Store the state in the spellbook and synchronize it between players when multiple are viewing the same book

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/DynamicContent.java

@@ -8,18 +8,18 @@ import com.minelittlepony.common.client.gui.dimension.Bounds;
 import com.minelittlepony.unicopia.client.gui.DrawableUtil;
 import com.minelittlepony.unicopia.client.gui.spellbook.SpellbookChapterList.Content;
 import com.minelittlepony.unicopia.client.gui.spellbook.SpellbookChapterList.Drawable;
+import com.minelittlepony.unicopia.container.SpellbookState;
 import com.minelittlepony.unicopia.entity.player.Pony;
 import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.util.math.MatrixStack;
 import net.minecraft.text.Text;
 import net.minecraft.util.*;
-import net.minecraft.util.math.MathHelper;
 
 public class DynamicContent implements Content {
     private static final Text UNKNOWN = Text.of("???");
     private static final Text UNKNOWN_LEVEL = Text.literal("Level: ???").formatted(Formatting.DARK_GREEN);
 
-    private int offset = 0;
+    private SpellbookState.PageState state = new SpellbookState.PageState();
     private final List<Page> pages = new ArrayList<>();
 
     private Bounds bounds = Bounds.empty();
@@ -30,7 +30,7 @@ public class DynamicContent implements Content {
 
     @Override
     public void draw(MatrixStack matrices, int mouseX, int mouseY, IViewRoot container) {
-        int pageIndex = offset * 2;
+        int pageIndex = state.getOffset() * 2;
 
         getPage(pageIndex).ifPresent(page -> page.draw(matrices, mouseX, mouseY, container));
 
@@ -45,7 +45,7 @@ public class DynamicContent implements Content {
     @Override
     public void copyStateFrom(Content old) {
         if (old instanceof DynamicContent o) {
-            offset = o.offset;
+            state = o.state;
             setBounds(o.bounds);
         }
     }
@@ -67,10 +67,11 @@ public class DynamicContent implements Content {
     }
 
     @Override
-    public void init(SpellbookScreen screen) {
+    public void init(SpellbookScreen screen, Identifier pageId) {
+        state = screen.getState().getState(pageId);
         setBounds(screen.getFrameBounds());
         screen.addPageButtons(187, 30, 350, incr -> {
-            offset = MathHelper.clamp(offset + incr, 0, (int)Math.ceil(pages.size() / 2F) - 1);
+            state.swap(incr, (int)Math.ceil(pages.size() / 2F));
         });
     }
 

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/SpellbookChapterList.java

@@ -1,7 +1,7 @@
 package com.minelittlepony.unicopia.client.gui.spellbook;
 
 import java.util.*;
-import java.util.function.Consumer;
+import java.util.function.BiConsumer;
 import java.util.stream.Stream;
 
 import com.minelittlepony.common.client.gui.IViewRoot;
@@ -15,13 +15,14 @@ public class SpellbookChapterList {
     public static final Identifier PROFILE_ID = Unicopia.id("profile");
     public static final Identifier TRAIT_DEX_ID = Unicopia.id("traits");
 
-    private final Chapter craftingChapter;
+    private final SpellbookScreen screen;
 
-    private Optional<Identifier> currentChapter = Optional.empty();
+    private final Chapter craftingChapter;
 
     private final Map<Identifier, Chapter> chapters = new HashMap<>();
 
-    public SpellbookChapterList(Chapter craftingChapter, Chapter... builtIn) {
+    public SpellbookChapterList(SpellbookScreen screen, Chapter craftingChapter, Chapter... builtIn) {
+        this.screen = screen;
         this.craftingChapter = craftingChapter;
         SpellbookChapterLoader.INSTANCE.getChapters().forEach(chapter -> {
             chapters.put(chapter.id(), chapter);
@@ -45,11 +46,8 @@ public class SpellbookChapterList {
                 chapters.put(chapter.id(), chapter);
             });
         }
-        return currentChapter.map(chapters::get).orElse(craftingChapter);
-    }
 
-    public void setCurrentChapter(Chapter chapter) {
+        return screen.getState().getCurrentPageId().map(chapters::get).orElse(craftingChapter);
-        currentChapter = Optional.of(chapter.id());
     }
 
     public record Chapter (
@@ -70,15 +68,19 @@ public class SpellbookChapterList {
     }
 
     public interface Content extends Drawable {
-        void init(SpellbookScreen screen);
+        void init(SpellbookScreen screen, Identifier pageId);
 
         default void copyStateFrom(Content old) {}
 
-        static Optional<Content> of(Consumer<SpellbookScreen> init, Drawable obj) {
+        default boolean showInventory() {
+            return false;
+        }
+
+        static Optional<Content> of(BiConsumer<SpellbookScreen, Identifier> init, Drawable obj) {
             return Optional.of(new Content() {
                 @Override
-                public void init(SpellbookScreen screen) {
+                public void init(SpellbookScreen screen, Identifier pageId) {
-                    init.accept(screen);
+                    init.accept(screen, pageId);
                 }
 
