目标：在相机预览上绘制一个可调整大小的矩形。
对于空指针异常问题，我找不到任何明确的答案。所以我希望通过发布问题来解决我的问题。我已经拥有的：
- 正确的 Android 清单文件（相机权限等）
- 正确的xml文件
类似问题的答案之一需要添加我尝试过的 getBestPreviewSize() 方法
这是代码：
预览.java
public class Preview extends SurfaceView implements SurfaceHolder.Callback {
private SurfaceHolder mHolder;
private Camera mCamera;
public Preview(Context context, Camera camera) {
super(context);
mCamera = camera;
mHolder = getHolder();
mHolder.addCallback(this);
// mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}
// without this constructor, xml which uses this class, will throw this
// error: class cannot be instantiated
public Preview(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format, int w, int h) {
// TODO Auto-generated method stub
Camera.Parameters params = mCamera.getParameters();
Camera.Size size = getBestPreviewSize(params, w, h);
if (size != null) {
params.setPreviewSize(size.width, size.height);
}
if (mHolder.getSurface() == null) {
return;
}
mCamera.startPreview();
try {
mCamera.setPreviewDisplay(mHolder);
mCamera.startPreview();
} catch (IOException e) {
// TODO Auto-generated catch block
Log.d("CameraView",
"Error setting camera preview: " + e.getMessage());
}
}
private Size getBestPreviewSize(Parameters params, int w, int h) {
// TODO Auto-generated method stub
Camera.Size result = null;
for (Camera.Size size : params.getSupportedPreviewSizes()) {
if (size.width <= w && size.height <= h) {
if (result == null) {
result = size;
} else {
int resultDelta = w - result.width + h - result.height;
int newDelta = w - size.width + h - size.height;
if (newDelta < resultDelta) {
result = size;
}
}
}
}
return result;
}
public void onPause() {
mCamera.release();
mCamera = null;
}
@Override
public void surfaceCreated(SurfaceHolder arg0) {
if (mCamera == null) {
mCamera = Camera.open();
try {
mCamera.setPreviewDisplay(mHolder);
mCamera.startPreview();
} catch (IOException e) {
// TODO Auto-generated catch block
Log.d("", "Error setting camera preview: " + e.getMessage());
}
}
}
@Override
public void surfaceDestroyed(SurfaceHolder arg0) {
// TODO Auto-generated method stub
if (mCamera != null) {
mCamera.stopPreview();
mCamera.release();
mCamera = null;
}
}
}
CameraPreview.java 与 MainActivity()
public class CameraPreview extends Activity {
private Preview mPreview;
private ImageView mTakePicture, cameraIcon;
private DrawView dV;
private boolean mAutoFocus = true;
private boolean mFlashBoolean = false;
private Button bDisplayText;
private RelativeLayout rl;
private FrameLayout fl;
Camera c;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
requestWindowFeature(Window.FEATURE_NO_TITLE);
getWindow().setFlags(WindowManager.LayoutParams.MATCH_PARENT,
WindowManager.LayoutParams.MATCH_PARENT);
setContentView(R.layout.activity_main);
mTakePicture = (ImageView) findViewById(R.id.cameraButton);
bDisplayText = (Button) findViewById(R.id.bDispText);
fl = (FrameLayout) findViewById(R.id.preview);
rl = (RelativeLayout) findViewById(R.id.rl);
c = getCameraInstance();
dV = new DrawView(this);
dV.invalidate();
rl.addView(dV);
mPreview = new Preview(this, c);
fl.addView(mPreview);
}
@Override
protected void onPause() {
// TODO Auto-generated method stub
super.onPause();
if (mPreview != null) {
mPreview.onPause();
mPreview = null;
}
}
@Override
protected void onResume() {
// TODO Auto-generated method stub
super.onResume();
}
private static Camera getCameraInstance() {
// TODO Auto-generated method stub
Camera c = null;
c = Camera.open();
return c;
}
}
编辑：LogCat
09-06 20:23:15.739: E/AndroidRuntime(2352): FATAL EXCEPTION: main
09-06 20:23:15.739: E/AndroidRuntime(2352): java.lang.NullPointerException
09-06 20:23:15.739: E/AndroidRuntime(2352): at com.example.textrecognize.Preview.surfaceCreated(Preview.java:94)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.SurfaceView.updateWindow(SurfaceView.java:543)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.SurfaceView.access$000(SurfaceView.java:81)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:169)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:671)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1820)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1000)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:4214)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:725)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.Choreographer.doCallbacks(Choreographer.java:555)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.Choreographer.doFrame(Choreographer.java:525)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:711)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.os.Handler.handleCallback(Handler.java:615)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.os.Handler.dispatchMessage(Handler.java:92)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.os.Looper.loop(Looper.java:137)
09-06 20:23:15.739: E/AndroidRuntime(2352): at android.app.ActivityThread.main(ActivityThread.java:4778)
09-06 20:23:15.739: E/AndroidRuntime(2352): at java.lang.reflect.Method.invokeNative(Native Method)
09-06 20:23:15.739: E/AndroidRuntime(2352): at java.lang.reflect.Method.invoke(Method.java:511)
09-06 20:23:15.739: E/AndroidRuntime(2352): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
09-06 20:23:15.739: E/AndroidRuntime(2352): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
09-06 20:23:15.739: E/AndroidRuntime(2352): at dalvik.system.NativeStart.main(Native Method)