feat(util): add value change notifiers

src/main/java/frc/lib/listeners/BaseChangeNotifier.java

@@ -0,0 +1,47 @@
+package frc.lib.listeners;
+
+import static edu.wpi.first.util.ErrorMessages.requireNonNullParam;
+
+import java.util.HashSet;
+import java.util.Set;
+
+public abstract class BaseChangeNotifier {
+  private static Set<BaseChangeNotifier> changeNotifiers;
+
+  protected static void registerChangeNotifier(BaseChangeNotifier changeNotifier) {
+    requireNonNullParam(changeNotifier, "changeNotifier", "registerChangeNotifier");
+
+    if (changeNotifiers == null) {
+      changeNotifiers = new HashSet<>();
+    }
+
+    changeNotifiers.add(changeNotifier);
+  }
+
+  protected static void removeChangeListener(BaseChangeNotifier notifier) {
+    if (changeNotifiers == null) {
+      return;
+    }
+
+    changeNotifiers.remove(notifier);
+  }
+
+  public static final void updateAllChangeNotifiers() {
+    if (changeNotifiers != null) {
+      changeNotifiers.forEach(x -> x.update());
+    }
+  }
+
+  protected abstract void updateInternalState();
+
+  protected abstract boolean shouldNotify();
+
+  protected abstract void notifyConsumers();
+
+  private void update() {
+    updateInternalState();
+    if (shouldNotify()) {
+      notifyConsumers();
+    }
+  }
+}

src/main/java/frc/lib/listeners/ChangeNotifier.java

@@ -0,0 +1,70 @@
+package frc.lib.listeners;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.function.Consumer;
+import java.util.function.Supplier;
+
+public class ChangeNotifier<T> extends BaseChangeNotifier {
+  private final Supplier<T> m_supplier;
+  private final List<Consumer<T>> m_consumers;
+  private T m_currentValue;
+  private T m_previousValue;
+
+  protected ChangeNotifier(Supplier<T> value) {
+    m_supplier = value;
+    m_consumers = new ArrayList<>();
+
+    registerChangeNotifier(this);
+  }
+
+  @Override
+  protected void updateInternalState() {
+    m_previousValue = m_currentValue;
+    m_currentValue = m_supplier.get();
+  }
+
+  @Override
+  protected boolean shouldNotify() {
+    return m_currentValue != null && !m_currentValue.equals(m_previousValue);
+  }
+
+  @Override
+  protected void notifyConsumers() {
+    m_consumers.forEach(x -> x.accept(m_currentValue));
+  }
+
+  public ChangeNotifier<T> addListener(Consumer<T> onUpdated) {
+    m_consumers.add(onUpdated);
+    return this;
+  }
+
+  public ChangeNotifier<T> addListener(Runnable onUpdated) {
+    m_consumers.add(x -> onUpdated.run());
+    return this;
+  }
+
+  public ChangeNotifier<T> addListenerIf(Consumer<T> onUpdated, boolean condition) {
+    if (!condition) {
+      return this;
+    }
+
+    return addListener(onUpdated);
+  }
+
+  public ChangeNotifier<T> listenIf(Consumer<T> onUpdated, Supplier<Boolean> condition) {
+    return addListenerIf(onUpdated, condition.get());
+  }
+
+  public static <T> ChangeNotifier<T> of(Supplier<T> value) {
+    return new ChangeNotifier<>(value);
+  }
+
+  public static <T> ChangeNotifier<T> ofLogged(String key, Supplier<T> value) {
+    return new LoggedChangeNotifier<>(key, value);
+  }
+
+  public T getValue() {
+    return m_currentValue;
+  }
+}

src/main/java/frc/lib/listeners/LoggedChangeNotifier.java

@@ -0,0 +1,20 @@
+package frc.lib.listeners;
+
+import java.util.function.Supplier;
+
+import org.littletonrobotics.junction.Logger;
+
+public class LoggedChangeNotifier<T> extends ChangeNotifier<T> {
+    private final String m_key;
+
+    protected LoggedChangeNotifier(String key, Supplier<T> value) {
+        super(value);
+        m_key = key;
+    }
+
+    @Override
+    protected void updateInternalState() {
+        super.updateInternalState();
+        Logger.getInstance().recordOutput(this.m_key, this.getValue().toString());
+    }
+}

src/main/java/frc/robot/Robot.java

@@ -9,6 +9,7 @@
 import edu.wpi.first.wpilibj2.command.Command;
 import edu.wpi.first.wpilibj2.command.CommandScheduler;
 import frc.lib.advantagekit.LoggerUtil;
+import frc.lib.listeners.BaseChangeNotifier;
 
 /**
  * The VM is configured to automatically run this class, and to call the
@@ -43,6 +44,7 @@ public void robotPeriodic() {
     // finished or interrupted commands, and running subsystem periodic() methods.
     // This must be called from the robot's periodic block in order for anything in
     // the Command-based framework to work.
+    BaseChangeNotifier.updateAllChangeNotifiers();
     CommandScheduler.getInstance().run();
   }