                 @Override

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/SpellbookCraftingPageContent.java

@@ -6,15 +6,19 @@ import com.minelittlepony.common.client.gui.element.Label;
 import com.minelittlepony.unicopia.ability.magic.spell.crafting.SpellbookRecipe;
 import com.minelittlepony.unicopia.client.gui.DrawableUtil;
 import com.minelittlepony.unicopia.container.SpellbookPage;
+import com.minelittlepony.unicopia.container.SpellbookState;
 import com.minelittlepony.unicopia.item.URecipes;
 import com.mojang.blaze3d.systems.RenderSystem;
 
 import net.minecraft.client.util.math.MatrixStack;
 import net.minecraft.text.Text;
+import net.minecraft.util.Identifier;
 
 public class SpellbookCraftingPageContent extends ScrollContainer implements SpellbookChapterList.Content, SpellbookScreen.RecipesChangedListener {
     private final SpellbookScreen screen;
 
+    private SpellbookState.PageState state = new SpellbookState.PageState();
+
     public SpellbookCraftingPageContent(SpellbookScreen screen) {
         this.screen = screen;
         backgroundColor = 0xFFf9efd3;
@@ -22,8 +26,11 @@ public class SpellbookCraftingPageContent extends ScrollContainer implements Spe
     }
 
     @Override
-    public void init(SpellbookScreen screen) {
+    public void init(SpellbookScreen screen, Identifier pageId) {
-        screen.addPageButtons(187, 300, 350, SpellbookPage::swap);
+        state = screen.getState().getState(pageId);
+        screen.addPageButtons(187, 300, 350, incr -> {
+            state.swap(incr, SpellbookPage.VALUES.length);
+        });
         initContents();
         screen.addDrawable(this);
         ((IViewRoot)screen).getChildElements().add(this);
@@ -34,9 +41,9 @@ public class SpellbookCraftingPageContent extends ScrollContainer implements Spe
 
         int headerColor = mouseY % 255;
 
-        DrawableUtil.drawScaledText(matrices, SpellbookPage.getCurrent().getLabel(), screen.getFrameBounds().left + screen.getFrameBounds().width / 2 + 20, SpellbookScreen.TITLE_Y, 1.3F, headerColor);
+        DrawableUtil.drawScaledText(matrices, SpellbookPage.VALUES[state.getOffset()].getLabel(), screen.getFrameBounds().left + screen.getFrameBounds().width / 2 + 20, SpellbookScreen.TITLE_Y, 1.3F, headerColor);
 
-        Text pageText = Text.translatable("%s/%s", SpellbookPage.getCurrent().ordinal() + 1, SpellbookPage.VALUES.length);
+        Text pageText = Text.translatable("%s/%s", state.getOffset() + 1, SpellbookPage.VALUES.length);
         textRenderer.draw(matrices, pageText, 337 - textRenderer.getWidth(pageText) / 2F, 190, headerColor);
     }
 
@@ -49,11 +56,16 @@ public class SpellbookCraftingPageContent extends ScrollContainer implements Spe
         init(this::initPageContent);
     }
 
+    @Override
+    public boolean showInventory() {
+        return SpellbookPage.VALUES[state.getOffset()] == SpellbookPage.INVENTORY;
+    }
+
     private void initPageContent() {
         getContentPadding().setVertical(10);
         getContentPadding().bottom = 30;
 
-        switch (SpellbookPage.getCurrent()) {
+        switch (SpellbookPage.VALUES[state.getOffset()]) {
             case INVENTORY:
                 // handled elsewhere
                 break;

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/SpellbookProfilePageContent.java

@@ -14,6 +14,7 @@ import net.minecraft.client.gui.DrawableHelper;
 import net.minecraft.client.resource.language.I18n;
 import net.minecraft.client.util.math.MatrixStack;
 import net.minecraft.text.Text;
+import net.minecraft.util.Identifier;
 import net.minecraft.util.math.MathHelper;
 
 public class SpellbookProfilePageContent extends DrawableHelper implements SpellbookChapterList.Content {
@@ -28,10 +29,15 @@ public class SpellbookProfilePageContent extends DrawableHelper implements Spell
     }
 
     @Override
-    public void init(SpellbookScreen screen) {
+    public void init(SpellbookScreen screen, Identifier pageId) {
 
     }
 
+    @Override
+    public boolean showInventory() {
+        return true;
+    }
+
     @Override
     public void draw(MatrixStack matrices, int mouseX, int mouseY, IViewRoot container) {
 

