diff --git a/AlgebraicDataflowArchitectureModel/src/simulator/Simulator.java b/AlgebraicDataflowArchitectureModel/src/simulator/Simulator.java
index 7528793..7e222cb 100644
--- a/AlgebraicDataflowArchitectureModel/src/simulator/Simulator.java
+++ b/AlgebraicDataflowArchitectureModel/src/simulator/Simulator.java
@@ -51,12 +51,12 @@
 				// root resource
 				Resource resource = new Resource(res);
 				curState.addResource(resource);
-				Expression initilValue = res.getInitialValue();
-				if (initilValue != null) {
-					if (initilValue instanceof Term) {
-						initilValue = ((Term) initilValue).reduce();
+				Expression initialValue = res.getInitialValue();
+				if (initialValue != null) {
+					if (initialValue instanceof Term) {
+						initialValue = ((Term) initialValue).reduce();
 					}
-					curState.updateResourceState(resource.getResourceIdentifier(), initilValue);
+					curState.updateResourceState(resource.getResourceIdentifier(), null, null, initialValue);
 				}
 			}
 		}
@@ -145,9 +145,12 @@
 	 */
 	private void fireEvent(final Event event, final SystemState curSystemState, final SystemState nextSystemState)
 			throws ParameterizedIdentifierIsFutureWork, ResolvingMultipleDefinitionIsFutureWork, InvalidMessage, UnificationFailed, ValueUndefined {
+		final ChannelMember[] outTarget = new ChannelMember[1];
+		final Variable[] outResVar = new Variable[1];
 		IResourceStateAccessor resouceStateAccessor = new IResourceStateAccessor() {
 			@Override
 			public Expression getCurrentStateAccessorFor(ChannelMember target, ChannelMember from) {
+				if (target == outTarget[0]) return outResVar[0];		// the current state of each output resource is not to be replaced with its value.
 				ResourceIdentifier resId = event.getResourceIdentifier(target.getResource());
 				Resource res = curSystemState.getResource(resId);
 				if (res == null) return null;
@@ -176,7 +179,9 @@
 			// For each output resource, calculate the next state.
 			for (ChannelMember out: channel.getOutputChannelMembers()) {
 				// Calculate the next state expression.
-				Expression nextResState = null; 
+				Expression nextResState = null;
+				outTarget[0] = out;
+				outResVar[0] = new Variable(channel.getChannelName() + "$" + out.getResource().toString() + "$this");		// A special variable to represent the current state of each output resource.
 				if (!event.isInput()) {
 					nextResState = channel.deriveUpdateExpressionOf(out, resouceStateAccessor).getKey();
 				} else {
@@ -189,18 +194,25 @@
 						nextResState = ((Term) nextResState).substitute((Variable) selExp, selestorAndVal.getValue());
 					}
 				}
-				if (nextResState instanceof Term) {
-					nextResState = ((Term) nextResState).reduce();
-				}
 				// Update the resource state.
 				ResourceIdentifier outResId = event.getOutputResourceIdentifier(out.getResource());
-				ResourceIdentifier updatedOutResId = nextSystemState.updateResourceState(outResId, nextResState);
-				while (updatedOutResId != null) {		// In addition to the target state, its descendants' states are also changed.
-					for (Event nextEvent: getNextEvents(updatedOutResId, curSystemState, nextSystemState)) {
-						fireEvent(nextEvent, curSystemState, nextSystemState);
+				Expression curResState = null;
+				if (curSystemState.getResource(outResId) != null) {
+					curResState = curSystemState.getResource(outResId).getState().getValue();
+				}
+				List<ResourceIdentifier> updatedOutResIds = nextSystemState.updateResourceState(outResId, outResVar[0], curResState, nextResState);
+				if (updatedOutResIds != null) {
+					Set<ResourceIdentifier> ancestors = new HashSet<>();
+					for (ResourceIdentifier updatedOutResId: updatedOutResIds) {
+						while (updatedOutResId != null && !ancestors.contains(updatedOutResId)) {		// In addition to the target state, its ancestors' states are also changed.
+							ancestors.add(updatedOutResId);
+							for (Event nextEvent: getNextEvents(updatedOutResId, curSystemState, nextSystemState)) {
+								fireEvent(nextEvent, curSystemState, nextSystemState);
+							}
+							if (channel.isNative()) break;		// To avoid multiple updates of the same resource.
+							updatedOutResId = (ResourceIdentifier) updatedOutResId.getParent();
+						}
 					}
-					if (channel.isNative()) break;		// To avoid multiple updates of the same resource.
-					updatedOutResId = (ResourceIdentifier) updatedOutResId.getParent();
 				}
 			}
 		} else if (channel.isNative()) {