SerialUpdatingValue

Thread-safe access to a lazily retrieved value, with optional validity checking

Motivation

Swift's Structured Concurrency provides many primitives, but lacks higher-level mechanics such as serial queues. This package is an investigation into how to reimplement that.

A recent Twitter thread inspired the investigation: https://twitter.com/layoutSubviews/status/1486925222137954304

Swift Twitter, help me, you’re my only hope: With Swift Concurrency, is there an equivalent to a dispatch serial queue, i.e. something on which @Sendable () async -> Void operations can be enqueued, and which executes them serially?

Goals

Locally cache an auth token, providing concurrent access, lazy fetching, and re-fetching if no longer valid.

• Using an actor for thread-safety
• Generic over a Value
• Has a single getter method/property to get an up to date Value
• The value is fetched by a caller-defined async function
• Lazily fetch the first value upon first request
• While fetching, all subsequent requests must also await the fetch value
• Once fetched, cache it in-memory for faster performance in subsequent requests
• Include a way to invalidate the locally cached value, requiring the value to be re-fetched if invalid
• No concurrency warnings!

Discussion

More discussion about this on the Swift Forums: https://forums.swift.org/t/rfc-using-swift-concurrency-as-a-serial-blocking-queue/55135