Detect Shaking Motion on Windows Phone 7

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The other day on the MSDN forums, someone asked about how to detect a shaking motion on Windows Phone 7. I've been playing with the accelerometer lately, so I took great joy in answering this along with providing a working implementation. The question was asking about shaking motion in a left-right direction. I made a class that detects left-right and up-down motion (totally ignoring the Z-axis all together for now). Though extending it to consider the Z-axis wouldn't be hard.

The code for detecting the motion has been abstracted in a class called ShakeDetector. The algorithm used has a few variables/constants defined that can be modified to tune the behaviour of the class. The classes constructor accepts an [optional] parameter of how many times the phone should be shaken before the motion is considered acceptable. <codeminimumaccelerationmagnitude> can be raised or lowered to control how hard the device needs to be shaken to be considered acceptable. And MinimumShakeTime takes a time span that defines the maximum length of time over which a shake sequence must occur to be considered acceptable. Once the user moves the phone in a way that meets the requirements for the type of shake we wanted to detect a ShakeDetected event is raised.

I've reduced the direction in which the device is moving to one of 8 directions (North, East, South, West, and the directions in between those). I could have kept the direction as an angle and just ensured that there was atleast a minimum difference between the angles but I thought using the directions on a map would make it easier for someone else to understand.

void _accelerometer_ReadingChanged(object sender, AccelerometerReadingEventArgs e)
{
    //Does the current acceleration vector meet the minimum magnitude that we
    //care about?
    if ((e.X*e.X + e.Y*e.Y) > MinimumAccelerationMagnitudeSquared)
    {
        //I prefer to work in radians. For the sake of those reading this code
        //I will work in degrees. In the following direction will contain the direction
        // in which the device was accelerating in degrees. 
        double degrees = 180.0*Math.Atan2(e.Y, e.X)/Math.PI;
        Direction direction = DegreesToDirection(degrees);

        //If the shake detected is in the same direction as the last one then ignore it
        if ((direction & _shakeRecordList[_shakeRecordIndex].ShakeDirection) 
		!= Direction.None)
            return;
        //This is a shake we care about. save in our list
        ShakeRecord record = new ShakeRecord();
        record.EventTime = DateTime.Now;
        record.ShakeDirection = direction;
        _shakeRecordIndex = (_shakeRecordIndex + 1)%_minimumShakes;
        _shakeRecordList[_shakeRecordIndex] = record;

            CheckForShakes();
    }
}
void CheckForShakes()
{
    int startIndex = (_shakeRecordIndex - 1);
    if (startIndex < 0) startIndex = _minimumShakes - 1;
    int endIndex = _shakeRecordIndex;

    if ((_shakeRecordList[endIndex].EventTime.Subtract
	(_shakeRecordList[startIndex].EventTime)) <= MinimumShakeTime)
    {
        OnShakeEvent();
    }
}

The example code can be found in my SkyDrive account here. If you want to see the program in action, there is a video on YouTube.

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