// 按 x, y 居中显示文本,关键是按 y 竖直居中显示文本
private void drawNumCenter(Canvas canvas, String numStr, Paint paint, int x, int y){
int textWidth = (int)(paint.measureText(numStr) + 1);
Paint.FontMetrics fontMetrics = paint.getFontMetrics();
float distance = Math.abs(fontMetrics.top + fontMetrics.bottom)/2;
canvas.drawText(numStr, x-textWidth/2, y + distance, paint);
}