Touch based focus with Camera API2 on android

I am trying to implement touch based focus on Android using cameraApi2. It work sort of but it doesn't seem to work always for me. My implementation is as follows:

First I create Camera PreviewSession with the following steps:

mPreviewRequestBuilder = mCameraDevice.createCaptureRequest(CameraDevice.TEMPLATE_PREVIEW);
mPreviewRequestBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AE_MODE, CameraMetadata.CONTROL_AE_MODE_ON);
mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AWB_MODE, CameraMetadata.CONTROL_AWB_MODE_AUTO);

Then I call createCaptureSession and in that I call

mCaptureSession.setRepeatingRequest( mPreviewRequestBuilder.build(), mCaptureCallback, mBackgroundHandler);

I have implement OnTouch Listener to capture the touched area of the screen and generate AF Region

    mPreviewRequestBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
    mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_AUTO);
    mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_REGIONS, meteringRectangleArr);

    mCaptureSession.setRepeatingRequest(mPreviewRequestBuilder.build(), mCaptureCallback, mBackgroundHandler);

In the captureCallback, I do following steps in onCaptureCompleted

  mPreviewRequestBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
  mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_START);
  mCaptureSession.capture(mPreviewRequestBuilder.build(), null, mBackgroundHandler);


  mPreviewRequestBuilder.set(CaptureRequest.CONTROL_MODE, CameraMetadata.CONTROL_MODE_AUTO);
  mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_IDLE);
  mCaptureSession.setRepeatingRequest(mPreviewRequestBuilder.build(), null, mBackgroundHandler);

Now upon touch the Camera focus works sometimes and sometimes the preview is completely blur.

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