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/SpellbookScreen.java

@@ -10,9 +10,10 @@ import com.minelittlepony.common.client.gui.element.Button;
 import com.minelittlepony.common.client.gui.sprite.TextureSprite;
 import com.minelittlepony.unicopia.Unicopia;
 import com.minelittlepony.unicopia.client.gui.spellbook.SpellbookChapterList.*;
-import com.minelittlepony.unicopia.container.SpellbookPage;
+import com.minelittlepony.unicopia.container.*;
-import com.minelittlepony.unicopia.container.SpellbookScreenHandler;
 import com.minelittlepony.unicopia.container.SpellbookScreenHandler.*;
+import com.minelittlepony.unicopia.network.Channel;
+import com.minelittlepony.unicopia.network.MsgSpellbookStateChanged;
 import com.mojang.blaze3d.platform.GlStateManager;
 import com.mojang.blaze3d.systems.RenderSystem;
 
@@ -25,6 +26,7 @@ import net.minecraft.client.texture.NativeImage;
 import net.minecraft.client.util.math.MatrixStack;
 import net.minecraft.entity.player.PlayerInventory;
 import net.minecraft.screen.slot.Slot;
+import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.sound.SoundEvents;
 import net.minecraft.text.Text;
 import net.minecraft.util.Identifier;
@@ -42,11 +44,12 @@ public class SpellbookScreen extends HandledScreen<SpellbookScreenHandler> imple
 
     private final RecipeBookWidget recipeBook = new RecipeBookWidget();
 
-    private final Chapter craftingChapter = new Chapter(SpellbookChapterList.CRAFTING_ID, TabSide.LEFT, 0, 0, Optional.of(new SpellbookCraftingPageContent(this)));
+    private final Chapter craftingChapter;
-    private final Chapter profileChapter = new Chapter(SpellbookChapterList.PROFILE_ID, TabSide.LEFT, 1, 0, Optional.of(new SpellbookProfilePageContent(this)));
+    private final SpellbookChapterList chapters = new SpellbookChapterList(this,
-    private final Chapter traitdexChapter = new Chapter(SpellbookChapterList.TRAIT_DEX_ID, TabSide.LEFT, 3, 0, Optional.of(new SpellbookTraitDexPageContent(this)));
+        craftingChapter = new Chapter(SpellbookChapterList.CRAFTING_ID, TabSide.LEFT, 0, 0, Optional.of(new SpellbookCraftingPageContent(this))),
-
+        new Chapter(SpellbookChapterList.PROFILE_ID, TabSide.LEFT, 1, 0, Optional.of(new SpellbookProfilePageContent(this))),
-    private final SpellbookChapterList chapters = new SpellbookChapterList(craftingChapter, profileChapter, traitdexChapter);
+        new Chapter(SpellbookChapterList.TRAIT_DEX_ID, TabSide.LEFT, 3, 0, Optional.of(new SpellbookTraitDexPageContent(this)))
+    );
     private final SpellbookTabBar tabs = new SpellbookTabBar(this, chapters);
 
     private Bounds contentBounds = Bounds.empty();
@@ -59,11 +62,17 @@ public class SpellbookScreen extends HandledScreen<SpellbookScreenHandler> imple
 
         handler.addSlotShowingCondition(slotType -> {
             if (slotType == SlotType.INVENTORY) {
-                return chapters.getCurrentChapter() == profileChapter
+                return chapters.getCurrentChapter().content().filter(Content::showInventory).isPresent();
-                   || (chapters.getCurrentChapter() == craftingChapter && SpellbookPage.getCurrent() == SpellbookPage.INVENTORY);
             }
             return chapters.getCurrentChapter() == craftingChapter;
         });
+        handler.getSpellbookState().setSynchronizer(state -> {
+            Channel.CLIENT_SPELLBOOK_UPDATE.send(new MsgSpellbookStateChanged<ServerPlayerEntity>(handler.syncId, state));
+        });
+    }
+
+    public SpellbookState getState() {
+        return handler.getSpellbookState();
     }
 
     public void addPageButtons(int buttonY, int prevX, int nextX, IntConsumer pageAction) {
@@ -100,7 +109,7 @@ public class SpellbookScreen extends HandledScreen<SpellbookScreenHandler> imple
     public void init() {
         super.init();
         tabs.init();
-        chapters.getCurrentChapter().content().ifPresent(content -> content.init(this));
+        chapters.getCurrentChapter().content().ifPresent(content -> content.init(this, chapters.getCurrentChapter().id()));
     }
 
     @Override
@@ -196,9 +205,8 @@ public class SpellbookScreen extends HandledScreen<SpellbookScreenHandler> imple
     @Override
     public boolean mouseClicked(double mouseX, double mouseY, int button) {
         return tabs.getAllTabs().anyMatch(tab -> {
-
             if (tab.bounds().contains(mouseX, mouseY) && chapters.getCurrentChapter() != tab.chapter()) {
-                chapters.setCurrentChapter(tab.chapter());
+                getState().setCurrentPageId(tab.chapter().id());
                 GameGui.playSound(SoundEvents.ITEM_BOOK_PAGE_TURN);
                 clearAndInit();
                 return true;

src/main/java/com/minelittlepony/unicopia/client/gui/spellbook/SpellbookTraitDexPageContent.java

