How to ignore parts of a .dump (handling expected differences)

[jklingen]

As a user of the SnapshotTesting library, I'd like to be able to ignore parts of a .dump snapshot, so that I can snapshot test a data structure containing (expectedly) varying properties.

In my specific use case, the data structure is not entirely under my control and contains generated UUIDs. I guess other use cases might include data like generated time stamps, so I think being able to regex-replace expected differences would be a good way to go.

Is there a way to extend the existing strategy in a way that I can make modifications to the snapped string before diffing? (ideally without copypasting all of Any.swift into my code base).
If yes, I'd love to learn about it. If not, I'd be happy to provide a PR enhancing Any.swift to accept an optional regex pattern.

[stephencelis]

@jklingen I'm going to convert this to a discussion since it's a question and not a bug report.

There are a couple ways to override a portion of a dump:

1. Under the hood, the dump strategy uses Mirror, which means if you can conform the type to CustomReflectable you can control the dump's hierarchy and return only the fields you want to preserve.
2. SnapshotTesting also ships with a protocol that allows you to override that portion of the dump with a string:

   swift-snapshot-testing/Sources/SnapshotTesting/Snapshotting/Any.swift

   Lines 66 to 75 in 9e28725

   	/// A type with a customized snapshot dump representation.
   	///
   	/// Types that conform to the `AnySnapshotStringConvertible` protocol can provide their own representation to be used when converting an instance to a `dump`-based snapshot.
   	public protocol AnySnapshotStringConvertible {
   	/// Whether or not to dump child nodes (defaults to `false`).
   	static var renderChildren: Bool { get }
   	/// A textual snapshot dump representation of this instance.
   	var snapshotDescription: String { get }
   	}

Our newer library swift-custom-dump offers even more features and flexibility here, which we may use in the future here. But if you need those features now, you could write your own strategy that uses CustomDump under the hood.

[jklingen]

Thanks so much for the quick and detailed response, looks very promising - I am pretty sure these options are a great fit for our use case. I will give it a try tomorrow 🙇