iOS导航栏按钮图片不同大小在表现上的区别

测试工具:
图片一张，正方形

测试环境:
6s模拟器

1.图片大小为20*20PX
<UINavigationButton: 0x7fa47d735c40; frame = (338 6; 32 30); opaque = NO; layer = <CALayer: 0x610000234b20>>

计算得出：centerY=21

2.图片大小为40*40PX
<UINavigationButton: 0x7fb9cf7451c0; frame = (328 6; 42 30); opaque = NO; layer = <CALayer: 0x60000042f6c0>>

计算得出：centerY=21

3.图片大小为60*60PX
<UINavigationButton: 0x7fa263a463d0; frame = (318 6; 52 30); opaque = NO; layer = <CALayer: 0x600000421040>>

计算得出：centerY=21

4.图片大小为80*80PX
<UINavigationButton: 0x7fb6f335ea80; frame = (308 1; 62 40); opaque = NO; layer = <CALayer: 0x61800022dba0>>

计算得出：centerY=21

5.图片大小为100*100PX
<UINavigationButton: 0x7f9009e51ad0; frame = (298 -4; 72 50); opaque = NO; layer = <CALayer: 0x6100004226e0>>

计算得出：centerY=21

得出结论: 导航栏左右按钮的centerY恒等于21，而titleView的centerY恒等于22