https://www.cnblogs.com/qynprime/p/8026672.html https://blog.csdn.net/u012735483/article/details/52902047 https://juejin.im/entry/5993f4d96fb9a024865cdf5d
top ... -32
ascent ... -28
descent ... 7
bottom ... 9
leading ... 0
可以发现top、ascent值为负，是因为这里是按照baseline为0的标准进行计算的
