public class ObservableScrollView extends ScrollView {
    private ArrayList<Callbacks> mCallbacks = new ArrayList<Callbacks>();

    public ObservableScrollView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        for (Callbacks c : mCallbacks) {
            c.onScrollChanged(l - oldl, t - oldt);
        }
    }

    @Override
    public int computeVerticalScrollRange() {
        return super.computeVerticalScrollRange();
    }

    public void addCallbacks(Callbacks listener) {
        if (!mCallbacks.contains(listener)) {
            mCallbacks.add(listener);
        }
    }

    public static interface Callbacks {
        public void onScrollChanged(int deltaX, int deltaY);
    }
}

public class ImageryHeaderFragment extends Fragment
        implements ObservableScrollView.Callbacks {
	...
	@Override
	public View onCreateView(LayoutInflater inflater, ViewGroup container,
	                         Bundle savedInstanceState) {
		... findViewByID ...
	    setupCustomScrolling(rootView);
	    return rootView;
	}
	
	private void setupCustomScrolling(View rootView) {
	    scrollView = (ObservableScrollView) rootView.findViewById(R.id.scroll_view);
	    scrollView.addCallbacks(this);
	    ...
	}

    private ViewTreeObserver.OnGlobalLayoutListener globalLayoutListener
            = new ViewTreeObserver.OnGlobalLayoutListener() {
        @Override
        public void onGlobalLayout() {
            recomputeHeaderImageAndScrollingMetrics();
        }
    };

    private void setupCustomScrolling(View rootView) {
        ...
        ViewTreeObserver vto = scrollView.getViewTreeObserver();
        if (vto.isAlive()) {
            vto.addOnGlobalLayoutListener(globalLayoutListener);
        }
    }

    private void recomputeHeaderImageAndScrollingMetrics() {
        final int actionBarSize = calculateActionBarSize();
        headerBarTopClearance = actionBarSize - headerBarContents.getPaddingTop();
        headerBarContentsHeightPixels = headerBarContents.getHeight();

        headerImageHeightPixels = headerBarTopClearance;
        if (showHeaderImage) {
            headerImageHeightPixels =
		            (int) (headerImage.getWidth() / HEADER_IMAGE_ASPECT_RATIO);
            headerImageHeightPixels = Math.min(headerImageHeightPixels,
		            rootView.getHeight() * 2 / 3);
        }

        ViewGroup.LayoutParams lp;
        lp = headerImageContainer.getLayoutParams();
        if (lp.height != headerImageHeightPixels) {
            lp.height = headerImageHeightPixels;
            headerImageContainer.setLayoutParams(lp);
        }

        lp = headerBarBackground.getLayoutParams();
        if (lp.height != headerBarContentsHeightPixels) {
            lp.height = headerBarContentsHeightPixels;
            headerBarBackground.setLayoutParams(lp);
        }

        ViewGroup.MarginLayoutParams mlp = (ViewGroup.MarginLayoutParams)
                bodyContainer.getLayoutParams();
        if (mlp.topMargin
		         != headerBarContentsHeightPixels + headerImageHeightPixels) {
            mlp.topMargin = headerBarContentsHeightPixels + headerImageHeightPixels;
            bodyContainer.setLayoutParams(mlp);
        }

        onScrollChanged(0, 0); // trigger scroll handling
    }

    // GapFillアニメ開始位置の調整. 開始位置に"遊び"を持たせる.
    private static final float GAP_FILL_DISTANCE_MULTIPLIER = 1.5f;

    // ヘッダ画像スクロール時のパララックスエフェクト係数
    private static final float HEADER_IMAGE_BACKGROUND_PARALLAX_EFFECT_MULTIPLIER
		    = 0.5f;

    @Override
    public void onScrollChanged(int deltaX, int deltaY) {
        final Activity activity = getActivity();
        if (activity == null || activity.isFinishing()) {
            return;
        }

        // Reposition the header bar -- it's normally anchored to the
        // top of the content, but locks to the top of the screen on scroll
        int scrollY = scrollView.getScrollY();

        float newTop = Math.max(headerImageHeightPixels,
		        scrollY + headerBarTopClearance);
        headerBarContainer.setTranslationY(newTop);
        headerBarBackground.setPivotY(headerBarContentsHeightPixels);

        int gapFillDistance = 
		        (int) (headerBarTopClearance * GAP_FILL_DISTANCE_MULTIPLIER);
        boolean showGapFill = !showHeaderImage || 
		        (scrollY > (headerImageHeightPixels - gapFillDistance));
        float desiredHeaderScaleY = showGapFill ?
                ((headerBarContentsHeightPixels + gapFillDistance + 1) * 1f 
		                / headerBarContentsHeightPixels)
                : 1f;
        if (!showHeaderImage) {
            headerBarBackground.setScaleY(desiredHeaderScaleY);
        } else if (gapFillShown != showGapFill) {
            headerBarBackground.animate()
                    .scaleY(desiredHeaderScaleY)
                    .setInterpolator(new DecelerateInterpolator(2f))
                    .setDuration(250)
                    .start();
        }
        gapFillShown = showGapFill;

        // Make a shadow. TODO: Do not need if running on AndroidL
        headerBarShadow.setVisibility(View.VISIBLE);

        if (headerBarTopClearance != 0) {
            // Fill the gap between status bar and header bar with color
            float gapFillProgress = Math.min(Math.max(getProgress(scrollY,
                    headerImageHeightPixels - headerBarTopClearance * 2,
                    headerImageHeightPixels - headerBarTopClearance), 0), 1);
            // TODO: Set elevation prope