Android Two finger rotation

android, gesture-recognition, rotation

Solution

Improvements of the class:

- angle returned is total since rotation has begun

- removing unnecessary functions

- simplification

- get position of first pointer only after second pointer is down

public class RotationGestureDetector {
    private static final int INVALID_POINTER_ID = -1;
    private float fX, fY, sX, sY;
    private int ptrID1, ptrID2;
    private float mAngle;

    private OnRotationGestureListener mListener;

    public float getAngle() {
        return mAngle;
    }

    public RotationGestureDetector(OnRotationGestureListener listener){
        mListener = listener;
        ptrID1 = INVALID_POINTER_ID;
        ptrID2 = INVALID_POINTER_ID;
    }

    public boolean onTouchEvent(MotionEvent event){
        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_DOWN:
                ptrID1 = event.getPointerId(event.getActionIndex());
                break;
            case MotionEvent.ACTION_POINTER_DOWN:
                ptrID2 = event.getPointerId(event.getActionIndex());
                sX = event.getX(event.findPointerIndex(ptrID1));
                sY = event.getY(event.findPointerIndex(ptrID1));
                fX = event.getX(event.findPointerIndex(ptrID2));
                fY = event.getY(event.findPointerIndex(ptrID2));
                break;
            case MotionEvent.ACTION_MOVE:
                if(ptrID1 != INVALID_POINTER_ID && ptrID2 != INVALID_POINTER_ID){
                    float nfX, nfY, nsX, nsY;
                    nsX = event.getX(event.findPointerIndex(ptrID1));
                    nsY = event.getY(event.findPointerIndex(ptrID1));
                    nfX = event.getX(event.findPointerIndex(ptrID2));
                    nfY = event.getY(event.findPointerIndex(ptrID2));

                    mAngle = angleBetweenLines(fX, fY, sX, sY, nfX, nfY, nsX, nsY);

                    if (mListener != null) {
                        mListener.OnRotation(this);
                    }
                }
                break;
            case MotionEvent.ACTION_UP:
                ptrID1 = INVALID_POINTER_ID;
                break;
            case MotionEvent.ACTION_POINTER_UP:
                ptrID2 = INVALID_POINTER_ID;
                break;
            case MotionEvent.ACTION_CANCEL:
                ptrID1 = INVALID_POINTER_ID;
                ptrID2 = INVALID_POINTER_ID;
                break;
        }
        return true;
    }

    private float angleBetweenLines (float fX, float fY, float sX, float sY, float nfX, float nfY, float nsX, float nsY)
    {
        float angle1 = (float) Math.atan2( (fY - sY), (fX - sX) );
        float angle2 = (float) Math.atan2( (nfY - nsY), (nfX - nsX) );

        float angle = ((float)Math.toDegrees(angle1 - angle2)) % 360;
        if (angle < -180.f) angle += 360.0f;
        if (angle > 180.f) angle -= 360.0f;
        return angle;
    }

    public static interface OnRotationGestureListener {
        public void OnRotation(RotationGestureDetector rotationDetector);
    }
}

How to use it:

- Put the above class in a separate file `RotationGestureDetector.java`

- create a private field `mRotationDetector` of type `RotationGestureDetector` in your activity class and create a new instance of the detector during the initialization (`onCreate` method for example) and give as parameter a class implementing the `onRotation` method (here the `activity = this`).

- In the method `onTouchEvent`, send the touch events received to the gesture detector with '`mRotationDetector.onTouchEvent(event);`'

- Implements `RotationGestureDetector.OnRotationGestureListener` in your activity and add the method '`public void OnRotation(RotationGestureDetector rotationDetector)`' in the activity. In this method, get the angle with `rotationDetector.getAngle()`

Example:

public class MyActivity extends Activity implements RotationGestureDetector.OnRotationGestureListener {
    private RotationGestureDetector mRotationDetector;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mRotationDetector = new RotationGestureDetector(this);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event){
        mRotationDetector.onTouchEvent(event);
        return super.onTouchEvent(event);
    }

    @Override
    public void OnRotation(RotationGestureDetector rotationDetector) {
        float angle = rotationDetector.getAngle();
        Log.d("RotationGestureDetector", "Rotation: " + Float.toString(angle));
    }

}

Note:

You can also use the `RotationGestureDetector` class in a `View` instead of an `Activity`.

Problem

Original source

Related problems

• Get MotionEvent.getRawX/getRawY of other pointers