@Override
    public void addDataListener(Runnable runnable) {
        dataList.addListener(runnable);
    }
Returns a reference to the list backing this. The list may be modified freely,
unless this is frozen. If frozen, the only permissible write operations are those that
add new items to the end of the list.
container-core/src/main/java/com/yahoo/processing/response/ArrayDataList.java