Jul 31 2010

Detect Shaking Motion on Windows Phone 7

Category: MobileJoel Ivory Johnson @ 10:12

Download the code

The other day on the MSDN forums some one 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. 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 overwhich a shake sequence must occur to be considered acceptable. One 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 raise

I've reduced the direction in which the device is moving to one of 8 directions (North, East, South, West, and the directions inbetween 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 some one else to understand.

void _accelerometer_ReadingChanged(object sender, AccelerometerReadingEventArgs e)
    //Does the currenet 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)
        //This is a shake we care about. save in in our list
        ShakeRecord record = new ShakeRecord();
        record.EventTime = DateTime.Now;
        record.ShakeDirection = direction;
        _shakeRecordIndex = (_shakeRecordIndex + 1)%_minimumShakes;
        _shakeRecordList[_shakeRecordIndex] = record;



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

    if ((_shakeRecordList[endIndex].EventTime.Subtract(_shakeRecordList[startIndex].EventTime)) <= MinimumShakeTime)

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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Jacob Gable Jacob Gable United States says:

That's really cool stuff. I'm gonna have to find a way to mock up some shaking in the emulator.

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