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* @file scopeProxy.js
* @description The evaluation scope handed to template expressions, computed
* values and action bodies.
*
* ## What was wrong with building it as an object
*
* The scope used to be assembled with a spread:
*
* ```js
* const scope = { state: this.state, ...this.bridges, ...this.state, ...methods, … };
* ```
*
* `...this.state` reads every key on the reactive proxy, eagerly, every time a
* scope is built — which is once per render, per action call and per computed
* evaluation. Three separate defects followed from that one line.
*
* **Bare identifiers were not reactive.** The documented computed form
* `<computed value="count * 2" />` reads `count` out of the *snapshot* the
* spread produced, not out of the proxy, so no dependency was registered for
* the expression itself. `state.count * 2` worked; the form in the README,
* quickstart and state-management guide did not, and failed silently — the
* first render was correct and the value then never changed again.
*
* **Computed values reported false cycles.** Spreading the state proxy reads
* the computed key currently being evaluated. That re-entered the computed's
* own watcher mid-collection, tripping the re-entrancy guard: every app using
* computed properties logged `AVX_R04 Circular dependency detected` on every
* re-render, for a cycle that did not exist. Worse, the re-entrant
* `watcher.evaluate()` reset the watcher's dependency set part-way through
* collection, which is what actually lost the `count` dependency above. The
* bogus warning and the missing dependency were the same event.
*
* **Reactivity was component-granular.** Because the spread touched every key,
* the render watcher depended on all of them. Changing a key no expression
* mentioned still re-rendered the component, so the per-key precision the Proxy
* layer computes was discarded one level above it.
*
* ## What this does instead
*
* Nothing is read until an expression actually names it. The scope is a Proxy
* over ordered layers; a `get` resolves through them and, for a state or
* computed key, reads the live reactive proxy — inside whichever watcher is
* currently evaluating. So `count` in `count * 2` registers a dependency on
* `count` and on nothing else.
*
* ## Derivation instead of spreading
*
* Several call sites layer extra names onto a scope — a `<@for>` loop variable,
* an `event`, an action's `args`. Written as `{ ...scope, index }` that would
* reintroduce the eager read this module exists to remove, so scopes derive
* instead: {@link deriveScope} pushes another layer without materialising
* anything. It falls back to a spread for plain objects, so external code that
* passes an ordinary object still works.
* @module lib/core/reactive/scopeProxy
*/
/**
* Marks a value as one of this module's scope proxies.
* @type {symbol}
*/
export const IS_REACTIVE_SCOPE = Symbol.for('avenx.reactiveScope');
/**
* Derives a child scope from a scope proxy.
* @type {symbol}
*/
export const SCOPE_DERIVE = Symbol.for('avenx.scopeDerive');
/**
* Reports whether a value is a scope proxy created here.
* @param {any} value - The value to test.
* @returns {boolean} True for a reactive scope proxy.
*/
export function isReactiveScope(value) {
return !!(value && typeof value === 'object' && value[IS_REACTIVE_SCOPE]);
}
/**
* Layers extra bindings on top of an existing scope.
*
* Prefer this over `{ ...scope, extra }` anywhere a scope gains names. A spread
* of a scope proxy reads every key it can enumerate, which is exactly the eager
* read this module removes.
* @param {object} scope - The scope to extend. May be a scope proxy or a plain object.
* @param {object} extras - The bindings to layer on top.
* @returns {object} A scope with the extras applied.
*/
export function deriveScope(scope, extras) {
if (!extras) {
return scope;
}
if (isReactiveScope(scope)) {
return scope[SCOPE_DERIVE](extras);
}
return { ...scope, ...extras };
}
/**
* @typedef {object} ScopeLayer
* @property {function(string): boolean} has - Whether the layer binds a name.
* @property {function(string): any} get - The value bound to a name.
* @property {function(string, any): boolean} [set] - Assigns to a bound name.
* @property {function(): string[]} keys - The names the layer binds.
*/
/**
* Builds a layer over a plain object.
* @param {object} source - The object to expose.
* @returns {ScopeLayer} The layer.
*/
export function objectLayer(source) {
const target = source || {};
return {
/**
* @param {string} key - The name to test.
* @returns {boolean} Whether the object binds it.
*/
has(key) {
return Object.prototype.hasOwnProperty.call(target, key);
},
/**
* @param {string} key - The name to read.
* @returns {any} The bound value.
*/
get(key) {
return target[key];
},
/**
* @param {string} key - The name to write.
* @param {any} value - The value to assign.
* @returns {boolean} Always true.
*/
set(key, value) {
target[key] = value;
return true;
},
/**
* @returns {string[]} The names bound.
*/
keys() {
return Object.keys(target);
},
};
}
/**
* Builds a layer over the component's reactive state.