@@ -10,6 +10,7 @@ import com.minelittlepony.common.client.gui.element.Label;
 import com.minelittlepony.common.client.gui.sprite.TextureSprite;
 import com.minelittlepony.unicopia.ability.magic.spell.trait.*;
 import com.minelittlepony.unicopia.client.gui.spellbook.SpellbookScreen.ImageButton;
+import com.minelittlepony.unicopia.container.SpellbookState;
 import com.minelittlepony.unicopia.entity.player.Pony;
 import com.mojang.blaze3d.systems.RenderSystem;
 
@@ -18,13 +19,13 @@ import net.minecraft.client.gui.DrawableHelper;
 import net.minecraft.client.util.math.MatrixStack;
 import net.minecraft.item.*;
 import net.minecraft.text.Text;
+import net.minecraft.util.Identifier;
 import net.minecraft.util.collection.DefaultedList;
-import net.minecraft.util.math.MathHelper;
 
 public class SpellbookTraitDexPageContent extends DrawableHelper implements SpellbookChapterList.Content, SpellbookScreen.RecipesChangedListener {
 
     private final Trait[] traits = Trait.values();
-    private int offset;
+    private SpellbookState.PageState state = new SpellbookState.PageState();
 
     private final DexPage leftPage = new DexPage();
     private final DexPage rightPage = new DexPage();
@@ -41,12 +42,14 @@ public class SpellbookTraitDexPageContent extends DrawableHelper implements Spel
     }
 
     @Override
-    public void init(SpellbookScreen screen) {
+    public void init(SpellbookScreen screen, Identifier pageId) {
-        int page = offset * 2;
+        state = screen.getState().getState(pageId);
+
+        int page = state.getOffset() * 2;
         leftPage.init(screen, page);
         rightPage.init(screen, page + 1);
         screen.addPageButtons(187, 30, 350, incr -> {
-            offset = MathHelper.clamp(offset + incr, 0, (int)Math.ceil(traits.length / 2F) - 1);
+            state.swap(incr, (int)Math.ceil(traits.length / 2F));
             leftPage.scrollbar.scrollBy(leftPage.scrollbar.getVerticalScrollAmount());
             rightPage.scrollbar.scrollBy(rightPage.scrollbar.getVerticalScrollAmount());
         });
@@ -54,7 +57,7 @@ public class SpellbookTraitDexPageContent extends DrawableHelper implements Spel
 
     @Override
     public void onRecipesChanged() {
-        init(screen);
+        init(screen, SpellbookChapterList.TRAIT_DEX_ID);
     }
 
     private final class DexPage extends ScrollContainer {

src/main/java/com/minelittlepony/unicopia/container/SpellbookPage.java

@@ -1,26 +1,16 @@
 package com.minelittlepony.unicopia.container;
 
 import net.minecraft.text.Text;
-import net.minecraft.util.math.MathHelper;
 
 public enum SpellbookPage {
     INVENTORY,
     RECIPES;
 
     public static final SpellbookPage[] VALUES = values();
-    private static int current;
 
     private final Text label = Text.translatable("gui.unicopia.spellbook.page." + name().toLowerCase());
 
     public Text getLabel() {
         return label;
     }
-
-    public static SpellbookPage getCurrent() {
-        return VALUES[current];
-    }
-
-    public static void swap(int increment) {
-        current = MathHelper.clamp(current + increment, 0, VALUES.length - 1);
-    }
 }

src/main/java/com/minelittlepony/unicopia/container/SpellbookScreenHandler.java

@@ -3,6 +3,8 @@ package com.minelittlepony.unicopia.container;
 import java.util.*;
 import java.util.function.Predicate;
 
+import org.jetbrains.annotations.Nullable;
+
 import com.minelittlepony.unicopia.EquinePredicates;
 import com.minelittlepony.unicopia.USounds;
 import com.minelittlepony.unicopia.ability.magic.spell.crafting.SpellbookRecipe;
@@ -23,6 +25,7 @@ import net.minecraft.inventory.CraftingInventory;
 import net.minecraft.inventory.CraftingResultInventory;
 import net.minecraft.inventory.Inventory;
 import net.minecraft.item.ItemStack;
+import net.minecraft.network.PacketByteBuf;
 import net.minecraft.network.packet.s2c.play.ScreenHandlerSlotUpdateS2CPacket;
 import net.minecraft.screen.PlayerScreenHandler;
 import net.minecraft.screen.ScreenHandler;
@@ -59,12 +62,19 @@ public class SpellbookScreenHandler extends ScreenHandler {
 
     private Predicate<SlotType> canShowSlots;
 
-    protected SpellbookScreenHandler(int syncId, PlayerInventory inv) {
+    private final SpellbookState state;
-        this(syncId, inv, ScreenHandlerContext.EMPTY);
+
+    @Nullable
+    public UUID entityId;
+
+    protected SpellbookScreenHandler(int syncId, PlayerInventory inv, PacketByteBuf buf) {
+        this(syncId, inv, ScreenHandlerContext.EMPTY, new SpellbookState().fromPacket(buf), null);
     }
 
