Android: Using SharedPreferences in a Shake Listener Class

Question

First, I'm pretty new to Android and programming in general, so please go easy ;-) Second, I think this site is a lifesaver! OK, now on to the meat of the issue...

In my efforts to try to learn more about Android development and using user preferences I've created a little application that works fine and has a number of options that the user can change to make it look different. However, I've run into a little snag trying to get it setup to allow customization of the sensitivity of the shake detection and time between shake detection.

I have everything working so that if a user shakes the phone it does what I want. Here are the basics of what I've done for the Shake Detection (based off of a great post here on stackoverflow(Android: I want to shake it)):

Created a new class named ShakeDetector and put the following code in it:

import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.util.FloatMath;

public class ShakeDetector implements SensorEventListener {

/*
 * The gForce that is necessary to register as shake.
 * Must be greater than 1G (one earth gravity unit).
 * You can install "G-Force", by Blake La Pierre
 * from the Google Play Store and run it to see how
 *  many G's it takes to register a shake
 */
private static final float SHAKE_THRESHOLD_GRAVITY = 2.7F;
private static final int SHAKE_SLOP_TIME_MS = 500;
private static final int SHAKE_COUNT_RESET_TIME_MS = 3000;

private OnShakeListener mListener;
private long mShakeTimestamp;
private int mShakeCount;

public void setOnShakeListener(OnShakeListener listener) {
    this.mListener = listener;
}

public interface OnShakeListener {
    public void onShake(int count);
}

@Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {
    // ignore
}

@Override
public void onSensorChanged(SensorEvent event) {

    if (mListener != null) {
        float x = event.values[0];
        float y = event.values[1];
        float z = event.values[2];

        float gX = x / SensorManager.GRAVITY_EARTH;
        float gY = y / SensorManager.GRAVITY_EARTH;
        float gZ = z / SensorManager.GRAVITY_EARTH;

        // gForce will be close to 1 when there is no movement.
        float gForce = FloatMath.sqrt(gX * gX + gY * gY + gZ * gZ);

        if (gForce > SHAKE_THRESHOLD_GRAVITY) {
            final long now = System.currentTimeMillis();
            // ignore shake events too close to each other (500ms)
            if (mShakeTimestamp + SHAKE_SLOP_TIME_MS > now) {
                return;
            }

            // reset the shake count after 3 seconds of no shakes
            if (mShakeTimestamp + SHAKE_COUNT_RESET_TIME_MS < now) {
                mShakeCount = 0;
            }

            mShakeTimestamp = now;
            mShakeCount++;

            mListener.onShake(mShakeCount);
        }
    }
}
}

Next I put the following in my onCreate method of my main activity:

        // ShakeDetector initialization
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
    mAccelerometer = mSensorManager
            .getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    mShakeDetector = new ShakeDetector();
    mShakeDetector.setOnShakeListener(new OnShakeListener() {

        @Override
        public void onShake(int count) {
            /*
             * The following method, "handleShakeEvent(count):" is a stub //
             * method you would use to setup whatever you want done once the
             * device has been shook.
             */
            handleShakeEvent(count);
        }
    });

Again, at this point everything works as desired. However, what I'm not sure how to do is setup SHAKE_THRESHOLD_GRAVITY and SHAKE_COUNT_RESET_TIME_MS in the ShakeDetector class to use the shared preferences I already have setup in the application. Everything is configured in the application (and works) to allow the user to go in and select a different value for both of those settings, but I'm not sure how to pass the SharedPreferences settings from the activity to this class. I've tried a number of different things with no luck and I can't seem to find anything online that details how to do this. It's probably something rather simple, but I'm not having much luck figuring it out.

In a nutshell, I've tried to create another method in the ShakeDetector class (e.g. setShakePreferences()) and then put code in it that looks similar to the following:

    public void setShakePreferences() {

    shake_sensitivity = myPrefs.getString(SHAKE_THRESHOLD_GRAVITY2, "2.7");
    shake_time_between = myPrefs.getString(SHAKE_COUNT_RESET_TIME_MS2, "3000");
    float shake_threshold_gravity = Float.valueOf(shake_sensitivity);
    int shake_count_reset_time_ms = Integer.valueOf(shake_time_between);

}

I've tried passing in SharedPreferences (e.g. setShakePreferences(SharedPreferences myPrefs)) various different ways, however, everything I've tried to do to get myPrefs to actually contain the SharedPreferences only ever seems to end up with a null value for myPrefs. I have some settings in my main activity that are controlled through the user preferences and I have methods similar to what is above that work just fine. I'm just not sure how to set myPrefs to the actual SharedPreferences values in the ShakeDetector class.

