android - 在整个 Activity 生命周期中管理相机预览 SurfaceView？

Question

我正在尝试将 CameraPreview 定义在 Android Developer Guide 中到 ApiDemos/OS/Sensors 应用程序屏幕上。一半时间它按预期工作。但是，每隔一段时间我恢复此测试应用程序(前提是我通过按设备的“主页”图标暂停它)时，会记录以下异常:

06-13 14:10:17.369: D/SENSORS_TEST(11888): supported preview width x height: 640 x 480 06-13 14:10:17.369: D/SENSORS_TEST(11888): supported preview width x height: 320 x 240 06-13 14:10:17.369: D/SENSORS_TEST(11888): supported preview width x height: 176 x 144 06-13 14:10:17.600: D/dalvikvm(11888): GC_FOR_ALLOC freed 56K, 3% free 9091K/9347K, paused 22ms 06-13 14:10:17.600: I/dalvikvm-heap(11888): Grow heap (frag case) to 9.610MB for 695056-byte allocation 06-13 14:10:17.631: D/dalvikvm(11888): GC_CONCURRENT freed 1K, 3% free 9768K/10055K, paused 2ms+2ms 06-13 14:10:31.510: D/AndroidRuntime(11888): Shutting down VM 06-13 14:10:31.510: W/dalvikvm(11888): threadid=1: thread exiting with uncaught exception (group=0x40a351f8) 06-13 14:10:31.518: E/AndroidRuntime(11888): FATAL EXCEPTION: main 06-13 14:10:31.518: E/AndroidRuntime(11888): java.lang.RuntimeException: Method called after release() 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.hardware.Camera.setPreviewDisplay(Native Method) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.hardware.Camera.setPreviewDisplay(Camera.java:405) 06-13 14:10:31.518: E/AndroidRuntime(11888): at com.example.sensor.Sensors10Activity$CameraPreview.surfaceCreated(Sensors10Activity.java:221)
06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.SurfaceView.updateWindow(SurfaceView.java:533) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.SurfaceView.onWindowVisibilityChanged(SurfaceView.java:226) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.View.dispatchWindowVisibilityChanged(View.java:5839) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewGroup.dispatchWindowVisibilityChanged(ViewGroup.java:945) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:965) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.view.ViewRootImpl.handleMessage(ViewRootImpl.java:2442) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.os.Handler.dispatchMessage(Handler.java:99) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.os.Looper.loop(Looper.java:137) 06-13 14:10:31.518: E/AndroidRuntime(11888): at android.app.ActivityThread.main(ActivityThread.java:4424) 06-13 14:10:31.518: E/AndroidRuntime(11888): at java.lang.reflect.Method.invokeNative(Native Method) 06-13 14:10:31.518: E/AndroidRuntime(11888): at java.lang.reflect.Method.invoke(Method.java:511) 06-13 14:10:31.518: E/AndroidRuntime(11888): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784) 06-13 14:10:31.518: E/AndroidRuntime(11888): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551) 06-13 14:10:31.518: E/AndroidRuntime(11888): at dalvik.system.NativeStart.main(Native Method)

这是代码，其中大部分直接取自开发指南的相机示例和 ApiDemos/OS/Sensors 示例。有没有人看到在 Activity 生命周期中应该做些什么来避免第 221 行发生的异常(如代码中所注释)？

提前感谢您的检查，
格雷格

public class Sensors10Activity extends Activity {
private final String TAG = "SENSORS_TEST";
private SensorManager mSensorManager;
private GraphView mGraphView;
private Camera mCamera;
private CameraPreview mCameraPreview;

public class GraphView extends View implements SensorEventListener {
    private Bitmap  mBitmap;
    private Paint   mPaint = new Paint();
    private Canvas  mCanvas = new Canvas();
    private Path    mPath = new Path();
    private RectF   mRect = new RectF();
    private float   mLastValues[] = new float[3*2];
    private float   mOrientationValues[] = new float[3];
    private int     mColors[] = new int[3*2];
    private float   mLastX;
    private float   mScale[] = new float[2];
    private float   mYOffset;
    private float   mMaxX;
    private float   mSpeed = 1.0f;
    private float   mWidth;
    private float   mHeight;
    public GraphView(Context context) {
        super(context);
        mColors[0] = Color.argb(192, 255, 64, 64);
        mColors[1] = Color.argb(192, 64, 128, 64);
        mColors[2] = Color.argb(192, 64, 64, 255);
        mColors[3] = Color.argb(192, 64, 255, 255);
        mColors[4] = Color.argb(192, 128, 64, 128);
        mColors[5] = Color.argb(192, 255, 255, 64);

