An Equatable, value-semantic container for NSAttributedString attributes

TextAttributes.swift

import Foundation

/// An Equatable container for storing and accessing just the attributes of an NSAttributedString without
/// caring about the string itself.
struct TextAttributes: Equatable {
	private var storage = NSMutableAttributedString(string: "")

	mutating func setAttributes(_ attributes: [NSAttributedString.Key: Any], range: NSRange) {
		preserveUniqueReferenceToStorageIfNecessary()
		extendStringRangeIfNecessary(range: range)
		storage.setAttributes(attributes, range: range)
	}

	mutating func addAttributes(_ attributes: [NSAttributedString.Key: Any], range: NSRange) {
		preserveUniqueReferenceToStorageIfNecessary()
		extendStringRangeIfNecessary(range: range)
		storage.addAttributes(attributes, range: range)
	}

	func allAttributes() -> [(range: NSRange, attributes: [NSAttributedString.Key: Any])] {
		var result: [(NSRange, [NSAttributedString.Key: Any])] = []
		storage.enumerateAttributes(in: NSRange(location: 0, length: storage.length), options: []) { attributes, range, stop in
			result.append((range, attributes))
		}
		return result
	}

	func attributes(at location: Int) -> [NSAttributedString.Key: Any]? {
		guard location < storage.length else { return nil }
		return storage.attributes(at: location, effectiveRange: nil)
	}

	// MARK: - Private helpers

	private mutating func preserveUniqueReferenceToStorageIfNecessary() {
		if isKnownUniquelyReferenced(&storage) == false {
			// preserve value semantics
			storage = storage.mutableCopy() as! NSMutableAttributedString
		}
	}

	private mutating func extendStringRangeIfNecessary(range: NSRange) {
		if range.upperBound > storage.length {
			let extraLength = range.upperBound - storage.length
			storage.append(NSAttributedString(string: String(repeating: " ", count: extraLength)))
		}
	}
}

extension NSMutableAttributedString {
	func apply(attributes: TextAttributes) {
		for next in attributes.allAttributes() {
			guard next.range.upperBound <= self.length else { continue }
			self.setAttributes(next.attributes, range: next.range)
		}
	}
}