-    public SpellbookScreenHandler(int syncId, PlayerInventory inv, ScreenHandlerContext context) {
+    public SpellbookScreenHandler(int syncId, PlayerInventory inv, ScreenHandlerContext context, SpellbookState state, UUID entityId) {
         super(UScreenHandlers.SPELL_BOOK, syncId);
+        this.entityId = entityId;
+        this.state = state;
         inventory = inv;
         this.context = context;
 
@@ -136,6 +146,10 @@ public class SpellbookScreenHandler extends ScreenHandler {
         onContentChanged(input);
     }
 
+    public SpellbookState getSpellbookState() {
+        return state;
+    }
+
     public void addSlotShowingCondition(Predicate<SlotType> canShowSlots) {
         this.canShowSlots = canShowSlots;
     }

src/main/java/com/minelittlepony/unicopia/container/SpellbookState.java

@@ -0,0 +1,122 @@
+package com.minelittlepony.unicopia.container;
+
+import java.util.*;
+import java.util.function.Consumer;
+
+import com.minelittlepony.unicopia.network.datasync.Synchronizable;
+import com.minelittlepony.unicopia.util.NbtSerialisable;
+
+import net.minecraft.nbt.NbtCompound;
+import net.minecraft.nbt.NbtElement;
+import net.minecraft.network.PacketByteBuf;
+import net.minecraft.util.Identifier;
+import net.minecraft.util.math.MathHelper;
+
+public class SpellbookState extends Synchronizable<SpellbookState> implements NbtSerialisable {
+    public static final SpellbookState INSTANCE = new SpellbookState();
+
+    private Optional<Identifier> currentPageId = Optional.empty();
+
+    private final Map<Identifier, PageState> states = new HashMap<>();
+
+    public Optional<Identifier> getCurrentPageId() {
+        return currentPageId;
+    }
+
+    public void setCurrentPageId(Identifier pageId) {
+        currentPageId = Optional.ofNullable(pageId);
+        synchronize();
+    }
+
+    public PageState getState(Identifier pageId) {
+        return states.computeIfAbsent(pageId, i -> new PageState(page -> synchronize()));
+    }
+
+    @Override
+    public void copyFrom(SpellbookState state) {
+        currentPageId = state.currentPageId;
+        state.states.forEach((id, page) -> getState(id).copyFrom(page));
+    }
+
+    public void toPacket(PacketByteBuf buf) {
+        buf.writeOptional(currentPageId, PacketByteBuf::writeIdentifier);
+        buf.writeMap(states, PacketByteBuf::writeIdentifier, (b, v) -> v.toPacket(b));
+    }
+
+    public SpellbookState fromPacket(PacketByteBuf buf) {
+        currentPageId = buf.readOptional(PacketByteBuf::readIdentifier);
+        buf.readMap(PacketByteBuf::readIdentifier, b -> new PageState(page -> synchronize()).fromPacket(b)).forEach((id, state) -> {
+            getState(id).copyFrom(state);
+        });
+        return this;
+    }
+
+    @Override
+    public void toNBT(NbtCompound compound) {
+        currentPageId.ifPresent(id -> compound.putString("current_page", id.toString()));
+        NbtCompound states = new NbtCompound();
+        compound.put("states", states);
+        this.states.forEach((id, page) -> {
+            states.put(id.toString(), page.toNBT());
+        });
+    }
+
+    @Override
+    public void fromNBT(NbtCompound compound) {
+        currentPageId = compound.contains("current_page", NbtElement.STRING_TYPE) ? Optional.ofNullable(Identifier.tryParse(compound.getString("current_page"))) : Optional.empty();
+        NbtCompound states = compound.getCompound("states");
+        states.getKeys().stream().forEach(key -> {
+            Identifier id = Identifier.tryParse(key);
+            if (id != null) {
+                getState(id).fromNBT(states.getCompound(key));
+            }
+        });
+    }
+
+    public static class PageState extends Synchronizable<PageState> implements NbtSerialisable {
+        private int offset;
+
+        public PageState() {}
+
+        PageState(Consumer<PageState> synchronizer) {
+            setSynchronizer(synchronizer);
+        }
+
+        @Override
+        public void copyFrom(PageState other) {
+            offset = other.offset;
+        }
+
+        public void setOffset(int offset) {
+            this.offset = offset;
+            synchronize();
+        }
+
+        public int getOffset() {
+            return offset;
+        }
+
+        public void swap(int incr, int max) {
+            setOffset(MathHelper.clamp(getOffset() + incr, 0, max - 1));
+        }
+
+        public void toPacket(PacketByteBuf buf) {
+            buf.writeInt(offset);
+        }
+
+        public PageState fromPacket(PacketByteBuf buf) {
+            offset = buf.readInt();
+            return this;
+        }
+
+        @Override
+        public void toNBT(NbtCompound compound) {
+            compound.putInt("offset", offset);
+        }
+
+        @Override
+        public void fromNBT(NbtCompound compound) {
+            offset = compound.getInt("offset");
+        }
+    }
+}

src/main/java/com/minelittlepony/unicopia/container/UScreenHandlers.java

@@ -2,15 +2,16 @@ package com.minelittlepony.unicopia.container;
 
 import com.minelittlepony.unicopia.Unicopia;
 
