How to implement Touch-to-Focus in Android using Camera2 APIs

AndroidCamera2TouchToFocus.java

    //Override in your touch-enabled view (this can be differen than the view you use for displaying the cam preview)
    @Override
    public boolean onTouch(View view, MotionEvent motionEvent) {
        final int actionMasked = motionEvent.getActionMasked();
        if (actionMasked != MotionEvent.ACTION_DOWN) {
            return false;
        }
        if (mManualFocusEngaged) {
            Log.d(TAG, "Manual focus already engaged");
            return true;
        }

        final Rect sensorArraySize = mCameraInfo.get(CameraCharacteristics.SENSOR_INFO_ACTIVE_ARRAY_SIZE);

        //TODO: here I just flip x,y, but this needs to correspond with the sensor orientation (via SENSOR_ORIENTATION)
        final int y = (int)((motionEvent.getX() / (float)view.getWidth())  * (float)sensorArraySize.height());
        final int x = (int)((motionEvent.getY() / (float)view.getHeight()) * (float)sensorArraySize.width());
        final int halfTouchWidth  = 150; //(int)motionEvent.getTouchMajor(); //TODO: this doesn't represent actual touch size in pixel. Values range in [3, 10]...
        final int halfTouchHeight = 150; //(int)motionEvent.getTouchMinor();
        MeteringRectangle focusAreaTouch = new MeteringRectangle(Math.max(x - halfTouchWidth,  0),
                                                                 Math.max(y - halfTouchHeight, 0),
                                                                 halfTouchWidth  * 2,
                                                                 halfTouchHeight * 2,
                                                                 MeteringRectangle.METERING_WEIGHT_MAX - 1);

        CameraCaptureSession.CaptureCallback captureCallbackHandler = new CameraCaptureSession.CaptureCallback() {
            @Override
            public void onCaptureCompleted(CameraCaptureSession session, CaptureRequest request, TotalCaptureResult result) {
                super.onCaptureCompleted(session, request, result);
                mManualFocusEngaged = false;

                if (request.getTag() == "FOCUS_TAG") {
                    //the focus trigger is complete -
                    //resume repeating (preview surface will get frames), clear AF trigger
                    mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, null);
                    mCameraOps.setRepeatingRequest(mPreviewRequestBuilder.build(), null, null);
                }
            }

            @Override
            public void onCaptureFailed(CameraCaptureSession session, CaptureRequest request, CaptureFailure failure) {
                super.onCaptureFailed(session, request, failure);
                Log.e(TAG, "Manual AF failure: " + failure);
                mManualFocusEngaged = false;
            }
        };

        //first stop the existing repeating request
        mCameraOps.stopRepeating();

        //cancel any existing AF trigger (repeated touches, etc.)
        mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_CANCEL);
        mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_MODE, CaptureRequest.CONTROL_AF_MODE_OFF);
        mCameraOps.capture(mPreviewRequestBuilder.build(), captureCallbackHandler, mBackgroundHandler);

        //Now add a new AF trigger with focus region
        if (isMeteringAreaAFSupported()) {
            mPreviewRequestBuilder.set(CaptureRequest.CONTROL_AF_REGIONS, new MeteringRectangle[]{focusAreaTouch});
        }
        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_TRIGGER, CameraMetadata.CONTROL_AF_TRIGGER_START);
        mPreviewRequestBuilder.setTag("FOCUS_TAG"); //we'll capture this later for resuming the preview

        //then we ask for a single request (not repeating!)
        mCameraOps.capture(mPreviewRequestBuilder.build(), captureCallbackHandler, mBackgroundHandler);
        mManualFocusEngaged = true;

        return true;
    }

    private boolean isMeteringAreaAFSupported() {
        return mCameraInfo.get(CameraCharacteristics.CONTROL_MAX_REGIONS_AF) >= 1;
    }

@Fiooodooor it's still not correct, first we need to convert Touch coordinateness of View Size to Camera Preview Size coordinates and only after that we can convert it to Camera Sensor Size - https://stackoverflow.com/a/33181620/7767664 (the guy which worked in Google on camera2 API):

The units for them are in the sensor active array coordinate system, so you'll have to translate from your UI touch coordinates to coordinates relative to your preview view, and from there, to the active array coordinates.

@anonym24 great point, that is correct. I've made couple of subconscious assumptions, including working with full-screen, no icons, no menus preview of camera sensor. Thanks for correction!