NSMutableAttributedString

NSMutableAttributedString is a mutable subclass of NSAttributedString in the Foundation framework. It represents a string together with per-range attributes, allowing both the characters and their styling to be edited after creation. This makes it the primary data structure for building and modifying richly formatted text in Cocoa and Cocoa Touch applications.

Like NSAttributedString, an NSMutableAttributedString stores attributes as key–value pairs associated with specific ranges of the string. Attribute keys identify the kind of styling, such as font, color, or paragraph style, and values convey the corresponding settings. Typical keys include font, foreground color, and paragraph style (in older Objective-C APIs as NSFontAttributeName, NSForegroundColorAttributeName, NSParagraphStyleAttributeName; in modern Swift as NSAttributedString.Key.font, .foregroundColor, .paragraphStyle). Attributes can be added, replaced, or removed over chosen ranges using methods such as addAttribute:value:range:, addAttributes:range:, setAttributes:range:, or removeAttribute:range:.

Common operations include appending or inserting attributed substrings, replacing characters, and changing attributes for selected ranges. Because both the text and its attributes are mutable, developers can iteratively build complex styling, search and modify formatting, or enumerate attributes across ranges for processing or rendering.

Usage contexts include rendering rich text in UI elements such as UILabel’s attributedText or UITextView, as well as in AppKit controls like NSTextView. NSMutableAttributedString integrates with drawing and layout systems to produce styled text across different platforms and locales.