Add change tracking support for complex collections

[AndriySvyryd]

Part of #31237

Overview

The implementation introduces change-tracking support for complex type collections that enables EF Core to detect modifications, additions, and deletions within (nested) complex type collections. This design leverages ordinal-based tracking and runtime-generated property accessors to efficiently manage state changes in complex type collections.

Change tracking entries

This change introduces InternalComplexEntry, like InternalEntityEntry it directly tracks the original values and changes to the contained properties and non-collection complex properties. But unlike the latter, it doesn't contain a reference to the tracked object as it could be of a value type.

To access nested complex entries for a particular complex type collection the current or original ordinal for the corresponding collection needs to be used.

The change-tracking system employs a dual-ordinal approach to maintain element positioning:

• Original Ordinals: Preserve the initial ordering of elements as they existed when first tracked by the context. These serve as immutable identifiers that remain constant throughout the entity's lifetime in the change tracker.
• Current Ordinals: Reflect the present ordering of elements after any modifications. These values update dynamically as elements are added, removed, or reordered within collections.

This dual-ordinal system enables the change tracker to correlate elements between their original and current states, facilitating accurate detection of positional changes and element migrations within collections.

Value-Type Support for Observable Collections

The architecture is designed with forward compatibility for value-type collections. The ordinal-based tracking mechanism abstracts away the reference semantics typically required for change detection, allowing the same infrastructure to support both reference-type complex objects and value-type collections. This is achieved through the indices array system that provides positional addressing regardless of the underlying element type.

DetectChanges Algorithm

The DetectChanges process for complex type collections implements a matching algorithm that relies on snapshots:

1. Snapshot Comparison: The system maintains snapshots of collection states and compares current values against these snapshots to identify modifications.
2. Element Matching: Deleted and added elements are matched using a combination of:
   • Ordinal position correlation
   • Reference comparison for identifying moved elements for reference types
   • Null-matching heuristics that mark a property as modified if it transitions to or from a null value

Property Accessor Architecture

The property accessor system employs runtime code generation to create efficient access patterns for nested complex type collections:

Indices Array System

Each complex type collection maintains an indices array that contains index values for each level of nesting. For a deeply nested structure like Order.Details[i].LineItems[j].Properties[k], the indices array would contain [i, j, k], representing the position at each collection level.