Base subclass of NSTextStorage in Swift

NSTextStorageSubclass.swift

import Cocoa

@objc
class SomeTextStorage: NSTextStorage {
    
    private var storage: NSMutableAttributedString
    
    override init() {
        storage = NSMutableAttributedString(string: "", attributes: nil)
        super.init()
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("\(__FUNCTION__) is not supported")
    }
    
    required init?(pasteboardPropertyList propertyList: AnyObject, ofType type: String) {
        fatalError("\(__FUNCTION__) is not supported")
    }
    
    // MARK: NSTextStorage Primitive Methods
    // https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/TextStorageLayer/Tasks/Subclassing.html
    
    override var string: String {
        return storage.string
    }
    
    override func attributesAtIndex(location: Int, effectiveRange range: NSRangePointer) -> [String : AnyObject] {
        return storage.attributesAtIndex(location, effectiveRange: range)
    }
    
    override func replaceCharactersInRange(range: NSRange, withString str: String) {
        beginEditing()
        storage.replaceCharactersInRange(range, withString: str)
        edited(.EditedCharacters, range: range, changeInLength: (str as NSString).length - range.length)
        endEditing()
    }
    
    override func setAttributes(attrs: [String : AnyObject]?, range: NSRange) {
        beginEditing()
        storage.setAttributes(attrs, range: range)
        edited(.EditedAttributes, range: range, changeInLength: 0)
        endEditing()
    }

}

Great, thanks! Wrapping the mutating methods in beginEditing/endEditing even though it's not strictly necessary solves a lot of delegate callback problems.

But I also notice that this minimal replacement type is enough to let memory consumption spike when editing if you also replace the layout manager, like with textView.textContainer?.replaceLayoutManager(NSLayoutManager()):

• put 14k words in text view
• scroll down a bit
• hit enter a couple of times

According to instruments, I can get 8 GB of allocations in 8 seconds, mostly due to the string read-only property.

The memory issue is gone when you delegate to a NSTextStorage instead of a NSAttributedString, though:
https://stackoverflow.com/questions/37952726/sub-classing-nstextstorage-causes-significant-memory-issues

class SomeTextStorage: NSTextStorage {

    private var storage = NSTextStorage()
        
    // MARK: NSTextStorage Primitive Methods
    // https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/TextStorageLayer/Tasks/Subclassing.html
    
    override var string: String {
        return storage.string
    }
    
    override func attributes(at location: Int, effectiveRange range: NSRangePointer?) -> [String : Any] {
        return storage.attributes(at: location, effectiveRange: range)
    }
    
    override func replaceCharacters(in range: NSRange, with str: String) {
        beginEditing()
        storage.replaceCharacters(in: range, with: str)
        edited(.editedCharacters, range: range, changeInLength: (str as NSString).length - range.length)
        endEditing()
    }
    
    override func setAttributes(_ attrs: [String : Any]?, range: NSRange) {
        beginEditing()
        storage.setAttributes(attrs, range: range)
        edited(.editedAttributes, range: range, changeInLength: 0)
        endEditing()
    }
}

Since beginEditing/endEditing is not needed here but works nicely, I do not call these methods on storage. Note that I am not 100% certain if the behavior will change for the better or worse in some circumstances if you do.