using System.Net.Http.Json;
using DataTransfer.Client.Shared;

namespace DataTransfer.Client.Services;

/// <summary>Periodically fetches the load of the relay server; shared by all components of the page.</summary>
public sealed class ServerStatusMonitor(HttpClient http, ILogger<ServerStatusMonitor> logger) : IDisposable
{
    private static readonly TimeSpan PollInterval = TimeSpan.FromSeconds(5);

    private readonly CancellationTokenSource _stopping = new();
    private Task? _polling;

    public event Action? Changed;

    /// <summary>The latest status, or <c>null</c> while it is unknown.</summary>
    public ServerStatus? Current { get; private set; }

    public void EnsureStarted() => _polling ??= PollAsync(_stopping.Token);

    public async Task RefreshAsync(CancellationToken cancellationToken = default)
    {
        ServerStatus? status;
        try
        {
            status = await http.GetFromJsonAsync<ServerStatus>(TransferProtocol.StatusPath.TrimStart('/'), cancellationToken);
        }
        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
        {
            return;
        }
        catch (Exception ex)
        {
            logger.LogDebug(ex, "Fetching the server status failed");
            status = null;
        }

        if (status != Current)
        {
            Current = status;
            Changed?.Invoke();
        }
    }

    public void Dispose()
    {
        _stopping.Cancel();
        _stopping.Dispose();
    }

    private async Task PollAsync(CancellationToken cancellationToken)
    {
        using var timer = new PeriodicTimer(PollInterval);
        try
        {
            do
            {
                await RefreshAsync(cancellationToken);
            } while (await timer.WaitForNextTickAsync(cancellationToken));
        }
        catch (OperationCanceledException) { }
    }
}