+import net.fabricmc.fabric.api.screenhandler.v1.ExtendedScreenHandlerType;
 import net.minecraft.screen.ScreenHandler;
 import net.minecraft.screen.ScreenHandlerType;
 import net.minecraft.util.registry.Registry;
 
 public interface UScreenHandlers {
-    ScreenHandlerType<SpellbookScreenHandler> SPELL_BOOK = register("spell_book", SpellbookScreenHandler::new);
+    ScreenHandlerType<SpellbookScreenHandler> SPELL_BOOK = register("spell_book", new ExtendedScreenHandlerType<>(SpellbookScreenHandler::new));
 
-    static <T extends ScreenHandler> ScreenHandlerType<T> register(String name, ScreenHandlerType.Factory<T> factory) {
+    static <T extends ScreenHandler> ScreenHandlerType<T> register(String name, ScreenHandlerType<T> type) {
-        return Registry.register(Registry.SCREEN_HANDLER, Unicopia.id(name), new ScreenHandlerType<>(factory));
+        return Registry.register(Registry.SCREEN_HANDLER, Unicopia.id(name), type);
     }
 
     static void bootstrap() { }

src/main/java/com/minelittlepony/unicopia/entity/SpellbookEntity.java

@@ -4,8 +4,12 @@ import org.jetbrains.annotations.Nullable;
 
 import com.minelittlepony.unicopia.EquinePredicates;
 import com.minelittlepony.unicopia.container.SpellbookScreenHandler;
+import com.minelittlepony.unicopia.container.SpellbookState;
 import com.minelittlepony.unicopia.item.UItems;
+import com.minelittlepony.unicopia.network.Channel;
+import com.minelittlepony.unicopia.network.MsgSpellbookStateChanged;
 
+import net.fabricmc.fabric.api.screenhandler.v1.ExtendedScreenHandlerFactory;
 import net.fabricmc.fabric.api.util.TriState;
 import net.minecraft.entity.Entity;
 import net.minecraft.entity.EntityType;
@@ -15,14 +19,17 @@ import net.minecraft.entity.data.TrackedData;
 import net.minecraft.entity.data.TrackedDataHandlerRegistry;
 import net.minecraft.entity.mob.MobEntity;
 import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.entity.player.PlayerInventory;
 import net.minecraft.item.ItemStack;
 import net.minecraft.nbt.NbtCompound;
+import net.minecraft.network.PacketByteBuf;
 import net.minecraft.particle.ParticleTypes;
-import net.minecraft.screen.ScreenHandlerContext;
+import net.minecraft.screen.*;
-import net.minecraft.screen.SimpleNamedScreenHandlerFactory;
+import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.sound.BlockSoundGroup;
 import net.minecraft.sound.SoundCategory;
 import net.minecraft.sound.SoundEvents;
+import net.minecraft.text.Text;
 import net.minecraft.util.ActionResult;
 import net.minecraft.util.Hand;
 import net.minecraft.util.math.Vec3d;
@@ -39,10 +46,23 @@ public class SpellbookEntity extends MobEntity {
 
     private int activeTicks = TICKS_TO_SLEEP;
 
+    private final SpellbookState state = new SpellbookState();
+
     public SpellbookEntity(EntityType<SpellbookEntity> type, World world) {
         super(type, world);
         setPersistent();
         setAltered(world.random.nextInt(3) == 0);
+        if (!world.isClient) {
+            state.setSynchronizer(state -> {
+                getWorld().getPlayers().forEach(player -> {
+                    if (player instanceof ServerPlayerEntity recipient
+                            && player.currentScreenHandler instanceof SpellbookScreenHandler book
+                            && getUuid().equals(book.entityId)) {
+                        Channel.SERVER_SPELLBOOK_UPDATE.send(recipient, new MsgSpellbookStateChanged<>(player.currentScreenHandler.syncId, state));
+                    }
+                });
+            });
+        }
     }
 
     @Override
@@ -54,9 +74,15 @@ public class SpellbookEntity extends MobEntity {
         dataTracker.startTracking(ALTERED, false);
     }
 
+    public SpellbookState getSpellbookState() {
+        return state;
+    }
+
     @Override
     public ItemStack getPickBlockStack() {
-        return new ItemStack(UItems.SPELLBOOK);
+        ItemStack stack = UItems.SPELLBOOK.getDefaultStack();
+        stack.getOrCreateNbt().put("spellbookState", state.toNBT());
+        return stack;
     }
 
