re-frame/docs/Interceptors.md

This tutorial is about Interceptors.

Why Interceptors?

Two reasons.

First, we want simple event handlers.

Interceptors can look after "cross-cutting" concerns like undo, tracing and validation. They help us to factor out commonality, hide complexity and introduce further steps into the "Derived Data, Flowing" story promoted by re-frame.

So, you'll want to use Interceptors - they're helpful.

Second, under the covers, Interceptors are the means by which event handlers are executed (when you dispatch). You'll want to understand how that happens.

What Do Interceptors Do?

They wrap.

Specifically, they wrap event handlers.

Imagine your event handler is like a piece of ham. An interceptor would be
like bread either side of your ham, which makes a sandwich.

And two Interceptors, in a chain, would be like you put another pair of bread slices around the outside of the existing sandwich to make a sandwich of the sandwich. Now it is a very thick sandwich.

Interceptors wrap on both sides of a handler, layer after layer.

Wait, I know That Pattern!

Interceptors implement middleware. But differently.

Traditional middleware - often seen in web servers - creates a data processing pipeline via the nested composition of higher order functions. The result is a "stack" of functions. Data flows through this pipeline, first forwards from one end to the other, and then backwards.

Interceptors achieve the same outcome by assembling functions, as data, in a collection (a chain, rather than a stack). Data can then be iteratively pipelined, first forwards through the functions in the chain, and then backwards along the same chain.

Because the interceptor pipeline is composed via data, rather than higher order functions, it is a more flexible arrangement.

What's In The Pipeline?

Data. It flows through the pipeline being progressively transformed.

Fine. But what data?

With a web server, the middleware "stack" progressively transforms a request in one direction, and, then in the backwards sweep, it progressively produces a response.

In re-frame, the forwards sweep progressively creates the coeffects (inputs to the handler), while the backwards sweep processes the effects (outputs from the handler).

I'll pause while you read that sentence again. That's the key concept, right there.

Show Me

You can provide a chain of interceptors when you register an event handler.

Using a 3-arity registration function:

(reg-event-db          
   :some-id
   [in1 in2]       ;; <--- a chain of 2 interceptors  
   (fn [db v]      ;; <-- the handler here, as before
      ....)))

Each Event Handler can have its own tailored interceptor chain.

Handlers Are Interceptors Too

You might see that registration above as associating :some-id with two things: (1) a chain of interceptors and (2) a handler.

Except, the handler is turned into an interceptor too. (We'll see how later)

So :some-id is only associated with one thing: a 3-chain of interceptors, with the handler wrapped in an interceptor and put on the end of the other two.

Except, except, the registration function itself, reg-event-db, actually takes this 3-chain and adds its own interceptors (which do useful things) at the front (more on this soon too), so ACTUALLY, there's about 5 interceptors in the chain that is finally registered for :some-id.

So, ultimately, event registration associates an event id with a chain of interceptors.

Later, when a dispatch for :some-id is done, that 5-chain of interceptors will be "executed". That's how events get handled.

Brass Tacks

Each interceptor has this form:

{:id      :something             ;; decorative only
 :before  (fn [context] ...)     ;; returns possibly modified context
 :after   (fn [context] ...)}    ;; `identity` would be a noop

That's essentially a map of two functions. Now imagine a vector of these maps - that's an
an interceptor chain. Simple isn't it?

To "execute" an interceptor chain:

1. create a context (a map, described below)
2. iterate forwards over the chain, calling the :before function on each interceptor
3. iterate over the chain in the opposite direction calling the :after function on each interceptor

Remember that the last interceptor in the chain is the handler itself (wrapped up to be the :before).

That's it. That's how an event gets handled.

What Is Context?

Some data called a context is threaded through all the calls.

This value is passed as the argument to every :before and :after function and they returned it, possibly modified.

A context is a map with this structure:

{:coeffects {:event [:some-id :some-param]
             :db    <original contents of app-db>}
             
 :effects   {:db    <new value for app-db>
             :dispatch  [:an-event-id :param1]}
             
 :queue     <a collection of further interceptors>
 :stack     <a collection of interceptors already walked>}

context has :coeffects and :effects which, if this was a web server, would be somewhat analogous to request and response respectively.

:coeffects will contain the inputs required by the event handler (sitting presumably on the end of the chain). So that's data like the :event being processed, and the initial state of db. These are .

The handler-returned side effects are put into :effects including, but not limited to, new values for db.

The first few interceptors in a chain (inserted by reg-event-db) have :before functions which prime the :coeffects by adding in :event, and :db. Of course, other interceptors can add further to :coeffect. Perhaps the event handler needs data from localstore, or a random number, or a DataScript connection. Interceptors can build up the coeffect, via their :before.

