Swift Concurrency Adoption and Migration Plan

Role: Staff iOS Systems Architect specializing in Swift Concurrency runtime internals and legacy codebase modernizations.

Context

• Target application: {{app_name}}
• Deployment target: iOS {{target_ios_version}}
• Legacy concurrency patterns in use: {{legacy_concurrency_patterns}}
• High-risk business subsystems: {{critical_subsystems}}
• Compiler strict concurrency level target: {{strict_concurrency_level}}

Task

Generate a comprehensive, phased technical migration plan to transition {{app_name}} from {{legacy_concurrency_patterns}} to modern Swift structured concurrency, resolving data races across {{critical_subsystems}} while targeting {{strict_concurrency_level}} compliance on iOS {{target_ios_version}}.

Method

1. Analyze the existing codebase boundaries in {{critical_subsystems}} to isolate completion-handler-driven APIs and thread-locking primitives.
2. Establish an architectural baseline identifying Sendable protocol violations and shared mutable state.
3. Design a bottom-up refactoring sequence prioritizing data access layers before UI coordinator bindings.
4. Define custom global actors or isolated actor types to encapsulate mutable state in {{critical_subsystems}}.
5. Specify bridging mechanisms using withCheckedThrowingContinuation to modernize {{legacy_concurrency_patterns}} safely.
6. Formulate compiler diagnostic flags roadmap to advance progressively to {{strict_concurrency_level}}.
7. Detail regression testing protocols, thread sanitizer (TSan) execution, and performance benchmarking for context-switch overhead.

Constraints

• MUST prioritize zero regression on main-thread responsiveness across all migrated paths.
• MUST NOT recommend unilateral rewrite approaches; migrations must remain backward-compatible across phases.
• MUST include explicit fallback handling for legacy completion callbacks during migration phases.
• All actor boundaries must be explicitly declared and justified.

Output format

Provide a technical plan structured into four mandatory sections:

1. Architectural Gap Analysis (max 250 words)
2. Phased Migration Schedule (3 phased milestones with entry/exit criteria)
3. Critical Path Actor Design (code patterns and isolation boundaries)
4. Thread Safety & Validation Protocol (TSan and regression steps, max 200 words)

Self-review

• Ensure every variable from context ({{app_name}}, {{target_ios_version}}, {{legacy_concurrency_patterns}}, {{critical_subsystems}}, {{strict_concurrency_level}}) is specifically addressed.
• Confirm that exactly 4 named output sections are produced.
• Verify all migration steps maintain binary stability and do not block the main runloop.