#if NET
namespace DigitalData.Core.Abstractions;

/// <summary>
/// A deferred implementation of <see cref="IServiceProvider"/> that allows the <see cref="IServiceProvider"/> instance 
/// to be provided at a later time via a factory callback.
/// </summary>
/// <remarks>
/// This is particularly useful when service registration requires an <see cref="IServiceProvider"/> instance,
/// but the service provider is not yet available at registration time. 
/// By using <see cref="DeferredServiceProvider"/>, a service can safely resolve dependencies once
/// the application's service provider has been built, ensuring a single consistent scope.
/// </remarks>
public class DeferredServiceProvider : IServiceProvider
{
    private Lazy<IServiceProvider>? _serviceProvider;

    /// <summary>
    /// Sets the factory that will be used to lazily initialize the <see cref="IServiceProvider"/> instance.
    /// </summary>
    public Func<IServiceProvider> Factory
    {
        set => _serviceProvider = new(value);
    }

    /// <summary>
    /// Retrieves the requested service object from the deferred <see cref="IServiceProvider"/>.
    /// </summary>
    /// <param name="serviceType">The type of service object to retrieve.</param>
    /// <returns>The requested service object, or <c>null</c> if the service is not available.</returns>
    /// <exception cref="InvalidOperationException">
    /// Thrown if the service provider factory has not been set before calling <see cref="GetService"/>.
    /// </exception>
    public object? GetService(Type serviceType)
    {
        if (_serviceProvider is null)
            throw new InvalidOperationException("The service provider has not been initialized. Make sure 'Factory' is set before calling 'GetService'.");

        return _serviceProvider.Value.GetService(serviceType);
    }
}
#endif