Variable Listeners to Custom Shadow Variables

This section explains how to update your planning model to use the new declarative custom shadow variable approach instead of the custom VariableListener pattern.

Custom shadow variables replace imperative update logic with declarative, side-effect-free supplier methods. Timefold Solver automatically recalculates shadow values when their source variables change.

Why migrate

In earlier versions of Timefold Solver, shadow variables were updated using a VariableListener. This required:

  • Writing and maintaining listener classes.

  • Manually calling ScoreDirector.beforeVariableChanged() and afterVariableChanged().

  • Handling entity lifecycle events explicitly.

This approach has now been deprecated and will be removed in a future version.

For example:

@ShadowVariable(
        variableListenerClass = MyVariableListener.class,
        sourceVariableName = "someGenuineVariable"
)
private SomeType myShadow;

With custom shadow variables, you declare:

  • A shadow field annotated with @ShadowVariable.

  • A supplier method annotated with @ShadowSources that computes the value.

This approach:

  • Removes the need for listener classes.

  • Eliminates manual ScoreDirector calls.

  • Makes dependencies explicit and easier to reason about.

Migration steps

1. Add the supplier method

Add a method to the planning entity that computes the shadow value. Annotate the method with @ShadowSources and list all planning variables the computation depends on.

@ShadowSources("someVariable")
public SomeType computeMyShadow() {
    if (someVariable == null) {
        return null;
    }
    // Compute shadow value based on source variable(s).
    return ...;
}

  • The return type must match the type of the shadow field.

  • The method must be pure and deterministic.

  • Do not modify any fields inside the supplier method.

  • List every source variable that affects the result.

2. Update the shadow variable annotation

Replace the variableListenerClass reference with a supplierName that points to the new method.

@ShadowVariable(supplierName = "computeMyShadow")
private SomeType myShadow;

Timefold Solver invokes the supplier automatically whenever one of the source variables changes and assigns the returned value to the shadow field.

3. Remove the VariableListener class

Delete the custom VariableListener implementation. It is no longer needed.

Example: before and after

Before: using VariableListener

@PlanningEntity
public class Job {
    private int durationInDays;
    @PlanningVariable
    private LocalDate startDate;
    @ShadowVariable(variableListenerClass = EndDateUpdatingVariableListener.class,
                    sourceVariableName = "startDate")
    private LocalDate endDate;

    // ...
}
public class EndDateUpdatingVariableListener implements VariableListener<MaintenanceSchedule, Job> {

    @Override
    public void beforeEntityAdded(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        // Do nothing
    }

    @Override
    public void afterEntityAdded(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        updateEndDate(scoreDirector, job);
    }

    @Override
    public void beforeVariableChanged(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        // Do nothing
    }

    @Override
    public void afterVariableChanged(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        updateEndDate(scoreDirector, job);
    }

    @Override
    public void beforeEntityRemoved(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        // Do nothing
    }

    @Override
    public void afterEntityRemoved(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        // Do nothing
    }

    protected void updateEndDate(ScoreDirector<MaintenanceSchedule> scoreDirector, Job job) {
        scoreDirector.beforeVariableChanged(job, "endDate");
        job.setEndDate(calculateEndDate(job.getStartDate(), job.getDurationInDays()));
        scoreDirector.afterVariableChanged(job, "endDate");
    }

    public static LocalDate calculateEndDate(LocalDate startDate, int durationInDays) {
        if (startDate == null) {
            return null;
        } else {
            return startDate.plusDays(durationInDays);
        }
    }
}

After: declarative custom shadow variable

@PlanningEntity
public class Job {
    private int durationInDays;
    @PlanningVariable
    private LocalDate startDate;
    @ShadowVariable(supplierName = "endDateSupplier")
    private LocalDate endDate;

    @ShadowSources("startDate")
    public LocalDate endDateSupplier() {
        return calculateEndDate(startDate, durationInDays);
    }

    public static LocalDate calculateEndDate(LocalDate startDate, int durationInDays) {
        if (startDate == null) {
            return null;
        } else {
            return startDate.plusDays(durationInDays);
        }
    }
    // ...
}

In this version:

  • The supplier method computes the shadow value.

  • Timefold Solver updates endDate automatically when startDate changes.

Advanced migration scenarios

Shadow variables with multiple dependencies

If the shadow value depends on multiple planning or shadow variables, list all dependencies in @ShadowSources.

