iOS 字体权重weight
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)