Black screen when restart zbar barcode scanner sample app

Question

I'm running a sample code for barcode scanning but after pressing home button, then when i' m restarting the application camera is not working and i'm getting black screen. Does anybody understands the reason?

code:

public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.main);

        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

        autoFocusHandler = new Handler();
        mCamera = getCameraInstance();

        /* Instance barcode scanner */
        scanner = new ImageScanner();
        scanner.setConfig(0, Config.X_DENSITY, 3);
        scanner.setConfig(0, Config.Y_DENSITY, 3);

        mPreview = new CameraPreview(this, mCamera, previewCb, autoFocusCB);
        FrameLayout preview = (FrameLayout)findViewById(R.id.cameraPreview);
        preview.addView(mPreview);

        scanText = (TextView)findViewById(R.id.scanText);

        scanButton = (Button)findViewById(R.id.ScanButton);

        scanButton.setOnClickListener(new OnClickListener() {
                public void onClick(View v) {
                    if (barcodeScanned) {
                        barcodeScanned = false;
                        scanText.setText("Scanning...");
                        mCamera.setPreviewCallback(previewCb);
                        mCamera.startPreview();
                        previewing = true;
                        mCamera.autoFocus(autoFocusCB);
                    }
                }
            });
    }

    public void onPause() {
        super.onPause();
        releaseCamera();
    }

    /** A safe way to get an instance of the Camera object. */
    public static Camera getCameraInstance(){
        Camera c = null;
        try {
            c = Camera.open();
        } catch (Exception e){
        }
        return c;
    }

    private void releaseCamera() {
        if (mCamera != null) {
            previewing = false;
            mCamera.setPreviewCallback(null);
            mPreview.getHolder().removeCallback(mPreview);
            mCamera.release();
            mCamera = null;
        }
    }

    private Runnable doAutoFocus = new Runnable() {
            public void run() {
                if (previewing)
                    mCamera.autoFocus(autoFocusCB);
            }
        };

    PreviewCallback previewCb = new PreviewCallback() {
            public void onPreviewFrame(byte[] data, Camera camera) {
                Camera.Parameters parameters = camera.getParameters();
                Size size = parameters.getPreviewSize();

                Image barcode = new Image(size.width, size.height, "Y800");
                barcode.setData(data);

                int result = scanner.scanImage(barcode);

                if (result != 0) {
                    previewing = false;
                    mCamera.setPreviewCallback(null);
                    mCamera.stopPreview();

                    SymbolSet syms = scanner.getResults();
                    for (Symbol sym : syms) {
                        scanText.setText("barcode result " + sym.getData());
                        barcodeScanned = true;
                    }
                }
            }
        };

    // Mimic continuous auto-focusing
    AutoFocusCallback autoFocusCB = new AutoFocusCallback() {
            public void onAutoFocus(boolean success, Camera camera) {
                autoFocusHandler.postDelayed(doAutoFocus, 1000);
            }
        };

Answer 1

Always do things like that in pairs. If you're releasing the camera in onPause(), acquire and start the preview in onResume(), not onCreate(). When you hit home your Activity will be paused but not destroyed. So you release the camera, and when you come back your Activity is resumed. Since you never reinitialize the camera except in onCreate(), that's why you don't see a preview.