In reagent, the fundamental building block is a component.

Your reagent app will typically have many components - say, more than 5, but less than 100 - and the overall UI of a reagent app is the stitched-together-output from all of them, each contributing part of the overall HTML, typically in a hierarchical arrangement.

So they're important and this document describes how to create them.

Not An Absolute Introduction

Although I stay as basic as possible, this document isn't an introductory tutorial. You should read it after you have already digested The Official Introduction.

This document is useful because it clarifies the basics. It represents an extra bit of learning which might save you from some annoying paper cuts.

Contains Lies And Distortions

I care more about providing a useful mental model than bogging down with the full truth. Some white lies and distortions follow.

The Core Of A Component

At the core of any component is a render function.

A render function is the backbone, mandatory part of a component. In fact, as you'll soon see, components will often collapse down to be nothing more than a render function.

A render function turns data into HTML. Data is supplied via the function parameters, and HTML is the return value.

Data in, HTML out.

Much of the time, a render function will be a pure function. If you pass the same data into a render function, then it will return the same HTML, and it won't side effect.

Note: ultimately, the surrounding reagent/React framework will cause non-pure side-effects because the returned HTML will be spliced into the DOM (mutating global state!), but here, for the moment, all we care about is the pureness of the render function itself)

The Three Ways

There are three ways to create a component.

Ordered by increasing complexity, they are:

1. via a simple render function - data in as parameters, and it returns HTML.
2. via a function which returns the render function - the returned function is the render function.
3. via a map of functions, one of which is the render the rest of the functions are React lifecycle methods which allow for some more advanced interventions.

In all three cases, a render function is provided -- that's the backbone. The three creation methods differ only in terms of what they supply over and above a renderer.

Form-1: A Simple Function

In the simplest case, a component collapses down to only be a render function. You supply nothing else.

Although a simple approach, in my experience, you'll probably use Form-1 components about 40% of the time, perhaps more. Simple and useful.

You just write a regular clojurescript function which takes data as parameters and produces HTML.

(defn greet
   [name]                    ;; data coming in is a string
   [:div "Hello " name])     ;; returns Hiccup (HTML)

Until now, I've talked about render functions returning HTML. That isn't strictly speaking true, of course, as you've seen in the The Offical Introduction. Instead, renderers always return clojurescript data structures which specify HTML via Hiccup format.

Hiccup uses vectors to represent HTML elements, and maps to represent an element's attributes.

So this clojurescript data structure:

[:div {:style {:background "blue"}} "hello " "there"]

is simply a clojurescript vector, containing a keyword, map and two strings. But when processed as hiccup, this data structure will produce the HTML:

<div style="background:blue;">hello there</div>

To understand more about Hiccup see this Wiki.

Rookie mistake

At some point, you'll probably try to return sibling HTML elements in a normal cljs vector:

(defn wrong-component
   [name]              
   [[:div "Hello"] [:div name]])     ;; a vec of 2 [:div] 

That isn't valid Hiccup and you'll get a slightly baffling error. You'll have to correct this mistake by wrapping the two siblings in a parent [:div]:

(defn right-component
   [name]              
   [:div 
     [:div "Hello"] 
     [:div name]])     ;; [:div] containing two nested [:divs]

Form-2: A Function Returning A Function

Now, let's take one step up in complexity. Sometimes, a component requires:

• some setup; or
• some local state; and of course
• a renderer

The first two are optional, the last is not.

Form-2 components are written as an outer function which returns an inner render.

This example is taken from the tutorial:

(defn timer-component []
  (let [seconds-elapsed (reagent/atom 0)]     ;; setup, and local state
    (fn []        ;; inner, render function is returned
      (js/setTimeout #(swap! seconds-elapsed inc) 1000)
      [:div "Seconds Elapsed: " @seconds-elapsed])))

Here timer-component is the outer function, and it returns an inner, anonymous render function which closes over the initialised, local state seconds-elapsed.

As before, the job of the render function is to turn data into HTML. That's the backbone. Its just that Form-2 allows your renderer to close over some state created and initialised by the outer.

In my experience, you'll use Form-2 components at least 50% of the time.

Let's be quite clear what is going on here:

• timer-component is called once per component instance (and will create the state for that instance)
• the render function it returns will potentially be called many, many times. In fact, it will be called each time Reagent detects a possible difference in that component's inputs.

Rookie mistake

When starting out, everyone makes this mistake with the Form-2 construct: they forget to repeat the parameters in the inner, anonymous render function.

(defn outer 
  [a b c]            ;; <--- parameters
  ;;  ....
  (fn [a b c]        ;; <--- forgetting to repeat them, is a rookie mistake
    [:div
      (str a b c)]))

So the rookie mistake is to forget to put in the [a b c] parameters on the inner render function.

Remember, outer is called once per component instance. Each time, the parameters to outer will hold the initial parameter values. The renderer on the other hand, will be called by Reagent many times and, each time, potentially with alternative parameter values, but unless you repeat the parameters on the renderer it will close over those initial values in outer. As a result, the component renderer will stubbornly only ever render the original parameter values, not the updated ones, which can be baffling for a beginner.

Form-3: A Class With Life Cycle Methods

Now, for the final step in complexity.

In my experience, you'll probably use Form-3 components less than 1% of the time, perhaps only when you want to use a js library like D3 or introduce some hand-crafted optimisations. Maybe. While you'll ignore Form-3 components most of the time, when you do need them, you need them bad. So pay attention, because they'll save your bacon one day.

While the critical part of a component is its render function, sometimes we need to perform actions at various critical moments in a component's lifetime, like when it is first created, or when its about to be destroyed (removed from the DOM), or when its about to be updated, etc.