*
* The names are known without reading anything: they come from the raw target
* and the compiler's computed list. A value is read from the proxy only when an
* expression asks for it, which is what makes the read land inside the watcher
* that is currently evaluating.
* @param {object} state - The reactive state proxy.
* @param {function(): string[]} listKeys - Returns the bound names.
* @returns {ScopeLayer} The layer.
*/
export function stateLayer(state, listKeys) {
let cached = null;
return {
/**
* @param {string} key - The name to test.
* @returns {boolean} Whether state binds it.
*/
has(key) {
if (!cached) {
cached = new Set(listKeys());
}
return cached.has(key);
},
/**
* @param {string} key - The name to read.
* @returns {any} The value, read through the reactive proxy.
*/
get(key) {
return state[key];
},
/**
* @param {string} key - The name to write.
* @param {any} value - The value to assign.
* @returns {boolean} Always true.
*/
set(key, value) {
state[key] = value;
return true;
},
/**
* @returns {string[]} The names bound.
*/
keys() {
if (!cached) {
cached = new Set(listKeys());
}
return [...cached];
},
};
}
/**
* Builds a layer of lazily evaluated bindings.
*
* Used for resources, whose value is produced by calling `read()` and must not
* be produced merely because a scope was built.
* @param {Object<string, function(): any>} getters - Name to getter.
* @returns {ScopeLayer} The layer.
*/
export function getterLayer(getters) {
const source = getters || {};
return {
/**
* @param {string} key - The name to test.
* @returns {boolean} Whether a getter is bound.
*/
has(key) {
return Object.prototype.hasOwnProperty.call(source, key);
},
/**
* @param {string} key - The name to read.
* @returns {any} The getter's result.
*/
get(key) {
return source[key]();
},
/**
* @returns {string[]} The names bound.
*/
keys() {
return Object.keys(source);
},
};
}
/**
* Creates the evaluation scope.
*
* Layers are given in precedence order, highest first: the first layer that
* binds a name wins. That mirrors the object-literal precedence the previous
* implementation had, where later spreads overwrote earlier ones.
* @param {ScopeLayer[]} layers - The layers, highest precedence first.
* @returns {object} A scope proxy.
*/
export function createReactiveScope(layers) {
const active = layers.filter(Boolean);
/**
* Finds the layer that binds a name.
* @param {string} key - The name to resolve.
* @returns {ScopeLayer|null} The binding layer, or null.
*/
function resolve(key) {
for (const layer of active) {
if (layer.has(key)) {
return layer;
}
}
return null;
}
const handler = {
/**
* @param {object} target - Unused backing object.
* @param {string|symbol} key - The name to test.
* @returns {boolean} Whether the scope binds it.
*/
has(target, key) {
if (key === IS_REACTIVE_SCOPE || key === SCOPE_DERIVE) return true;
if (typeof key === 'symbol') return false;
return resolve(key) !== null;
},
/**
* @param {object} target - Unused backing object.
* @param {string|symbol} key - The name to read.
* @returns {any} The bound value, or undefined.
*/
get(target, key) {
if (key === IS_REACTIVE_SCOPE) return true;
if (key === SCOPE_DERIVE) {
return (extras) => createReactiveScope([objectLayer(extras), ...active]);
}
if (typeof key === 'symbol') return undefined;
const layer = resolve(key);
return layer ? layer.get(key) : undefined;
},
/**
* @param {object} target - Unused backing object.
* @param {string|symbol} key - The name to write.
* @param {any} value - The value to assign.
* @returns {boolean} Whether the write was accepted.
*/
set(target, key, value) {
if (typeof key === 'symbol') return false;
const layer = resolve(key);
if (layer && typeof layer.set === 'function') {
return layer.set(key, value);
}
// An undeclared name assigned from an action lands on the highest layer
// that can hold it, matching the previous behaviour where
// `scope[key] = value` created a key on the scope object.
for (const candidate of active) {
if (typeof candidate.set === 'function') {
return candidate.set(key, value);
}
}
return false;
},
/**
* Enumerating a scope materialises it, which is what spreading one does.
* Present so that external code holding a scope still behaves; internal
* call sites use {@link deriveScope} instead.
* @returns {string[]} Every bound name.
*/
ownKeys() {
const names = new Set();
for (const layer of active) {
for (const key of layer.keys()) {
names.add(key);
}
}
return [...names];
},
/**
* @param {object} target - Unused backing object.
* @param {string|symbol} key - The name to describe.
* @returns {object|undefined} A descriptor for a bound name.
*/
getOwnPropertyDescriptor(target, key) {
if (typeof key === 'symbol') return undefined;
if (!resolve(key)) return undefined;
return { configurable: true, enumerable: true, writable: true, value: this.get(target, key) };
},
};
return new Proxy(Object.create(null), handler);
}
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