re-frame/README.md

## re-frame

re-frame is a lightweight [reagent] framework for writing  [SPAs] using ClojureScript.

It proposes a pattern for structuring an app, and provides a small library
implementing one version of this pattern.

In another context, re-frame might be called an MVC framework, except
it is instead a functional RACES framework - Reactive-Atom Component Event Subscription
(I love the smell of acronym in the morning).

## Claims

Nothing about re-frame is the slightest bit original or clever. 
You'll find no ingenious use of functional zippers, transducers or core.async. 
This is a good thing (although, for the record, one day I'd love to develop
something original and clever).

Using re-frame, you can break your application code into distinct pieces. 
Each of these pieces can be easily described, understood and tested independently. 
These pieces will (mostly) be pure functions.

At small scale, any framework seems like pesky overhead. The 
explanatory examples in here are small scale, so you'll need to
squint a little to see the benefit.

## Shaping Beliefs

Above all we believe in the one true [Dan Holmsand] (creator of reagent), 
and his divine instrument the `ratom`.  We genuflect towards Sweden once a day. 

Second, because paradigm is worth 80 points of IQ, we think you'll only 
really "get" Reagent once you view it as an [FRP] library, and not simply a 
ReactJS wrapper. To put that another way, we think that Reagent is closer in 
nature to [Hoplon] or [Elm] than it is [OM]. This wasn't obvious to us initially - we
knew we liked reagent, but it took a while for the penny to drop as to why.

Finally, we believe in one way data flow.  We don't like read/write `cursors` which
allow for the two way flow of data. 

re-frame implements the two way flow of data into and 
out of views by using two, one-way flows. 

---------------------------------
## The Parts

To explain re-frame, we'll now incrementally 
develop a diagram.  We'll explain each part as it is added in.

##### The Big Ratom 

<blockquote class="twitter-tweet" lang="en"><p>Well-formed Data at rest is as close to perfection in programming as it gets. All the crap that had to happen to put it there however...</p>&mdash; Fogus (@fogus) <a href="https://twitter.com/fogus/status/454582953067438080">April 11, 2014</a></blockquote>
<script async src="//platform.twitter.com/widgets.js" charset="utf-8"></script>

Our re-frame diagram starts with the "well formed data at rest" bit: 
```
ratom
```

So, re-frame says that you should put your data into one, dirty great
big atom. Structure the  data in that  atom, of course, but put it
all in the one place.

Now, its not the slightest bit controversial to use databases, right? And they
encourage you to put your well formed data all in one place.  But if you have
background in OO, this data-in-one-place is a hard one to swallow.  You've
spent your life breaking systems into pieces, organised around behaviour and trying
to hide the data.  I still wake up in a sweat some nights thinking about all
that clojure data lying around exposed and passive.

But, as @Fogus says, data is the easy bit. 

From here on, we'll assume that this part of the framework looks like this:
```
(def ratom  (reagent/atom {}))    ;; a reagent atom, containing a map
```

It is useful to actively imagine this ratom as an (in memory) database.
It will contain structured data (perhaps with a formal [Prismatic Schema] spec).
You will need to query that data. You will perform CRUD 
and other transformations on it. You'll often want to transact on this
database atomically, etc.  So "in-memory database"
seems a more useful paradigm than plain old atom. In our implementation, we actually
use the name `db` to drive home the point.

Finally, `ratom` doesn't actually have to be a ratom containing a map. In theory, re-frame
imposes no requirement here.  It could be a [datascript] database.  Its just a reactive datastore
of some description.

