Code Shogun

Gesture/Motion-based detection is fast becoming an integral part of many mobile applications.

One neat usage of it is to shake to erease.  The first one I’ve encountered is SketchUp from the early days of jailbreak iPhone.  Then there are shake to delete done-items on your todo/shopping list, etc.

There are many approaches to detect shake motion, one is from ClingMarks.  Basically it calculates the total delta (changes) in all 3 axises: x, y and z between the current even time and the previous one, and divide by the time elapsed to find its psudo-velocity.  (I call it psudo because it’s not actually a velocity in any direction; it’s more like a mixed-up velocity from 3 axises).  If this velocity is greater than a pre-defined threshold, it’s been viewed as a shake motion.

From the code point of view, you need to implement the SensorListener:

public class ShakeActivity extends Activity implements SensorListener


You will need to acquire a SensorManager:

sensorMgr = (SensorManager) getSystemService(SENSOR_SERVICE);


And register this sensor with desired flags:

ensorMgr.registerListener(this,
SensorManager.SENSOR_ACCELEROMETER,
SensorManager.SENSOR_DELAY_GAME);


In your onSensorChange() method, you determine whether it’s a shake or not:

public void onSensorChanged(int sensor, float[] values) {
if (sensor == SensorManager.SENSOR_ACCELEROMETER) {
long curTime = System.currentTimeMillis();
// only allow one update every 100ms.
if ((curTime - lastUpdate) > 100) {
long diffTime = (curTime - lastUpdate);
lastUpdate = curTime;

x = values[SensorManager.DATA_X];
y = values[SensorManager.DATA_Y];
z = values[SensorManager.DATA_Z];

float speed = Math.abs(x+y+z – last_x – last_y – last_z) / diffTime * 10000;

if (speed > SHAKE_THRESHOLD) {
Log.d(“sensor”, “shake detected w/ speed: ” + speed);
Toast.makeText(this, “shake detected w/ speed: ” + speed, Toast.LENGTH_SHORT).show();
}
last_x = x;
last_y = y;
last_z = z;
}
}
}

The shake threshold is defined as:

private static final int SHAKE_THRESHOLD = 800;


This is pretty straightforward. So if your app just simply needs a shake motion detection, w/o needing to know on which direction it’s shaken, this code is good enough. However, there are situations where a non-shake motion would be detected as shake due to the fact that the formula is not actually the correct or should I say a precise formula for determining the shake motion. I call this a coarse-grain approach.

In my next post, I will show you a more fine-grain approach, something I learned from iPhone SDK programming

You can download this sample here

Happy Coding!

Android Shogun Android, Mobile, iPhone Android, iPhone, motion, shake

To implement swipe action in Android, such as the one used in iPhone homescreen unlock, actually Android SDK provides native support for gestures. Swipe is actually called Fling ^_^

You will need to extend SimpleOnGestureListener to implement your own handling on swipe/fling action:

class MyGestureDetector extends SimpleOnGestureListener {
@Override
public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) {
}


To determine if it’s a valid swipe, you will need to make sure the fling falls into an almost straight path, with at least certain length of touch duration, so it’s a continuous touch, and with certain velocity of course.

So let’s define some constants:

private static final int SWIPE_MIN_DISTANCE = 120;
private static final int SWIPE_MAX_OFF_PATH = 250;
private static final int SWIPE_THRESHOLD_VELOCITY = 200;


The max off path is to make sure the fling still falls within a somewhat straight path.

If onFling() method, you can do this:

if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
viewFlipper.setInAnimation(slideLeftIn);
viewFlipper.setOutAnimation(slideLeftOut);
viewFlipper.showNext();
} else if (e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY) {
viewFlipper.setInAnimation(slideRightIn);
viewFlipper.setOutAnimation(slideRightOut);
viewFlipper.showPrevious();
}


The viewFlipper is used to show how fling/swipe make the view animation/transition from one to the other.

At last, you need to make sure in your activity, you catch the gesture event by overriding onTouch() method:

@Override
public boolean onTouchEvent(MotionEvent event) {
if (gestureDetector.onTouchEvent(event))
return true;
else
return false;
}


Here’s a screenshot of the view in transition:

Android Swipe View Flipper in Action

The sample source is attached here.

Android Shogun Android, Mobile Android, gesture, motion, swipe, view flipper