feat: Lazy

packages/runed/src/lib/utilities/Lazy/Lazy.spec.ts

@@ -0,0 +1,25 @@
+import { describe, vi } from "vitest";
+import { Lazy } from "./Lazy.svelte.js";
+
+describe("lazy", () => {
+	it("calls the initialization function only when `current` is first accessed", () => {
+		const init = vi.fn(() => 0);
+		const counter = new Lazy(init);
+		expect(init).toHaveBeenCalledTimes(0);
+
+		expect(counter.current).toBe(0);
+		expect(init).toHaveBeenCalledTimes(1);
+
+		expect(counter.current).toBe(0);
+		expect(init).toHaveBeenCalledTimes(1);
+	});
+
+	it("does not call the initialization function when `current` is set", () => {
+		const init = vi.fn(() => 0);
+		const counter = new Lazy(init);
+
+		counter.current = 1;
+		expect(counter.current).toBe(1);
+		expect(init).toHaveBeenCalledTimes(0);
+	});
+});

packages/runed/src/lib/utilities/Lazy/Lazy.svelte.ts

@@ -0,0 +1,27 @@
+import { untrack } from "svelte";
+
+export class Lazy<T> {
+	#init: () => T;
+
+	constructor(init: () => T) {
+		this.#init = init;
+	}
+
+	#current: T = $state()!;
+	#initialized = false;
+
+	get current() {
+		if (!this.#initialized) {
+			untrack(() => {
+				this.#current = this.#init();
+				this.#initialized = true;
+			});
+		}
+		return this.#current;
+	}
+
+	set current(value) {
+		this.#current = value;
+		this.#initialized = true;
+	}
+}

packages/runed/src/lib/utilities/Lazy/index.ts

@@ -0,0 +1 @@
+export { Lazy } from "./Lazy.svelte.js";

packages/runed/src/lib/utilities/index.ts

@@ -20,3 +20,4 @@ export * from "./AnimationFrames/index.js";
 export * from "./useIntersectionObserver/index.js";
 export * from "./IsFocusWithin/index.js";
 export * from "./FiniteStateMachine/index.js";
+export * from "./Lazy/index.js";