Add empty-subcription detection to frptools

bump to v1.1.0

README.md

@@ -40,10 +40,12 @@ inViewport(false);
 ### Subscribe to changes
 
 Call the `subscribe` method with a callback that will be called when the observable is changed to a
-different value. The returned function can be called to unsubscribe from the observable.
+different value. The returned function can be called to unsubscribe from the observable. When called
+it will provide the count of remaining subscriptions.
 
 ```js
 const unsubscribe = inViewport.subscribe(console.log.bind(console));
+const remainingSubscriptionCount = unsubscribe();
 ```
 
 # [computed](./src/computed.js)
@@ -81,10 +83,12 @@ Call it to receive the stored value, recomputing if necessary.
 ### Subscribe to changes
 
 Call the subscribe method with a callback that will be called when the computed result changes to a
-different value. The returned function can be called to unsubscribe from the observable.
+different value. The returned function can be called to unsubscribe from the observable. When called
+it will provide the count of remaining subscriptions.
 
 ```js
 const unsubscribe = inViewport.subscribe(console.log.bind(console));
+const remainingSubscriptionCount = unsubscribe();
 ```
 
 **NOTE**: Subscribing to a computed forces it to recompute every time an upstream dependency

package.json

@@ -1,6 +1,6 @@
 {
 	"name": "frptools",
-	"version": "1.0.0",
+	"version": "1.1.0",
 	"description": "Observable and Computed data streams",
 	"main": "lib/index.js",
 	"jsnext:main": "src/index.js",

spec/computed.spec.js

@@ -40,17 +40,107 @@ describe('computed', () => {
 
 		a(1);
 		expect(runCount).toEqual(0);
+		// b evaluates
 		expect(b()).toEqual(1);
 		expect(runCount).toEqual(1);
+		// b does not evaluate
 		expect(b()).toEqual(1);
 		expect(runCount).toEqual(1);
 		currentValue = 3;
+		// b does not evaluate
 		a(3);
 		expect(runCount).toEqual(1);
+		// b evaluates
 		expect(b()).toEqual(9);
 		expect(runCount).toEqual(2);
 	});
 
+	it('computes automatically when subscribed', () => {
+		let runCount = 0;
+		let subRunCount = 0;
+		let currentValue = 1;
+		const a = observable(0);
+		const b = computed(
+			val => {
+				runCount += 1;
+				expect(val).toEqual(currentValue);
+				return val * val;
+			},
+			[a]
+		);
+
+		// b does not evaluate
+		a(1);
+		expect(runCount).toEqual(0);
+		// b evaluates
+		expect(b()).toEqual(1);
+		expect(runCount).toEqual(1);
+		// b does not evaluate
+		expect(b()).toEqual(1);
+		expect(runCount).toEqual(1);
+
+		const cancelSubscription = b.subscribe(val => {
+			subRunCount += 1;
+			expect(val).toEqual(currentValue * currentValue);
+		});
+
+		currentValue = 3;
+		// b evaluates
+		a(3);
+		expect(runCount).toEqual(2);
+		expect(subRunCount).toEqual(1);
+		// b does not evaluate
+		expect(b()).toEqual(9);
+		expect(runCount).toEqual(2);
+		expect(subRunCount).toEqual(1);
+	});
+
+	it('honors cancelled subscriptions', () => {
+		let runCount = 0;
+		let subRunCount = 0;
+		let currentValue = 1;
+		const a = observable(0);
+		const b = computed(
+			val => {
+				runCount += 1;
+				expect(val).toEqual(currentValue);
+				return val * val;
+			},
+			[a]
+		);
+		const cancelSubscription = b.subscribe(val => {
+			subRunCount += 1;
+			expect(val).toEqual(currentValue * currentValue);
+		});
+
+		const cancelSubscription2 = b.subscribe(val => {
+			subRunCount += 1;
+		});
+
+		// b evaluates
+		a(1);
+		expect(runCount).toEqual(1);
+		expect(subRunCount).toEqual(2);
+		// b does not evaluate
+		expect(b()).toEqual(1);
+		expect(runCount).toEqual(1);
+		expect(subRunCount).toEqual(2);
+
+		expect(cancelSubscription()).toEqual(1);
+
+		currentValue = 3;
+		// b evaluates
+		a(3);
+		expect(runCount).toEqual(2);
+		expect(subRunCount).toEqual(3);
+		// b does not evaluate
+		expect(b()).toEqual(9);
+		expect(runCount).toEqual(2);
+		expect(subRunCount).toEqual(3);
+
+		expect(cancelSubscription2()).toEqual(0);
+	});
+
 	it('can be detached', () => {
 		const a = observable(2);
 		const b = computed(square, [a]);

spec/observable.spec.js

@@ -44,12 +44,21 @@ describe('observable', () => {
 			runCount += 1;
 			expect(val).toEqual(currentValue);
 		});
+		const cancelSubscription2 = a.subscribe(val => {
+			runCount += 1;
+			expect(val).toEqual(currentValue);
+		});
 		expect(a(1)).toEqual(1);
-		expect(runCount).toEqual(1);
+		expect(runCount).toEqual(2);
 		expect(a(1)).toEqual(1);
-		expect(runCount).toEqual(1);
+		expect(runCount).toEqual(2);
-		cancelSubscription();
+		expect(cancelSubscription()).toEqual(1);
+		currentValue = 3;
 		expect(a(3)).toEqual(3);
-		expect(runCount).toEqual(1);
+		expect(runCount).toEqual(3);
+		expect(cancelSubscription2()).toEqual(0);
+		currentValue = 4;
+		expect(a(4)).toEqual(4);
+		expect(runCount).toEqual(3);
 	});
 });

src/computed.js

@@ -31,7 +31,10 @@ export function computed(fn, dependencies = []) {
 
 	accessor.subscribe = fn => {
 		subscribers.add(fn);
-		return () => subscribers.delete(fn);
+		return () => {
+			subscribers.delete(fn);
+			return subscribers.size;
+		};
 	};
 
 	accessor._d = fn => {

src/observable.js

@@ -11,7 +11,10 @@ export function observable(store) {
 
 	accessor.subscribe = accessor._d = fn => {
 		subscribers.add(fn);
-		return () => subscribers.delete(fn);
+		return () => {
+			subscribers.delete(fn);
+			return subscribers.size;
+		};
 	};
 
 	accessor.unsubscribeAll = () => subscribers.clear();