Please help. Detailed code examples of solutions would work best to help me understand how I can accomplish this. Thanks!

score 0 · Accepted Answer · answered Feb 05 '13 at 04:11

Figured this one out after reading up a little more on how to implement interfaces. It was actually pretty simple once I figured it out (not sure why I didn't catch it to begin with).

Basically, I modified onCreate() to pass the sharedPreferences as follows (just added "this.myPrefs" as a parameter):

        // ShakeDetector initialization
    mSensorManager = (SensorManager) getSystemService(Context.SENSOR_SERVICE);
    mAccelerometer = mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER);
    mShakeDetector = new ShakeDetector();
    mShakeDetector.setOnShakeListener(new OnShakeListener() {

        @Override
        public void onShake(int count) {
            /*
             * The following method, "handleShakeEvent(count):" is a stub //
             * method you would use to setup whatever you want done once the
             * device has been shook.
             */
            handleShakeEvent(count);
        }
    }, this.myPrefs); // <<<--- ADDED IT HERE

Then I had to modify the setOnShakeListener in the ShakeDetector class to accept myPrefs as a parameter and added everything so that it would correctly handle the variables. Here's what I ended up with:

import android.content.SharedPreferences;
import android.hardware.Sensor;
import android.hardware.SensorEvent;
import android.hardware.SensorEventListener;
import android.hardware.SensorManager;
import android.util.FloatMath;
import android.util.Log;

public class ShakeDetector implements SensorEventListener {

/*
 * The gForce that is necessary to register as shake.
 * Must be greater than 1G (one earth gravity unit).
 * You can install "G-Force", by Blake La Pierre
 * from the Google Play Store and run it to see how
 *  many G's it takes to register a shake
 */
private static final String SHAKE_THRESHOLD_GRAVITY = "<replace this with the android:key value you have configured in you preferences XML file.>";
private static final String SHAKE_COUNT_RESET_TIME_MS = "<replace this with the android:key value you have configured in you preferences XML file.>";
private static final String SHAKE_SLOP_TIME_MS = "<replace this with the android:key value you have configured in you preferences XML file.>";

private OnShakeListener mListener;
private long mShakeTimestamp;
private int mShakeCount;
private SharedPreferences myPrefs;

public void setOnShakeListener(OnShakeListener listener, SharedPreferences myPrefs) {
    this.mListener = listener;
    this.myPrefs = myPrefs;
}

public interface OnShakeListener {
    public void onShake(int count);
}

@Override
public void onAccuracyChanged(Sensor sensor, int accuracy) {
    // ignore
}

@Override
public void onSensorChanged(SensorEvent event) {

    if (mListener != null) {
        float x = event.values[0];
        float y = event.values[1];
        float z = event.values[2];

        float gX = x / SensorManager.GRAVITY_EARTH;
        float gY = y / SensorManager.GRAVITY_EARTH;
        float gZ = z / SensorManager.GRAVITY_EARTH;

        // gForce will be close to 1 when there is no movement.
        float gForce = FloatMath.sqrt(gX * gX + gY * gY + gZ * gZ);

        if (gForce > Float.parseFloat(myPrefs.getString(SHAKE_THRESHOLD_GRAVITY, "2.7F"))) {
            final long now = System.currentTimeMillis();
            // ignore shake events too close to each other (500ms)
            if (mShakeTimestamp + Integer.parseInt(myPrefs.getString(SHAKE_SLOP_TIME_MS, "500")) > now) {
                return;
            }

            // reset the shake count after 3 seconds of no shakes
            if (mShakeTimestamp + Integer.parseInt(myPrefs.getString(SHAKE_COUNT_RESET_TIME_MS, "3000")) < now) {
                mShakeCount = 0;
            }

            mShakeTimestamp = now;
            mShakeCount++;

            mListener.onShake(mShakeCount);
        }
    }
}
}

Hope that helps if anyone else was wondering.

Android: Using SharedPreferences in a Shake Listener Class

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