First try

- Implement instance hiding by deleting/stealing
- Work around instancer persistence by storing a recreation supplier in
  the instance handle
- Rework instancer ctors to just take an InstancerKey
- Parameterize InstanceHandle by I extends Instance so the steal method
  and the supplier can be safely assigned

common/src/api/java/dev/engine_room/flywheel/api/instance/InstanceHandle.java

@@ -7,4 +7,8 @@ public interface InstanceHandle {
 	void setChanged();
 
 	void setDeleted();
+
+	void setVisible(boolean visible);
+
+	boolean isVisible();
 }

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

@@ -1,6 +1,7 @@
 package dev.engine_room.flywheel.backend.engine;
 
 import java.util.ArrayList;
+import java.util.function.Supplier;
 
 import org.jetbrains.annotations.Nullable;
 
@@ -13,26 +14,32 @@ import dev.engine_room.flywheel.backend.util.AtomicBitSet;
 public abstract class AbstractInstancer<I extends Instance> implements Instancer<I> {
 	public final InstanceType<I> type;
 	public final Environment environment;
+	private final Supplier<AbstractInstancer<I>> recreate;
 
 	// Lock for all instances, only needs to be used in methods that may run on the TaskExecutor.
 	protected final Object lock = new Object();
 	protected final ArrayList<I> instances = new ArrayList<>();
-	protected final ArrayList<InstanceHandleImpl> handles = new ArrayList<>();
+	protected final ArrayList<InstanceHandleImpl<I>> handles = new ArrayList<>();
 
 	protected final AtomicBitSet changed = new AtomicBitSet();
 	protected final AtomicBitSet deleted = new AtomicBitSet();
 
-	protected AbstractInstancer(InstanceType<I> type, Environment environment) {
-		this.type = type;
-		this.environment = environment;
+	protected AbstractInstancer(InstancerKey<I> key, Supplier<AbstractInstancer<I>> recreate) {
+		this.type = key.type();
+		this.environment = key.environment();
+		this.recreate = recreate;
 	}
 
 	@Override
 	public I createInstance() {
 		synchronized (lock) {
 			var i = instances.size();
-			var handle = new InstanceHandleImpl(this, i);
+			var handle = new InstanceHandleImpl<I>();
+			handle.instancer = this;
+			handle.recreate = recreate;
+			handle.index = i;
 			I instance = type.create(handle);
+			handle.instance = instance;
 
 			addLocked(instance, handle);
 			return instance;
@@ -47,12 +54,15 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 
 		var instanceHandle = instance.handle();
 
-		if (!(instanceHandle instanceof InstanceHandleImpl handle)) {
+		if (!(instanceHandle instanceof InstanceHandleImpl<?>)) {
 			// UB: do nothing
 			return;
 		}
 
-		if (handle.instancer == this) {
+		// Should InstanceType have an isInstance method?
+		var handle = (InstanceHandleImpl<I>) instanceHandle;
+
+		if (handle.instancer == this && handle.visible) {
 			return;
 		}
 
@@ -65,19 +75,23 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 		// is filtering deleted instances later, so is safe.
 		handle.setDeleted();
 
-		// Only lock now that we'll be mutating our state.
-		synchronized (lock) {
-			// Add the instance to this instancer.
-			handle.instancer = this;
-			handle.index = instances.size();
-			addLocked(instance, handle);
+		// Add the instance to this instancer.
+		handle.instancer = this;
+		handle.recreate = recreate;
+
+		if (handle.visible) {
+			// Only lock now that we'll be mutating our state.
+			synchronized (lock) {
+				handle.index = instances.size();
+				addLocked(instance, handle);
+			}
 		}
 	}
 
 	/**
 	 * Calls must be synchronized on {@link #lock}.
 	 */
-	private void addLocked(I instance, InstanceHandleImpl handle) {
+	private void addLocked(I instance, InstanceHandleImpl<I> handle) {
 		instances.add(instance);
 		handles.add(handle);
 		changed.set(handle.index);
@@ -163,7 +177,7 @@ public abstract class AbstractInstancer<I extends Instance> implements Instancer
 	 * Clear all instances without freeing resources.
 	 */
 	public void clear() {
-		for (InstanceHandleImpl handle : handles) {
+		for (InstanceHandleImpl<I> handle : handles) {
 			// Only clear instances that belong to this instancer.
 			// If one of these handles was stolen by another instancer,
 			// clearing it here would cause significant visual artifacts and instance leaks.

