Attributed Strings

Attributed string objects manage character strings and associated sets of attributes (for example, font and kerning) that apply to individual characters or ranges of characters in the string. The classes NSAttributedString and NSMutableAttributedString declare the programmatic interface for read-only attributed strings and modifiable attributed strings, respectively. The Foundation Kit defines the basic functionality, while additional Objective-C methods are defined in the Application Kit. The Application Kit also uses a subclass of NSMutableAttributedString, called NSTextStorage, to provide the storage for the extended text-handling system (see Text System Storage Layer Overview).

NSAttributedString and NSMutableAttributedString are toll-free bridged to their Core Foundation counterparts, CFAttributedString and CFMutableAttributedString respectively. This means that a Foundation attributed string is interchangeable in function or method calls with the corresponding bridged Core Foundation type. Therefore, in a method where you see an NSMutableAttributedString * parameter, you can pass in a variable of type CFMutableAttributedStringRef, and in a function where you see a CFAttributedStringRef parameter, you can pass in an instance of NSAttributedString (or NSMutableAttributedString).

NSAttributedString is not a subclass of NSString. It contains an NSString object to which it applies attributes. This protects users of attributed strings from ambiguities caused by the semantic differences between simple and attributed strings. For example, equality can’t be simply defined between an NSString and an attributed string. The attributed string classes adopt the NSCopying and NSMutableCopying protocols, making it convenient to convert an attributed string from one type to the other.

NSAttributedString and NSMutableAttributedString add a number of features to the basic content storage of NSString:

• Association of arbitrary, programmer-defined attributes with ranges of characters.

• Preservation of attribute-to-character mapping after changes (NSMutableAttributedString).

• Support for RTF, including file attachments and graphics.

• Drawing in NSView objects (note that the Application Kit adds drawing methods to NSString as well)

• Linguistic unit (word) and line calculation.

An attributed string identifies attributes by name, storing their values as opaque ids in an NSDictionary object. For example, the text font is stored as an NSFont object under the name given by NSFontAttributeName. You can associate any object value, by any name, with a given range of characters in the attributed string.

A mutable attributed string keeps track of the attribute mapping as characters are added to and deleted from it and as attributes are changed. It allows you to group batches of edits with the beginEditing and endEditing methods, and to consolidate changes to the attribute-to-character mapping with the fix... methods.