feat: Add AsyncSequence.flatMapLatest operator #382
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This introduces a simplified version of the flatMapLatest operator for AsyncSequence, along with a basic unit test. Note that this implementation uses unstructured concurrency and may have race conditions regarding task cancellation.
- Replaces naive implementation with a thread-safe approach using ManagedCriticalState. - Introduces generation tracking to prevent race conditions where cancelled inner sequences could yield stale values. - Adds test_interleaving_race_condition to verify correctness under concurrent load. - Ensures Swift 6 Sendable compliance.
- Replaces AsyncThrowingStream implementation with a custom AsyncSequence, Storage, and StateMachine. - Implements explicit state management using Lock for thread safety. - Handles concurrency between outer and inner sequences robustly. - Ensures correct cancellation propagation and error handling. - Verified with test_interleaving_race_condition.
- Moves AsyncFlatMapLatestSequence.swift to Sources/AsyncAlgorithms/FlatMapLatest/ - Extracts FlatMapLatestStateMachine and FlatMapLatestStorage into their own files. - Aligns project structure with other complex operators like CombineLatest and Debounce.
- Added test_outer_throwing to verify outer sequence error propagation - Added test_inner_throwing to verify inner sequence error propagation - Added test_cancellation to verify proper cancellation handling - Added test_empty_outer to verify empty outer sequence handling - Added test_empty_inner to verify empty inner sequence handling - Fixed test_simple_sequence to be more robust against timing issues
- Changed from synchronous map with throwIf to AsyncThrowingStream - Added delay to ensure proper error propagation timing - Fixes intermittent test failures
- Removed LLM-style thinking comments - Kept only essential, professional explanations - No functional changes, all tests pass
- Added FlatMapLatest.md documentation guide - Authored by Peter Friese - Includes introduction, code samples, and use cases - Covers search-as-you-type, location-based data, and dynamic config examples - Compares with similar operators in ReactiveX and Combine
- Created SAA-00nn proposal for flatMapLatest operator - Includes motivation, detailed design, and examples - Covers implementation strategy with state machine - Compares with ReactiveX switchMap and Combine switchToLatest
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This PR introduces the
flatMapLatestoperator forAsyncSequence.What's included:
flatMapLatestoperator: A new operator that transforms elements of an asynchronous sequence into new asynchronous sequences, automatically cancelling previous inner sequences when a new element arrives from the outer sequence.switchToLatest).
Motivation:
The
flatMapLatestoperator addresses the common problem of managing asynchronous operations where only the result of the most recent operation is relevant. This prevents wasted resources from stale operations and ensures users always see up-to-date information, particularly in dynamic user interfaces.This PR is an implementation of #381