Resolves #73.

src/tink/state/internal/AutoObservable.hx

@@ -19,7 +19,7 @@ class AutoObservable<T> extends Dispatcher
   #end
   public var hot(default, null) = false;
   final annex:Annex<{}>;
-  var status = Dirty;
+  var status = Fresh;
   var last:T = null;
   var subscriptions:Array<Subscription>;
   var dependencies = new ObjectMap<ObservableObject<Dynamic>, Subscription>();
@@ -54,8 +54,14 @@ class AutoObservable<T> extends Dispatcher
     return true;
   }
 
+  public function swapComputation(c:Computation<T>) {
+    this.computation = c;
+    this.status = Fresh;
+    fire(this);
+  }
+
   public function isValid()
-    return status != Dirty && (hot || subsValid());
+    return status == Computed && (hot || subsValid());
 
   public function getComparator()
     return comparator;
@@ -155,11 +161,9 @@ class AutoObservable<T> extends Dispatcher
       #if tink_state.debug
       logger.revalidating(this);
       #end
-      var prevSubs = subscriptions;
-
       if (++count == Observable.MAX_ITERATIONS)
         throw 'no result after ${Observable.MAX_ITERATIONS} attempts';
-      else if (subscriptions != null) {
+      else if (status != Fresh) {
         var valid = true;
 
         for (s in subscriptions)
@@ -279,4 +283,5 @@ private class Subscription {
 private enum abstract AutoObservableStatus(Int) {
   var Dirty;
   var Computed;
+  var Fresh;
 }

tests/RunTests.hx

@@ -19,6 +19,7 @@ class RunTests {
       new issues.Issue60(),
       new issues.Issue61(),
       new issues.Issue63(),
+      new issues.Issue73(),
     ])).handle(Runner.exit);
   }
 }

tests/issues/Issue73.hx

@@ -0,0 +1,43 @@
+package issues;
+
+import tink.state.internal.AutoObservable;
+import tink.state.*;
+
+@:asserts
+class Issue73 {
+  public function new() {}
+  public function test() {
+
+    var log = '';
+
+    final s1 = new State(2, (active) -> log += (if (active) '+' else '-') + '1'),
+          s2 = new State(3, (active) -> log += (if (active) '+' else '-') + '2');
+
+    final sum = Observable.auto(() -> s1.value + s2.value),
+          product = Observable.auto(() -> s1.value * s2.value);
+
+    final a = new AutoObservable(() -> product.value - sum.value);
+
+    (a:Observable<Int>).bind(() -> {});
+
+    asserts.assert(log == '+1+2');
+    asserts.assert(a.getValue() == 1);
+
+    a.swapComputation(() -> product.value + sum.value);
+
+    asserts.assert(log == '+1+2');
+    asserts.assert(a.getValue() == 11);
+
+    a.swapComputation(() -> product.value);
+
+    asserts.assert(a.getValue() == 6);
+    asserts.assert(log == '+1+2');
+
+    a.swapComputation(() -> s1.value);
+
+    asserts.assert(a.getValue() == 2);
+    asserts.assert(log == '+1+2-2');
+
+    return asserts.done();
+  }
+}