Cache Me If You Can

What?

Lightweight, type-safe caching based on URLs in Swift.

Why?

We're focusing on something we have to do a lot: Load data from a URL that will not change, such as a PDF file or an image, so that it can be cached locally. In cases like this, if the image or file is changed by a user or an admin, then a new URL is returned rather than new data being returned by the same URL.

Far more often than not, we've found you can make the assumption that the data at a given URL will not change, but most existing libraries for caching on iOS don't make this assumption. If you can't assume data at the same URL will always be the same, you should use another library for caching.

However, if you can make that assumption, caching logic can be way simpler. That's why we built this library.

We do:

• Caching objects in memory or to the file system based on a URL
• Specification and enforcement of callback DispatchQueues
• Individual or wholesale item removal from the cache

We don't do:

• HTTP-based caching
• Time-based expiration
• Automatic re-fetching if the contents of the URL have changed
• Automatic invalidation if the contents of the URL have been removed

How?

Generics and Enums and Protocols! This is basically not usable from Obj-C without writing a wrapper. 😜

The main useful classes are:

FileSystemDataCache

A cache which stores data from DataConvertible types on the filesystem - ie, anything which you declare can be optionally converted to Data.

Also includes:

• Downloading items from the given URL if they are not present in the filesystem
• The DataConvertibleCodable protocol to make it really easy to store any Codable objects using one of these caches. See UserCache in the tests for an example of how to implement this.

ImageCache

Since this is one of the main things we built this for, we've included a UIImage-specific version of the FileSystemDataCache to save everyone the trouble of making their own.

InMemoryCache

A cache which stores AnyObject and any conforming types in-memory using an underlying NSCache.

Note: This cache does not automatically handle downloading since Data is actually a struct.

DownloadHelper

A simple URLSession-based wrapper to fetch Data from a single URL and call back on a given queue, and throw basic errors if it fails.

When?

RIGHT NOW!

Who?

This project is sponsored by Bakken & Bæck, but specifically, contributors include:

• Ellen Shapiro
• You? Open a PR!

Shout out to Colin Dodd for naming this library.