Proposal: Property observers (willset/didset)

[antonsynd]

Summary

Explore adding Swift-inspired property observers to Sharpy auto-properties. This would allow reacting to property changes without converting to fully custom getter/setter syntax.

Motivation

Currently, to add side effects (validation, logging, notification) to a property, you must convert from auto-property to fully custom function-style property — a significant syntactic rewrite. Property observers would allow "auto + side effects" without changing the property's fundamental form.

Proposed Syntax (tentative)

property health: int
    didset:
        print(f"health changed from {oldvalue} to {self.health}")
    willset(new_value):
        assert new_value >= 0

Design Considerations

• willset fires before the value changes, receives the new value as a parameter
• didset fires after the value changes, oldvalue is a contextual keyword referring to the previous value
• Observers are only valid on auto-properties (not custom getter/setter)
• Maps to C# property setter with validation/notification code before/after field assignment
• Orthogonal to events — observers are per-property side effects, events are pub/sub
• Consider interaction with @dataclass auto-generated properties

Prior Art

• Swift: willSet / didSet blocks on stored properties
• Kotlin: Delegates.observable / Delegates.vetoable
• C#: INotifyPropertyChanged (library-level, verbose)

Context

Identified during syntax consolidation analysis (conversation: dataclass-property-syntax-analysis). Lower priority than @dataclass and mixed auto+custom property support — this is a future consideration once the property system foundation is solid.

Status

Proposal — needs RFC-level design before implementation.