Enhancement: Write conversion policy for locale-aware date formatting on write operations #17

Problem

SOSO-439 solved locale-aware validation (reading/accepting locale-formatted dates). However, the library currently sends dates in ISO 8601 format when writing (add/update). While AppSheet tolerates this when Properties.Locale is set, there is no active conversion to the locale-specific format.

Proposed Solution

Implement a WriteConversionPolicyInterface (Strategy Pattern) that optionally converts field values to locale-specific formats before sending them to the AppSheet API.

Architecture

• Strategy Pattern analogous to UnknownFieldPolicyInterface
• Interface: WriteConversionPolicyInterface with apply() method
• Built-in Implementations:
  • NoOpWriteConversionPolicy (default, backward-compatible)
  • LocaleWriteConversionPolicy (converts ISO dates to locale format)
• Injection: Via DynamicTable/DynamicTableFactory constructor
• Write Path: unknownFieldPolicy → validate → writeConversion → client.add()

Scope

Phase 1: Date, DateTime, ChangeTimestamp fields
Phase 2 (future): Percent, Price, Time, Duration (requires API research)

Key Decisions

1. Strategy Pattern over simple flag — consistent with existing architecture, extensible
2. Conversion happens after validation — ensures only valid values are converted
3. Default is NoOp — no breaking change, explicit opt-in required
4. Builds on getLocaleDateFormat() from SOSO-439 for conversion logic

Related

• Follows from SOSO-439 (locale-aware validation)
• Uses architectural patterns from SOSO-435 (UnknownFieldPolicyInterface)