se-launcher/PluginLoader/SteamAPI.cs

using NLog;
using Steamworks;

namespace PluginLoader;

public static class SteamAPI
{
    public static bool IsSubscribed(ulong id)
    {
        var state = (EItemState)SteamUGC.GetItemState(new(id));
        return (state & EItemState.k_EItemStateSubscribed) == EItemState.k_EItemStateSubscribed;
    }

    public static void SubscribeToItem(ulong id)
    {
        SteamUGC.SubscribeItem(new(id));
    }

    public static void Update(IEnumerable<ulong> ids)
    {
        var enumerable = ids as ulong[] ?? ids.ToArray();
        
        LogFile.Log.Info("Updating {Count} workshop items", enumerable.Length);

        try
        {
            UpdateInternal(enumerable);
        }
        catch (Exception e)
        {
            LogFile.Log.Error(e, "An error occurred while updating workshop items");
            throw;
        }
    }

    private static void UpdateInternal(IEnumerable<ulong> ids)
    {
        var count = 0;
        using var callback = Callback<DownloadItemResult_t>.Create(t =>
        {
            if (t.m_eResult == EResult.k_EResultOK)
                Interlocked.Increment(ref count);

            LogFile.Log.Log(t.m_eResult == EResult.k_EResultOK ? LogLevel.Info : LogLevel.Error,
                "Download finished for {Id} with {State}", t.m_nPublishedFileId.m_PublishedFileId, t.m_eResult);
        });

        //items could have other flags besides installed
        var toDownload = ids.Where(b =>
            ((EItemState)SteamUGC.GetItemState(new(b)) & EItemState.k_EItemStateInstalled) == 0).ToArray();
        foreach (var id in toDownload)
        {
            LogFile.Log.Info("Updating workshop item {Id}", id);

            SteamUGC.DownloadItem(new(id), true);
        }
        
        while (count < toDownload.Length)
        {
            Steamworks.SteamAPI.RunCallbacks();
            Thread.Sleep(10);
        }
    }
}