如何在android TextView中将html文本水平和垂直居中？

public class CustomTagHandler implements Html.TagHandler {
    @Override
    public void handleTag(boolean opening, String tag, Editable output, XMLReader xmlReader) {
        if (tag.equalsIgnoreCase("center")) {
            if (opening) {
                startCenterTag(output);
            } else {
                endCenterTag(output);
            }
        }
    }

    private void startCenterTag(Editable output) {
        output.setSpan(new AlignmentSpan.Standard(Layout.Alignment.ALIGN_CENTER), output.length(), output.length(), Spannable.SPAN_MARK_MARK);
    }

    private void endCenterTag(Editable output) {
        Object obj = getLast(output, AlignmentSpan.Standard.class);
        int where = output.getSpanStart(obj);
        output.removeSpan(obj);
        if (where != output.length()) {
            output.setSpan(new AlignmentSpan.Standard(Layout.Alignment.ALIGN_CENTER), where, output.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
        }
    }

    private Object getLast(Editable text, Class kind) {
        Object[] objs = text.getSpans(0, text.length(), kind);
        if (objs.length == 0) {
            return null;
        } else {
            for (int i = objs.length; i > 0; i--) {
                if (text.getSpanFlags(objs[i - 1]) == Spannable.SPAN_MARK_MARK) {
                    return objs[i - 1];
                }
            }
            return null;
        }
    }
}

TextView textView = findViewById(R.id.textView);
String htmlText = "<center><h1>This is a centered heading</h1></center>";
SpannableStringBuilder spannableStringBuilder = (SpannableStringBuilder) Html.fromHtml(htmlText);
spannableStringBuilder.setSpan(new CustomTagHandler(), 0, spannableStringBuilder.length(), Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
textView.setText(spannableStringBuilder);