{"id":44086,"date":"2024-07-15T13:43:53","date_gmt":"2024-07-15T17:43:53","guid":{"rendered":"https:\/\/mjtsai.com\/blog\/?p=44086"},"modified":"2024-07-15T13:43:53","modified_gmt":"2024-07-15T17:43:53","slug":"nscopying-in-a-swift-world","status":"publish","type":"post","link":"https:\/\/mjtsai.com\/blog\/2024\/07\/15\/nscopying-in-a-swift-world\/","title":{"rendered":"NSCopying in a Swift World"},"content":{"rendered":"

Douglas Hill<\/a>:<\/p>\n

\n

This crash happens because, behind the scenes, the Swift compiler synthesises overrides of a superclass’s designated initialisers. These overridden initialisers crash to prevent objects from being incorrectly initialised from Objective-C.<\/p>\n

[…]<\/p>\n

From a quick look on Stack Overflow, it seems [self.class alloc]<\/code> is often a recommended way to create a copy in Objective-C. However, the problem is that the use of self.class<\/code> dynamically looks up the subclass SocialDocument<\/code>, but the code here in our framework has no idea that SocialDocument<\/code> has changed the initialisation requirements.<\/p>\n

[…]<\/p>\n

If Document<\/code> were a simpler type where all state that should be copied was public, then subclasses that required copying to create instances of the subclass could override copy(with:)<\/code> without calling super<\/code>[…]<\/p>\n

[…]<\/p>\n

There isn’t a nice way to make copying subclasses work while still adhering to Swift’s principle of reducing the amount of mutable state by using let<\/code> to create read-only properties.<\/p>\n<\/blockquote>\n\n

Adrian Kashivskyy<\/a>:<\/p>\n

PSA: Don’t cast values to NSCopying in Swift or you risk a crash<\/a> at runtime. Learned this the hard way. 🤠<\/p>

[…]<\/p>

Because all values that were originally bridged from Objective-C will pass the alone NSCopying<\/code> cast, even when they don’t actually conform to the protocol. Such values immediately become instances of _SwiftValue<\/code> and, like trojans, they will sit there\npretending to be innocent but will crash at runtime as soon as they’re accessed.<\/p><\/blockquote>\n\n

Previously:<\/p>\n