##### Magic Ratoms

ratoms have a key feature.  They act like normal ClojureScript atoms, **plus** they allow
you to create reactive functions similar to `lift` in [Elm] or `defc=` in [hoplon].
You create these reactive functions via the reagent macro `reaction` (or `run!`).

```clojure
(ns example1
  (:require-macros [reagent.ratom :refer [reaction]])
  (:require
    [reagent.core   :as    r]))
    
(def ratom1  (r/atom {:a 1}))

(def ratom2  (reaction {:b (:a @ratom1)}))           ;; notice use of "reaction"
(def ratom3  (reaction (cond = (:a @ratom1)          ;; notice use of "reaction"
                             0 "World"
                             1 "Hello")))         

(println @ratom2)    ;; ==>  {:b 1}
(println @ratom3)    ;; ==> "Hello"
(reset!  ratom1  {:a 0})
(println @ratom2)    ;; ==>  {:b 0}    ;; ratom2 is automatically updated.
(println @ratom3)    ;; ==> "World"    ;; ratom3 is automatically updated.
    
;; cleanup 
(dispose ratom2)
(dispose ratom3)
```

`reaction` is a macro which turns returns a ratom. It takes the
the forms supplied and turns them into the body of a reactive
function. This function will be rerun each
time one of the ratoms referenced in the forms changes,
and the returned atom will be reset! with the new result.
Over time, ratom2 and ratom3 are auto-updated each
time ratom1 changes. This enables [FRP] (its similar in affect to `lift` in Elm).

### The Components

Extending the diagram a bit, we introduce the beginnings of one way data flow:
```
ratom -->  components --> hiccup
```
When using reagent, you write one or more `components`.  Think about
`components` as `pure functions` - data in, hiccup out.  [`hiccup`] is
ClojureScript data structures which represent DOM.

These components are a bit like the templates you'd find in frameworks
like Django or Rails or Mustache, except for two massive differences: 

- you have available the full power of ClojureScript.
   The downside tradeoff is that these are not "designer friendly" HTML templates.
- these components are reactive.  When their inputs change, they
  are automatically rerun, producing new hiccup. reagent adroitly shields you from
  the details, but `components` (functions you write) are wrapped by
  `reaction` in such a way that they re-run when the ratoms they
  dereference change. This is FRP. This is why reagent feels slightly magic.

So `components` are pure, **reactive** functions.  Change the inputs
and automatically, new hiccup is produced.

This is not a tutorial on how to write reagent components, but let's
talk briefly about the "data in" bit. Turns out there are two ways
data flows into components:

1. the data is supplied as component parameters, typically from a parent
   component.  There tends to be a
   hierarchy of components and data often flows from parent to child
   via function parameters.

2. a component can 'subscribe' to some aspect of the data.  As a result, it becomes an 
   observer of that state, and it gets an stream of data updates as that part
   of the ratom changes.

Either way, when the "data in" changes, the component function is rerun,
and it produces new hiccup, which is then stitched into the DOM.

Subscriptions are a significant part of re-frame ... more on them soon.

### ReactJS

The complete data flow from data to DOM is:

```
ratom  -->  components --> Hiccup --> Reagent --> VDOM  -->  ReactJS  --> DOM
```

Best to imagine this process as a pipeline of 3 functions.  Each
function takes data from the
previous step, and produces data for the next step.  In the next
diagram, the three functions are marked. The unmarked nodes are data,
produced by one step, becoming the input to the next step.  hiccup,
VDOM and DOM are all various forms of HTML markup (in our world that's data).

```
ratom  -->  components --> hiccup --> Reagent --> VDOM  -->  ReactJS  --> DOM
               f1                      f2                     f3
```

The combined three-function pipeline should be seen as a pure
function `P` which takes a ratom as input, and produces DOM.
One way data flow:
```
ratom  -->  P  --> DOM
```

This arrangement is:
1.  Easy to test.  We put data into the ratom, allow the DOM to be rendered, and check that the right divs are in place.  This would typically be done via phantomjs. 
2.  Easily understood.  Generally, components can be understood in isolation.  In almost all cases, a component is genuinely a pure fucntion, or is conceptually a pure function. 
 
The whole thing might be a multistep process, but we only have to bother ourselves with the writing of the components.  Reagent/ReactJS looks after the rest. XXXX

If the ratom changes, the DOM changes.  The DOM is a function of the ratom (state of the app).  XXXX


### Event Flow

The previous data flow is the first half of the story. So now we need to consider the data flow in the opposite direction.

