android用缩略图拍摄和保存照片

Question

我是非常新的android并尝试一些复杂的东西。这就是我的意图：

在http://developer.android.com的帮助下，我创建了以下预览：

public class Preview extends ViewGroup implements SurfaceHolder.Callback {

    SurfaceView mSurfaceView;
    SurfaceHolder mHolder;
    //CAM INSTANCE ================================
    private Camera mCamera;
    List<Size> mSupportedPreviewSizes;
    //=============================================

    /**
     * @param context
     */
    public Preview(Context context) {
        super(context);

        mSurfaceView = new SurfaceView(context);
        addView(mSurfaceView);

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = mSurfaceView.getHolder();
        mHolder.addCallback(this);
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    /**
     * @param context
     * @param attrs
     */
    public Preview(Context context, AttributeSet attrs) {
        super(context, attrs);
        // TODO Auto-generated constructor stub
    }

    /**
     * @param context
     * @param attrs
     * @param defStyle
     */
    public Preview(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        // TODO Auto-generated constructor stub
    }

    /* (non-Javadoc)
     * @see android.view.ViewGroup#onLayout(boolean, int, int, int, int)
     */
    @Override
    protected void onLayout(boolean arg0, int arg1, int arg2, int arg3, int arg4) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width,
            int height) {
        // Now that the size is known, set up the camera parameters and begin
        // the preview.
        Camera.Parameters parameters = mCamera.getParameters();
        parameters.setPreviewSize(width, height);
        requestLayout();
        mCamera.setParameters(parameters);

        /*
          Important: Call startPreview() to start updating the preview surface. Preview must be
          started before you can take a picture.
        */
        mCamera.startPreview();

    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        // TODO Auto-generated method stub

    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
        // Surface will be destroyed when we return, so stop the preview.
        if (mCamera != null) {
            /*
              Call stopPreview() to stop updating the preview surface.
            */
            mCamera.stopPreview();
        }

    }



    /**
      * When this function returns, mCamera will be null.
      */
    private void stopPreviewAndFreeCamera() {

        if (mCamera != null) {
            /*
              Call stopPreview() to stop updating the preview surface.
            */
            mCamera.stopPreview();

            /*
              Important: Call release() to release the camera for use by other applications. 
              Applications should release the camera immediately in onPause() (and re-open() it in
              onResume()).
            */
            mCamera.release();

            mCamera = null;
        }
    }



    public void setCamera(Camera camera) {
        if (mCamera == camera) { return; }

        stopPreviewAndFreeCamera();

        mCamera = camera;

        if (mCamera != null) {
            List<Size> localSizes = mCamera.getParameters().getSupportedPreviewSizes();
            mSupportedPreviewSizes = localSizes;
            requestLayout();

            try {
                mCamera.setPreviewDisplay(mHolder);
            } catch (IOException e) {
                e.printStackTrace();
            }

            /*
              Important: Call startPreview() to start updating the preview surface. Preview must 
              be started before you can take a picture.
              */
            mCamera.startPreview();
        }
    }


    public boolean safeCameraOpen(int id) {
        boolean qOpened = false;

        try {
            releaseCameraAndPreview();
            mCamera = Camera.open(id);
            qOpened = (mCamera != null);
        } catch (Exception e) {
          //  Log.e(R.string.app_name, "failed to open Camera");
            e.printStackTrace();
        }

        return qOpened;    
    }

    private void releaseCameraAndPreview() {
        this.setCamera(null);
        if (mCamera != null) {
            mCamera.release();
            mCamera = null;
        }
    }

}

但我不知道如何使用。我试过这个：

public class HeadActivity extends Activity {

....
     public void click_button(View view) {
        Preview prev = new Preview(this);
     prev.safeCameraOpen(0);
}
....
}

但是没有任何事情发生，我必须实现PictureCallback？为什么我在创建它的实例时看不到预览？