UIFontWeightUltraLight - 超细字体
UIFontWeightThin - 纤细字体
UIFontWeightLight - 亮字体
UIFontWeightRegular - 常规字体
UIFontWeightMedium - 介于Regular和Semibold之间
UIFontWeightSemibold - 半粗字体
UIFontWeightBold - 加粗字体
UIFontWeightHeavy - 介于Bold和Black之间
UIFontWeightBlack - 最粗字体(理解)