In response to user interaction, a DOM will generate
events like "the user clicked the delete button on item 42" or
"the user unticked the checkbox for 'send me spam'".

These events have to "handled".  The code doing this handling might
 mutate the ratom, or requrest more data from thet server, or POST somewhere, etc. 

An app will have many handlers, and collectively
they represent the **control layer of the application**.

The backward data flow happens in one (conveyor belt) hop:

```
ratom  -->  components --> Hiccup --> Reagent --> VDOM  -->  ReactJS  --> DOM
  ^                                                                        |
  |                                                                        v
  handlers <---------------  events  ---------------------------------------
                           a "conveyor belt" takes
                           events from the DOM to
                           the handlers.
```


Generally, the user manipulates the GUI because they want to change the state of the
application.  They don't think that way, of course.  They think in terms of "deleting this blah" 
or "swapping to foo".  But all these actions invariably mean the ratom will be changed.
After all the ratom **is** the state.  The DOM presented to the user is a function of that state.
The GUI doesn't change until the ratom changes.

So handlers are the part of the system which does ratom mutation. You
could almost imagine them as a "stored procedure" in a
database. Almost. Stretching it?  We do like our in-memory
database analogies.

### What are events?

Events are data. You choose the format.

Our implementation chooses to represent events as vectors. For example:

    [:delete-item 42]

The first item in the vector identifies the event and
the rest of the vector is the optional parameters -- in this cse, the id (42) of the item to delete.

Here are some other example events:
```clojure
    [:set-spam-wanted false]
    [[:complicated :multi :part :key] "a parameter" "another one"  45.6]
```

### Back To The DOM

Events start in the DOM.  They are `dispatched`. 

For example, a button component might look like this: 
```clojure
    (defn yes-button
        []
        [:div  {:class "button-class"
                :on-click  #(dispatch [:yes-button-clicked])}
                "Yes"])
```

Notice the `on-click` handler:
```clojure
    #(dispatch [:yes-button-clicked])
```

With re-frame, we try to keep the DOM as passive as possible.  It is simply a rendering of the data. 

It is Event Handlers which interpret the events and respond.  `dispatch` is how the DOM passes off events to handlers. And, just to be clear, there is a signle `dsipatch` fucntion.  Not one dispatch functions for each event. 

### Routing

Once they are dispatched, events have to be routed to their eventual handler. 

`dispatch` can be implemtned in There's a bunch of ways to implement "dispatch events".  You could push the events into a core.asyc channel. Or you could call a `dispatch` multimethod, and find the right event handler by inspection of `first` on the event vectory. 

We use a technique in which the 




Events are then routed to the right handler via a conveyor belt. Remember,
an event is just data.  And it is flowing in the opposite direction
to the data which causes the DOm to be rendered in the first place.


 and routed to the right handler.



So here's what a

When an event reaches the end of the conveyor belt, it has to be routed to the right handler -- the one which handles this kind of event.  

So something has to look at `first` in event vector cand know how to call the right handler.

The conveyor belt could easily be done via core.async, but we do it the simplest possible way. 



[SPAs]:http://en.wikipedia.org/wiki/Single-page_application
[reagent]:http://reagent-project.github.io/
[Dan Holmsand]:https://github.com/holmsand
[Hiccup]:https://github.com/weavejester/hiccup
[FRP]:https://gist.github.com/staltz/868e7e9bc2a7b8c1f754
[Elm]:http://elm-lang.org/
[OM]:https://github.com/swannodette/om
[Prismatic Schema]:https://github.com/Prismatic/schema
[InterViews]:http://www.softwarepreservation.org/projects/c_plus_plus/library/index.html#InterViews
[datascript]:https://github.com/tonsky/datascript
[Hoplon]:http://hoplon.io/
[Pedestal App]:https://github.com/pedestal/pedestal-app