Key difference between Dictionary and NSDictionary

from blog The Desolation of Blog, 4 Dec 2017 | ↗ original

The recent article Strings in Swift 4 by Ole Begemann talked about how Swift String equality is implemented as Unicode canonical equivalence. As a result, two String instances can be equal even if they contain different Unicode code units. This behavior made me worried about my Swift project Bonjeff, a Mac app that shows you a live display of the Bonjour services published on your network, because Bonjour service names are UTF-8 strings. When I audited the equality checks in the source code, I discovered that Bonjour does not compare service names by Unicode canonical equivalence, and thus it's possible to publish multiple NetService instances with canonically equivalent names. Unfortunately, this revealed a bug where Bonjeff did not properly save the individual outline view disclosure states of Bonjour services with canonically equivalent names. The problem was that UserDefaults.standard.dictionary(forKey:) returns [String:Any]?. Since the dictionary uses Swift String keys, it cannot contain more than one value for canonically equivalent Unicode strings. Thus, multiple NetService instances with canonically equivalent names would fail to be distinguished.

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Key difference between Dictionary and NSDictionary

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