Android RotateAnimation, how to perform more rotations?

I have to rotate an ImageView on a Button click. At the first click it have to rotate to right, at the second to left etc.

The problem is that when I try to rotate for second time the "just rotated" image, the rotation start from original point and not from "post first rotation" point.

I need to rotate the image resulting from previous rotation. Below I past the code.

public class Rotate extends Activity {
    boolean mDirRight = true;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.rotate);
        final ImageView imageArray = (ImageView) findViewById(R.id.ImageViewArray);
        imageArray.setImageResource(R.drawable.array01);
        imageArray.setAdjustViewBounds(true);
        final Button btnRotate = (Button) findViewById (R.id.ButtonRotate);
        btnRotate.setOnClickListener(new OnClickListener() {
            @Override
            pu开发者_如何学JAVAblic void onClick(View v) {
                doRotation();
            }
        });    
    }

    private void doRotation(){
        final int rotationRight = 30;
        final int rotationLeft = -20;
        final RotateAnimation rAnim;
        int degree;
        if (mDirRight) {
            degree = rotationRight;
            mDirRight = false;
        } else {
            degree = rotationLeft;
            mDirRight = true;
        }
        final ImageView image = (ImageView) findViewById(R.id.ImageViewArray);
        rAnim = new RotateAnimation(0f, degree, RotateAnimation.RELATIVE_TO_SELF, 0.5f, RotateAnimation.RELATIVE_TO_SELF, 0.5f);
        rAnim.setStartOffset(0);
        rAnim.setDuration(2000);
        rAnim.setFillAfter(true);
        rAnim.setFillEnabled(true);
        image.startAnimation(rAnim);
    }
}

In this line

rAnim = new RotateAnimation(0f, degree, RotateAnimation.RELATIVE_TO_SELF, 0.5f, RotateAnimation.RELATIVE_TO_SELF, 0.5f);

change 0f to the desired starting angle.