fixed dependency tracking

ObservableTests.pas

@@ -13,25 +13,64 @@   TObservableTests = class(TTestCase)
     procedure ObservableSetValueChangesValue;
 
     procedure DependentObservableReturnsValue;
-//    procedure DependentObservableEvaluatesOnlyOnceAfterChange; // not yet implemented
+    procedure DependentObservableEvaluatesOnlyOnceAfterChange;
     procedure DependentObservableUpdatesValueWhenDependencyChanges;
+
+    procedure DependentObservableClearsOldDependencies;
   end;
 
 implementation
 
+type
+  KO = Observable;
+
 { TObservableTests }
 
-//procedure TObservableTests.DependentObservableEvaluatesOnlyOnceAfterChange;
-//var
-//  o: IObservable<string>;
-//  count: Integer;
-//begin
-//  count := 0;
-//  o := TDependentObservable<string>.Create(function: string begin Inc(count); Result := 'test' end);
-//  CheckEquals('test', o.Value);
-//  CheckEquals('test', o.Value);
-//  CheckEquals(1, count);
-//end;
+procedure TObservableTests.DependentObservableClearsOldDependencies;
+var
+  a: IObservable<Boolean>;
+  b, c: IObservable<string>;
+  o: IObservable<string>;
+  count: Integer;
+begin
+  a := KO.Create(False);
+  b := KO.Create('true');
+  c := KO.Create('false');
+  o := KO.Computed<string>(
+    function: string
+    begin
+      Inc(count);
+      if a.Value then
+        Result := b.Value
+      else
+        Result := c.Value
+    end);
+  count := 0;
+  CheckEquals('false', o.Value);
+  CheckEquals(0, count);
+  b.Value := 'TRUE';
+  CheckEquals(0, count);
+  c.Value := 'FALSE';
+  CheckEquals('FALSE', o.Value);
+  CheckEquals(1, count);
+  a.Value := True;
+  CheckEquals('TRUE', o.Value);
+  CheckEquals(2, count);
+  c.Value := 'false';
+  CheckEquals(2, count);
+end;
+
+procedure TObservableTests.DependentObservableEvaluatesOnlyOnceAfterChange;
+var
+  o: IObservable<string>;
+  count: Integer;
+begin
+  count := 0;
+  o := TDependentObservable<string>.Create(function: string begin Inc(count); Result := 'test' end);
+  CheckEquals('test', o.Value);
+  CheckEquals('test', o.Value);
+  CheckEquals(1, count);
+end;
 
 procedure TObservableTests.DependentObservableReturnsValue;
 var