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Asked: 2025-06-18T22:14:11+05:30

How can I update the server about my hotbar changes in a FabricMC mod?

I’m working on this mod for FabricMC that allows players to switch between saved creative hotbars easily, but I’m running into a bit of a snag. Everything’s functioning well on the client side — I can switch hotbars and see the items update perfectly. But when I try to place blocks after switching, they just vanish! I’m pretty sure this has to do with the server not knowing that I’ve updated my inventory.

Here’s the deal: I have this method called `endHotbarPreview()`, where I handle the hotbar switching. It retrieves the items from my saved hotbar and updates the player’s inventory using `setStack()` for each slot. I also call `clickCreativeStack()` for each item to inform the interaction manager that I want to update the creative inventory. However, even though the items appear correctly on my screen, it feels like the server isn’t aware of these changes, which is why I’m thinking it doesn’t allow me to place blocks.

I like the idea of the hotbar being fully client-side, but this leads to a significant issue because players can’t actually place the blocks they want. So, I’m stuck wondering how to notify the server that the hotbar has been updated. What’s the proper way to do this?

It seems like I could be missing a crucial step to sync the client and server regarding the inventory state. I’ve considered sending a packet to the server after I update the inventory, but I’m not sure how to best implement that in this context. If anyone has experience with this sort of thing or has come across similar challenges, I would really appreciate your insights!

In short, I need a way to ensure the server knows when the inventory changes due to the hotbar switch. Any suggestions on what I should do? Or tips on how to structure the networking part of my mod to handle the synchronization properly? Thanks in advance!

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    2 Answers

    1. Okay, so I think I get what you’re going through! It sounds like your client-side stuff is all cool and shows the right blocks in the hotbar, but the server isn’t aware of these changes. That’s a classic issue when working with client-server setups.

      From what you’ve described, it seems like the server needs to know whenever you make a change to the player’s inventory. One way you could do this is by sending a packet from the client to the server after you call endHotbarPreview(). This packet could tell the server something like, “Hey, I’ve just switched my hotbar, and here’s what’s in it now!”

      You can create a custom packet using Fabric’s networking system. It would contain the details of the items you’ve set in your hotbar. Then, when the server receives this packet, you can update the player’s inventory on the server side accordingly. This way, when you go to place a block, the server knows exactly what items you have available.

      Here’s a rough idea of how you might structure that:

              1. Create a custom packet class.
        2. Send this packet after your endHotbarPreview() method updates the inventory.
        3. Handle the packet on the server side to update the inventory.

      I would also suggest checking how the built-in creative inventory works. There might be methods already in place that you can hook into or mimic, which handle the synchronization between the client and server. That can save you some time!

      Lastly, don’t forget to test and see if other players can also place blocks after switching hotbars. If it works for them, then you’re definitely on the right track!

      Hope that helps! Just remember to take it step by step.

    2. The issue arises because directly using methods like setStack() and clickCreativeStack() on the client-side only updates the local inventory state, without properly synchronizing it with the server-side inventory handler. In Fabric, inventory changes—especially those involving creative mode inventories—aren’t automatically broadcasted when modified purely client-side. Therefore, any method modifying inventory must explicitly synchronize these changes through proper network packets or standard game actions that ensure the server acknowledges and updates the inventory accordingly.

      To resolve your problem, you should implement a custom packet that transmits your updated hotbar inventory from client to server whenever you invoke endHotbarPreview(). Using Fabric’s built-in networking utilities such as ClientPlayNetworking.send() and ServerPlayNetworking.registerGlobalReceiver(), you can create a dedicated packet handler. Immediately after applying the client-side changes, send a packet containing either the full hotbar snapshot or inventory-change details to the server. On receiving the packet server-side, explicitly update the player inventory state there, keeping everything synchronized. This approach ensures consistency between client and server, allowing you to place blocks normally after hotbar switching.