with OnScrimVisibilityChangedListener

MyCollapsingToolbarLayout.java

public class ECCollapsingToolbarLayout extends CollapsingToolbarLayout {

    private boolean mIsScrimShow;
    private OnScrimVisibilityChangedListener mOnScrimVisibilityChangedListener;

    private AppBarLayout.OnOffsetChangedListener mOnOffsetChangedListener;

    public ECCollapsingToolbarLayout(Context context) {
        super(context);
    }

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

    public ECCollapsingToolbarLayout(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    /**
     * @return Whether the scrim is shown.
     */
    public boolean isScrimShow() {
        return mIsScrimShow;
    }

    /**
     * Register a callback to be invoked when scrim visibility changed.
     */
    public void setOnScrimVisibilityChangedListener(@Nullable OnScrimVisibilityChangedListener listener) {
        mOnScrimVisibilityChangedListener = listener;
    }

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

        final ViewParent parent = getParent();
        if (parent instanceof AppBarLayout) {
            if (mOnOffsetChangedListener == null) {
                mOnOffsetChangedListener = new OffsetUpdateListener();
            }
            ((AppBarLayout) parent).addOnOffsetChangedListener(mOnOffsetChangedListener);
        }
    }

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

        final ViewParent parent = getParent();
        if (mOnOffsetChangedListener != null && parent instanceof AppBarLayout) {
            ((AppBarLayout) parent).removeOnOffsetChangedListener(mOnOffsetChangedListener);
        }
    }

    /**
     * Callback to be invoked when the visibility of scrim changes.
     */
    public interface OnScrimVisibilityChangedListener {

        void onShown(ECCollapsingToolbarLayout collapsingToolbarLayout);

        void onHidden(ECCollapsingToolbarLayout collapsingToolbarLayout);
    }

    private class OffsetUpdateListener implements AppBarLayout.OnOffsetChangedListener {

        OffsetUpdateListener() {
        }

        @Override
        public void onOffsetChanged(AppBarLayout appBarLayout, int verticalOffset) {
            if (isScrimVisible(verticalOffset)) {
                if (mOnScrimVisibilityChangedListener != null && !mIsScrimShow) {
                    mOnScrimVisibilityChangedListener.onShown(ECCollapsingToolbarLayout.this);
                }
                mIsScrimShow = true;
            } else {
                if (mOnScrimVisibilityChangedListener != null && mIsScrimShow) {
                    mOnScrimVisibilityChangedListener.onHidden(ECCollapsingToolbarLayout.this);
                }
                mIsScrimShow = false;
            }
        }

        private boolean isScrimVisible(int verticalOffset) {
            return getHeight() + verticalOffset < getScrimVisibleHeightTrigger();
        }
    }
}