Proposal: Add Decorators Support to QML for Enhanced Binding, Control Flow, and Property Behavior

Summary:
Introduce decorator support to QML syntax to enable modular extensions of properties and functions. This addresses recurring developer needs such as bi-directional bindings, custom getters/setters, debouncing of function calls, and value transformation — all in a declarative and maintainable way. Based on the ECMAScript Decorators Proposal (Stage 3).

Description:
QML benefits from a declarative syntax and JavaScript integration, but it lacks a first-class way to apply reusable logic to properties and functions without verbose boilerplate. This proposal introduces decorators to QML as a clean mechanism to attach behavior, metadata, and transformation logic in a modular way.

Decorators are increasingly popular in modern JavaScript/TypeScript (via the [ECMAScript Decorators Proposal](https://github.com/tc39/proposal-decorators)) and align well with QML’s component model and reactive paradigm.

Proposed Syntax (Hypothetical QML):

import QtQuick 2.15
import MyApp.Decorators 1.0
Item {
property alias sliderValue: slider.value
@bindTwoWay("sliderValue")
property int modelValue: 50
@getter(function()
{ return _internal * 2; }
)
@setter(function(val)
{ _internal = val / 2; }
)
property int doubled
property int _internal: 10
@transform(function(val)
{ return val.trim().toLowerCase(); }
)
property string username: "User123 "
@debounce(300)
function updateLayout()
{ console.log("Expensive layout updated"); }
onWidthChanged: updateLayout()
}

Key Use Cases:

• Bi-Directional Binding (@bindTwoWay)
  Keeps two properties (e.g. model ↔ UI) in sync. Highly requested by the QML community.

• Custom Getters/Setters (@getter, @setter)
  Enables computed properties and encapsulated logic without clutter.

• Value Transformation (@transform)
  Automatically formats or normalizes values, e.g. trimming strings or clamping numbers.

• Debounced Function Calls (@debounce)
  Improves performance for high-frequency triggers like `onTextChanged`, `onWidthChanged`, etc.

• Validation (@validate)
  Enforces runtime constraints such as ranges or patterns.

• Debugging (@logAccess, @trace)
  Adds automatic logging for diagnostics and development ease.

Why Now?

• Decorators solve multiple highly requested features with a consistent syntax.
• Aligns QML with modern JavaScript/TypeScript development patterns.
• Keeps QML declarative while enabling modular, reusable behavior.

Relevant Specification:

• ECMAScript Decorators Proposal (Stage 3)
  https://github.com/tc39/proposal-decorators([https://github.com/tc39/proposal-decorators])

Implementation Considerations:

• Can be implemented as syntactic sugar or runtime wrappers.
• Decorators may be defined in JS (as functions) or exposed via QML modules.
• Must remain fully backward-compatible and opt-in.

Benefits:

• Reduces boilerplate and increases expressiveness.
• Encourages reusable, declarative design patterns.
• Offers clean solutions to common UI + data synchronization challenges.
• Brings QML closer to the modern JavaScript ecosystem.