Before in Visit.java:

public class Visit {
    @InverseRelationShadowVariable(sourceVariableName = "visits")
    private Vehicle vehicle;

    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(
            variableListenerClass = ArrivalTimeUpdatingVariableListener.class,
            sourceVariableName = "vehicle")
    @ShadowVariable(
            variableListenerClass = ArrivalTimeUpdatingVariableListener.class,
            sourceVariableName = "previous")
    private LocalDateTime arrivalTime;

    // ...
}

After in Visit.java:

public class Visit {
    @InverseRelationShadowVariable(sourceVariableName = "visits")
    private Vehicle vehicle;

    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(supplierName = "computeArrivalTime")
    private LocalDateTime arrivalTime;

    @ShadowSources({"vehicle", "previous.arrivalTime"})
    public LocalDateTime computeArrivalTime() { … }

    // ...
}

Timefold Solver triggers the supplier whenever either vehicle or previous changes.

Read the full @ShadowSources reference here.

Variable listeners that updated multiple fields

A custom shadow variable updates exactly one field. If your listener updated multiple fields, split the logic into multiple shadow variables.

Before in Visit.java:

public class Visit {
    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(
            variableListenerClass = ArrivalTimeUpdatingVariableListener.class,
            sourceVariableName = "previous")
    private int totalFuelConsumptionSinceStart;

    @PiggybackShadowVariable(shadowVariableName = "totalFuelConsumptionSinceStart")
    private Duration totalTravelTimeSinceStart;

    // ...
}

After in Visit.java:

public class Visit {
    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(supplierName = "computeTotalFuelConsumption")
    private int totalFuelConsumptionSinceStart;

    @ShadowVariable(supplierName = "computeTotalTravelTime")
    private Duration totalTravelTimeSinceStart;

    @ShadowSources({"previous.totalFuelConsumptionSinceStart"})
    public int computeTotalFuelConsumption() { … }

    @ShadowSources({"previous.totalTravelTimeSinceStart"})
    public Duration computeTotalTravelTime() { … }

    // ...
}

Accessing the planning solution

In rare cases, a shadow computation needs access to data stored on the @PlanningSolution. You can add the solution as a parameter to the supplier method.

In the following example, the travel time matrix is stored on the planning solution.

Before in Visit.java and ArrivalTimeUpdatingVariableListener.java

public class Visit {
    private Location location;

    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(
            variableListenerClass = ArrivalTimeUpdatingVariableListener.class,
            sourceVariableName = "previous")
    private LocalDateTime arrivalTime;

    // ...
}

public class ArrivalTimeUpdatingVariableListener
        implements VariableListener<RoutingSolution, Visit> {

    @Override
    public void afterVariableChanged(
            ScoreDirector<RoutingSolution> scoreDirector, Visit visit) {
        updateArrivalTime(scoreDirector, visit);
    }

    @Override
    public void afterEntityAdded(
            ScoreDirector<RoutingSolution> scoreDirector, Visit visit) {
        updateArrivalTime(scoreDirector, visit);
    }

    protected void updateArrivalTime(
            ScoreDirector<RoutingSolution> scoreDirector, Visit visit) {
        RoutingSolution solution = scoreDirector.getWorkingSolution();

        Visit previous = visit.getPreviousStandstill();
        Long arrivalTime = null;

        if (previous != null) {
            // Global travel time matrix stored on the solution
            long travelTime = solution.getTravelTime(
                previous.getLocation(), visit.getLocation());

            Long previousDeparture = previous.getDepartureTime();
            if (previousDeparture != null) {
                arrivalTime = previousDeparture + travelTime;
            }
        }

        scoreDirector.beforeVariableChanged(visit, "arrivalTime");
        visit.setArrivalTime(arrivalTime);
        scoreDirector.afterVariableChanged(visit, "arrivalTime");
    }
}

After in Visit.java. ArrivalTimeUpdatingVariableListener.java is removed.

// This example keeps the "travel time matrix" on the solution
public class Visit {
    private Location location;

    @PreviousElementShadowVariable(sourceVariableName = "visits")
    private Visit previous;

    @ShadowVariable(supplierName = "computeArrivalTime")
    private LocalDateTime arrivalTime;

    @ShadowSources({"vehicle", "previous.arrivalTime"})
    public LocalDateTime computeArrivalTime(RoutingSolution planningSolution) {
        if (previous == null) {
            return null;
        }
        // Global travel time matrix stored on the solution
        long travelTime = solution.getTravelTime(
            previous.getLocation(), visit.getLocation());

        Long previousDeparture = previous.arrivalTime + solution.getDefaultDuration();
        return previousDeparture + travelTime;
    }

    // ...
}

Use this sparingly. In many cases, storing derived data directly on the planning entity leads to simpler and more testable models.

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