Animate Android PhotoView height

I am using https://github.com/chrisbanes/PhotoView and trying to animate its height.

I use ValueAnimator and update the layout height, so that triggers the internal PhotoViewAttacher and onGlobalLayoutwhich transforms the matrix.

Is there any workaround to prevent scale and y position to be unchanged, like could I somehow update the matrix myself to keep the image Y position and scaleX/scaleY unchanged? Now those are reset to scale 1.0 and y position center of image.

Animation code:

ValueAnimator animator = ValueAnimator.ofInt(start, end).setDuration(300);

animator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {
        @Override
        public void onAnimationUpdate(ValueAnimator animation) {
            mImageView.getLayoutParams().height = (int) animation.getAnimatedValue();
            mImageView.requestLayout();
        }
    });

animator.start();

This was more tricky than first thought. The library doesn't seem to have public API's to get this to work, so here is a workaround using private methods and fields. Following solution seems to work with my cases where I can change the actual height of PhotoView and scale + y position keeps unchanged during the animations.

First extend PhotoView and write following code:

// Private fields and methods
private Matrix mBaseMatrix;
private Field mAttacher;
private Method mGetDrawMatrix;
private Method mSetImageViewMatrix;
private Method mCheckMatrixBounds;

...

@Override
protected void init() {
    super.init();

    getViewTreeObserver().removeOnGlobalLayoutListener(
            (ViewTreeObserver.OnGlobalLayoutListener) getIPhotoViewImplementation());

    getViewTreeObserver().addOnGlobalLayoutListener(
            new ViewTreeObserver.OnGlobalLayoutListener() {
                @Override
                public void onGlobalLayout() {
                    updateBaseMatrix();
                }
            });
}

private void updateBaseMatrix() {
    try {
        if (mBaseMatrix == null) {
            mBaseMatrix = getMatrix("mBaseMatrix");
        }

        if (mBaseMatrix != null) {
            mBaseMatrix.setValues(new float[]{
                    1.0f, 0.0f, 0.0f,
                    0.0f, 1.0f, 0.0f,
                    0.0f, 0.0f, 1.0f
            });
        }

        if (mAttacher == null) {
            mAttacher = PhotoView.class.getDeclaredField("mAttacher");
            mAttacher.setAccessible(true);
        }

        if (mGetDrawMatrix == null) {
            mGetDrawMatrix = PhotoViewAttacher.class.getDeclaredMethod("getDrawMatrix");
            mGetDrawMatrix.setAccessible(true);
        }

        Matrix drawMatrix = (Matrix) mGetDrawMatrix.invoke(mAttacher.get(this));

        if (mSetImageViewMatrix == null) {
            mSetImageViewMatrix = PhotoViewAttacher.class
                    .getDeclaredMethod("setImageViewMatrix", Matrix.class);
            mSetImageViewMatrix.setAccessible(true);
        }

        mSetImageViewMatrix.invoke(mAttacher.get(this), drawMatrix);

        if (mCheckMatrixBounds == null) {
            mCheckMatrixBounds = PhotoViewAttacher.class.getDeclaredMethod("checkMatrixBounds");
            mCheckMatrixBounds.setAccessible(true);
        }

        mCheckMatrixBounds.invoke(mAttacher.get(this));
    } catch (Exception e) {
        e.printStackTrace();
    }
}

private Matrix getMatrix(String fieldName) {
    try {
        Field f = PhotoView.class.getDeclaredField("mAttacher");
        f.setAccessible(true);

        PhotoViewAttacher a = (PhotoViewAttacher) f.get(this);

        f = PhotoViewAttacher.class.getDeclaredField(fieldName);
        f.setAccessible(true);

        return (Matrix) f.get(a);
    } catch (Exception e) {
        e.printStackTrace();
    }

    return null;
}