Add observer for online servers.

Only for the servers in the Eindhoven city.

ServerManagement/Components/Controls/CityComponent.razor

@@ -1,13 +1,26 @@
+@using ServerManagement.StateStore
+
+@implements IDisposable
+
+@inject EindhovenOnlineServersStore EindhovenOnlineServersStore
+
 <div class="col">
   <div class="card @((city == selectedCity) ? "border-primary" : "")">
     <img src=@($"/images/{@city}.png") class="card-img-top" alt="@city">
     <div class="card-body @((city == selectedCity) ? "active" : "")">
       <button class="btn btn-primary" @onclick="@(() => { SelectCity(city); })">@city</button>
     </div>
+    <div>
+      @if (city.Equals("Eindhoven", StringComparison.OrdinalIgnoreCase))
+      {
+        <text>Online: @serversOnlineEindhoven</text>
+      }
+    </div>
   </div>
 </div>
 
 @code {
+  private int serversOnlineEindhoven;
   [Parameter]
   public string? city { get; set; } = "";
 
@@ -21,4 +34,26 @@
   {
     SelectCityCallBack.InvokeAsync(cityName);
   }
+
+  protected override void OnAfterRender(bool firstRender)
+  {
+    if (firstRender)
+    {
+      serversOnlineEindhoven = EindhovenOnlineServersStore.GetNumberServersOnline();
+      EindhovenOnlineServersStore.AddStateChangeListener(OnServersStatusChange);
+      StateHasChanged();
+    }
+  }
+
+  private void OnServersStatusChange()
+  {
+    serversOnlineEindhoven = EindhovenOnlineServersStore.GetNumberServersOnline();
+    StateHasChanged();
+  }
+
+  // Remove the listeners, otherwise it can cause memory leaks.
+  public void Dispose()
+  {
+    EindhovenOnlineServersStore.RemoveStateChangeListener(OnServersStatusChange);
+  }
 }

ServerManagement/Components/Controls/ServerComponent.razor

@@ -1,6 +1,7 @@
 @namespace ServerManagement.Components.Controls
 
 @inject NavigationManager NavigationManager
+@inject EindhovenOnlineServersStore EindhovenOnlineServersStore
 
 @if (Server != null)
 {
@@ -32,14 +33,14 @@
       @if (Server.IsOnline)
       {
         <button type="button" class="btn btn-outline-danger"
-          @onclick="@(() => { Server.IsOnline = false; })">
+          @onclick="@(() => { SetServerStatus(false); })">
         Turn off
         </button>
       }
       else
       {
         <button type="button" class="btn btn-outline-success"
-          @onclick="@(() => { Server.IsOnline = true; })">
+          @onclick="@(() => { SetServerStatus(true); })">
         Turn on
         </button>
       }
@@ -92,4 +93,27 @@
       return "white";
     }
   }
+
+  private void SetServerStatus(bool status)
+  {
+    if (this.Server != null)
+    {
+      if (this.Server.IsOnline != status)
+      {
+        if (this.Server.City.Equals("Eindhoven", StringComparison.OrdinalIgnoreCase))
+        {
+          var numberOnline = EindhovenOnlineServersStore.GetNumberServersOnline();
+          if (status)
+          {
+            EindhovenOnlineServersStore.SetNumberServersOnline(numberOnline + 1);
+          }
+          else if (numberOnline > 1)
+          {
+            EindhovenOnlineServersStore.SetNumberServersOnline(numberOnline - 1);
+          }
+        }
+        this.Server.IsOnline = status;
+      }
+    }
+  }
 }

ServerManagement/Components/Pages/Servers.razor

@@ -1,7 +1,10 @@
 @page "/servers"
 @page "/servers/back_from/{cityName}"
+
 @using ServerManagement.Components.Controls
+
 @inject NavigationManager NavigationManager
+@inject EindhovenOnlineServersStore EindhovenOnlineServersStore
 
 <h3>Servers</h3>
 <br/>
@@ -60,6 +63,12 @@
         selectedCity = CityName;
         StateHasChanged();
       }
+
+      var serversEindhoven = ServersRepository.GetServersByCity("Eindhoven");
+      if (serversEindhoven != null)
+      {
+        EindhovenOnlineServersStore.SetNumberServersOnline(serversEindhoven.Count(s => s.IsOnline));
+      }
     }
   }
 }

ServerManagement/Components/StateStore/EindhovenOnlineServersStore.cs

@@ -0,0 +1,18 @@
+namespace ServerManagement.StateStore
+{
+  public class EindhovenOnlineServersStore : Observer
+  {
+    private int _numberServersOnline;
+
+    public int GetNumberServersOnline()
+    {
+      return _numberServersOnline;
+    }
+
+    public void SetNumberServersOnline(int numbersOnline)
+    {
+      _numberServersOnline = numbersOnline;
+      base.BroadcastStateChange();
+    }
+  }
+}

ServerManagement/Components/StateStore/Observer.cs

@@ -0,0 +1,28 @@
+namespace ServerManagement.StateStore
+{
+  public class Observer
+  {
+    protected Action? _listeners;
+
+    public void AddStateChangeListener(Action? listener)
+    {
+      if (listener is not null)
+      {
+        _listeners += listener;
+      }
+    }
+
+    public void RemoveStateChangeListener(Action? listener)
+    {
+      if (listener is not null)
+      {
+        _listeners -= listener;
+      }
+    }
+
+    public void BroadcastStateChange()
+    {
+      _listeners?.Invoke();
+    }
+  }
+}

ServerManagement/Program.cs

@@ -15,6 +15,8 @@ builder.Services.AddTransient<SessionStorage>();
 // For a WebAssembly, you want to use AddSingleton, so that everything is locally downloaded in the browser.
 builder.Services.AddScoped<ContainerStorage>();
 
+builder.Services.AddScoped<EindhovenOnlineServersStore>();
+
 var app = builder.Build();
 
 // Configure the HTTP request pipeline.