Swift Concurrency Modernization and Actor Isolation Plan

Role: Principal Swift Systems Architect specializing in language runtime safety and modern multithreading architecture.

Context

• Target Subsystem: {{target_subsystem}}
• Existing Asynchronous Patterns: {{legacy_asynchronous_patterns}}
• Target Swift Concurrency Level: {{swift_concurrency_mode}}
• Minimum Supported OS Deployment Target: {{minimum_os_target}}
• Thread-Critical Dependencies: {{third_party_dependencies}}
• Critical Codebase Size / Module Count: {{module_scope_size}}

Task

Develop a comprehensive architectural migration plan to transition {{target_subsystem}} from {{legacy_asynchronous_patterns}} to modern Swift structured concurrency under {{swift_concurrency_mode}} without introducing regressions, data races, or UI stutter on {{minimum_os_target}}.

Method

1. Audit the dependency graph of {{target_subsystem}} across {{module_scope_size}} to identify leaf nodes and isolate non-Sendable types.
2. Classify shared mutable state across components and determine boundaries for @MainActor, custom global actors, and isolated actor types.
3. Map current completion-handler APIs, Combine pipelines, and DispatchQueues in {{legacy_asynchronous_patterns}} to equivalent async/await and AsyncSequence contracts.
4. Design compatibility shims using @preconcurrency and continuation bridges to support un-migrated code in {{third_party_dependencies}}.
5. Sequence the migration into phased pull requests, starting with leaf domain models before refactoring view models and coordinator layers.
6. Establish runtime validation workflows leveraging Xcode Thread Sanitizer (TSan) and concurrency runtime diagnostics.
7. Formulate a fallback and regression rollback protocol targeting specific subsystem cutovers.

Constraints

• MUST eliminate all compiler warnings generated by {{swift_concurrency_mode}}.
• MUST NOT introduce priority inversion or deadlocks caused by unconstrained Task instantiation.
• Plan MUST maintain backward runtime compatibility with {{minimum_os_target}}.
• Keep implementation guidance strictly tailored to Swift native concurrency primitives.

Output format

Provide a structured migration plan divided into:

1. Architecture & Model Sendability Strategy (bulleted type classifications)
2. Phased Rollout Sequence (numbered phases with entry/exit criteria)
3. Dependency & Compatibility Bridge Plan (table format with component, approach, risk)
4. Verification & Testing Matrix (test type, tooling, pass condition) Total length must be between 400 and 800 words.

Self-review

• Did I define clear Actor isolation boundaries for all shared mutable state?
• Are all listed steps compliant with the constraints of {{minimum_os_target}}?
• Does the plan isolate high-risk third-party calls using @preconcurrency or continuations?