Instead of using AdvancedPosition objects with a value for spectatedEntityID, use SpectatorData objects for Spectator Keyframes. This allows us to add more options to Spectator Keyframes.

src/main/java/eu/crushedpixel/replaymod/holders/AdvancedPosition.java

@@ -18,30 +18,27 @@ public class AdvancedPosition extends Position {
     @Interpolate
     public double pitch, yaw, roll;
 
-    private Integer spectatedEntityID;
-
-    public AdvancedPosition(double x, double y, double z, double pitch, double yaw, double roll, Integer spectatedEntityID) {
+    public AdvancedPosition(double x, double y, double z, double pitch, double yaw, double roll) {
         super(x, y, z);
         this.pitch = pitch;
         this.yaw = yaw;
         this.roll = roll;
-        this.spectatedEntityID = spectatedEntityID;
     }
 
-    public AdvancedPosition(int entityID, boolean spectate) {
-        this(Minecraft.getMinecraft().theWorld.getEntityByID(entityID), spectate);
+    public AdvancedPosition(int entityID) {
+        this(Minecraft.getMinecraft().theWorld.getEntityByID(entityID));
     }
 
-    public AdvancedPosition(Entity e, boolean spectate) {
-        this(e.posX, e.posY, e.posZ, e.rotationPitch, e.rotationYaw, spectate ? e.getEntityId() : null);
-    }
-
-    public AdvancedPosition(double x, double y, double z, float pitch, float yaw, Integer spectatedEntityID) {
-        this(x, y, z, pitch, yaw, 0, spectatedEntityID);
+    public AdvancedPosition(Entity e) {
+        this(e.posX, e.posY, e.posZ, e.rotationPitch, e.rotationYaw);
     }
 
     public AdvancedPosition(double x, double y, double z, float pitch, float yaw) {
-        this(x, y, z, pitch, yaw, 0, null);
+        this(x, y, z, pitch, yaw, 0);
+    }
+
+    public SpectatorData asSpectatorData(int entityID) {
+        return new SpectatorData(x, y, z, pitch, yaw, roll, entityID);
     }
 
     public double distanceSquared(double x, double y, double z) {
@@ -62,7 +59,7 @@ public class AdvancedPosition extends Position {
 
         Vector3d position = new Vector3d(x, y, z);
         position.add(direction);
-        return new AdvancedPosition(position.x, position.y, position.z, pitch, yaw, roll, null);
+        return new AdvancedPosition(position.x, position.y, position.z, pitch, yaw, roll);
     }
 
     @Override

src/main/java/eu/crushedpixel/replaymod/holders/SpectatorData.java

@@ -0,0 +1,47 @@
+package eu.crushedpixel.replaymod.holders;
+
+import eu.crushedpixel.replaymod.interpolation.AdvancedPositionLinearInterpolation;
+import eu.crushedpixel.replaymod.interpolation.AdvancedPositionSplineInterpolation;
+import eu.crushedpixel.replaymod.interpolation.Interpolation;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+import lombok.NoArgsConstructor;
+import net.minecraft.client.Minecraft;
+import net.minecraft.entity.Entity;
+
+@Data
+@NoArgsConstructor
+@EqualsAndHashCode(callSuper=true)
+public class SpectatorData extends AdvancedPosition {
+
+    private int spectatedEntityID;
+
+    public SpectatorData(double x, double y, double z, double pitch, double yaw, double roll, int entityID) {
+        super(x, y, z, pitch, yaw, roll);
+        this.spectatedEntityID = entityID;
+    }
+
+    public SpectatorData(int entityID) {
+        this(Minecraft.getMinecraft().theWorld.getEntityByID(entityID));
+    }
+
+    public SpectatorData(Entity e) {
+        super(e.posX, e.posY, e.posZ, e.rotationPitch, e.rotationYaw);
+        this.spectatedEntityID = e.getEntityId();
+    }
+
+    @Override
+    public SpectatorData newInstance() {
+        return new SpectatorData();
+    }
+
+    @Override
+    public Interpolation getCubicInterpolator() {
+        return new AdvancedPositionSplineInterpolation();
+    }
+
+    @Override
+    public Interpolation getLinearInterpolator() {
+        return new AdvancedPositionLinearInterpolation();
+    }
+}