Add .subscribe- method to signals to add support for Svelte

.changeset/large-news-report.md

@@ -0,0 +1,5 @@
+---
+"@preact/signals-core": minor
+---
+
+Add `.subscribe()`-method to signals to add support for natively using signals with Svelte

packages/core/src/index.ts

@@ -123,6 +123,10 @@ export class Signal<T = any> {
 		};
 	}
 
+	subscribe(fn: (value: T) => () => void) {
+		return effect(() => fn(this.value));
+	}
+
 	/**
 	 * A custom update routine to run when this Signal's value changes.
 	 * @internal

packages/core/test/signal.test.tsx

@@ -47,6 +47,28 @@ describe("signal", () => {
 			expect(b.peek()).to.equal(2);
 		});
 	});
+
+	describe(".subscribe()", () => {
+		it("should subscribe to a signal", () => {
+			const spy = sinon.spy();
+			const a = signal(1);
+
+			a.subscribe(spy);
+			expect(spy).to.be.calledWith(1);
+		});
+
+		it("should unsubscribe from a signal", () => {
+			const spy = sinon.spy();
+			const a = signal(1);
+
+			const dispose = a.subscribe(spy);
+			dispose();
+			spy.resetHistory();
+
+			a.value = 2;
+			expect(spy).not.to.be.called;
+		});
+	});
 });
 
 describe("effect()", () => {