I'm using Android's Camera2 API and I currently want the camera to perform a certain action whenever it prepares to flash.

When building the CaptureRequest, the following line:

```
captureRequest.set(CaptureRequest.CONTROL_AE_MODE,CaptureRequest.CONTROL_AE_MODE_ON_AUTO_FLASH);

```

allows the camera to flash under low-lighting conditions. However, I am at a loss as to how I can detect whether the camera is prepped to flash or not. It seems like the literature online about this particular action is pretty sparse.

I have tried checking if `FLASH_STATE` is in `FLASH_STATE_READY` while processing a partialresult in the camera's `CaptureCallback`, but it seems like the key wasn't available - it kept returning `null`. Perhaps I'm not checking in the right place?

The camera's `CaptureCallback`, shown below (based off Google's `Camera2Basic` code sample):

```
private CameraCaptureSession.CaptureCallback mCaptureCallback
        = new CameraCaptureSession.CaptureCallback()    {
    private void process(CaptureResult result)  {
        switch(mState)  {
            case STATE_PREVIEW: break;
            case STATE_WAITING_LOCK:
                // checking if result.get(CaptureResult.FLASH_STATE) == 
                // CaptureResult.FLASH_READY over here didn't work because
                // null was returned
                int afState = result.get(CaptureResult.CONTROL_AF_STATE);
                if (CaptureResult.CONTROL_AF_STATE_FOCUSED_LOCKED == afState ||
                        CaptureResult.CONTROL_AF_STATE_NOT_FOCUSED_LOCKED == afState) {
                    Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
                    if (aeState == null ||
                            aeState == CaptureResult.CONTROL_AE_STATE_CONVERGED) {
                        mState = STATE_WAITING_NON_PRECAPTURE;
                        captureStillPicture();
                    } else {
                        runPrecaptureSequence();
                    }
                }
                break;
            case STATE_WAITING_PRECAPTURE:
                Integer aeState = result.get(CaptureResult.CONTROL_AE_STATE);
                if (aeState == null ||
                        aeState == CaptureResult.CONTROL_AE_STATE_PRECAPTURE ||
                        aeState == CaptureRequest.CONTROL_AE_STATE_FLASH_REQUIRED) {
                    mState = STATE_WAITING_NON_PRECAPTURE;
                }
                break;
            case STATE_WAITING_NON_PRECAPTURE:
                Integer aeState1 = result.get(CaptureResult.CONTROL_AE_STATE);
                if (aeState1 == null || aeState1 != CaptureResult.CONTROL_AE_STATE_PRECAPTURE) {
                    mState = STATE_PICTURE_TAKEN;
                    captureStillPicture();
                }
                break;
        }
    }

    @Override
    public void onCaptureProgressed(CameraCaptureSession session,
                                    CaptureRequest request, CaptureResult partialResult) {
        super.onCaptureProgressed(session, request, partialResult);
        process(partialResult);
    }

    @Override
    public void onCaptureCompleted(CameraCaptureSession session,
                                   CaptureRequest request, TotalCaptureResult result) {
        super.onCaptureCompleted(session, request, result);
        process(result);
    }
};

```