        mPaint.setFlags(Paint.ANTI_ALIAS_FLAG);
        mRect.set(-0.5f, -0.5f, 0.5f, 0.5f);
        mPath.arcTo(mRect, 0, 180);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.RGB_565);
        mCanvas.setBitmap(mBitmap);
        mCanvas.drawColor(0xFFFFFFFF);
        mYOffset = h * 0.5f;
        mScale[0] = - (h * 0.5f * (1.0f / (SensorManager.STANDARD_GRAVITY * 2)));
        mScale[1] = - (h * 0.5f * (1.0f / (SensorManager.MAGNETIC_FIELD_EARTH_MAX)));
        mWidth = w;
        mHeight = h;
        if (mWidth < mHeight) {
            mMaxX = w;
        } else {
            mMaxX = w-50;
        }
        mLastX = mMaxX;
        super.onSizeChanged(w, h, oldw, oldh);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        synchronized (this) {
            if (mBitmap != null) {
                final Paint paint = mPaint;
                final Path path = mPath;
                final int outer = 0xFFC0C0C0;
                final int inner = 0xFFff7010;

                if (mLastX >= mMaxX) {
                    mLastX = 0;
                    final Canvas cavas = mCanvas;
                    final float yoffset = mYOffset;
                    final float maxx = mMaxX;
                    final float oneG = SensorManager.STANDARD_GRAVITY * mScale[0];
                    paint.setColor(0xFFAAAAAA);
                    cavas.drawColor(0xFFFFFFFF);
                    cavas.drawLine(0, yoffset,      maxx, yoffset,      paint);
                    cavas.drawLine(0, yoffset+oneG, maxx, yoffset+oneG, paint);
                    cavas.drawLine(0, yoffset-oneG, maxx, yoffset-oneG, paint);
                }
                canvas.drawBitmap(mBitmap, 0, 0, null);

                float[] values = mOrientationValues;
                if (mWidth < mHeight) {
                    float w0 = mWidth * 0.333333f;
                    float w  = w0 - 32;
                    float x = w0*0.5f;
                    for (int i=0 ; i<3 ; i++) {
                        canvas.save(Canvas.MATRIX_SAVE_FLAG);
                        canvas.translate(x, w*0.5f + 4.0f);
                        canvas.save(Canvas.MATRIX_SAVE_FLAG);
                        paint.setColor(outer);
                        canvas.scale(w, w);
                        canvas.drawOval(mRect, paint);
                        canvas.restore();
                        canvas.scale(w-5, w-5);
                        paint.setColor(inner);
                        canvas.rotate(-values[i]);
                        canvas.drawPath(path, paint);
                        canvas.restore();
                        x += w0;
                    }
                } else {
                    float h0 = mHeight * 0.333333f;
                    float h  = h0 - 32;
                    float y = h0*0.5f;
                    for (int i=0 ; i<3 ; i++) {
                        canvas.save(Canvas.MATRIX_SAVE_FLAG);
                        canvas.translate(mWidth - (h*0.5f + 4.0f), y);
                        canvas.save(Canvas.MATRIX_SAVE_FLAG);
                        paint.setColor(outer);
                        canvas.scale(h, h);
                        canvas.drawOval(mRect, paint);
                        canvas.restore();
                        canvas.scale(h-5, h-5);
                        paint.setColor(inner);
                        canvas.rotate(-values[i]);
                        canvas.drawPath(path, paint);
                        canvas.restore();
                        y += h0;
                    }
                }
            }
        }
    }

    public void onSensorChanged(SensorEvent event) {
        //Log.d(TAG, "sensor: " + sensor + ", x: " + values[0] + ", y: " + values[1] + ", z: " + values[2]);
        synchronized (this) {
            if (mBitmap != null) {
                final Canvas canvas = mCanvas;
                final Paint paint = mPaint;
                if (event.sensor.getType() == Sensor.TYPE_ORIENTATION) {
                    for (int i=0 ; i<3 ; i++) {
                        mOrientationValues[i] = event.values[i];
                    }
                } else {
                    float deltaX = mSpeed;
                    float newX = mLastX + deltaX;

                    int j = (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD) ? 1 : 0;
                    for (int i=0 ; i<3 ; i++) {
                        int k = i+j*3;
                        final float v = mYOffset + event.values[i] * mScale[j];
                        paint.setColor(mColors[k]);
                        canvas.drawLine(mLastX, mLastValues[k], newX, v, paint);
                        mLastValues[k] = v;
                    }
                    if (event.sensor.getType() == Sensor.TYPE_MAGNETIC_FIELD)
                        mLastX += mSpeed;
                }
                invalidate();
            }
        }
    }

    public void onAccuracyChanged(Sensor sensor, int accuracy) {
    }
}

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
    private SurfaceHolder mHolder;
    private Camera mCamera;

    public CameraPreview(Context context, Camera camera) {
        super(context);
        mCamera = camera;

