Tasteful stateful handles

- Use state machine interface in InstanceHandleImpl
- 3 states: deleted, visible, hidden
- Visible is directly implemented by AbstractInstancer
- Hidden stores the instancer supplier to recreate an instancer

common/src/backend/java/dev/engine_room/flywheel/backend/engine/AbstractInstancer.java

@@ -11,10 +11,10 @@ import dev.engine_room.flywheel.api.instance.Instancer;
 import dev.engine_room.flywheel.backend.engine.embed.Environment;
 import dev.engine_room.flywheel.backend.util.AtomicBitSet;
 
-public abstract class AbstractInstancer<I extends Instance> implements Instancer<I> {
+public abstract class AbstractInstancer<I extends Instance> implements Instancer<I>, InstanceHandleImpl.State<I> {
 	public final InstanceType<I> type;
 	public final Environment environment;
-	private final Supplier<AbstractInstancer<I>> recreate;
+	private final InstanceHandleImpl.Hidden<I> hidden;
 
 	// Lock for all instances, only needs to be used in methods that may run on the TaskExecutor.
 	protected final Object lock = new Object();
@@ -27,20 +27,45 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 	protected AbstractInstancer(InstancerKey<I> key, Supplier<AbstractInstancer<I>> recreate) {
 		this.type = key.type();
 		this.environment = key.environment();
-		this.recreate = recreate;
+		this.hidden = new InstanceHandleImpl.Hidden<>(recreate);
+	}
+
+	@Override
+	public InstanceHandleImpl.State<I> setChanged(int index) {
+		notifyDirty(index);
+		return this;
+	}
+
+	@Override
+	public InstanceHandleImpl.State<I> setDeleted(int index) {
+		notifyRemoval(index);
+		return InstanceHandleImpl.Deleted.instance();
+	}
+
+	@Override
+	public InstanceHandleImpl.State<I> setVisible(int index, I instance, boolean visible) {
+		if (visible) {
+			return this;
+		}
+
+		notifyRemoval(index);
+
+		return hidden;
+	}
+
+	@Override
+	public InstanceHandleImpl.Status status() {
+		return InstanceHandleImpl.Status.VISIBLE;
 	}
 
 	@Override
 	public I createInstance() {
-		synchronized (lock) {
-			var i = instances.size();
-			var handle = new InstanceHandleImpl<I>();
-			handle.instancer = this;
-			handle.recreate = recreate;
-			handle.index = i;
+		var handle = new InstanceHandleImpl<>(this);
 		I instance = type.create(handle);
 		handle.instance = instance;
 
+		synchronized (lock) {
+			handle.index = instances.size();
 			addLocked(instance, handle);
 			return instance;
 		}
@@ -62,7 +87,8 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 		// Should InstanceType have an isInstance method?
 		var handle = (InstanceHandleImpl<I>) instanceHandle;
 
-		if (handle.instancer == this && handle.visible) {
+		// Should you be allowed to steal deleted instances?
+		if (handle.state == this || handle.state.status() == InstanceHandleImpl.Status.DELETED) {
 			return;
 		}
 
@@ -76,15 +102,18 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 		handle.setDeleted();
 
 		// Add the instance to this instancer.
-		handle.instancer = this;
-		handle.recreate = recreate;
-
-		if (handle.visible) {
+		switch (handle.state.status()) {
+		case VISIBLE:
+			handle.state = this;
 			// Only lock now that we'll be mutating our state.
 			synchronized (lock) {
 				handle.index = instances.size();
 				addLocked(instance, handle);
 			}
+			break;
+		case HIDDEN:
+			handle.state = hidden;
+			break;
 		}
 	}
 
@@ -187,8 +216,9 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 			// clearing it here would cause significant visual artifacts and instance leaks.
 			// At the same time, we need to clear handles we own to prevent
 			// instances from changing/deleting positions in this instancer that no longer exist.
-			if (handle.instancer == this) {
+			if (handle.state == this || handle.state == hidden) {
 				handle.clear();
+				handle.state = InstanceHandleImpl.Deleted.instance();
 			}
 		}
 		instances.clear();

common/src/backend/java/dev/engine_room/flywheel/backend/engine/DrawManager.java

@@ -115,8 +115,11 @@ public abstract class DrawManager<N extends AbstractInstancer<?>> {
 					continue;
 				}
 
