FontMetrics对象 它以四个基本坐标为基准，分别为：
・FontMetrics.top
・FontMetrics.ascent
・FontMetrics.descent
・FontMetrics.bottom
//获取文字的宽高
Rect rect = new Rect();
wordsPaint.getTextBounds(string, 0, 1, rect);
Paint.FontMetrics fontMetrics = wordsPaint.getFontMetrics();
int wordWidth = rect.width();
//绘制字母
float wordX = itemWidth / 2 - wordWidth / 2;
// float wordY = i * itemHeight+1/2 * itemHeight + 1/2 * (fontMetrics.descent- fontMetrics.ascent) - fontMetrics.bottom;
float wordY = i * itemHeight + itemHeight / 2 +
((fontMetrics.bottom - fontMetrics.top) / 2 - fontMetrics.bottom);
