Filters are the middleware – they are individual functions that make up the request processing pipeline. They execute all of the framework’s functionality.
The filter type is a simple function:
type Filter func(c *Controller, filterChain []Filter)
Each filter is responsible for pulling the next filter off of the filter chain and invoking it. Here is the default filter stack:
// Filters is the default set of global filters. // It may be set by the application on initialization. var Filters = []Filter{ PanicFilter, // Recover from panics and display an error page instead. RouterFilter, // Use the routing table to select the right Action FilterConfiguringFilter, // A hook for adding or removing per-Action filters. ParamsFilter, // Parse parameters into Controller.Params. SessionFilter, // Restore and write the session cookie. FlashFilter, // Restore and write the flash cookie. ValidationFilter, // Restore kept validation errors and save new ones from cookie. I18nFilter, // Resolve the requested language InterceptorFilter, // Run interceptors around the action. ActionInvoker, // Invoke the action. }
Applications may configure the filter chain by re-assigning the revel.Filters
variable in init()
(by default this will be in app/init.go
for newly
generated apps).
func init() { // Filters is the default set of global filters. revel.Filters = []Filter{ PanicFilter, // Recover from panics and display an error page instead. RouterFilter, // Use the routing table to select the right Action FilterConfiguringFilter, // A hook for adding or removing per-Action filters. ParamsFilter, // Parse parameters into Controller.Params. SessionFilter, // Restore and write the session cookie. FlashFilter, // Restore and write the flash cookie. ValidationFilter, // Restore kept validation errors and save new ones from cookie. I18nFilter, // Resolve the requested language InterceptorFilter, // Run interceptors around the action. ActionInvoker, // Invoke the action. } }
Every request is sent down this chain, top to bottom.
Although all requests are sent down the revel.Filters
chain, Revel also
provides a
FilterConfigurator
,
which allows the developer to add, insert, or remove filter stages based on the
Action or Controller.
This functionality is implemented by the FilterConfiguringFilter
, itself a
filter stage.
Filters are responsible for invoking the next filter to continue the request processing. This is generally done with an expression as shown here:
var MyFilter = func(c *revel.Controller, fc []revel.Filter) { // .. do some pre-processing .. fc[0](c, fc[1:]) // Execute the next filter stage. // .. do some post-processing .. }
Filters receive the base *Controller
type as an
argument, rather than the actual Controller type that was invoked. If your
filter requires access to the actual Controller type that was invoked, it may
grab it with the following trick:
var MyFilter = func(c *revel.Controller, fc []revel.Filter) { if ac, err := c.AppController.(*MyController); err == nil { // Have an instance of *MyController... } fc[0](c, fc[1:]) // Execute the next filter stage. }
Note: this pattern is frequently an indicator that interceptors may be a better mechanism to accomplish the desired functionality.