API Availability (Swift)

Swift gates new APIs behind availability checks at the call site and availability attributes at the declaration. #available branches at runtime; @available documents the contract to the compiler.

Checking at the Call Site

#available runs the new branch when the platform supports it; #unavailable flips the test.

if #available(iOS 16, *) {
    // Use new APIs
} else {
    // Use old APIs
}

if #unavailable(iOS 16) {
    // Use old APIs
}

Declaring Availability

@available marks the platforms a declaration needs. This is the pattern for backporting: the wrapper is available early and branches to the real API when it exists.

extension View {
    @available(iOS 14, macOS 11, *)
    func backportedTask<Value: Equatable>(
        id: Value,
        task: @Sendable @escaping () async -> Void
    ) -> some View {
        if #available(iOS 15, macOS 12, *) {
            return self.task(id: id, task)
        } else {
            return self.onChange(of: id) { _ in
                Task { await task() }
            }
        }
    }
}

Deprecation Lifecycle

The long form tracks a lifecycle per platform. A deprecated version produces a compiler warning at that target; an obsoleted version produces a compiler error. message adds text to the diagnostic, and renamed gives Xcode a fix-it that rewrites the call site.

@available(iOS, introduced: 14, deprecated: 16, obsoleted: 17, renamed: "task")
@available(macOS, introduced: 11, deprecated: 12, obsoleted: 13)

Back Deployment

@backDeployed, from Swift 5.8, ships a fallback implementation to older OS versions while the new OS uses its own copy. Library authors use it so callers get one API name on every version.

extension View {
    @available(iOS 14.0, macOS 11.0, tvOS 14.0, watchOS 7.0, *)
    @backDeployed(before: iOS 16.0, macOS 13.0, tvOS 16.0, watchOS 9.0)
    func task<Value: Equatable>(
        id: Value,
        closure: @Sendable @escaping () async -> Void
    ) -> some View {
        self.onChange(of: id) { _ in
            Task { await closure() }
        }
    }
}