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

@@ -11,7 +11,6 @@ import java.util.concurrent.ConcurrentLinkedQueue;
 import dev.engine_room.flywheel.api.backend.Engine;
 import dev.engine_room.flywheel.api.instance.Instance;
 import dev.engine_room.flywheel.api.instance.InstanceType;
-import dev.engine_room.flywheel.api.instance.Instancer;
 import dev.engine_room.flywheel.api.model.Model;
 import dev.engine_room.flywheel.api.visualization.VisualType;
 import dev.engine_room.flywheel.backend.FlwBackend;
@@ -36,9 +35,13 @@ public abstract class DrawManager<N extends AbstractInstancer<?>> {
 	 */
 	protected final Queue<UninitializedInstancer<N, ?>> initializationQueue = new ConcurrentLinkedQueue<>();
 
+	public <I extends Instance> AbstractInstancer<I> getInstancer(Environment environment, InstanceType<I> type, Model model, VisualType visualType, int bias) {
+		return getInstancer(new InstancerKey<>(environment, type, model, visualType, bias));
+	}
+
 	@SuppressWarnings("unchecked")
-	public <I extends Instance> Instancer<I> getInstancer(Environment environment, InstanceType<I> type, Model model, VisualType visualType, int bias) {
-		return (Instancer<I>) instancers.computeIfAbsent(new InstancerKey<>(environment, type, model, visualType, bias), this::createAndDeferInit);
+	public <I extends Instance> AbstractInstancer<I> getInstancer(InstancerKey<I> key) {
+		return (AbstractInstancer<I>) instancers.computeIfAbsent(key, this::createAndDeferInit);
 	}
 
 	public void flush(LightStorage lightStorage, EnvironmentStorage environmentStorage) {
@@ -94,8 +97,8 @@ public abstract class DrawManager<N extends AbstractInstancer<?>> {
 		return false;
 	}
 
-	protected static <I extends AbstractInstancer<?>> Map<GroupKey<?>, Int2ObjectMap<List<Pair<I, InstanceHandleImpl>>>> doCrumblingSort(Class<I> clazz, List<Engine.CrumblingBlock> crumblingBlocks) {
-		Map<GroupKey<?>, Int2ObjectMap<List<Pair<I, InstanceHandleImpl>>>> byType = new HashMap<>();
+	protected static <I extends AbstractInstancer<?>> Map<GroupKey<?>, Int2ObjectMap<List<Pair<I, InstanceHandleImpl<?>>>>> doCrumblingSort(Class<I> clazz, List<Engine.CrumblingBlock> crumblingBlocks) {
+		Map<GroupKey<?>, Int2ObjectMap<List<Pair<I, InstanceHandleImpl<?>>>>> byType = new HashMap<>();
 		for (Engine.CrumblingBlock block : crumblingBlocks) {
 			int progress = block.progress();
 
@@ -107,7 +110,7 @@ public abstract class DrawManager<N extends AbstractInstancer<?>> {
 				// Filter out instances that weren't created by this engine.
 				// If all is well, we probably shouldn't take the `continue`
 				// branches but better to do checked casts.
-				if (!(instance.handle() instanceof InstanceHandleImpl impl)) {
+				if (!(instance.handle() instanceof InstanceHandleImpl<?> impl)) {
 					continue;
 				}
 

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

@@ -1,16 +1,22 @@
 package dev.engine_room.flywheel.backend.engine;
 
+import java.util.function.Supplier;
+
+import org.jetbrains.annotations.UnknownNullability;
+
+import dev.engine_room.flywheel.api.instance.Instance;
 import dev.engine_room.flywheel.api.instance.InstanceHandle;
 
-public class InstanceHandleImpl implements InstanceHandle {
-	public AbstractInstancer<?> instancer;
+public class InstanceHandleImpl<I extends Instance> implements InstanceHandle {
+	@UnknownNullability
+	public AbstractInstancer<I> instancer;
+	@UnknownNullability
+	public I instance;
+	@UnknownNullability
+	public Supplier<AbstractInstancer<I>> recreate;
+	public boolean visible = true;
 	public int index;
 
-	public InstanceHandleImpl(AbstractInstancer<?> instancer, int index) {
-		this.instancer = instancer;
-		this.index = index;
-	}
-
 	@Override
 	public void setChanged() {
 		instancer.notifyDirty(index);
@@ -23,6 +29,27 @@ public class InstanceHandleImpl implements InstanceHandle {
 		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();
+		}
+	}
+
+	@Override
+	public boolean isVisible() {
+		return visible;
+	}
+
 	public void clear() {
 		index = -1;
 	}

common/src/backend/java/dev/engine_room/flywheel/backend/engine/indirect/IndirectDrawManager.java

