2.2SignInAuthenticationHandler【SignOutAuthenticationHandler、IAuthenticationSignInHandler】

using System.Security.Claims;
using System.Text.Encodings.Web;
using System.Threading.Tasks;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;

namespace Microsoft.AspNetCore.Authentication
{
    /// <summary>
    /// Adds support for SignInAsync
    /// </summary>
    public abstract class SignInAuthenticationHandler<TOptions> : SignOutAuthenticationHandler<TOptions>, IAuthenticationSignInHandler
        where TOptions : AuthenticationSchemeOptions, new()
    {
        /// <summary>
        /// Initializes a new instance of <see cref="SignInAuthenticationHandler{TOptions}"/>.
        /// </summary>
        /// <param name="options">The monitor for the options instance.</param>
        /// <param name="logger">The <see cref="ILoggerFactory"/>.</param>
        /// <param name="encoder">The <see cref="UrlEncoder"/>.</param>
        /// <param name="clock">The <see cref="ISystemClock"/>.</param>
        public SignInAuthenticationHandler(IOptionsMonitor<TOptions> options, ILoggerFactory logger, UrlEncoder encoder, ISystemClock clock) : base(options, logger, encoder, clock)
        { }

        /// <inheritdoc/>
        public virtual Task SignInAsync(ClaimsPrincipal user, AuthenticationProperties? properties)
        {
            var target = ResolveTarget(Options.ForwardSignIn);
            return (target != null)
                ? Context.SignInAsync(target, user, properties)
                : HandleSignInAsync(user, properties ?? new AuthenticationProperties());
        }

        /// <summary>
        /// Override this method to handle SignIn.
        /// </summary>
        /// <param name="user"></param>
        /// <param name="properties"></param>
        /// <returns>A Task.</returns>
        protected abstract Task HandleSignInAsync(ClaimsPrincipal user, AuthenticationProperties? properties);

    }
}