     @Override
@@ -180,7 +206,7 @@ public class SpellbookEntity extends MobEntity {
             world.playSound(getX(), getY(), getZ(), sound.getBreakSound(), SoundCategory.BLOCKS, sound.getVolume(), sound.getPitch(), true);
 
             if (world.getGameRules().getBoolean(GameRules.DO_TILE_DROPS)) {
-                dropItem(UItems.SPELLBOOK, 1);
+                dropStack(getPickBlockStack(), 1);
             }
         }
         return false;
@@ -198,7 +224,22 @@ public class SpellbookEntity extends MobEntity {
 
         if (isOpen() && EquinePredicates.PLAYER_UNICORN.test(player)) {
             setBored(false);
-            player.openHandledScreen(new SimpleNamedScreenHandlerFactory((syncId, inv, ply) -> new SpellbookScreenHandler(syncId, inv, ScreenHandlerContext.create(world, getBlockPos())), getDisplayName()));
+            player.openHandledScreen(new ExtendedScreenHandlerFactory() {
+                @Override
+                public Text getDisplayName() {
+                    return SpellbookEntity.this.getDisplayName();
+                }
+
+                @Override
+                public ScreenHandler createMenu(int syncId, PlayerInventory inv, PlayerEntity player) {
+                    return new SpellbookScreenHandler(syncId, inv, ScreenHandlerContext.create(world, getBlockPos()), state, getUuid());
+                }
+
+                @Override
+                public void writeScreenOpeningData(ServerPlayerEntity player, PacketByteBuf buf) {
+                    state.toPacket(buf);
+                }
+            });
             player.playSound(SoundEvents.ITEM_BOOK_PAGE_TURN, 2, 1);
             return ActionResult.SUCCESS;
         }
@@ -213,6 +254,8 @@ public class SpellbookEntity extends MobEntity {
         setBored(compound.getBoolean("bored"));
         setAltered(compound.getBoolean("altered"));
         setLocked(compound.contains("locked") ? TriState.of(compound.getBoolean("locked")) : TriState.DEFAULT);
+
+        state.fromNBT(compound.getCompound("spellbookState"));
     }
 
     @Override
@@ -226,5 +269,7 @@ public class SpellbookEntity extends MobEntity {
         if (locked != TriState.DEFAULT) {
             compound.putBoolean("locked", locked.get());
         }
+
+        compound.put("spellbookState", state.toNBT());
     }
 }

src/main/java/com/minelittlepony/unicopia/item/SpellbookItem.java

@@ -12,6 +12,7 @@ import net.minecraft.entity.player.PlayerEntity;
 import net.minecraft.item.BookItem;
 import net.minecraft.item.ItemStack;
 import net.minecraft.item.ItemUsageContext;
+import net.minecraft.nbt.NbtCompound;
 import net.minecraft.util.ActionResult;
 import net.minecraft.util.TypedActionResult;
 import net.minecraft.util.math.BlockPointer;
@@ -31,7 +32,7 @@ public class SpellbookItem extends BookItem implements Dispensable {
         BlockPos pos = source.getPos().offset(facing);
 
         float yaw = facing.getOpposite().asRotation();
-        placeBook(source.getWorld(), pos.getX(), pos.getY(), pos.getZ(), yaw);
+        placeBook(stack, source.getWorld(), pos.getX(), pos.getY(), pos.getZ(), yaw);
         stack.decrement(1);
 
         return new TypedActionResult<>(ActionResult.SUCCESS, stack);
@@ -46,7 +47,7 @@ public class SpellbookItem extends BookItem implements Dispensable {
         if (!context.getWorld().isClient && EquinePredicates.PLAYER_UNICORN.test(player)) {
             BlockPos pos = context.getBlockPos().offset(context.getSide());
 
-            placeBook(context.getWorld(), pos.getX(), pos.getY(), pos.getZ(), context.getPlayerYaw() + 180);
+            placeBook(context.getStack(), context.getWorld(), pos.getX(), pos.getY(), pos.getZ(), context.getPlayerYaw() + 180);
 
             if (!player.getAbilities().creativeMode) {
                 player.getStackInHand(context.getHand()).decrement(1);
@@ -57,12 +58,19 @@ public class SpellbookItem extends BookItem implements Dispensable {
         return ActionResult.PASS;
     }
 