        // Install a SurfaceHolder.Callback so we get notified when the
        // underlying surface is created and destroyed.
        mHolder = getHolder();
        mHolder.addCallback(this);
        // deprecated setting, but required on Android versions prior to 3.0
        mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
    }

    public void surfaceCreated(SurfaceHolder holder) {
        // The Surface has been created, now tell the camera where to draw the preview.
        try {
            mCamera.setPreviewDisplay(holder);  // !!! LINE 221: exception occurs here.
            mCamera.startPreview();
        } catch (IOException e) {
            Log.d(TAG, "Error setting camera preview: " + e.getMessage());
        }
    }

    public void surfaceDestroyed(SurfaceHolder holder) {
        // empty. Take care of releasing the Camera preview in your activity.
    }

    public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
        // If your preview can change or rotate, take care of those events here.
        // Make sure to stop the preview before resizing or reformatting it.

        if (mHolder.getSurface() == null){
          // preview surface does not exist
          return;
        }

        // stop preview before making changes
        try {
            mCamera.stopPreview();
        } catch (Exception e){
          // ignore: tried to stop a non-existent preview
        }

        // set preview size and make any resize, rotate or
        // reformatting changes here

        // start preview with new settings
        try {
            mCamera.setPreviewDisplay(mHolder);
            mCamera.startPreview();

        } catch (Exception e){
            Log.d(TAG, "Error starting camera preview: " + e.getMessage());
        }
    }
}

//////////////
// LIFE CYCLE
//

/**
 * Initialization of the Activity after it is first created.  Must at least
 * call {@link android.app.Activity#setContentView setContentView()} to
 * describe what is to be displayed in the screen.
 */
@Override
protected void onCreate(Bundle savedInstanceState) {
    // Be sure to call the super class.
    super.onCreate(savedInstanceState);

    mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
    mCamera = Camera.open(1);
    mCameraPreview = new CameraPreview(this, mCamera);
    mGraphView = new GraphView(this);
    // setContentView(mGraphView);

    // Create RelativeLayout for layout root.
    RelativeLayout relativeLayout = new RelativeLayout(this);
    RelativeLayout.LayoutParams rlp = new RelativeLayout.LayoutParams(
            RelativeLayout.LayoutParams.FILL_PARENT,
            RelativeLayout.LayoutParams.FILL_PARENT);

    // Add GraphView to layout.
    RelativeLayout.LayoutParams lpGraph = new RelativeLayout.LayoutParams(
            RelativeLayout.LayoutParams.FILL_PARENT,
            RelativeLayout.LayoutParams.FILL_PARENT);
    mGraphView.setLayoutParams(lpGraph);
    relativeLayout.addView(mGraphView);

    // Add SurfaceView to layout.
    List<Camera.Size> ls = mCamera.getParameters().getSupportedPreviewSizes();
    int n = ls.size();
    int widthMin = 10000;
    int imin = -1;
    for (int i=0; i<n; i++) {
        Log.d(TAG, "supported preview width x height: " + ls.get(i).width + " x " + ls.get(i).height);
        if (widthMin > ls.get(i).width) {
            widthMin = ls.get(i).width;
            imin = i;
        }
    }
    if (imin >= 0) {
        RelativeLayout.LayoutParams lpSurface = new RelativeLayout.LayoutParams(
                ls.get(imin).width, ls.get(imin).height);
        lpSurface.addRule(RelativeLayout.ALIGN_PARENT_BOTTOM);
        lpSurface.addRule(RelativeLayout.CENTER_HORIZONTAL);
        mCameraPreview.setLayoutParams(lpSurface);
        relativeLayout.addView(mCameraPreview);
    }

    // Provide Android framework with layout root.
    setContentView(relativeLayout, rlp);
}

@Override
protected void onResume() {
    super.onResume();
    mSensorManager.registerListener(mGraphView,
            mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_FASTEST);
    mSensorManager.registerListener(mGraphView,
            mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD),
            SensorManager.SENSOR_DELAY_FASTEST);
    mSensorManager.registerListener(mGraphView, 
            mSensorManager.getDefaultSensor(Sensor.TYPE_ORIENTATION),
            SensorManager.SENSOR_DELAY_FASTEST);

    if (mCamera == null)
        mCamera = Camera.open(1);
    if (mCameraPreview == null) 
        mCameraPreview = new CameraPreview(this, mCamera);
}


@Override
protected void onPause() {
    super.onPause();
    if (mCamera != null) {
        mCamera.stopPreview();
        mCamera.release();        // release the camera for other applications
        mCamera = null;
    }
    if (mCameraPreview != null) {
        mCameraPreview = null;
    }
}

