DigitalData.MessagingService/src/presentation/DigitalData.MessagingService.Client/EmailSender.cs

using DigitalData.MessagingService.Publisher;
using DigitalData.MessagingService.Publisher.Abstraction;
using DigitalData.MessagingService.RabbitMQ;
using Microsoft.Extensions.DependencyInjection;

namespace DigitalData.MessagingService.Client;

/// <summary>
/// Provides a static, self-contained client for sending emails via RabbitMQ
/// without requiring a host-level dependency injection container.
/// </summary>
/// <remarks>
/// This class manages its own internal <see cref="IServiceProvider"/> using a
/// <see cref="Lazy{T}"/> pattern so the DI container is only built once,
/// on the first call to <see cref="ConnectRabbitMq(Action{RabbitMqConfiguration}, OnReconnect)"/>.
/// </remarks>
public static class EmailSender
{
    /// <summary>
    /// Internal event used to accumulate service registrations before the
    /// <see cref="IServiceProvider"/> is built. Handlers are added by
    /// <see cref="ConnectRabbitMq(Action{RabbitMqConfiguration}, OnReconnect)"/> and invoked exactly once during
    /// lazy initialization.
    /// </summary>
    private static event Action<IServiceCollection> ConfigureServices = delegate { };

    /// <summary>
    /// The lazily-initialized internal service provider.
    /// Built on first access by invoking all registered
    /// <see cref="ConfigureServices"/> handlers.
    /// </summary>
    private static readonly Lazy<IServiceProvider> LazyProvider = new(() =>
    {
        var services = new ServiceCollection();
        services.AddLogging();
        ConfigureServices?.Invoke(services);
        return services.BuildServiceProvider();
    });

    /// <summary>
    /// Gets a value indicating whether the messaging service has been connected
    /// and the internal <see cref="IServiceProvider"/> has been initialized.
    /// </summary>
    /// <value>
    /// <see langword="true"/> if <see cref="ConnectRabbitMq(Action{RabbitMqConfiguration}, OnReconnect)"/> has been called
    /// and the provider is built; otherwise <see langword="false"/>.
    /// </value>
    public static bool IsConnected => LazyProvider.IsValueCreated;

    /// <summary>
    /// Configures and establishes a connection to RabbitMQ, then initializes
    /// the internal dependency injection container.
    /// </summary>
    /// <param name="configure">
    /// A delegate used to configure the <see cref="RabbitMqConfiguration"/>,
    /// such as host, port, credentials, and exchange settings.
    /// </param>
    /// <param name="onReconnect">
    /// Controls the behavior when this method is called while already connected.
    /// Defaults to <see cref="OnReconnect.ThrowException"/>.
    /// </param>
    /// <exception cref="InvalidOperationException">
    /// Thrown when the service is already connected and
    /// <paramref name="onReconnect"/> is <see cref="OnReconnect.ThrowException"/>.
    /// </exception>
    public static void ConnectRabbitMq(Action<RabbitMqConfiguration> configure, OnReconnect onReconnect = OnReconnect.ThrowException)
    {
        if(IsConnected && onReconnect == OnReconnect.ThrowException)
            throw new InvalidOperationException("Messaging service is already connected.");

        ConfigureServices += services => services.AddMessagingServicePublisher(configure);
        _ = LazyProvider.Value; // Force initialization
    }

    /// <summary>
    /// Configures and establishes a connection to RabbitMQ using a URL, then initializes
    /// the internal dependency injection container.
    /// </summary>
    /// <param name="url">
    /// The RabbitMQ server URL (e.g. <c>amqp://hostname:5672/virtualhost</c>).
    /// The host, port, and virtual host are extracted from this URL.
    /// </param>
    /// <param name="username">The username used to authenticate with RabbitMQ.</param>
    /// <param name="password">The password used to authenticate with RabbitMQ.</param>
    /// <param name="onReconnect">
    /// Controls the behavior when this method is called while already connected.
    /// Defaults to <see cref="OnReconnect.ThrowException"/>.
    /// </param>
    /// <exception cref="InvalidOperationException">
    /// Thrown when the service is already connected and
    /// <paramref name="onReconnect"/> is <see cref="OnReconnect.ThrowException"/>.
    /// </exception>
    public static void ConnectRabbitMq(string url, string username, string password, OnReconnect onReconnect = OnReconnect.ThrowException)
    {
        var uri = new Uri(url);

        ConnectRabbitMq(cfg =>
        {
            cfg.HostName = uri.Host;
            cfg.Port = uri.IsDefaultPort ? 5672 : uri.Port;
            cfg.UserName = username;
            cfg.Password = password;
            if (!string.IsNullOrEmpty(uri.AbsolutePath) && uri.AbsolutePath != "/")
                cfg.VirtualHost = Uri.UnescapeDataString(uri.AbsolutePath.TrimStart('/'));
        }, onReconnect);
    }

    /// <summary>
    /// Enqueues the specified email to the RabbitMQ messaging pipeline.
    /// </summary>
    /// <param name="email">The outgoing email data to enqueue.</param>
    /// <exception cref="InvalidOperationException">
    /// Thrown when <see cref="ConnectRabbitMq(Action{RabbitMqConfiguration}, OnReconnect)"/> has not been called prior to sending.
    /// </exception>
    /// <remarks>
    /// This method maps <see cref="Email"/> to <see cref="OutgoingEmailEvent"/>,
    /// then resolves <see cref="IOutgoingEmailPublisher"/> from the internal
    /// service provider and calls <c>EnqueueAsync</c> in a fire-and-forget manner.
    /// Ensure that any unhandled exceptions from the async operation are handled
    /// at the publisher level.
    /// </remarks>
    public static void Send(Email email)
    {
        if(!IsConnected)
            throw new InvalidOperationException("Messaging service is not connected. Call ConnectRabbitMq first.");

        var publisher = LazyProvider.Value.GetRequiredService<IOutgoingEmailPublisher>();
        publisher.EnqueueAsync(email.ToEvent());
    }
}