How does `NSAttibutedString` equate attribute values of type `Any`?

Question

The enumerateAttribute(_:in:options:using:) method of NSAttributedString appears to equate arbitrary instances of type Any. Of course, Any does not conform to Equatable, so that should not be possible.

Question: How does the method compare one instance of Any to another?

Context: In Swift, I am subclassing NSTextStorage, and have need to provide my own implementation of this method in Swift.

Observations:

• NSAttributedString attributes come in key-value pairs, with the keys being instances of type NSAttributedString.Key and the values being instances of type Any?, with each pair being associated with one or more ranges of characters in the string. At least, that is how the data structure appears from the outside; the internal implementation is opaque.
• The enumerateAttribute method walks through the entire range of an NSAttributedString, effectively identifying each different value corresponding to a specified key, and with the ranges over which that value applies.
• The values corresponding to a given key could be of multiple different types.
• NSAttributedString seemingly has no way of knowing what underlying types the Any values might be, and thus seemingly no way of type casting in order to make a comparison of two given Any values.
• Yet, the method somehow is differentiating among ranges of the string based on differences in the Any values.
• Interestingly, the method is able to differentiate between values even when the underlying type does not conform to Equatable. I take this to be a clue that the method may be using some sort of reflection to perform the comparison.
• Even more interesting, the method goes so far as to differentiate between values when the underlying type does conform to Equatable and the difference between two values is a difference that the specific implementation of Equatable intentionally ignores. In other words, even if a == b returns true, if there is a difference in opaque properties of a and b that are ignored by ==, the method will treat the values as being different, not the same.
• I assume the method bridges to an implementation in ObjC.

Is the answer: It cannot be done in Swift?

Answer 1

As you know, Cocoa is Objective-C, so these are Objective-C NSDictionary objects, not Swift Dictionary objects. So equality comparison between them uses Objective-C isEqual, not Swift ==. We are not bound by Swift strict typing, the Equatable protocol, or anything else from Swift.

To illustrate, here's a slow and stupid but effective implementation of style run detection:

let s = NSMutableAttributedString(
    string: "howdy", attributes: [.foregroundColor:UIColor.red])
s.addAttributes([.foregroundColor:UIColor.blue], 
    range: NSRange(location: 2, length: 1))
var lastatt = s.attributes(at: 0, effectiveRange: nil)
for ix in 1..<5 {
    let newatt = s.attributes(at:ix, effectiveRange:nil)
    if !(newatt as NSDictionary).isEqual(to: lastatt) {
        print("style run ended at \(ix)")
        lastatt = newatt
    }
}

That correctly prints:

style run ended at 2
style run ended at 3

So since it is always possible to compare the attributes at any index with the attributes at another, it is possible to implement attribute enumeration in Swift. (Whether that's a good idea is another question.)