SE-0385: Custom Reflection Metadata

[hborla (Holly Borla)]

@filip-sakel

The stored-properties use case is another part of the proposal I had some trouble understanding. I think the text has some motivating examples spread throughout but the actual “Motivation” section mainly focuses on tests. Since this is a novel and quite broad feature, I think it’d be more effective to discuss specific cases.

Sure thing, I can add a code example involving property wrapper "metadata" to the Motivation section!

@filip-sakel

Coming back to property wrappers, my understanding of the proposal is that property wrappers used to store metadata are inefficient. Are there any examples the authors are considering where the current inefficiency has significant impacts on code size or performance?

Yes, but it's not just about performance. It's a serious expressivity limitation that you must have an instance in order to retrieve property wrapper metadata. I've seen use cases that will create dummy instances of types just to gather the metadata from stored property wrappers. I'll include this in the code example I'll add to the Motivation section.

EDIT: I remembered a very recent example posted on the forums over here:

@tgoyne

The other is something not directly related to this proposal: I don't think keyPathToColumnId is currently implementable. _enclosingInstance has a similar problem where we need to turn a keypath into something that can identify which database column to access. There we take advantage of that the target of the keypath is a property wrapper we define, and store the column id on each of the property wrappers (initializing these property wrappers involves ivar_getOffset() and pointer math, and is the part that I'm not sure is actually valid).

This use case in Realm is using a very brittle approach that turns a key-path into an identifier used for database access. This use-case could instead use a custom metadata attribute that provides an identifier for a given key-path that is provided by the programmer, or we could have #function in a metadata attribute grab the name of the function it's attached to (which also works for properties today).