An NSAttributedString object manages character strings and associated sets of attributes (for example, font and kerning) that apply to individual characters or ranges of characters in the string.
这句话就是对这个类的一个最简明扼要的概括。NSAttributedString管理一个字符串，以及与该字符串中的单个字符或某些范围的字符串相关的属性。比如这个字符串“我爱北京天安门”，“我”跟其他字符的颜色不一样，而“北京”与其他的字体和大小不一样，等等。NSAttributedString就是用来存储这些信息的，具体实现时，NSAttributedString维护了一个NSString，用来保存最原始的字符串，另有一个NSDictionary用来保存各个子串/字符的属性。
Create Attributed String
有3种方法创建Attributed String。
1. 使用initWithString:, initWithString:attributes:, 或者 initWithAttributedString: ，下面是一个实例代码：
NSFont *font = [NSFont fontWithName:@"Palatino-Roman" size:14.0];
NSDictionary *attrsDictionary = [NSDictionary dictionaryWithObject:font
forKey:NSFontAttributeName];
NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:@"strigil"
attributes:attrsDictionary];可以看到上面创建的整个字符串关联了Font属性。如果希望只是对某一范围的字符串施加某个属性应该使用NSMutableAttributedString的 setAttributes:range:方法。这里例子是使用了Font属性，在Appkit中特殊定义了若干属性，这些属性被用于Core Text中。其他的属性包括前景色、背景色、是否有shadow等，具体可见本文。
2. 使用initWithRTF:documentAttributes:, initWithRTFD:documentAttributes:, and initWithRTFDFileWrapper:documentAttributes:从rich text (RTF) 或者 rich text with attachments (RTFD) 数据中创建。
NSData *rtfData = ...; // assume rtfData is an NSData object containing valid RTF data
NSDictionary *docAttributes;
NSSize paperSize;
NSAttributedString *attrString;
if ((attrString = [[NSAttributedString alloc]
initWithRTF: rtfData documentAttributes: &docAttributes])) {
NSValue *value = [docAttrs objectForKey:@"PaperSize"];
paperSize = [value sizeValue];
// implementation continues...3. 使用initWithHTML:documentAttributes: 和 initWithHTML:baseURL:documentAttributes:从HTML数据中创建。有线程安全问题，使用时需要注意。
对RTF和HTML的支持都是AppKit对NSAttributedString的扩展。
Accessing Attributes
从上面对这个类的介绍可以知道，如果我们要访问某个子串/字符的属性，需要提供子串的位置和属性的名字，而如果不提供属性名字，那就把所有属性都返回。下面就是其对应的APIs：
attributesAtIndex:effectiveRange:
attributesAtIndex:longestEffectiveRange:inRange:
attribute:atIndex:effectiveRange:
attribute:atIndex:longestEffectiveRange:inRange:
fontAttributesInRange:
rulerAttributesInRange:fontAttributesInRange: 和 rulerAttributesInRange: 是由AppKit扩展的属性。
The first four methods also return by reference the effective range and the longest effective range of the attributes. These ranges allow you to determine the extent of attributes. Conceptually, each character in an attributed string has its own collection of attributes; however, it’s often useful to know when the attributes and values are the same over a series of characters. This allows a routine to progress through an attributed string in chunks larger than a single character. In retrieving the effective range, an attributed string simply looks up information in its attribute mapping, essentially the dictionary of attributes that apply at the index requested. In retrieving the longest effective range, the attributed string continues checking characters past this basic range as long as the attribute values are the same. This extra comparison increases the execution time for these methods but guarantees a precise maximal range for the attributes requested.
Methods that return an effective range by reference are not guaranteed to return the maximal range to which the attribute(s) apply; they are merely guaranteed to return some range over which they apply. In practice they will return whatever range is readily available from the attributed string's internal storage mechanisms, which may depend on the implementation and on the precise history of modifications to the attributed string.
那些用reference返回有效范围的方法并不保证一定返回attribute应用的最大范围。它们只保证返回那些attribute有效的一些范围。实际上，它们只是返回attributed string中容易返回的那些信息，是否容易与内部实现，已经对attributed string的精确修改历史有关。
Methods that return a longest effective range by reference, on the other hand, are guaranteed to return the longest range containing the specified index to which the attribute(s) in question apply (constrained by the value of the argument passed in forinRange:). For efficiency, it is important that the inRange: argument should be as small as appropriate for the range of interest to the client.
那些返回最长有效范围的方法时能够保证返回制定attribute有效的最长的range的。为了效率，inRange: 参数应该尽量小，能满足客户需要就好。
When you iterate over an attributed string by attribute ranges, either sort of method may be appropriate depending on the situation. If there is some processing to be done for each range, and you know that the full range for a given attribute is going to have to be handled eventually, it may be more efficient to use the longest-effective-range variant, so as not to have to handle the range in pieces. However, you should use the longest-effective-range methods with caution, because the longest effective range could be quite long—potentially the entire length of the document, if the inRange: argument is not constrained.
Changing an Attributed String
NSMutableAttributedString提供若干方法，即可以修改字符串，又可以修改字符串的属性。经过多次修改后，有些信息可能变的不一致了，为了让信息保持一致，可以使用下面的方法：
fixAttributesInRange:
fixAttachmentAttributeInRange:
fixFontAttributeInRange:
fixParagraphStyleAttributeInRange:
beginEditing
endEditing
这些方法都是AppKit的扩展功能。
Reference：
1. https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/AttributedStrings/AttributedStrings.html#//apple_ref/doc/uid/10000036-BBCCGDBG
2. https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSAttributedString_AppKitAdditions/Reference/Reference.html#//apple_ref/doc/uid/20000167-BAJJCCFC
