Overview

• Routing uses routes (implementations of **IRouter**) to:
  • Map incoming requests to route handlers
  • Generate URLs used in responses.
• Generally, when a request arrives, the route collection is processed in order.
  • The incoming request looks for a route that matches the request URL by calling the RouteAsync method on each available route in the route collection.
  • By contrast, a response can use routing to generate URLs (for example, for redirection or links) based on route information, and thus avoid having to hard-code URLs, which helps maintainability.
• Routing is connected to the middleware pipeline by the RouterMiddleware class.

URL Matching

• URL matching is the process by which routing dispatches an incoming request to a handler.
  • This process is generally based on data in the URL path, but can be extended to consider any data in the request. Incoming requests enter the RouterMiddleware, which calls the RouteAsync method on each route in sequence.
  • The IRouter instance chooses whether to handle the request by setting the RouteContext.Handler to a non-null RequestDelegate.
  • If a route sets a handler for the request, route processing stops and the handler will be invoked to process the request.
  • If all routes are tried and no handler is found for the request, the middleware calls next and the next middleware in the request pipeline is invoked.
• The primary input to RouteAsync is the RouteContext.HttpContext associated with the current request.
  • The RouteContext.Handler and RouteContext.RouteData are outputs that will be set after a route matches.
• A match during RouteAsync will also set the properties of the RouteContext.RouteData to appropriate values based on the request processing done so far.
  • If a route matches a request, the RouteContext.RouteData will contain important state information about the result.
• RouteData.Values is a dictionary of route values produced from the route.
  • These values are usually determined by tokenizing the URL, and can be used to accept user input, or to make further dispatching decisions inside the application.
• RouteData.DataTokens is a property bag of additional data related to the matched route.
  • DataTokens are provided to support associating state data with each route so the application can make decisions later based on which route matched.
  • These values are developer-defined and do not affect the behavior of routing in any way.
  • Additionally, values stashed in data tokens can be of any type, in contrast to route values, which must be easily convertible to and from strings.
• RouteData.Routers is a list of the routes that took part in successfully matching the request.
  • Routes can be nested inside one another, and the Routers property reflects the path through the logical tree of routes that resulted in a match.
  • Generally the first item in Routers is the route collection, and should be used for URL generation.
  • The last item in Routers is the route handler that matched.

URL Generation

• URL generation is the process by which routing can create a URL path based on a set of route values.
  • This allows for a logical separation between your handlers and the URLs that access them.
• URL generation follows a similar iterative process, but starts with user or framework code calling into the GetVirtualPath method of the route collection.
  • Each route will then have its GetVirtualPath method called in sequence until a non-null VirtualPathData is returned.
  • The primary inputs to GetVirtualPath are:
  1. VirtualPathContext.HttpContext
  2. VirtualPathContext.Values
  3. VirtualPathContext.AmbientValues
• Routes primarily use the route values provided by the Values and AmbientValues to decide where it's possible to generate a URL and what values to include.
• The AmbientValues are the set of route values that were produced from matching the current request with the routing system.
• In contrast, Values are the route values that specify how to generate the desired URL for the current operation.
• The HttpContext is provided in case a route needs to get services or additional data associated with the current context.
• Think of Values as being a set of overrides for the AmbientValues.
  • URL generation tries to reuse route values from the current request to make it easy to generate URLs for links using the same route or route values.
• The output of GetVirtualPath is a VirtualPathData. VirtualPathData is a parallel of RouteData; it contains the VirtualPath for the output URL as well as the some additional properties that should be set by the route.
  • The VirtualPathData.VirtualPath property contains the virtual path produced by the route.
    • Depending on your needs you may need to process the path further.
    • For instance, if you want to render the generated URL in HTML you need to prepend the base path of the application.
  • The VirtualPathData.Router is a reference to the route that successfully generated the URL.
  • The VirtualPathData.DataTokens properties is a dictionary of additional data related to the route that generated the URL.
    • This is the parallel of RouteData.DataTokens.