Fast, concrete text storage intended to be subclassed.

BaseTextStorage.swift

import UIKit

/// Fast, concrete text storage intended to be subclassed.
class BaseTextStorage: NSTextStorage {

    // MARK: - Properties

    private let storage = NSMutableAttributedString()

    // MARK: - NSTextStorage

    override var string: String {
        return storage.string
    }

    override func attributes(at location: Int, effectiveRange range: NSRangePointer?) -> [NSAttributedString.Key: Any] {
        assert(location < length,
               "Tried to access attributes at out of bounds index \(location). Length: \(length)")
        return storage.attributes(at: location, effectiveRange: range)
    }

    override func replaceCharacters(in range: NSRange, with string: String) {
        assert(NSMaxRange(range) <= length,
               "Tried to replace characters at out of bounds range \(range). Length: \(length)")

        let stringLength = (string as NSString).length
        if range.length == 0 && stringLength == 0 {
            return
        }

        beginEditing()
        storage.replaceCharacters(in: range, with: string)
        edited(.editedCharacters, range: range, changeInLength: stringLength - range.length)
        endEditing()
    }

    override func setAttributes(_ attributes: [NSAttributedString.Key: Any]?, range: NSRange) {
        assert(NSMaxRange(range) <= self.length,
               "Tried to set attributes at out of bounds range \(range). Length: \(length)")

        beginEditing()
        storage.setAttributes(attributes, range: range)
        edited(.editedAttributes, range: range, changeInLength: 0)
        endEditing()
    }
}

Why import UIKit / AppKit here?

@dasmer so it works on both :)

Keep in mind that replacing the NSMutableAttributedString with a NSTextStorage instance as backing storage speeds up things tremendously.

@DivineDominion you can't initialize one directly per the documentation. That's why this class exists.

How did you measure speed up here? From what I see, NSMutableAttributedString is significantly slower than NSTextStorage. Especially when using with Swift (See https://bugs.swift.org/browse/SR-6197)

@krzyzanowskim interesting. I talked with the Swift folks about this years ago and my understanding was this was fixed. If that's not the case, use the Objective-C version instead.