How to add a SeekBar to resize Android webview text?

SeekBar seekbar = (SeekBar)root.findViewById(R.id.seekBar);

seekbar.setOnSeekBarChangeListener(new OnSeekBarChangeListener() {
    public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
        webview.getSettings().setTextZoom(progress);
    webview.loadData(abst.getAbstract_detail(), "text/html", "UTF-8");
    }

    public void onStartTrackingTouch(SeekBar seekBar) {
        // TODO Auto-generated method stub
    }

    public void onStopTrackingTouch(SeekBar seekBar) {

    }
});