Add a post on alerts in ASP.NET Core

src/posts/AspNetCoreAlerts.md

@@ -0,0 +1,197 @@
+---
+title: ASP.NET Core Alerts
+description: Tools to display alerts across redirects
+tags: [ Programming, C#, ASP.NET Core ]
+---
+
+This code allows you to store alerts that will be displayed the next time the user is served a page. This is useful for when you want to display an alert after a redirect.
+
+## How To Use
+
+To add an alert use one of the extension methods to add an alert to the session:
+
+```csharp
+return this.RedirectToPage("/Things/Index")
+	.WithSuccess("Did the thing!");
+```
+
+Then to display the alerts, add something like this to the `_Layout.cshtml`:
+
+```csharp
+@{
+	Alert[] alerts = this.TempData.GetAlerts();
+}
+
+@if (alerts != null && alerts.Any())
+{
+	foreach (Alert alert in alerts)
+	{
+		...
+	}
+}
+```
+
+Note that calling `TempData.GetAlerts()` will also remove all current alerts from the session so they won't be displayed on following page loads.
+
+## Code
+
+The alerts class:
+
+```csharp
+public class Alert
+{
+	public AlertType AlertType { get; set; }
+	public string Text { get; set; }
+
+	public Alert(AlertType alertType, string text)
+	{
+		this.AlertType = alertType;
+		this.Text = text;
+	}
+}
+
+public enum AlertType
+{
+	Success,
+	Info,
+	Warning,
+	Error
+}
+```
+
+The `AlertActionResult` To wrap the return `ActionResult` and actually add the alert to the session:
+
+```csharp
+/// <summary>
+/// This is a wrapper around another action result. When executed, this adds the given alert to
+/// TempData and executes the inner ActionResult
+/// </summary>
+public class AlertActionResult : ActionResult
+{
+	public ActionResult InnerResult { get; set; }
+	public Alert Alert { get; set; }
+
+	public AlertActionResult(ActionResult innerResult, Alert alert)
+	{
+		this.InnerResult = innerResult;
+		this.Alert = alert;
+	}
+
+	public AlertActionResult(ActionResult innerResult, AlertType alertType, string alertText)
+	{
+		this.InnerResult = innerResult;
+		this.Alert = new Alert(alertType, alertText);
+	}
+
+	public override void ExecuteResult(ActionContext context)
+	{
+		ITempDataDictionary tempData = context.HttpContext.RequestServices
+			.GetService<ITempDataDictionaryFactory>()
+			.GetTempData(context.HttpContext);
+
+		tempData.AddAlert(this.Alert);
+
+		this.InnerResult.ExecuteResult(context);
+	}
+
+	public override async Task ExecuteResultAsync(ActionContext context)
+	{
+		ITempDataDictionary tempData = context.HttpContext.RequestServices
+			.GetService<ITempDataDictionaryFactory>()
+			.GetTempData(context.HttpContext);
+
+		tempData.AddAlert(this.Alert);
+
+		if (this.InnerResult is PageResult pageResult)
+		{
+			// Need to do some additional setup for Razor Pages
+			// See https://stackoverflow.com/questions/55989209/
+
+			PageContext pageContext = context as PageContext
+				?? throw new ArgumentException("The context must be a PageContext for Razor Pages",
+					nameof(context));
+
+			Func<PageContext, ViewContext, object> pageFactory = pageContext.HttpContext.RequestServices
+				.GetRequiredService<IPageFactoryProvider>()
+				.CreatePageFactory(pageContext.ActionDescriptor);
+
+			ViewContext viewContext = new(
+				pageContext,
+				NullView.Instance,
+				pageContext.ViewData,
+				tempData,
+				TextWriter.Null,
+				new HtmlHelperOptions())
+			{
+				ExecutingFilePath = pageContext.ActionDescriptor.RelativePath
+			};
+
+			pageResult.ViewData = viewContext.ViewData;
+			pageResult.Page = (PageBase)pageFactory(pageContext, viewContext);
+		}
+
+		await this.InnerResult.ExecuteResultAsync(context);
+	}
+
+	private class NullView : IView
+	{
+		public static readonly NullView Instance = new();
+
+		public string Path => string.Empty;
+
+		public Task RenderAsync(ViewContext context)
+		{
+			ArgumentNullException.ThrowIfNull(context, nameof(context));
+
+			return Task.CompletedTask;
+		}
+	}
+}
+```
+
+And some extension methods for adding and getting alerts:
+
+```csharp
+public static class AspNetExtensions
+{
+	private static readonly string _alertsTempDataKey = "atticAlerts";
+
+	public static Alert[] GetAlerts(this ITempDataDictionary tempData)
+	{
+		if (!tempData.ContainsKey(_alertsTempDataKey))
+			tempData[_alertsTempDataKey] = JsonSerializer.Serialize(Array.Empty<Alert>());
+
+		return JsonSerializer.Deserialize<Alert[]>(tempData[_alertsTempDataKey] as string);
+	}
+
+	public static void AddAlert(this ITempDataDictionary tempData, Alert alert)
+	{
+		Alert[] currentAlerts = tempData.GetAlerts();
+
+		currentAlerts = currentAlerts
+			.Concat(new Alert[] { alert })
+			.ToArray();
+		tempData[_alertsTempDataKey] = JsonSerializer.Serialize(currentAlerts);
+	}
+
+	public static ActionResult WithSuccess(this ActionResult actionResult, string alertText)
+	{
+		return new AlertActionResult(actionResult, AlertType.Success, alertText);
+	}
+
+	public static ActionResult WithInfo(this ActionResult actionResult, string alertText)
+	{
+		return new AlertActionResult(actionResult, AlertType.Info, alertText);
+	}
+
+	public static ActionResult WithWarning(this ActionResult actionResult, string alertText)
+	{
+		return new AlertActionResult(actionResult, AlertType.Warning, alertText);
+	}
+
+	public static ActionResult WithError(this ActionResult actionResult, string alertText)
+	{
+		return new AlertActionResult(actionResult, AlertType.Error, alertText);
+	}
+}
+```