@tanstack/query-core/build/legacy/subscribable.js

The framework agnostic core that powers TanStack Query

//#region src/subscribable.ts
/**
* The base class behind everything in Query that you can subscribe to: `QueryCache`, `MutationCache`,
* the observers, and the `FocusManager`/`OnlineManager` behind `focusManager` and `onlineManager`.
* Subclasses decide what a listener receives and when it is called.
*/
var Subscribable = class {
	constructor() {
		this.listeners = /* @__PURE__ */ new Set();
		this.subscribe = this.subscribe.bind(this);
	}
	/**
	* Registers a listener to be called on every update this object notifies about. Returns a function
	* that removes the listener again — call it to stop listening. The base class never drops a listener
	* on its own, though some subclasses clear all of theirs in `destroy()`.
	* @param listener - Called on each update, with whatever the subclass passes to its subscribers.
	* @returns A function that removes the listener.
	* @example
	* ```ts
	* const unsubscribe = subscribable.subscribe(() => {
	*   // react to the update
	* })
	*
	* unsubscribe()
	* ```
	*/
	subscribe(listener) {
		this.listeners.add(listener);
		this.onSubscribe();
		return () => {
			this.listeners.delete(listener);
			this.onUnsubscribe();
		};
	}
	/**
	* Returns `true` while at least one listener is registered, `false` once they have all unsubscribed.
	* @returns `true` if at least one listener is registered.
	*/
	hasListeners() {
		return this.listeners.size > 0;
	}
	/**
	* Called after a listener is added. Does nothing here; subclasses override it, e.g. to start
	* tracking what they observe once the first listener subscribes.
	*/
	onSubscribe() {}
	/**
	* Called after a listener is removed. Does nothing here; subclasses override it, e.g. to clean up
	* once the last listener unsubscribes.
	*/
	onUnsubscribe() {}
};
//#endregion
export { Subscribable };

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