从字体中删除字符

Question

据this WWDC doc我所知，我的代码应该有用，但它没有：

NSString *fontName = [@"MaisonNeue-" stringByAppendingString:@"Light"];
UIFont *originalFont = [UIFont fontWithName:fontName size:size];
UIFontDescriptor *originalDescriptor = [originalFont fontDescriptor];

NSMutableCharacterSet *cset = [[originalDescriptor objectForKey:UIFontDescriptorCharacterSetAttribute] mutableCopy];
[cset formIntersectionWithCharacterSet:[[NSCharacterSet decimalDigitCharacterSet] invertedSet]];

NSDictionary *newAttributes = @{UIFontDescriptorCharacterSetAttribute: cset};

UIFontDescriptor *monospaceDescriptor = [originalDescriptor fontDescriptorByAddingAttributes:newAttributes];
UIFont *newFont = [UIFont fontWithDescriptor:monospaceDescriptor size:0.0];

出了什么问题？新创建的字体仍包含我专门删除的字符：

(lldb) p [cset characterIsMember:'2']
(bool) $0 = false
(lldb) po [(NSCharacterSet *)[(id)monospaceDescriptor objectForKey:UIFontDescriptorCharacterSetAttribute] characterIsMember:'2']
false
(lldb) po [(NSCharacterSet *)[[newFont fontDescriptor] objectForKey:UIFontDescriptorCharacterSetAttribute] characterIsMember:'2']
true