code: package me.dm7.barcodescanner.core;
  
  import android.content.Context;
  import android.content.res.Configuration;
  import android.graphics.Point;
  import android.hardware.Camera;
  import android.os.Handler;
  import android.util.AttributeSet;
  import android.util.Log;
  import android.view.Display;
  import android.view.Surface;
  import android.view.SurfaceHolder;
  import android.view.SurfaceView;
  import android.view.View;
  import android.view.ViewGroup;
  import android.view.WindowManager;
  
  import java.util.List;
  
  public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {
      private static final String TAG = "CameraPreview";
  
      private CameraWrapper mCameraWrapper;
      private Handler mAutoFocusHandler;
      private boolean mPreviewing = true;
      private boolean mAutoFocus = true;
      private boolean mSurfaceCreated = false;
      private boolean mShouldScaleToFill = true;
      private Camera.PreviewCallback mPreviewCallback;
      private float mAspectTolerance = 0.1f;
  
      public CameraPreview(Context context, CameraWrapper cameraWrapper, Camera.PreviewCallback previewCallback) {
          super(context);
          init(cameraWrapper, previewCallback);
      }
  
      public CameraPreview(Context context, AttributeSet attrs, CameraWrapper cameraWrapper, Camera.PreviewCallback previewCallback) {
          super(context, attrs);
          init(cameraWrapper, previewCallback);
      }
  
      public void init(CameraWrapper cameraWrapper, Camera.PreviewCallback previewCallback) {
          setCamera(cameraWrapper, previewCallback);
          mAutoFocusHandler = new Handler();
          getHolder().addCallback(this);
          getHolder().setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
      }
  
      public void setCamera(CameraWrapper cameraWrapper, Camera.PreviewCallback previewCallback) {
          mCameraWrapper = cameraWrapper;
          mPreviewCallback = previewCallback;
      }
  
      public void setShouldScaleToFill(boolean scaleToFill) {
          mShouldScaleToFill = scaleToFill;
      }
  
      public void setAspectTolerance(float aspectTolerance) {
          mAspectTolerance = aspectTolerance;
      }
  
      @Override
      public void surfaceCreated(SurfaceHolder surfaceHolder) {
          mSurfaceCreated = true;
      }
  
      @Override
      public void surfaceChanged(SurfaceHolder surfaceHolder, int i, int i2, int i3) {
          if(surfaceHolder.getSurface() == null) {
              return;
          }
          stopCameraPreview();
          showCameraPreview();
      }
  
      @Override
      public void surfaceDestroyed(SurfaceHolder surfaceHolder) {
          mSurfaceCreated = false;
          stopCameraPreview();
      }
  
      public void showCameraPreview() {
          if(mCameraWrapper != null) {
              try {
                  getHolder().addCallback(this);
                  mPreviewing = true;
                  setupCameraParameters();
                  mCameraWrapper.mCamera.setPreviewDisplay(getHolder());
                  mCameraWrapper.mCamera.setDisplayOrientation(getDisplayOrientation());
                  mCameraWrapper.mCamera.setOneShotPreviewCallback(mPreviewCallback);
                  mCameraWrapper.mCamera.startPreview();
                  if(mAutoFocus) {
                      if (mSurfaceCreated) { // check if surface created before using autofocus
                          safeAutoFocus();
                      } else {
                          scheduleAutoFocus(); // wait 1 sec and then do check again
                      }
                  }
              } catch (Exception e) {
                  Log.e(TAG, e.toString(), e);
              }
          }
      }
  
      public void safeAutoFocus() {
          try {
              mCameraWrapper.mCamera.autoFocus(autoFocusCB);
          } catch (RuntimeException re) {
              // Horrible hack to deal with autofocus errors on Sony devices
              // See https://github.com/dm77/barcodescanner/issues/7 for example
              scheduleAutoFocus(); // wait 1 sec and then do check again
          }
      }
  
      public void stopCameraPreview() {
          if(mCameraWrapper != null) {
              try {
                  mPreviewing = false;
                  getHolder().removeCallback(this);
                  mCameraWrapper.mCamera.cancelAutoFocus();
                  mCameraWrapper.mCamera.setOneShotPreviewCallback(null);
                  mCameraWrapper.mCamera.stopPreview();
              } catch(Exception e) {
                  Log.e(TAG, e.toString(), e);
              }
          }
      }
  
      public void setupCameraParameters() {
          Camera.Size optimalSize = getOptimalPreviewSize();
          Camera.Parameters parameters = mCameraWrapper.mCamera.getParameters();
          parameters.setPreviewSize(optimalSize.width, optimalSize.height);
          mCameraWrapper.mCamera.setParameters(parameters);
          adjustViewSize(optimalSize);
      }
  
      private void adjustViewSize(Camera.Size cameraSize) {
          Point previewSize = convertSizeToLandscapeOrientation(new Point(getWidth(), getHeight()));
          float cameraRatio = ((float) cameraSize.width) / cameraSize.height;
          float screenRatio = ((float) previewSize.x) / previewSize.y;
  
          if (screenRatio > cameraRatio) {
              setViewSize((int) (previewSize.y * cameraRatio), previewSize.y);
          } else {
              setViewSize(previewSize.x, (int) (previewSize.x / cameraRatio));
          }
      }
  
