Tell me about Sendable types and how they fit into the concurrency world

Sendable types are an important concept in Swift concurrency, designed to ensure safe data sharing across concurrent tasks. The Sendable protocol in Swift marks types that are safe to share between different threads or tasks. When a type conforms to Sendable, it guarantees that its instances can be safely used in concurrent contexts without causing data races.

Here are some key points about Sendable types and their role in Swift concurrency:

1. Value Types: Many basic Swift types, such as strings, integers, and dates, are value types and inherently safe to share across threads. Collections like arrays and dictionaries are also value types and can be sendable when their elements are sendable.

2. Conditional Conformance: Collections like arrays define conditional conformances to Sendable, meaning they are sendable when their elements are sendable.

3. Actors: Actors in Swift are designed to protect mutable state by ensuring that only one task can access the state at a time. Actors are inherently sendable, allowing actor objects to be shared safely across concurrent tasks.

4. Main Actor: Types associated with the main actor are implicitly sendable, which means you don't need to explicitly mark them as sendable.

5. Concurrency Safety: The Swift compiler checks for sendability whenever data is passed between actors and tasks, helping to prevent data races and ensuring concurrency safety.

6. Classes and Closures: Classes and closures can involve mutable state that is not safe to share concurrently. Therefore, they should be used from one task at a time unless they are explicitly made sendable.

For more detailed information on Sendable types and their use in Swift concurrency, you can refer to the session Embracing Swift concurrency (16:56) from WWDC 2025.