Introducing Swift RunOnce

Daniel Roth from Thumbtack's iOS team just solved a problem every Swift developer has faced: ensuring code runs exactly once without the brittle boolean flags that break at scale.

Thumbtack Engineering open-sourced Swift-RunOnce, a thread-safe utility that eliminates an entire category of bugs around one-time code execution. The solution uses Objective-C runtime hacks and Swift's literal expressions to create truly declarative, inline one-time code blocks that can't be invalidated.

Key Takeaways

• Traditional boolean flag approach fails at scale with multiple engineers working in parallel
• Solution uses objc_setAssociatedObject and source code location (#fileId, #line, #column) as unique identifiers
• Thread-safe, reentrant implementation completely hides state from developers
• Enables trailing closure syntax: self.runOnce { /* code here */ }
• Available on GitHub under Apache 2.0 license

The article reveals why even Swift's lazy var syntax fails the thread-safety test and how the team worked around fundamental language limitations.