Platform-Agnostic API for Text Notifications

XAF 17.1+
Text notification display (e.g. confirmation messages, warnings, errors) has been simplified for desktop and Web XAF applications with the help of the platform-agnostic DevExpress.ExpressApp > ShowViewStrategyBase > ShowMessage  method. You can quickly learn more about it from this blog as a preview of this functionality was already available in XAF v16.2.5+.
With v17.1, this functionality was greatly extended based on customer feedback and you can research it further in the FeatureCenter and SimpleProjectManager demos:

C#
...  
                MessageOptions options = new MessageOptions();  
                options.Duration = 2000;  
                options.Message = string.Format("{0} task(s) have been successfully updated!", e.SelectedObjects.Count);  
                options.Type = InformationType.Success;  
                options.Web.Position = InformationPosition.Right;  
                options.Win.Caption = "Success";  
                options.Win.Type = WinMessageType.Flyout;  
                Application.ShowViewStrategy.ShowMessage(options);  
...  

We also look forward to hearing from you once you have had an opportunity to try this extended ShowMessage functionality in v17.1.

Hi James,

FOR OLDER VERSIONS
1. Display a message in a panel embedded at the view top/bottom
1.1. Introduce a new class, for example, with a static property to store text you want to show. You can also use the Session to store this text;
1.2. Create a WebControl descendant and override its Render method. In this method, create a new Literal control with the text from your store used on step 1 and add it to the Controls collection. Check the source code of our default ErrorInfoControl (…\DevExpress.ExpressApp.Web\Templates\ErrorInfoControl.cs) for more inspiration, if necessary.
1.3. Create a custom TemplateContent, add a new XafUpdatePanel control onto the TemplateContent and place the created control into the XafUpdatePanel control. For more details, refer to the How to: Customize an ASP.NET Template article.

In addition, see our FeatureCenter demo source as it shows another way of providing custom messages:
"%public%\Documents\DevExpress Demos 1X.X\Components\eXpressApp Framework\FeatureCenter\CS\FeatureCenter.Module\ShowHintController.cs"
"%public%\Documents\DevExpress Demos 1X.X\Components\eXpressApp Framework\FeatureCenter\CS\FeatureCenter.Module\HintAttribute.cs"
"%public%\Documents\DevExpress Demos 1X.X\Components\eXpressApp Framework\FeatureCenter\CS\FeatureCenter.Web\Default.aspx.cs"

2. Display a message via a custom popup window or window.alert in JavaScript
See an example at Displaying modal pop-up conformation dialog/message box.

3. Display a popup using the XafApplication.ShowViewStrategy.ShowViewInPopupWindow method
With v16.2,  the display of Dialog Views (configurable confirmation messages, reminders, etc.) has been simplified for both Windows and Web based XAF applications. We've implelemented a new platform-independent XafApplication.ShowViewStrategy.ShowViewInPopupWindow method. Refer to the How to show complex dialogs on the client-side in XAF ASP.NET applications (similar to the MessageBox in Windows Forms) ticket for more details.

Thanks,
Dennis