      @SuppressWarnings("SuspiciousNameCombination")
      private Point convertSizeToLandscapeOrientation(Point size) {
          if (getDisplayOrientation() % 180 == 0) {
              return size;
          } else {
              return new Point(size.y, size.x);
          }
      }
  
      @SuppressWarnings("SuspiciousNameCombination")
      private void setViewSize(int width, int height) {
          ViewGroup.LayoutParams layoutParams = getLayoutParams();
          int tmpWidth;
          int tmpHeight;
          if (getDisplayOrientation() % 180 == 0) {
              tmpWidth = width;
              tmpHeight = height;
          } else {
              tmpWidth = height;
              tmpHeight = width;
          }
  
          if (mShouldScaleToFill) {
              int parentWidth = ((View) getParent()).getWidth();
              int parentHeight = ((View) getParent()).getHeight();
              float ratioWidth = (float) parentWidth / (float) tmpWidth;
              float ratioHeight = (float) parentHeight / (float) tmpHeight;
  
              float compensation;
  
              if (ratioWidth > ratioHeight) {
                  compensation = ratioWidth;
              } else {
                  compensation = ratioHeight;
              }
  
              tmpWidth = Math.round(tmpWidth * compensation);
              tmpHeight = Math.round(tmpHeight * compensation);
          }
  
          layoutParams.width = tmpWidth;
          layoutParams.height = tmpHeight;
          setLayoutParams(layoutParams);
      }
  
      public int getDisplayOrientation() {
          if (mCameraWrapper == null) {
              //If we don't have a camera set there is no orientation so return dummy value
              return 0;
          }
  
          Camera.CameraInfo info = new Camera.CameraInfo();
          if(mCameraWrapper.mCameraId == -1) {
              Camera.getCameraInfo(Camera.CameraInfo.CAMERA_FACING_BACK, info);
          } else {
              Camera.getCameraInfo(mCameraWrapper.mCameraId, info);
          }
  
          WindowManager wm = (WindowManager) getContext().getSystemService(Context.WINDOW_SERVICE);
          Display display = wm.getDefaultDisplay();
  
          int rotation = display.getRotation();
          int degrees = 0;
          switch (rotation) {
              case Surface.ROTATION_0: degrees = 0; break;
              case Surface.ROTATION_90: degrees = 90; break;
              case Surface.ROTATION_180: degrees = 180; break;
              case Surface.ROTATION_270: degrees = 270; break;
          }
  
          int result;
          if (info.facing == Camera.CameraInfo.CAMERA_FACING_FRONT) {
              result = (info.orientation + degrees) % 360;
              result = (360 - result) % 360;  // compensate the mirror
          } else {  // back-facing
              result = (info.orientation - degrees + 360) % 360;
          }
          return result;
      }
  
      private Camera.Size getOptimalPreviewSize() {
          if(mCameraWrapper == null) {
              return null;
          }
  
          List<Camera.Size> sizes = mCameraWrapper.mCamera.getParameters().getSupportedPreviewSizes();
          int w = getWidth();
          int h = getHeight();
          if (DisplayUtils.getScreenOrientation(getContext()) == Configuration.ORIENTATION_PORTRAIT) {
              int portraitWidth = h;
              h = w;
              w = portraitWidth;
          }
  
          double targetRatio = (double) w / h;
          if (sizes == null) return null;
  
          Camera.Size optimalSize = null;
          double minDiff = Double.MAX_VALUE;
  
          int targetHeight = h;
  
          // Try to find an size match aspect ratio and size
          for (Camera.Size size : sizes) {
              double ratio = (double) size.width / size.height;
              if (Math.abs(ratio - targetRatio) > mAspectTolerance) continue;
              if (Math.abs(size.height - targetHeight) < minDiff) {
                  optimalSize = size;
                  minDiff = Math.abs(size.height - targetHeight);
              }
          }
  
          // Cannot find the one match the aspect ratio, ignore the requirement
          if (optimalSize == null) {
              minDiff = Double.MAX_VALUE;
              for (Camera.Size size : sizes) {
                  if (Math.abs(size.height - targetHeight) < minDiff) {
                      optimalSize = size;
                      minDiff = Math.abs(size.height - targetHeight);
                  }
              }
          }
          return optimalSize;
      }
  
      public void setAutoFocus(boolean state) {
          if(mCameraWrapper != null && mPreviewing) {
              if(state == mAutoFocus) {
                  return;
              }
              mAutoFocus = state;
              if(mAutoFocus) {
                  if (mSurfaceCreated) { // check if surface created before using autofocus
                      Log.v(TAG, "Starting autofocus");
                      safeAutoFocus();
                  } else {
                      scheduleAutoFocus(); // wait 1 sec and then do check again
                  }
              } else {
                  Log.v(TAG, "Cancelling autofocus");
                  mCameraWrapper.mCamera.cancelAutoFocus();
              }
          }
      }
  
      private Runnable doAutoFocus = new Runnable() {
          public void run() {
              if(mCameraWrapper != null && mPreviewing && mAutoFocus && mSurfaceCreated) {
                  safeAutoFocus();
              }
          }
      };
  
      // Mimic continuous auto-focusing
      Camera.AutoFocusCallback autoFocusCB = new Camera.AutoFocusCallback() {
          public void onAutoFocus(boolean success, Camera camera) {
              scheduleAutoFocus();
          }
      };
  
      private void scheduleAutoFocus() {
          mAutoFocusHandler.postDelayed(doAutoFocus, 1000);
      }
  }
  