Using Current Value in an Effect Model

Now that we have our magnetometer resources, we need to build an activity that changes the MagDataMode for us (since MagDataRate is a derived resource, we shouldn't have to touch it) and changes the overall SSR RecordingRate to reflect the magnetometer's data rate change. This activity, which we'll call ChangeMagMode, only needs one parameter of type MagDataCollectionMode to allow the user to request a change to the mode. Let's give that parameter a default value of LOW_RATE.

In the effect model for this activity (which we'll call run() by convention), we can use the set() method in the DiscreteEffects class to change the MagDataMode to the value provided by our mode parameter. The computation of the change to the RecordingRate caused by the mode change is a little tricky because we need to know both the value of the MagDataRate before and after the mode change. Once we know those value, we can subtract the old value from the new value to get the net increase to the RecordingRate. If the new value happens to be less than the old value, our answer will be negative, but that should be ok as long as we use the increase() method when effecting the RecordingRate resource.

We can get the current value of a resource with a static method called currentValue() available in the Resources class. For our case here, we want to get the current value of the MagDataRate before we actually change the mode to the requested value, so we have to be a little careful about the order of operations within our effect model. The resulting activity type and its effect model should look something like this:

package missionmodel;

import gov.nasa.jpl.aerie.contrib.streamline.modeling.discrete.DiscreteEffects;
import gov.nasa.jpl.aerie.merlin.framework.annotations.ActivityType;
import gov.nasa.jpl.aerie.merlin.framework.annotations.Export;

import static gov.nasa.jpl.aerie.contrib.streamline.core.Resources.currentValue;

@ActivityType("ChangeMagMode")
public class ChangeMagMode {

    @Export.Parameter
    public MagDataCollectionMode mode = MagDataCollectionMode.LOW_RATE;

    @ActivityType.EffectModel
    public void run(Mission model) {
         double currentRate = currentValue(model.dataModel.MagDataRate);
         double newRate = mode.getDataRate();
         // Divide by 10^3 for kbps->Mbps conversion
         DiscreteEffects.increase(model.dataModel.RecordingRate, (newRate-currentRate)/1.0e3);
         DiscreteEffects.set(model.dataModel.MagDataMode, mode);
    }
}

Looking at our new activity definition, you can see how we use the increase() effect on RecordingRate to "increase" the data rate based on the net data change from the old rate. You may also notice a magic number where we do a unit conversion from kbps to Mbps, which isn't ideal. Later on in this tutorial, we will introduce a "Unit Aware" resource framework that will help a bit with conversions like these if desired.

As a final step, make sure you add this new activity to our package-info.java file

@WithActivityType("ChangeMagMode.class")