-    private static void placeBook(World world, int x, int y, int z, float yaw) {
+    private static void placeBook(ItemStack stack, World world, int x, int y, int z, float yaw) {
         SpellbookEntity book = UEntities.SPELLBOOK.create(world);
 
         book.refreshPositionAndAngles(x + 0.5, y, z + 0.5, 0, 0);
         book.setHeadYaw(yaw);
         book.setYaw(yaw);
+
+        @Nullable
+        NbtCompound tag = stack.getSubNbt("spellbookState");
+        if (tag != null) {
+            book.getSpellbookState().fromNBT(tag);
+        }
+
         world.spawnEntity(book);
     }
 }

src/main/java/com/minelittlepony/unicopia/network/Channel.java

@@ -8,6 +8,8 @@ import com.minelittlepony.unicopia.util.network.C2SPacketType;
 import com.minelittlepony.unicopia.util.network.SimpleNetworking;
 
 import net.fabricmc.fabric.api.networking.v1.ServerPlayConnectionEvents;
+import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.server.network.ServerPlayerEntity;
 import net.minecraft.util.Identifier;
 
 public interface Channel {
@@ -25,6 +27,9 @@ public interface Channel {
     Identifier SERVER_SELECT_TRIBE_ID = Unicopia.id("select_tribe");
     S2CPacketType<MsgTribeSelect> SERVER_SELECT_TRIBE = SimpleNetworking.serverToClient(SERVER_SELECT_TRIBE_ID, MsgTribeSelect::new);
 
+    S2CPacketType<MsgSpellbookStateChanged<PlayerEntity>> SERVER_SPELLBOOK_UPDATE = SimpleNetworking.serverToClient(Unicopia.id("server_spellbook_update"), MsgSpellbookStateChanged::new);
+    C2SPacketType<MsgSpellbookStateChanged<ServerPlayerEntity>> CLIENT_SPELLBOOK_UPDATE = SimpleNetworking.clientToServer(Unicopia.id("client_spellbook_update"), MsgSpellbookStateChanged::new);
+
     Identifier SERVER_RESOURCES_SEND_ID = Unicopia.id("resources_send");
     S2CPacketType<MsgServerResources> SERVER_RESOURCES_SEND = SimpleNetworking.serverToClient(SERVER_RESOURCES_SEND_ID, MsgServerResources::new);
 

src/main/java/com/minelittlepony/unicopia/network/MsgSpellbookStateChanged.java

@@ -0,0 +1,52 @@
+package com.minelittlepony.unicopia.network;
+
+import com.minelittlepony.unicopia.container.SpellbookScreenHandler;
+import com.minelittlepony.unicopia.container.SpellbookState;
+import com.minelittlepony.unicopia.util.network.Packet;
+
+import net.minecraft.entity.player.PlayerEntity;
+import net.minecraft.network.PacketByteBuf;
+import net.minecraft.server.network.ServerPlayerEntity;
+
+/**
+ * Received by the server when a player changes their opened spellbook's state
+ * Received by the client when another player changes the shared spellbook's state
+ */
+public class MsgSpellbookStateChanged<T extends PlayerEntity> implements Packet<T> {
+
+    private final int syncId;
+    private final SpellbookState state;
+
+    public MsgSpellbookStateChanged(int syncId, SpellbookState state) {
+        this.syncId = syncId;
+        this.state = state;
+    }
+
+    public MsgSpellbookStateChanged(PacketByteBuf buffer) {
+        syncId = buffer.readInt();
+        state = new SpellbookState().fromPacket(buffer);
+    }
+
+    @Override
+    public void toBuffer(PacketByteBuf buffer) {
+        buffer.writeInt(syncId);
+        state.toPacket(buffer);
+    }
+
+    @Override
+    public void handle(T sender) {
+
+        if (sender.currentScreenHandler.syncId != syncId) {
+            return;
+        }
+
+        System.out.println("Spellbook update page=" + state.getCurrentPageId().get() + " offset=" + state.getState(state.getCurrentPageId().get()).getOffset() + " to " + sender);
+
+        if (sender.currentScreenHandler instanceof SpellbookScreenHandler spellbook) {
+            spellbook.getSpellbookState().copyFrom(state);
+            if (sender instanceof ServerPlayerEntity) {
+                spellbook.getSpellbookState().synchronize();
+            }
+        }
+    }
+}

src/main/java/com/minelittlepony/unicopia/network/datasync/Synchronizable.java

@@ -0,0 +1,20 @@
+package com.minelittlepony.unicopia.network.datasync;
+
+import java.util.Optional;
+import java.util.function.Consumer;
+
+public abstract class Synchronizable<T extends Synchronizable<T>> {
+
+    private Optional<Consumer<T>> synchronizer = Optional.empty();
+
+    @SuppressWarnings("unchecked")
+    public void synchronize() {
+        synchronizer.ifPresent(s -> s.accept((T)this));
+    }
+
+    public void setSynchronizer(Consumer<T> synchronizer) {
+        this.synchronizer = Optional.of(synchronizer);
+    }
+
+    public abstract void copyFrom(T state);
+}

src/main/java/com/minelittlepony/unicopia/util/network/S2CPacketType.java

@@ -8,7 +8,7 @@ import net.minecraft.util.Identifier;
 /**
  * A client packet type. Sent by the server to a specific player.
  */
-public interface S2CPacketType<T extends Packet<PlayerEntity>> {
+public interface S2CPacketType<T extends Packet<? extends PlayerEntity>> {
     Identifier getId();
 
     default void send(ServerPlayerEntity recipient, T packet) {