Pixplicity's

startActivityForResult(new Intent(context, BarcodeActivity.class), REQUEST_CODE);

...

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
  if (requestCode == REQUEST_CODE && resultCode == RESULT_OK) {
    String barcode = data.getStringExtra(BarcodeActivity.EXTRA_STR_QRCODE);
    ...
  }
}

package com.pixplicity.example.activities;

import android.Manifest.permission;
import android.content.Intent;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.util.Log;
import android.util.SparseArray;
import android.view.SurfaceHolder;
import android.view.View;
import android.widget.TextView;

import com.google.android.gms.vision.CameraSource;
import com.google.android.gms.vision.Detector;
import com.google.android.gms.vision.barcode.Barcode;
import com.google.android.gms.vision.barcode.BarcodeDetector;
import com.google.android.gms.vision.barcode.BarcodeDetector.Builder;
import com.pixplicity.example.R;
import com.pixplicity.example.ui.CameraView;
import com.pixplicity.example.util.CameraFocusFix;

import java.io.IOException;

import androidx.core.app.ActivityCompat;

/**
 * Barcode scanning activity. Returns the scanned value as soon as it is recognized, <em>without checking if it is a suitable URI or not!</em>
 * <p>
 * Possible results:
 * <ul>
 * <li>RESULT_OK -- Code scanned, it will contain it as the value for {@link #EXTRA_STR_QRCODE} in the result intent.</li>
 * <li>RESULT_CANCEL -- User canceled. Scanner closes.</li>
 * </ul>
 */
public class BarcodeActivity extends Activity {

    public static final String EXTRA_STR_QRCODE = "extra_qr";

    private static final String TAG = BarcodeActivity.class.getSimpleName();

    protected CameraView mCameraView;
    protected TextView mMessageView;

    @Override
    public void onCreate(Bundle icicle) {
        super.onCreate(icicle);
        setContentView(R.layout.activity_camera);

        mCameraView = findViewById(R.id.camera_surfaceview);
        mMessageView = findViewById(R.id.tv_camera_error);

        final BarcodeDetector barcodeDetector = new Builder(this)
                .setBarcodeFormats(Barcode.QR_CODE)
                .build();
        final CameraSource cameraSource = new CameraSource
                .Builder(this, barcodeDetector)
                .setAutoFocusEnabled(true)
                .setRequestedFps(30f)
                .build();

        mCameraView.getHolder().addCallback(new SurfaceHolder.Callback() {
            @Override
            public void surfaceCreated(SurfaceHolder holder) {
                try {
                    if (ActivityCompat.checkSelfPermission(BarcodeActivity.this, permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
                        return;
                    }

                    // Start preview
                    cameraSource.start(mCameraView.getHolder());

                    // Fix broken autofocus
                    CameraFocusFix.cameraFocus(cameraSource, android.hardware.Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE);

                    // Fix stretched preview
                    mCameraView.setCameraSource(cameraSource);

                } catch (IOException ie) {
                    mMessageView.setText(R.string.error_camera_open);
                    Log.e(TAG, ie.getMessage());
                }
            }

            @Override
            public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
            }

            @Override
            public void surfaceDestroyed(SurfaceHolder holder) {
                cameraSource.stop();
            }
        });

        barcodeDetector.setProcessor(new Detector.Processor<Barcode>() {
            @Override
            public void release() {
            }

            @Override
            public void receiveDetections(Detector.Detections<Barcode> detections) {
                final SparseArray<Barcode> barcodes = detections.getDetectedItems();
                if (barcodes.size() != 0) {
                    mMessageView.post(() -> {
                        Intent data = new Intent();
                        data.putExtra(EXTRA_STR_QRCODE, barcodes.valueAt(0).displayValue);
                        setResult(RESULT_OK, data);
                        finish();
                    });
                }
            }
        });
    }
}

package com.pixplicity.example.util;

import android.hardware.Camera;
import androidx.annotation.NonNull;
import androidx.annotation.StringDef;
import android.view.SurfaceHolder;

import com.google.android.gms.vision.CameraSource;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.reflect.Field;

/**
 * Source: https://gist.github.com/Gericop/7de0b9fdd7a444e53b5a
 */
public class CameraFocusFix {
    /**
     * Custom annotation to allow only valid focus modes.
     */
    @StringDef({
            Camera.Parameters.FOCUS_MODE_CONTINUOUS_PICTURE,
            Camera.Parameters.FOCUS_MODE_CONTINUOUS_VIDEO,
            Camera.Parameters.FOCUS_MODE_AUTO,
            Camera.Parameters.FOCUS_MODE_EDOF,
            Camera.Parameters.FOCUS_MODE_FIXED,
            Camera.Parameters.FOCUS_MODE_INFINITY,
            Camera.Parameters.FOCUS_MODE_MACRO
    })
    @Retention(RetentionPolicy.SOURCE)
    private @interface FocusMode {
    }

