Demo how to capture the user

Author: javiercn

BlazorServerCaptureUser/Pages/Index.razor

@@ -1,8 +1,8 @@
 @page "/"
-
+@inject UserService UserService
 <PageTitle>Index</PageTitle>
 
-<h1>Hello, world!</h1>
+<h1>Hello, @(UserService.GetUser().Identity.Name ?? "world!")</h1>
 
 Welcome to your new app.
 

BlazorServerCaptureUser/Program.cs

@@ -1,11 +1,11 @@
 using BlazorServerCaptureUser.Areas.Identity;
 using BlazorServerCaptureUser.Data;
-using Microsoft.AspNetCore.Components;
 using Microsoft.AspNetCore.Components.Authorization;
-using Microsoft.AspNetCore.Components.Web;
+using Microsoft.AspNetCore.Components.Server.Circuits;
 using Microsoft.AspNetCore.Identity;
-using Microsoft.AspNetCore.Identity.UI;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection.Extensions;
+using System.Security.Claims;
 
 var builder = WebApplication.CreateBuilder(args);
 
@@ -21,6 +21,9 @@
 builder.Services.AddScoped<AuthenticationStateProvider, RevalidatingIdentityAuthenticationStateProvider<IdentityUser>>();
 builder.Services.AddSingleton<WeatherForecastService>();
 
+builder.Services.AddScoped<UserService>();
+builder.Services.TryAddEnumerable(ServiceDescriptor.Scoped<CircuitHandler, UserCircuitHandler>());
+
 var app = builder.Build();
 
 // Configure the HTTP request pipeline.
@@ -48,3 +51,76 @@
 app.MapFallbackToPage("/_Host");
 
 app.Run();
+
+public class UserService
+{
+    private ClaimsPrincipal _currentUser = new ClaimsPrincipal(new ClaimsIdentity());
+
+    public ClaimsPrincipal GetUser()
+    {
+        return _currentUser;
+    }
+
+    internal void SetUser(ClaimsPrincipal user)
+    {
+        // This might be called more than once
+        if (_currentUser != user)
+        {
+            _currentUser = user;
+        }
+    }
+}
+
+internal sealed class UserCircuitHandler : CircuitHandler, IDisposable
+{
+    private readonly AuthenticationStateProvider _authenticationStateProvider;
+    private readonly UserService _userService;
+
+    public UserCircuitHandler(
+        AuthenticationStateProvider authenticationStateProvider,
+        UserService userService)
+    {
+        _authenticationStateProvider = authenticationStateProvider;
+        _userService = userService;
+    }
+
+    public void Dispose()
+    {
+        _authenticationStateProvider.AuthenticationStateChanged -= AuthenticationChanged;
+    }
+
+    // This and the method below are only needed if you want to update the user after the app has re-connected.
+    public override Task OnCircuitOpenedAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        // If you want to update the service, then you can attach an event handler as follows
+        _authenticationStateProvider.AuthenticationStateChanged += AuthenticationChanged;
+        return base.OnCircuitOpenedAsync(circuit, cancellationToken);
+    }
+
+    private void AuthenticationChanged(Task<AuthenticationState> task)
+    {
+        _ = UpdateAuthentication(task);
+
+        async Task UpdateAuthentication(Task<AuthenticationState> task)
+        {
+            try
+            {
+                var state = await task;
+                _userService.SetUser(state.User);
+            }
+            catch
+            {
+                // Swallow all exceptions here since there is no way to report them
+                // (and if they come from the task, they'll be reported somewhere else).
+            }
+        }
+    }
+
+    public override async Task OnConnectionUpAsync(Circuit circuit, CancellationToken cancellationToken)
+    {
+        // This gets called every time the circuit reconnects, setting the user for the lifetime of the connection.
+        // Unless you implement updates.
+        var state = await _authenticationStateProvider.GetAuthenticationStateAsync();
+        _userService.SetUser(state.User);
+    }
+}