Swift High Performance: Dictionary Keys

I improved Factory’s dictionary lookup performance by over 500%! Can you do the same in your app?

Photo by Chunli Ju on Unsplash

This article is an in-depth exploration of how to create dictionary keys that are fast and performant under all circumstances, and why String-based keys often fail in that regard.

It includes actual examples and performance data taken from Factory, my open-source dependency injection library.

Ready? Let’s get into it.

Caching

A common practice in any application is to create some expensive item, cache that item in a key-based dictionary, and then check later on to see if that item exists, returning the cached value if it does.

An extremely rudimentary implementation of that might be:

class ItemRepository {
    private var cache = [String:Item]()
    func item(forKey key: String) -> Item? {
        if let item = cache[key] {
            return item
        }
        let item = Item(key: key)
        cache[key] = item
        return item
    }
}

We’ve all seen or done some variation of this.

But the fundamental problem we have here is that using a Swift String as our dictionary key isn’t an optimal choice. Far…