@Override
protected void onStop() {
    mSensorManager.unregisterListener(mGraphView);
    super.onStop();
}

Answer 1

经过几个小时的反复试验，这些生命周期处理程序似乎起作用了(即，它们处理由电源按钮引起的暂停、由主页图标引起的停止、由后退图标引起的销毁，以及相机被释放 onPause() 以使其可用于系统的相机应用程序)。

@Override
protected void onCreate(Bundle savedInstanceState) {
    // Be sure to call the super class.
    super.onCreate(savedInstanceState);

    mSensorManager = (SensorManager) getSystemService(SENSOR_SERVICE);
    mCamera = Camera.open(1);
    mCameraPreview = new CameraPreview(this, mCamera);
    mGraphView = new GraphView(this);
    // setContentView(mGraphView);

    // Create RelativeLayout for layout root.
    mLayoutRoot = new RelativeLayout(this);
    RelativeLayout.LayoutParams rlp = new RelativeLayout.LayoutParams(
            RelativeLayout.LayoutParams.FILL_PARENT,
            RelativeLayout.LayoutParams.FILL_PARENT);

    // Add GraphView to layout.
    RelativeLayout.LayoutParams lpGraph = new RelativeLayout.LayoutParams(
            RelativeLayout.LayoutParams.FILL_PARENT,
            RelativeLayout.LayoutParams.FILL_PARENT);
    mGraphView.setLayoutParams(lpGraph);
    mLayoutRoot.addView(mGraphView);

    // Add SurfaceView to layout.
    List<Camera.Size> ls = mCamera.getParameters().getSupportedPreviewSizes();
    int n = ls.size();
    int widthMin = 10000;
    int imin = -1;
    for (int i=0; i<n; i++) {
        Log.d(TAG, "supported preview width x height: " + ls.get(i).width + " x " + ls.get(i).height);
        if (widthMin > ls.get(i).width) {
            widthMin = ls.get(i).width;
            mCameraPreviewSize = ls.get(i);
            imin = i;
        }
    }
    if (imin >= 0) {
        RelativeLayout.LayoutParams lpSurface = new RelativeLayout.LayoutParams(
                ls.get(imin).width, ls.get(imin).height);
        lpSurface.addRule(RelativeLayout.ALIGN_PARENT_BOTTOM);
        lpSurface.addRule(RelativeLayout.CENTER_HORIZONTAL);
        mCameraPreview.setLayoutParams(lpSurface);
        mLayoutRoot.addView(mCameraPreview);
    }

    // Provide Android framework with layout root.
    setContentView(mLayoutRoot, rlp);
    Log.d(TAG, "onCreate OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}

@Override
protected void onStart() {
    super.onStart();
    Log.d(TAG, "onStart OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}

@Override
protected void onResume() {
    super.onResume();
    mSensorManager.registerListener(mGraphView,
            mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_FASTEST);
    mSensorManager.registerListener(mGraphView,
            mSensorManager.getDefaultSensor(Sensor.TYPE_MAGNETIC_FIELD),
            SensorManager.SENSOR_DELAY_FASTEST);
    mSensorManager.registerListener(mGraphView, 
            mSensorManager.getDefaultSensor(Sensor.TYPE_ORIENTATION),
            SensorManager.SENSOR_DELAY_FASTEST);

    if (mCamera == null)
        mCamera = Camera.open(1);
    if (mCameraPreview == null) {
        mCameraPreview = new CameraPreview(this, mCamera);
        RelativeLayout.LayoutParams lpCameraPreview = new RelativeLayout.LayoutParams(
                mCameraPreviewSize.width, mCameraPreviewSize.height);
        lpCameraPreview.addRule(RelativeLayout.ALIGN_PARENT_BOTTOM);
        lpCameraPreview.addRule(RelativeLayout.CENTER_HORIZONTAL);
        mCameraPreview.setLayoutParams(lpCameraPreview);
        mLayoutRoot.addView(mCameraPreview);
    }
    Log.d(TAG, "onResume OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}


@Override
protected void onPause() {
    if (mCamera != null) {
        mCamera.stopPreview();
        mCamera.release();        // release the camera for other applications
        mCamera = null;
    }
    if (mCameraPreview != null) {
        mLayoutRoot.removeView(mCameraPreview);
        mCameraPreview = null;
    }
    super.onPause();
    Log.d(TAG, "onPause OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}

@Override
protected void onStop() {
    mSensorManager.unregisterListener(mGraphView);
    super.onStop();
    Log.d(TAG, "onStop OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}

@Override
protected void onDestroy() {
    super.onDestroy();
    Log.d(TAG, "onDestroy OUT mCamera, mCameraPreview: " + mCamera + ", " + mCameraPreview);
}