    /**
     * <p>
     * Sets the Mobile Vision API provided {@link com.google.android.gms.vision.CameraSource}'s
     * focus mode. Use {@link Camera.Parameters#FOCUS_MODE_CONTINUOUS_PICTURE} or
     * {@link Camera.Parameters#FOCUS_MODE_CONTINUOUS_VIDEO} for continuous autofocus.
     * </p>
     * <p>
     * Note that the CameraSource's {@link CameraSource#start()} or
     * {@link CameraSource#start(SurfaceHolder)} has to be called and the camera image has to be
     * showing prior using this method as the CameraSource only creates the camera after calling
     * one of those methods and the camera is not available immediately. You could implement some
     * kind of a callback method for the SurfaceHolder that notifies you when the imaging is ready
     * or use a direct action (e.g. button press) to set the focus mode.
     * </p>
     * <p>
     * Check out <a href="https://github.com/googlesamples/android-vision/blob/master/face/multi-tracker/app/src/main/java/com/google/android/gms/samples/vision/face/multitracker/ui/camera/CameraSourcePreview.java#L84">CameraSourcePreview.java</a>
     * which contains the method <code>startIfReady()</code> that has the following line:
     * <blockquote><code>mCameraSource.start(mSurfaceView.getHolder());</code></blockquote><br>
     * After this call you can use our <code>cameraFocus(...)</code> method because the camera is ready.
     * </p>
     *
     * @param cameraSource The CameraSource built with {@link com.google.android.gms.vision.CameraSource.Builder}.
     * @param focusMode    The focus mode. See {@link android.hardware.Camera.Parameters} for possible values.
     * @return true if the camera's focus is set; false otherwise.
     * @see com.google.android.gms.vision.CameraSource
     * @see android.hardware.Camera.Parameters
     */
    public static boolean cameraFocus(@NonNull CameraSource cameraSource, @FocusMode @NonNull String focusMode) {
        Field[] declaredFields = CameraSource.class.getDeclaredFields();

        for (Field field : declaredFields) {
            if (field.getType() == Camera.class) {
                field.setAccessible(true);
                try {
                    Camera camera = (Camera) field.get(cameraSource);
                    if (camera != null) {
                        Camera.Parameters params = camera.getParameters();

                        if (!params.getSupportedFocusModes().contains(focusMode)) {
                            return false;
                        }

                        params.setFocusMode(focusMode);
                        camera.setParameters(params);
                        return true;
                    }

                    return false;
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }

                break;
            }
        }

        return false;
    }
}

package com.pixplicity.example.ui;

import android.annotation.TargetApi;
import android.content.Context;
import android.os.Build;
import android.util.AttributeSet;
import android.view.SurfaceView;

import com.google.android.gms.common.images.Size;
import com.google.android.gms.vision.CameraSource;

/**
 * This SurfaceView ensures the preview is not stretched or squished.
 * This surface is only useful for the barcode scanner (it uses {@link CameraSource}
 * and should not be used for taking pictures (there are more elaborate examples for that,
 * which pick the best preview size, etc.)
 */
public class CameraView extends SurfaceView {

    private Size mPreviewSize;

    public CameraView(Context context) {
        super(context);
    }

    public CameraView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public CameraView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @TargetApi(Build.VERSION_CODES.LOLLIPOP)
    public CameraView(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context, attrs, defStyleAttr, defStyleRes);
    }

    public void setCameraSource(CameraSource source) {
        mPreviewSize = source.getPreviewSize();
        requestLayout();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int width = resolveSize(getSuggestedMinimumWidth(), widthMeasureSpec);
        int height = resolveSize(getSuggestedMinimumHeight(), heightMeasureSpec);
        int newWidth = width, newHeight = height;

        if (mPreviewSize != null) {

            float camRatio = (float) mPreviewSize.getWidth() / (float) mPreviewSize.getHeight();
            float screenRatio = (float) width / (float) height;

            if (camRatio < screenRatio) {
                newHeight = (int) (width / camRatio);
                setY(getY() - .5f * (newHeight - height));
            } else {
                newWidth = (int) (height / camRatio);
                setX(getX() - .5f * (newWidth - width));
            }
        }
        setMeasuredDimension(newWidth, newHeight);
    }
}