How to refresh app upon shaking the device?

Here is an example code. Put this into your activity class:

  /* put this into your activity class */
  private SensorManager mSensorManager;
  private float mAccel; // acceleration apart from gravity
  private float mAccelCurrent; // current acceleration including gravity
  private float mAccelLast; // last acceleration including gravity

  private final SensorEventListener mSensorListener = new SensorEventListener() {

    public void onSensorChanged(SensorEvent se) {
      float x = se.values[0];
      float y = se.values[1];
      float z = se.values[2];
      mAccelLast = mAccelCurrent;
      mAccelCurrent = (float) Math.sqrt((double) (x*x + y*y + z*z));
      float delta = mAccelCurrent - mAccelLast;
      mAccel = mAccel * 0.9f + delta; // perform low-cut filter
    }

    public void onAccuracyChanged(Sensor sensor, int accuracy) {
    }
  };

  @Override
  protected void onResume() {
    super.onResume();
    mSensorManager.registerListener(mSensorListener, mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER), SensorManager.SENSOR_DELAY_NORMAL);
  }

  @Override
  protected void onPause() {
    mSensorManager.unregisterListener(mSensorListener);
    super.onPause();
  }

And add this to your onCreate method:

    /* do this in onCreate */
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
    mSensorManager.registerListener(mSensorListener, mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER), SensorManager.SENSOR_DELAY_NORMAL);
    mAccel = 0.00f;
    mAccelCurrent = SensorManager.GRAVITY_EARTH;
    mAccelLast = SensorManager.GRAVITY_EARTH;

You can then ask "mAccel" wherever you want in your application for the current acceleration, independent from the axis and cleaned from static acceleration such as gravity. It will be approx. 0 if there is no movement, and, lets say >2 if the device is shaked.

Based on the comments - to test this:

if (mAccel > 12) {
    Toast toast = Toast.makeText(getApplicationContext(), "Device has shaken.", Toast.LENGTH_LONG);
    toast.show();
}

Notes:

The accelometer should be deactivated onPause and activated onResume to save resources (CPU, Battery). The code assumes we are on planet Earth ;-) and initializes the acceleration to earth gravity. Otherwise you would get a strong "shake" when the application starts and "hits" the ground from free-fall. However, the code gets used to the gravitation due to the low-cut filter and would work also on other planets or in free space, once it is initialized. (you never know how long your application will be in use...;-)

You should subscribe as a SensorEventListener, and get the accelerometer data. Once you have it, you should monitor for sudden change in direction (sign) of acceleration on a certain axis. It would be a good indication for the 'shake' movement of device.

I modify @peceps's answer and make kotlin version of it. And I also add LifecycleOwner parameter to make it lifecycle-aware.

import android.hardware.Sensor
import android.hardware.SensorEvent
import android.hardware.SensorEventListener
import android.hardware.SensorManager
import androidx.lifecycle.DefaultLifecycleObserver
import androidx.lifecycle.LifecycleOwner
import kotlin.math.abs


/**
 * Listener that detects shake gesture.
 */
class ShakeEventListener(
    lifecycleOwner: LifecycleOwner,
    private val sensorManager: SensorManager,
    private val onShake: () -> Unit = {}
) : SensorEventListener, DefaultLifecycleObserver {
    /** Time when the gesture started.  */
    private var mFirstDirectionChangeTime: Long = 0

    /** Time when the last movement started.  */
    private var mLastDirectionChangeTime: Long = 0

    /** How many movements are considered so far.  */
    private var mDirectionChangeCount = 0

    /** The last x position.  */
    private var lastX = 0f

    /** The last y position.  */
    private var lastY = 0f

    /** The last z position.  */
    private var lastZ = 0f

    init {
        sensorManager.registerListener(
            this,
            sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_UI
        )
        // observe lifecycle state
        lifecycleOwner.lifecycle.addObserver(this)
    }


    override fun onResume(owner: LifecycleOwner) {
        sensorManager.registerListener(
            this,
            sensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER),
            SensorManager.SENSOR_DELAY_UI
        )
    }


    override fun onPause(owner: LifecycleOwner) {
        sensorManager.unregisterListener(this)
    }

    override fun onSensorChanged(se: SensorEvent) {
        // get sensor data
        val x = se.values[0]
        val y = se.values[1]
        val z = se.values[2]

        // calculate movement
        val totalMovement = abs(x + y + z - lastX - lastY - lastZ)
        if (totalMovement > MIN_FORCE) {

            // get time
            val now = System.currentTimeMillis()

            // store first movement time
            if (mFirstDirectionChangeTime == 0L) {
                mFirstDirectionChangeTime = now
                mLastDirectionChangeTime = now
            }

            // check if the last movement was not long ago
            val lastChangeWasAgo = now - mLastDirectionChangeTime
            if (lastChangeWasAgo < MAX_PAUSE_BETWEEN_DIRECTION_CHANGE) {

                // store movement data
                mLastDirectionChangeTime = now
                mDirectionChangeCount++

                // store last sensor data
                lastX = x
                lastY = y
                lastZ = z

                // check how many movements are so far
                if (mDirectionChangeCount >= MIN_DIRECTION_CHANGE) {

                    // check total duration
                    val totalDuration = now - mFirstDirectionChangeTime
                    if (totalDuration < MAX_TOTAL_DURATION_OF_SHAKE) {
                        onShake()
                        resetShakeParameters()
                    }
                }
            } else {
                resetShakeParameters()
            }
        }
    }

    /**
     * Resets the shake parameters to their default values.
     */
    private fun resetShakeParameters() {
        mFirstDirectionChangeTime = 0
        mDirectionChangeCount = 0
        mLastDirectionChangeTime = 0
        lastX = 0f
        lastY = 0f
        lastZ = 0f
    }

    override fun onAccuracyChanged(sensor: Sensor, accuracy: Int) {}

    companion object {
        /** Minimum movement force to consider.  */
        private const val MIN_FORCE = 10

        /**
         * Minimum times in a shake gesture that the direction of movement needs to
         * change.
         */
        private const val MIN_DIRECTION_CHANGE = 3

        /** Maximum pause between movements.  */
        private const val MAX_PAUSE_BETWEEN_DIRECTION_CHANGE = 200

        /** Maximum allowed time for shake gesture.  */
        private const val MAX_TOTAL_DURATION_OF_SHAKE = 400
    }
}

In your activity, add this code to make it detecting shake event:

ShakeEventListener(this, sensorManager){
    // onShake logic
}