With Form-3 components, you can nominate lifecycle methods. reagent provides a very thin layer over React's own lifecycle methods. So, before going on, read all about React's lifecycle methods.

A Form-3 component definition looks like this:

(defn my-component
  [x y z]  
  (let [some (local but shared state)      ;; <-- closed over by lifecycle fns
        can  (go here)]   
     (reagent/create-class                 ;; <-- expects a map of functions 
       {:component-did-mount               ;; the name of a lifecycle function
        #(println "component-did-mount")   ;; your implementation
         
        :component-will-mount              ;; the name of a lifecycle function
        #(println "component-will-mount")  ;; your implementation
      
        ;; other lifecycle funcs can go in here
      
        :display-name  "my-component"  ;; for more helpful warnings & errors

        :reagent-render        ;; Note:  is not :render
         (fn [x y z]           ;; remember to repeat parameters
            [:div (str x " " y " " z)])})))

(reagent/render-component 
    [my-component 1 2 3]         ;; pass in x y z
    (.-body js/document))

;; or as a child in a larger Reagent component

(defn homepage []
  [:div
   [:h1 "Welcome"]
   [my-component 1 2 3]]) ;; Be sure to put the Reagent class in square brackets to force it to render!

At the time of writing, the official reagent tutorial doesn't show how to do Form-3 components in the way shown above, and instead suggests that you use with-meta, which is clumsy and inferior. So I won't show that method here, but be aware that an alternative way exists to achieve the same outcome.

Rookie mistake

In the code sample above, notice that the renderer function is identified via an odd keyword in the map given to reagent/create-class. It's called :reagent-render rather than the shorter, more obvious :render.

Its a trap to mistakenly use :render because you won't get any errors, except the function you supply will only ever be called with one parameter, and it won't be the one you expect. Some details here.

Note: prior to version 0.5.0 you had to use the key :component-function instead of :reagent-render.

Rookie mistake

While you can override component-should-update to achieve some performance improvements, you probably shouldn't unless you really, really know what you are doing. Resist the urge. Your current performance is just fine. :-)

Rookie mistake

Leaving out the :display-name entry. If you leave it out, Reagent and React have no way of knowing the name of the component causing a problem. As a result, the warnings and errors they generate won't be as informative.

Final Note

Above I used the terms Form-1, Form-2 and Form-3, but there's actually only one kind of component. It is just that there's 3 different ways to create a component.

At the end of the day, no matter how it is created, a component will end up with a render function and some life-cycle methods. A component created via Form-1 has the same basic structure as one created via Form-3 because underneath they are all just React components.

Appendix A - Lifting the Lid Slightly

Here's some further notes about Reagent's mechanics:

1. When you provide a function as the first element of a hiccup vector [my-func 1 2 3], Reagent will say "hey I have been given a render function". That function might be Form-1 or Form-2, but it doesn't know at that point. It just sees a function.
2. A render function by itself is not enough to make a React Component. So, Reagent takes this render function and "merges" it with default lifecycle functions to form a React component. (Form-3, of course, allows you to supply your own lifecycle functions)
3. Some time later, when Reagent first wants to render this component, it will, unsurprisingly, call the render function which you supplied (my-func in the snippet above). It will pass in the "props" (parameters) supplied by the rendering parent (1 2 3 in the snippet above).
4. If this first call to the render function returns hiccup (a vector of stuff):
   • Reagent will just interpret it. So this is what happens in the case of a Form-1 function.
   • If, however, this render function returns another function - ie. it is a Form-2 outer function returning the inner function - then Reagent knows to replace the Component's render function with the newly returned inner function forever thereafter. So the outer will have been called once but, from that point forward, the inner function will be used for all further rendering. In fact, Reagent will instantly call the inner function after the outer returns it, because Reagent wants a first rendering (hiccup) for the component.
5. So, in the case of Form-2, the outer function is called once and once only (with initial props/parameters), and the inner is called at least once (with initial props/parameters), but probably many, many times thereafter. Both will be called with the same arrangement of props/parameters - although the inner render function will see different values in those props/parameters, over time.

Appendix B - with-let macro

The with-let macro looks just like let – but the bindings only execute once, and it takes an optional finally clause, that runs when the component is no longer rendered. This can be particularly useful because it can prevent the need for a form-2 component in many instances (like creating a local reagent atom in your component).

For example: here's a component that sets up an event listener for mouse moves, and stops listening when the component is removed.

(defn mouse-pos-comp []
  (r/with-let [pointer (r/atom nil)
               handler #(swap! pointer assoc
                               :x (.-pageX %)
                               :y (.-pageY %))
               _ (.addEventListener js/document "mousemove" handler)]
    [:div
     "Pointer moved to: "
     (str @pointer)]
    (finally
      (.removeEventListener js/document "mousemove" handler))))

The same thing could of course be achieved with React lifecycle methods, but that would be a lot more verbose.

with-let can also be combined with track (and other Reactive contexts). For example, the component above could be written as:

(defn mouse-pos []
  (r/with-let [pointer (r/atom nil)
               handler #(swap! pointer assoc
                               :x (.-pageX %)
                               :y (.-pageY %))
               _ (.addEventListener js/document "mousemove" handler)]
    @pointer
    (finally
      (.removeEventListener js/document "mousemove" handler))))

(defn tracked-pos []
  [:div
   "Pointer moved to: "
   (str @(r/track mouse-pos))])

The finally clause will run when mouse-pos is no longer tracked anywhere, i.e in this case when tracked-pos is unmounted.