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useCallback,
useEffect,
useMemo,
useRef,
useSyncExternalStore,
} from "react";
import {
subscribe,
getSnapshot as getStoreSnapshot,
emit as storeEmit,
getSubscriberCount,
getEmitCount,
getTimestamp,
getData,
} from "./store";
/**
* Signal metadata object for debugging and monitoring.
*
* @remarks
* `info` is a stable object whose fields are backed by getters that read the
* store **on access**. The values are therefore render-time snapshots, not
* reactive state — reading `info.subscriberCount` (or any other field) does
* **not** subscribe the component to changes in that value. A field only
* reflects new data on renders that happen for another reason (e.g. after the
* `signal` version changes). Do not bind UI that must update live (such as a
* subscriber count badge) to these fields expecting them to re-render on their
* own; drive such UI from the `signal` version instead.
*/
export interface SignalInfo<T = unknown> {
/** Signal subscription name */
readonly name: string;
/** Current number of active subscribers (read-on-access, not reactive) */
readonly subscriberCount: number;
/** Timestamp of last emit (Date.now()) */
readonly timestamp: number;
/** Total number of times this signal has been emitted */
readonly emitCount: number;
/** Data passed with the last emit */
readonly data: T | undefined;
}
/**
* Options for useSignal hook
*/
export interface SignalOptions {
/** Automatically emit when component mounts */
emitOnMount?: boolean;
/** Callback executed when emit is called */
onEmit?: () => void;
/** Conditionally enable/disable subscription (default: true) */
enabled?: boolean;
/** Debounce emit calls in milliseconds */
debounce?: number;
}
/**
* Return type for useSignal hook
*/
export interface UseSignalReturn<T = unknown> {
/** Current signal version number - use in dependency arrays */
signal: number;
/** Function to emit the signal and notify all subscribers, optionally with data */
emit: (data?: T) => void;
/** Stable metadata object for debugging and monitoring */
info: SignalInfo<T>;
}
/**
* A hook for event-driven communication between components without prop drilling.
* Components subscribe to a shared signal by name. When any component emits,
* all subscribers receive a new version number.
*
* @param name - Unique identifier string for the signal channel
* @param options - Configuration options
* @returns Object containing signal value, emit function, and info metadata
*
* @example
* ```tsx
* // Parent Component - emits signal
* function ParentComponent() {
* const { emit } = useSignal("Dashboard Refresh");
*
* return <button onClick={emit}>Refresh All</button>;
* }
*
* // Child Component - subscribes to signal
* function DataTable() {
* const { signal } = useSignal("Dashboard Refresh");
*
* useEffect(() => {
* fetchTableData();
* }, [signal]);
*
* return <table>...</table>;
* }
* ```
*
* @example
* ```tsx
* // With debugging info
* function MonitoredComponent() {
* const { signal, emit, info } = useSignal("API Sync", {
* onEmit: () => console.log("Syncing...")
* });
*
* useEffect(() => {
* syncData();
* console.log(`Sync #${info.emitCount} with ${info.subscriberCount} listeners`);
* }, [signal]);
*
* return <button onClick={emit}>Sync</button>;
* }
* ```
*
* @example
* ```tsx
* // With data payload
* function DataEmitter() {
* const { emit } = useSignal<{ userId: string }>("user-action");
*
* const handleClick = (userId: string) => {
* emit({ userId });
* };
*
* return <button onClick={() => handleClick("123")}>Action</button>;
* }
*
* function DataReceiver() {
* const { signal, info } = useSignal<{ userId: string }>("user-action");
*
* useEffect(() => {
* if (info.data) {
* console.log("User action for:", info.data.userId);
* }
* }, [signal]);
*
* return <div>Listening...</div>;
* }
* ```
*/
export function useSignal<T = unknown>(
name: string,
options: SignalOptions = {}
): UseSignalReturn<T> {
const {
emitOnMount = false,
onEmit,
enabled = true,
debounce,
} = options;
// Store options in refs for stable references
const onEmitRef = useRef(onEmit);
onEmitRef.current = onEmit;
// Stable name ref for info object
const nameRef = useRef(name);
nameRef.current = name;
// Info object with stable reference using getters for live data
const infoRef = useRef<SignalInfo<T> | null>(null);
if (!infoRef.current) {
infoRef.current = {
get name() {
return nameRef.current;
},
get subscriberCount() {
return getSubscriberCount(nameRef.current);
},
get timestamp() {
return getTimestamp(nameRef.current);
},
get emitCount() {
return getEmitCount(nameRef.current);
},
get data() {
return getData(nameRef.current) as T | undefined;
},
} as SignalInfo<T>;
}
// Subscribe function for useSyncExternalStore
const subscribeToStore = useCallback(
(onStoreChange: () => void) => {
if (!enabled) {
return () => {};
}
return subscribe(name, onStoreChange);
},
[name, enabled]
);
// Get snapshot function
const getSnapshot = useCallback((): number => {
if (!enabled) {
return 0;
}
return getStoreSnapshot(name);
}, [name, enabled]);
// Server snapshot (always 0 for SSR)
const getServerSnapshot = useCallback((): number => {
return 0;
}, []);
// Use useSyncExternalStore for synchronized state
const signal = useSyncExternalStore(
subscribeToStore,
getSnapshot,
getServerSnapshot
);
// Base emit function
const baseEmit = useCallback(
(data?: T) => {
storeEmit(name, data);
onEmitRef.current?.();
},
[name]
);
// Debounce timer ref
const debounceTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
// Store the latest data for debounced emit
const pendingDataRef = useRef<T | undefined>(undefined);
// Debounced or regular emit
const emit = useMemo(() => {
if (!debounce || debounce <= 0) {
return baseEmit;
}
return (data?: T) => {
// Store the latest data
pendingDataRef.current = data;
if (debounceTimerRef.current) {
clearTimeout(debounceTimerRef.current);
}
debounceTimerRef.current = setTimeout(() => {
baseEmit(pendingDataRef.current);
pendingDataRef.current = undefined;
debounceTimerRef.current = null;
}, debounce);
};
}, [baseEmit, debounce]);
// Cancel any pending debounced emit when the timing or the target channel
// changes (and on unmount). Keying on [name, debounce] ensures a timer
// scheduled for a previous `name` never fires against the new channel — it is
// cleared before the debounced `emit` closure is recreated for the new name.
useEffect(() => {
return () => {
if (debounceTimerRef.current) {
clearTimeout(debounceTimerRef.current);
debounceTimerRef.current = null;
pendingDataRef.current = undefined;
}
};
}, [name, debounce]);
// Emit on mount if option is set.
// Guarded by a ref so React 18/19 StrictMode's mount → unmount → remount
// double-invoke fires the mount emit exactly once (the ref persists across
// the remount of the same component instance).
const didEmitOnMountRef = useRef(false);
useEffect(() => {
if (emitOnMount && !didEmitOnMountRef.current) {
didEmitOnMountRef.current = true;
baseEmit();
}
}, [emitOnMount, baseEmit]);
return {
signal,
emit,
info: infoRef.current!,
};
}
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