Disallow .listen inside computations

lib/src/computed.dart

@@ -80,7 +80,7 @@ class GlobalCtx {
   static ComputedImpl get currentComputation {
     if (_currentComputation == null) {
       throw StateError(
-          "`use` and `prev` are only allowed inside Computed expressions.");
+          "`use` and `prev` are only allowed inside computations.");
     }
     return _currentComputation!;
   }
@@ -208,6 +208,9 @@ class ComputedImpl<T> with Computed<T> {
   @override
   ComputedSubscription<T> listen(
       void Function(T event)? onData, Function? onError) {
+    if (GlobalCtx._currentComputation != null) {
+      throw StateError('`listen` is not allowed inside computations.');
+    }
     final sub = _ComputedSubscriptionImpl<T>(this, onData, onError);
     if (_novalue) {
       try {

test/computed_test.dart

@@ -832,19 +832,39 @@ void main() {
       s.use;
       fail("Should have thrown");
     } on StateError catch (e) {
-      expect(e.message,
-          "`use` and `prev` are only allowed inside Computed expressions.");
+      expect(
+          e.message, "`use` and `prev` are only allowed inside computations.");
     }
 
     try {
       s.prev;
       fail("Should have thrown");
     } on StateError catch (e) {
-      expect(e.message,
-          "`use` and `prev` are only allowed inside Computed expressions.");
+      expect(
+          e.message, "`use` and `prev` are only allowed inside computations.");
     }
   });
 
+  test('cannot call listen inside computations', () async {
+    final c = Computed(() => null);
+    final c2 = Computed(() => c.listen((event) {}, null));
+
+    var flag = false;
+
+    c2.listen((event) {
+      fail("Should not call the listener");
+    }, (e) {
+      expect(flag, false);
+      flag = true;
+      expect(e, isA<StateError>());
+      expect(e.message, '`listen` is not allowed inside computations.');
+    });
+
+    await Future.value();
+
+    expect(flag, true);
+  });
+
   group('react', () {
     test('can switch from use to react', () async {
       // Note that the other direction (react -> use) does not work.