@@ -66,7 +66,7 @@ public class IndirectDrawManager extends DrawManager<IndirectInstancer<?>> {
 
 	@Override
 	protected <I extends Instance> IndirectInstancer<?> create(InstancerKey<I> key) {
-		return new IndirectInstancer<>(key.type(), key.environment(), key.model());
+		return new IndirectInstancer<>(key, () -> getInstancer(key));
 	}
 
 	@SuppressWarnings("unchecked")

common/src/backend/java/dev/engine_room/flywheel/backend/engine/indirect/IndirectInstancer.java

@@ -2,17 +2,16 @@ package dev.engine_room.flywheel.backend.engine.indirect;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.function.Supplier;
 
 import org.jetbrains.annotations.UnknownNullability;
 import org.joml.Vector4fc;
 import org.lwjgl.system.MemoryUtil;
 
 import dev.engine_room.flywheel.api.instance.Instance;
-import dev.engine_room.flywheel.api.instance.InstanceType;
 import dev.engine_room.flywheel.api.instance.InstanceWriter;
-import dev.engine_room.flywheel.api.model.Model;
 import dev.engine_room.flywheel.backend.engine.AbstractInstancer;
-import dev.engine_room.flywheel.backend.engine.embed.Environment;
+import dev.engine_room.flywheel.backend.engine.InstancerKey;
 import dev.engine_room.flywheel.backend.util.AtomicBitSet;
 import dev.engine_room.flywheel.lib.math.MoreMath;
 
@@ -29,12 +28,12 @@ public class IndirectInstancer<I extends Instance> extends AbstractInstancer<I>
 	private int modelIndex = -1;
 	private int baseInstance = -1;
 
-	public IndirectInstancer(InstanceType<I> type, Environment environment, Model model) {
-		super(type, environment);
+	public IndirectInstancer(InstancerKey<I> key, Supplier<AbstractInstancer<I>> recreate) {
+		super(key, recreate);
 		instanceStride = MoreMath.align4(type.layout()
 				.byteSize());
 		writer = this.type.writer();
-		boundingSphere = model.boundingSphere();
+		boundingSphere = key.model().boundingSphere();
 	}
 
 	@Override

common/src/backend/java/dev/engine_room/flywheel/backend/engine/instancing/InstancedDrawManager.java

@@ -128,7 +128,7 @@ public class InstancedDrawManager extends DrawManager<InstancedInstancer<?>> {
 
 	@Override
 	protected <I extends Instance> InstancedInstancer<I> create(InstancerKey<I> key) {
-		return new InstancedInstancer<>(key.type(), key.environment());
+		return new InstancedInstancer<>(key, () -> getInstancer(key));
 	}
 
 	@Override

common/src/backend/java/dev/engine_room/flywheel/backend/engine/instancing/InstancedInstancer.java

@@ -2,14 +2,14 @@ package dev.engine_room.flywheel.backend.engine.instancing;
 
 import java.util.ArrayList;
 import java.util.List;
+import java.util.function.Supplier;
 
 import org.jetbrains.annotations.Nullable;
 
 import dev.engine_room.flywheel.api.instance.Instance;
-import dev.engine_room.flywheel.api.instance.InstanceType;
 import dev.engine_room.flywheel.api.instance.InstanceWriter;
 import dev.engine_room.flywheel.backend.engine.AbstractInstancer;
-import dev.engine_room.flywheel.backend.engine.embed.Environment;
+import dev.engine_room.flywheel.backend.engine.InstancerKey;
 import dev.engine_room.flywheel.backend.gl.TextureBuffer;
 import dev.engine_room.flywheel.backend.gl.buffer.GlBuffer;
 import dev.engine_room.flywheel.backend.gl.buffer.GlBufferUsage;
@@ -25,8 +25,8 @@ public class InstancedInstancer<I extends Instance> extends AbstractInstancer<I>
 
 	private final List<InstancedDraw> draws = new ArrayList<>();
 
-	public InstancedInstancer(InstanceType<I> type, Environment environment) {
-		super(type, environment);
+	public InstancedInstancer(InstancerKey<I> key, Supplier<AbstractInstancer<I>> recreate) {
+		super(key, recreate);
 		var layout = type.layout();
 		// Align to one texel in the texture buffer
 		instanceStride = MoreMath.align16(layout.byteSize());