Equally, some interceptors in the chain will have :after functions which process the side effects accumulated into :effects including but, not limited to, updates to app-db.

Fancy

Through both stages (before and after), context contains a :queue of interceptors yet to be processed, and a :stack of interceptors already done.

In advanced cases, these values can be modified by the functions through which the context is threaded.

What I'm saying is that interceptors can be dynamically added and removed from the :queue by the interceptors themselves.

Credit

All truths are easy to understand once they are discovered
-- Galileo Galilei

This elegant and flexible arrangement was originally designed by the talented Pedestal Team. Thanks!

Write An Interceptor

Dunno about you, but I'm easily offended by underscores.

If our components did this:

(dispatch [:delete-item 42])

We'd have to write this handler:

(def-event-db 
  :delete-item
  (fn 
     [db [_  key-to-delete]]     ;;  <---- Arrgggghhh underscore
     (dissoc db key-to-delete)))

Do you see it there? In the event destructing!!! Almost mocking us with that passive aggressive, understated thing it has going on!! Co-workers have said I'm "being overly sensitive", perhaps even horizontalist, but you can see it, right? Of course you can.

Let's remove the need for it. We'll write an interceptor: trim-event

Once we have written trim-event, our registration will change to look like this:

(def-event-db 
  :delete-item
  [trim-event]                ;;  <--- interceptor added
  (fn 
     [db [key-to-delete]]     ;;  <--- no leading underscore necessary
     (dissoc db key-to-delete)))

trim-event will need to change the :coeffects (of context). More specifically, it will be changing the :event value within the :coeffects.

:event will start off as [:delete-item 42], but will end up [42]. trim-event will remove that leading :delete-item because, by the time the event is being processed, we already know what id is has.

And, here it is:

(def trim-event
  (re-frame.core/->interceptor
    :id      :trim-event
    :before  (fn [context]
               (let [new-event (-> context 
                                   :coeffects 
                                   :event        ;; extra event from coeffects
                                   rest          ;; remove first element
                                   vec)          ;; list->vector
                  (assoc-in context [:coeffects :event] new-event)))))

As you read this, look back to what a context looks like.

Notes:

1. We use ->interceptor to create an interceptor (but it just a map)
2. Our interceptor only has a :before function
3. Our :before is given context. It modifies it and returns it.
4. There is no :after for this Interceptor. It has nothing to do with the backwards processing flow of effects. It is concerned only with coeffects in the forward flow.

Wrapping Handlers

We're going well. Let's do an advanced wrapping.

How would you wrap a handler in an interceptor?

There's two kinds of handler:

• the -db variety registered by reg-event-db
• the -fx variety registered by reg-event-fx

Let's do a -db variety. We'll be wrapping a function like this:

(fn [db event]               ;;  takes two params
  (assoc db :flag true))     ;; returns a new db

Here a function which turns a given handler into an interceptor:

(defn db-handler->interceptor
  [db-handler-fn]
  (->interceptor
    :id     :db-handler
    :before (fn [context]
              (let [{:keys [db event]} (:coeffects context)    ;; extract db and event from coeffects
                    new-db (db-handler-fn db event)]           ;; call the handler 
                 (assoc-in context [:effects :db] new-db)))))) ;; put db back into :effects

Summary

In this tutorial, we've learned:

1. When you register a handler, you can supply some interceptors:

 (reg-event-db          
    :some-id
    [in1 in2]       ;; <--- a chain of 2 interceptors  
    (fn [db v]      ;; <-- real handler here
       ....)))

2. That will associate :some-id with a chain of about 5 interceptors because:

• there are two interceptors supplied
• the handler is wrapped as an interceptor and added to the end
• the registration function adds a couple of interceptors of its own

3. Interceptors can do interesting things:

• add to coeffects (inputs to the handler)
• process effects (make side effects happen)
• produce logs

In the next Tutorial, we look at how you can add coeffects.

Free Beer

re-frame comes with some builtin Interceptors:

• debug: log each event as it is processed. Shows incremental clojure.data/diff reports.
• trim-v: a convenience. More readable handlers.

And some Interceptor factories (functions that return Interceptors):

• enrich: perform additional computations (validations?), after the handler has run. More derived data flowing.
• after: perform side effects, after a handler has run. Eg: use it to report if the data in app-db matches a schema.
• path: a convenience. Simplifies our handlers. Acts almost like update-in.

In addition, a Library like re-frame-undo provides an Interceptor factory called undoable which checkpoints app state.

To use them, first require them:

(ns my.core
  (:require
    [re-frame.core :refer [debug path]])