public static void onPlayerCloned(PlayerEvent.Clone event) {
        if (!event.getOriginal().world.isRemote() && !event.getPlayer().world.isRemote() && event.getOriginal() != null && event.getPlayer() != null) {
            LazyOptional<IMindCapability> oldMindCap = event.getOriginal().getCapability(LibCapabilities.MIND_CAPABILITY);
            LazyOptional<IMindCapability> newMindCap = event.getPlayer().getCapability(LibCapabilities.MIND_CAPABILITY);
            if (oldMindCap.isPresent() && newMindCap.isPresent()) {
                newMindCap.ifPresent(capability -> {
                    oldMindCap.ifPresent(capabilityOld -> {
                        event.getPlayer().sendMessage(new StringTextComponent("Before: " + capability.getMindStrength()), CommonProxy.IN_GAME_UUID);
                        capability.deserializeNBT(capabilityOld.serializeNBT());
                        event.getPlayer().sendMessage(new StringTextComponent("After: " + capability.getMindStrength()), CommonProxy.IN_GAME_UUID);
                    });
                  // This don't work, the value on the client side is still the initial value
                    SCPacketHandler.INSTANCE.send(PacketDistributor.PLAYER.with(() -> (ServerPlayerEntity) event.getPlayer()), new MSGMindCapabilitySync(capability.getMindStrength()));
                  // It works,the value on the client side is correct, but I don't know why, I think it's just a coincidence.
//                    SCPacketHandler.INSTANCE.sendToServer(new MSGMindCapabilitySync(capability.getMindStrength()));
                });
            }
        }
    }

 

This is my message code.

public class MSGMindCapabilitySync {
    private final int mindStrength;

    public MSGMindCapabilitySync(int mindStrength) {
        this.mindStrength = mindStrength;

    }

    public static void encodingMSG(MSGMindCapabilitySync msg, PacketBuffer buffer) {
        buffer.writeInt(msg.mindStrength);
    }

    public static MSGMindCapabilitySync decodingMSG(PacketBuffer buffer) {
        return new MSGMindCapabilitySync(buffer.readInt());
    }

    public static void handle(MSGMindCapabilitySync msg, Supplier<NetworkEvent.Context> ctx) {
        ctx.get().enqueueWork(() -> {
            // Form client to server
            ServerPlayerEntity serverPlayer = ctx.get().getSender();
            if (serverPlayer != null) {
                serverPlayer.getCapability(LibCapabilities.MIND_CAPABILITY).ifPresent((capability -> {
                    capability.setMindStrength(msg.mindStrength);
                }));
                SaintChapter.LOGGER.info("Form client to server. Done");
            }
            // From server to client
            DistExecutor.unsafeRunWhenOn(Dist.CLIENT, () -> () -> {
                if (Minecraft.getInstance().player == null) return;
                Minecraft.getInstance().player.getCapability(LibCapabilities.MIND_CAPABILITY).ifPresent((capability -> {
                    capability.setMindStrength(msg.mindStrength);
                }));
                SaintChapter.LOGGER.info("From server to client. Done");
            });
        });
        ctx.get().setPacketHandled(true);
    }
}

16 minutes ago, diesieben07 said:

PlayerEvent.Clone is only for copying data from the old player to the new player. It is not the place to send packets or do anything else.

thank you😀