-				AbstractInstancer<?> abstractInstancer = impl.instancer;
+				InstanceHandleImpl.State<?> abstractInstancer = impl.state;
+				// AbstractInstancer directly implement HandleState, so this check is valid.
 				if (!clazz.isInstance(abstractInstancer)) {
+					// This rejects instances that were created by a different engine,
+					// and also instances that are hidden or deleted.
 					continue;
 				}
 

common/src/backend/java/dev/engine_room/flywheel/backend/engine/InstanceHandleImpl.java

@@ -8,49 +8,110 @@ import dev.engine_room.flywheel.api.instance.Instance;
 import dev.engine_room.flywheel.api.instance.InstanceHandle;
 
 public class InstanceHandleImpl<I extends Instance> implements InstanceHandle {
-	@UnknownNullability
-	public AbstractInstancer<I> instancer;
+	public State<I> state;
 	@UnknownNullability
 	public I instance;
-	@UnknownNullability
-	public Supplier<AbstractInstancer<I>> recreate;
-	public boolean visible = true;
 	public int index;
 
+	public InstanceHandleImpl(State<I> state) {
+		this.state = state;
+	}
+
 	@Override
 	public void setChanged() {
-		instancer.notifyDirty(index);
+		state = state.setChanged(index);
 	}
 
 	@Override
 	public void setDeleted() {
-		instancer.notifyRemoval(index);
+		state = state.setDeleted(index);
 		// invalidate ourselves
 		clear();
 	}
 
 	@Override
 	public void setVisible(boolean visible) {
-		if (this.visible == visible) {
-			return;
-		}
-
-		this.visible = visible;
-
-		if (visible) {
-			recreate.get().stealInstance(instance);
-		} else {
-			instancer.notifyRemoval(index);
-			clear();
-		}
+		state = state.setVisible(index, instance, visible);
 	}
 
 	@Override
 	public boolean isVisible() {
-		return visible;
+		return state.status() == Status.VISIBLE;
 	}
 
 	public void clear() {
 		index = -1;
 	}
+
+	public enum Status {
+		HIDDEN,
+		DELETED,
+		VISIBLE
+	}
+
+	public interface State<I extends Instance> {
+		State<I> setChanged(int index);
+
+		State<I> setDeleted(int index);
+
+		State<I> setVisible(int index, I instance, boolean visible);
+
+		Status status();
+	}
+
+	public record Hidden<I extends Instance>(Supplier<AbstractInstancer<I>> supplier) implements State<I> {
+		@Override
+		public State<I> setChanged(int index) {
+			return this;
+		}
+
+		@Override
+		public State<I> setDeleted(int index) {
+			return this;
+		}
+
+		@Override
+		public State<I> setVisible(int index, I instance, boolean visible) {
+			if (!visible) {
+				return this;
+			}
+			var instancer = supplier.get();
+			instancer.stealInstance(instance);
+			return instancer;
+		}
+
+		@Override
+		public Status status() {
+			return Status.HIDDEN;
+		}
+	}
+
+	public record Deleted<I extends Instance>() implements State<I> {
+		private static final Deleted<?> INSTANCE = new Deleted<>();
+
+		@SuppressWarnings("unchecked")
+		public static <I extends Instance> Deleted<I> instance() {
+			return (Deleted<I>) INSTANCE;
+		}
+
+		@Override
+		public State<I> setChanged(int index) {
+			return this;
+		}
+
+		@Override
+		public State<I> setDeleted(int index) {
+			return this;
+		}
+
+		@Override
+		public State<I> setVisible(int index, I instance, boolean visible) {
+			return this;
+		}
+
+		@Override
+		public Status status() {
+			return Status.DELETED;
+		}
+	}
 }