Welcome to Simple Town, My Notes on ASP.NET Razor Pages
Razor Pages provides a simplified abstraction on top of MVC. No bloated controllers, just bite sized modular pages that are paired with a page model (think Codebehind if you’ve used Web Forms). You don’t have to fuss over routing because routing defaults to your folder/page structure with simple conventions.
Here are some of my notes on RazorPages. This is not meant to teach RazorPages just a way to reinforce whats learned by just diving into it. These notes are the results of my research on questions that I ended up digging through docs.microsoft.com, StackOverflow and Google. Remember I’m still a RazorPage newbie so I may not have properly grasped some of this yet.
A Page in Razor Pages is a cshtml file with the @page directive at the top of the page. Pages are basically content pages scripted as Razor templates. You have all the power of Razor and a simplified coding experience with Page Models. You can still use Razor layout pages to have a consistent master page template for your pages. You also get Razor partial pages that allow you to go super modular and build up your pages with reusable components (nice… thinking user controls keeping with my trip down Web Forms memory lane).
Page Models are like Codebehinds from Web Forms because there is a one-to-one relationship between Page and PageModel. In fact, in the Page you bind the Page to the PageModel with the @model directive.
The PageModel is like an MVC Controller because it is an abstraction of a controller. It is unlike an MVC Controller because the Controller can be related to many Views and the PageModel has a beautiful, simplified, easy to understand one-to-one relationship with a Page.
A PageModel is a simplified controller that you don’t have to worry about mapping routes to. You get to create methods that handle actions triggered by page requests. There is a well defined convention to map requests to handlers that I won’t go into because there are great sites that talk about the details of everything I bring up in my notes.
https://www.learnrazorpages.com is a great resource to start digging into the details.
BindingProperty is used when you want read-write two-way state binding between the PageModel and Page. Don’t get it twisted, Razor Pages is still stateless, but you have a way to easily bind state and pass state between the client and the server. Don’t worry, I know I keep saying Web Forms, but there is no View State, Sessions, or other nasties trying to force the stateless web to be stateful.
The BindingProperty is kind of like a communication channel between the Page and PageModel. The communication channel is not like a phone where communication can flow freely back and forth. Its more like a walkie talkie or CB radio where each side has to take turns clicking a button to talk where request and response are the button clicks. Simply place a BindingProperty attribute on a public property in the PageModel and the PageModel can send its current state to the Page and the Page can send its current state to the PageModel.
DIGRESS: As I dug into this I wondered if there was a way to do reactive one-way data flow like ReactJS. Maybe a BindingProperty that is immutable in the Page. The Page doesn’t update the BindingProperty when a BindingProperty is changed in the Page. Instead, when the Page wants to update a BindingProperty it would send some kind of change event to the PageModel. Then the PageModel handles the event by updating the BindingProperty which updates the Page state. We may need to use WebSockets, think SignalR, to provide an open communication channel to allow the free flow of change events and state changes.
What do you know, of course this has been done – https://www.codeproject.com/Articles/1254354/Turn-Your-Razor-Page-Into-Reactive-Form. Not sure if this is ready for prime time, but I loved the idea of reactive one way data flow when I started to learn about ReactJS. Maybe there is some real benefit that may encourage this to be built into Razor Pages.
ViewData is the same ViewData we’ve been using in MVC. It is used to maintain read only Page state between postback (haven’t written “postback” since web forms… it all comes back around). ViewData is used in scenarios where one-way data flow from PageModel to the Page is acceptable. The page state saved to ViewData is passed from the PageModel to the Page.
ViewData is a data structure, a dictionary of objects with a string key. ViewData does not live beyond the request that it is returned to the Page in. When a new request is issued or a redirect occurs the state of ViewData is not maintained.
Since ViewData is weakly typed, values are stored as objects, the values have to be cast to a concrete type to be used. This also means that using ViewData you loose the benefits of Intellisense and compile-time checking. There are benefits that offset the shortcomings of weak typing. ViewData can be shared with a content Page’s layout and partials.
In a PageModel you can use the ViewData Attribute on public property of the PageModel. This makes the property available in ViewData. The property name becomes the key for the property values in the ViewData.
TempData is use used to send single-use read-only data from PageModel to the Page. The most common use of TempData is to provide user feedback after post actions that results in a redirect where you want to inform the user of the results of the post (“Hey, such and such was deleted like you asked.”).
TempData is marked for deletion after it is read from the request. There are Keep and Peek methods that can be used to look at the data without deleting it and a Remove method to delete it (I haven’t figured out a scenario where I want to use these yet).
TempData is contained in a dictionary of objects with a string key.
Razor Pages Life Cycle
Lastly, I wanted to understand the life cycle of Razor Pages and how I can plug-in to it to customize it for my purpose. Back to Web Forms again, I remember there being a well documented life cycle that let me shoot myself in the foot all the time. Below is the life cycle as I have pieced it together so far. I know we still have MVC under the hood so we still have the Middlewear pipeline, but I couldn’t find documentation on the life cycle with respect to Razor Pages specifically. Maybe I will walk through the source code one day or someone from the Razor Page team or someone else will do it for us (like https://docs.microsoft.com/en-us/aspnet/mvc/overview/getting-started/lifecycle-of-an-aspnet-mvc-5-application).
- A request is made to a URL.
- The URL is routed to a Page based on convention.
- The handler method in the IPageModel is selected based on convention.
- The OnPageHandlerSelcted IPageFilter and OnPageHandlerSelctedAsync IPageFilterAsync methods are ran.
- The PageModel properties and parameters are bound.
- The OnPageHandlerExecuting IPageFilter and OnPageHandlerExecutionAsync IPageFilterAsync methods are ran.
- The handler method is executed.
- The handler method returns a response.
- The OnPageHandlerExecuted IPageFilter method is ran.
- The Page is rendered (I need more research on how this happens in Razor, I mean we have the content, layout, and partial pages how are they rendered and stitched together?)
The Page Filters are cool because you have access the the HttpContext (request, response, headers, cookies…) so you can do some interesting things like global logging, messing with the headers, etc. They allow you to inject your custom logic into the lifecycle. They are kind of like Middlewear, but you have HttpContext (how cool is that?… very).