Like and Unlike alpha view effect - kikoso/Swipeable-Cards

CardContainer.java

@Override
    public boolean onTouchEvent(MotionEvent event) {
        if (mTopCard == null) {
            return false;
        }
        if (mGestureDetector.onTouchEvent(event)) {
            return true;
        }
        Log.d("Touch Event", MotionEvent.actionToString(event.getActionMasked()) + " ");
        final int pointerIndex;
        final float x, y;
        final float dx, dy;
        View alphaView;
        switch (event.getActionMasked()) {
            case MotionEvent.ACTION_DOWN:
                mTopCard.getHitRect(childRect);

                pointerIndex = event.getActionIndex();
                x = event.getX(pointerIndex);
                y = event.getY(pointerIndex);

                if (!childRect.contains((int) x, (int) y)) {
                    return false;
                }
                mLastTouchX = x;
                mLastTouchY = y;
                mActivePointerId = event.getPointerId(pointerIndex);


                float[] points = new float[]{x - mTopCard.getLeft(), y - mTopCard.getTop()};
                mTopCard.getMatrix().invert(mMatrix);
                mMatrix.mapPoints(points);
                mTopCard.setPivotX(points[0]);
                mTopCard.setPivotY(points[1]);

                break;
            case MotionEvent.ACTION_MOVE:

                pointerIndex = event.findPointerIndex(mActivePointerId);
                x = event.getX(pointerIndex);
                y = event.getY(pointerIndex);

                dx = x - mLastTouchX;
                dy = y - mLastTouchY;

                if (Math.abs(dx) > mTouchSlop || Math.abs(dy) > mTouchSlop) {
                    mDragging = true;
                }

                if (!mDragging) {
                    return true;
                }

                mTopCard.setTranslationX(mTopCard.getTranslationX() + dx);
                mTopCard.setTranslationY(mTopCard.getTranslationY() + dy);

                mTopCard.setRotation(40 * mTopCard.getTranslationX() / (getWidth() / 2.f));

                // Alpha like or dislike view
                float distance = Math.abs(mTopCard.getTranslationX());
                float alpha = (distance / 100);
                if (mTopCard.getTranslationX() > 0)
                    alphaView = ((View) getChildAt(getChildCount() - 1).findViewById(R.id.tvLike));
                else
                    alphaView = ((View) getChildAt(getChildCount() - 1).findViewById(R.id.tvDislike));
                alphaView.setAlpha(alpha);

                mLastTouchX = x;
                mLastTouchY = y;
                break;
            case MotionEvent.ACTION_UP:
            case MotionEvent.ACTION_CANCEL:
                if (!mDragging) {
                    return true;
                }
                mDragging = false;
                mActivePointerId = INVALID_POINTER_ID;
                ValueAnimator animator = ObjectAnimator.ofPropertyValuesHolder(mTopCard,
                        PropertyValuesHolder.ofFloat("translationX", 0),
                        PropertyValuesHolder.ofFloat("translationY", 0),
                        PropertyValuesHolder.ofFloat("rotation", (float) Math.toDegrees(mRandom.nextGaussian() * DISORDERED_MAX_ROTATION_RADIANS)),
                        PropertyValuesHolder.ofFloat("pivotX", mTopCard.getWidth() / 2.f),
                        PropertyValuesHolder.ofFloat("pivotY", mTopCard.getHeight() / 2.f)
                ).setDuration(250);
                animator.setInterpolator(new AccelerateInterpolator());
                animator.start();

                // Animate to alpha 0
                if (mTopCard.getTranslationX() > 0)
                    alphaView = ((View) getChildAt(getChildCount() - 1).findViewById(R.id.tvLike));
                else
                    alphaView = ((View) getChildAt(getChildCount() - 1).findViewById(R.id.tvDislike));
                ValueAnimator animatorAlpha = ObjectAnimator.ofPropertyValuesHolder(alphaView,
                        PropertyValuesHolder.ofFloat("alpha", 0)
                ).setDuration(250);
                animatorAlpha.setInterpolator(new AccelerateInterpolator());
                animatorAlpha.start();

                break;
            case MotionEvent.ACTION_POINTER_UP:
                pointerIndex = event.getActionIndex();
                final int pointerId = event.getPointerId(pointerIndex);

                if (pointerId == mActivePointerId) {
                    final int newPointerIndex = pointerIndex == 0 ? 1 : 0;
                    mLastTouchX = event.getX(newPointerIndex);
                    mLastTouchY = event.getY(newPointerIndex);

                    mActivePointerId = event.getPointerId(newPointerIndex);
                }
                break;
        }

        return true;
    }

std_card_inner.xml

<TextView
        android:id="@+id/tvLike"
        android:layout_width="match_parent"
        android:layout_height="150dp"
        android:text="Gostei"
        android:textSize="30dp"
        android:layout_margin="15dp"
        android:textColor="#ffffff"
        android:gravity="left"
        android:alpha="0"/>

    <TextView
        android:id="@+id/tvDislike"
        android:layout_width="match_parent"
        android:layout_height="150dp"
        android:text="Não gostei"
        android:textSize="30dp"
        android:layout_margin="15dp"
        android:textColor="#ffffff"
        android:gravity="right"
        android:alpha="0"/>