如果有人在处理罗斯福的代码示例时遇到困难（我肯定是，也许是因为它是Xamarin.Android？），这是一个更基本的Android Java版本的翻译：

    public class RoundedBackgroundSpan extends ReplacementSpan {

        private static int CORNER_RADIUS = 8;
        private int backgroundColor = 0;
        private int textColor = 0;

        public RoundedBackgroundSpan(Context context) {
            super();
            backgroundColor = context.getResources().getColor(R.color.gray);
            textColor = context.getResources().getColor(R.color.white);
        }

        @Override
        public void draw(Canvas canvas, CharSequence text, int start, int end, float x, int top, int y, int bottom, Paint paint) {
            RectF rect = new RectF(x, top, x + measureText(paint, text, start, end), bottom);
            paint.setColor(backgroundColor);
            canvas.drawRoundRect(rect, CORNER_RADIUS, CORNER_RADIUS, paint);
            paint.setColor(textColor);
            canvas.drawText(text, start, end, x, y, paint);
        }

        @Override
        public int getSize(Paint paint, CharSequence text, int start, int end, Paint.FontMetricsInt fm) {
            return Math.round(paint.measureText(text, start, end));
        }

        private float measureText(Paint paint, CharSequence text, int start, int end) {
            return paint.measureText(text, start, end);
        }
    }

为了便于使用，以下代码段取自Activity，基本上在每个标签字符串周围放置了一个圆角背景，每个标签之间都有一个空格缓冲区。请注意，注释掉的行仅带有背景色，看起来效果不佳。

    SpannableStringBuilder stringBuilder = new SpannableStringBuilder();

    String between = "";
    for (String tag : eventListing.getTags()) {
       stringBuilder.append(between);
       if (between.length() == 0) between = "  ";
       String thisTag = "  "+tag+"  ";
       stringBuilder.append(thisTag);
       stringBuilder.setSpan(new RoundedBackgroundSpan(this), stringBuilder.length() - thisTag.length(), stringBuilder.length() - thisTag.length() + thisTag.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
       //stringBuilder.setSpan(new BackgroundColorSpan(getResources().getColor(R.color.gray)), stringBuilder.length() - thisTag.length(), stringBuilder.length(), Spanned.SPAN_EXCLUSIVE_EXCLUSIVE);
    }

    TextView tv = new TextView(this);
    tv.setText(stringBuilder);

根据pskink的建议，我设法解决了我的问题。这是我的课：

public class RoundedBackgroundSpan : ReplacementSpan
{
    public override void Draw(Canvas canvas, ICharSequence text, int start, int end, float x, int top, int y, int bottom, Paint paint)
    {
        var rect = new RectF(x, top, x + MeasureText(paint, text, start, end), bottom);
        paint.Color = Application.Context.Resources.GetColor(Resource.Color.nextTimeBackgroundColor);
        canvas.DrawRoundRect(rect, Application.Context.Resources.GetDimensionPixelSize(Resource.Dimension.localRouteDetailsRoundRectValue), Application.Context.Resources.GetDimensionPixelSize(Resource.Dimension.localRouteDetailsRoundRectValue), paint);
        paint.Color = Application.Context.Resources.GetColor(Resource.Color.nextTimeTextColor);
        canvas.DrawText(text, start, end, x, y, paint);
    }

    public override int GetSize(Paint paint, ICharSequence text, int start, int end, Paint.FontMetricsInt fm)
    {
        return Math.Round(MeasureText(paint, text, start, end));
    }

    private float MeasureText(Paint paint, ICharSequence text, int start, int end)
    {
        return paint.MeasureText(text, start, end);
    }
}

用法示例：

var stringBuilder = new SpannableStringBuilder();
var stringToAppend = "hello world";
stringBuilder.Append(stringToAppend);
stringBuilder.SetSpan(new RoundedBackgroundSpan(), stringBuilder.Length() - stringToAppend.Length, stringBuilder.Length(), SpanTypes.ExclusiveExclusive);