解开UIFontDescriptor

Question

我在应用程序中使用动态类型，并且在某些情况下，我希望更改字体的外观，例如，使其斜体或取消粗体。添加样式非常容易：

UIFontDescriptor *descriptor = [[UIFontDescriptor preferredFontDescriptorWithTextStyle:UIFontTextStyleHeadline]
                                fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitItalic];
UIFont *font = [UIFont fontWithDescriptor:descriptor size:descriptor.pointSize];

但是，没有明确的机制来移除样式。我可以尝试调整属性，但是使用完全没有文档的API，它们看起来更加令人生畏：

常规标题：{ NSCTFontUIUsageAttribute = UICTFontTextStyleHeadline；NSFontNameAttribute = ".AppleSystemUIHeadline"；NSFontSizeAttribute = 17；}意大利标题：{ NSCTFontUIUsageAttribute = UICTFontTextStyleItalicHeadline；NSFontNameAttribute = ".AppleSystemUIItalicHeadline"；NSFontSizeAttribute = 17；}

我错过了另一条路吗？我可以使用[UIFont systemFontWithSize:descriptor.pointSize]，但是我不想丢失任何由动态类型提供的绘图规则。

Answer 1

fontDescriptorWithSymbolicTraits:方法实际上能够做您想做的事情，除了在内置语义文本样式中支持字体特征的一些边缘情况之外。这里的关键概念是，该方法将前面描述符上的所有符号特征替换为新的特征。文献资料在这个问题上有点犹豫，只是简单地说，新的特征“优先于”旧的。

按位操作用于添加和删除特定的特征，但在使用由preferredFontDescriptorWithTextStyle:生成的描述符时，似乎需要特别小心。不是所有的字体都支持所有的特征。例如，标题字体根据用户喜欢的内容大小进行加权，即使可以去掉描述符的粗体特征，匹配的UIFont也将是粗体。不幸的是，这在任何地方都没有被记录下来，所以发现任何额外的细微差别都是留给读者的练习。

以下示例说明了这些问题：

// Start with a system font, in this case the headline font
// bold: YES italic: NO
UIFontDescriptor * originalDescriptor = [UIFontDescriptor preferredFontDescriptorWithTextStyle:UIFontTextStyleHeadline];
NSLog(@"originalDescriptor bold: %d italic: %d",
      isBold(originalDescriptor), isItalic(originalDescriptor));

// Try to set the italic trait. This may not be what you expected; the 
// italic trait is not added. On a normal UIFontDescriptor the italic
// trait would have been set and the bold trait unset.
// Ultimately it seems that there is no variant of the headline font that
// is italic but not bold.
// bold: YES italic: NO
UIFontDescriptor * italicDescriptor = [originalDescriptor fontDescriptorWithSymbolicTraits:UIFontDescriptorTraitItalic];
NSLog(@"italicDescriptor bold: %d italic: %d",
      isBold(italicDescriptor), isItalic(italicDescriptor));

// The correct way to make this font descriptor italic (and coincidentally
// the safe way to make any other descriptor italic without discarding its
// other traits) would be as follows:
// bold: YES italic: YES
UIFontDescriptor * boldItalicDescriptor = [originalDescriptor fontDescriptorWithSymbolicTraits:(originalDescriptor.symbolicTraits | UIFontDescriptorTraitItalic)];
NSLog(@"boldItalicDescriptor bold: %d italic: %d",
      isBold(boldItalicDescriptor), isItalic(boldItalicDescriptor));

// Your intention was to remove bold without affecting any other traits, which
// is also easy to do with bitwise logic.
// Using the originalDescriptor, remove bold by negating it then applying
// a logical AND to filter it out of the existing traits.
// bold: NO  italic: NO
UIFontDescriptor * nonBoldDescriptor = [originalDescriptor fontDescriptorWithSymbolicTraits:(originalDescriptor.symbolicTraits & ~UIFontDescriptorTraitBold)];
NSLog(@"nonBoldDescriptor bold: %d italic: %d",
      isBold(nonBoldDescriptor), isItalic(nonBoldDescriptor));

// Seems like it worked, EXCEPT there is no font that matches. Turns out
// there is no regular weight alternative for the headline style font.
// To confirm, test with UIFontDescriptorTraitsAttribute as the mandatory
// key and you'll get back a nil descriptor.
// bold: YES italic: NO
nonBoldDescriptor = [nonBoldDescriptor matchingFontDescriptorsWithMandatoryKeys:nil].firstObject;
NSLog(@"nonBoldDescriptor bold: %d italic: %d",
      isBold(nonBoldDescriptor), isItalic(nonBoldDescriptor));

为了简洁起见，上面使用的isBold和isItalic函数可以实现如下：

 BOOL isBold(UIFontDescriptor * fontDescriptor)
 {
    return (fontDescriptor.symbolicTraits & UIFontDescriptorTraitBold) != 0;
 }

 BOOL isItalic(UIFontDescriptor * fontDescriptor)
 {
    return (fontDescriptor.symbolicTraits & UIFontDescriptorTraitItalic) != 0;
 }