java:4745)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at java.lang.reflect.Method.invokeNative(Native Method)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at java.lang.reflect.Method.invoke(Method.java:511)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at dalvik.system.NativeStart.main(Native Method)
11-06 17:13:02.695: E/AndroidRuntime(17050): Caused by: java.lang.NullPointerException
11-06 17:13:02.695: E/AndroidRuntime(17050):    at com.example.torchnexus.MainActivity.onCreate(MainActivity.java:32)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.Activity.performCreate(Activity.java:5008)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
11-06 17:13:02.695: E/AndroidRuntime(17050):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
11-06 17:13:02.695: E/AndroidRuntime(17050):    ... 11 more

Answer 1

在添加回调之前初始化 mHolder

    mHolder = preview.getHolder();
    Log.v("test", "3");
    mHolder.addCallback(this);