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Declarative, functional and extensible wrapper of JavaFX inspired by better parts of react and re-frame


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Cljfx is a declarative, functional and extensible wrapper of JavaFX inspired by better parts of react and re-frame.


I wanted to have an elegant, declarative and composable UI library for JVM and couldn't find one. Cljfx is inspired by react, reagent, re-frame and fn-fx.

Like react, it allows to specify only desired layout, and handles all actual changes underneath. Unlike react (and web in general) it does not impose xml-like structure of everything possibly having multiple children, thus it uses maps instead of hiccup for describing layout.

Like reagent, it allows to specify component descriptions using simple constructs such as data and functions. Unlike reagent, it rejects using multiple stateful reactive atoms for state and instead prefers composing ui in more pure manner.

Like re-frame, it provides an approach to building large applications using subscriptions and events to separate view from logic. Unlike re-frame, it has no hard-coded global state, and subscriptions work on referentially transparent values instead of ever-changing atoms.

Like fn-fx, it wraps underlying JavaFX library so developer can describe everything with clojure data. Unlike fn-fx, it is more dynamic, allowing users to use maps and functions instead of macros and deftypes, and has more explicit and extensible lifecycle for components.

Installation and requirements

Cljfx uses tools.deps, so you can add this repo with latest sha as a dependency:

 cljfx {:git/url "" :sha "<insert-sha-here>"}

Cljfx is also published on Clojars, so you can add cljfx as a maven dependency, current version is on this badge:

Cljfx on Clojars

Minimum required version of clojure is 1.10.

When depending on git coordinates, minimum required Java version is 11. When using maven dependency, both Java 8 (assumes it has JavaFX provided in JRE) and Java 11 (via openjfx dependency) are supported. You don't need to configure anything in this regard: correct classifiers are picked up automatically.

Please note that JavaFX 8 is outdated and has problems some people consider severe: it does not support HiDPI scaling on Linux, and sometimes crashes JVM on macOS Mojave. You should prefer JDK 11.


Hello world

Components in cljfx are described by maps with :fx/type key. By default, fx-type can be:

  • a keyword corresponding to some JavaFX class
  • a function, which receives this map as argument and returns another description
  • an implementation of Lifecycle protocol (more on that in extending cljfx section)

Minimal example:

(ns example
  (:require [cljfx.api :as fx]))

    {:fx/type :stage
     :showing true
     :title "Cljfx example"
     :width 300
     :height 100
     :scene {:fx/type :scene
             :root {:fx/type :v-box
                    :alignment :center
                    :children [{:fx/type :label
                                :text "Hello world"}]}}}))

Evaluating this code will create and show a window:

The overall mental model of these descriptions is this:

  • whenever you need a JavaFX class, use map where :fx/type key has a value of a kebab-cased keyword derived from that class name
  • other keys in this map represent JavaFX properties of that class (also in kebab-case);
  • if prop x can be changed by user, there is a corresponding :on-x-changed prop for observing these changes


To be truly useful, there should be some state and changes over time, for this matter there is a renderer abstraction, which is a function that you may call whenever you want with new description, and cljfx will advance all the mutable state underneath to match this description. Example:

(def renderer

(defn root [{:keys [showing]}]
  {:fx/type :stage
   :showing showing
   :scene {:fx/type :scene
           :root {:fx/type :v-box
                  :padding 50
                  :children [{:fx/type :button
                              :text "close"
                              :on-action (fn [_]
                                           (renderer {:fx/type root
                                                      :showing false}))}]}}})

(renderer {:fx/type root
           :showing true})

Evaluating this code will show this:

Clicking close button will hide this window.

Renderer batches descriptions and re-renders views on fx thread only with last received description, so it is safe to call many times at once. Calls to renderer function return derefable that will contain component value with most recent description.


Example above works, but it's not very convenient: what we'd really like is to have a single global state as a value in an atom, derive our description of JavaFX state from this value, and change this atom's contents instead. Here is how it's done:

;; Define application state

(def *state
  (atom {:title "App title"}))

;; Define render functions

(defn title-input [{:keys [title]}]
  {:fx/type :text-field
   :on-text-changed #(swap! *state assoc :title %)
   :text title})

(defn root [{:keys [title]}]
  {:fx/type :stage
   :showing true
   :title title
   :scene {:fx/type :scene
           :root {:fx/type :v-box
                  :children [{:fx/type :label
                              :text "Window title input"}
                             {:fx/type title-input
                              :title title}]}}})

;; Create renderer with middleware that maps incoming data - description -
;; to component description that can be used to render JavaFX state.
;; Here description is just passed as an argument to function component.

(def renderer
    :middleware (fx/wrap-map-desc assoc :fx/type root)))

;; Convenient way to add watch to an atom + immediately render app

(fx/mount-renderer *state renderer)

Evaluating code above pops up this window:

Editing input then immediately updates displayed app title.

Map events

Consider this example:

(defn todo-view [{:keys [text id done]}]
  {:fx/type :h-box
   :children [{:fx/type :check-box
               :selected done
               :on-selected-changed #(swap! *state assoc-in [:by-id id :done] %)}
              {:fx/type :label
               :style {:-fx-text-fill (if done :grey :black)}
               :text text}]})

There are problems with using functions as event handlers:

  1. Performing mutation from these handlers requires coupling with that state, thus making todo-view dependent on mutable *state
  2. Updating state from listeners complects logic with view, making application messier over time
  3. There are unnecessary reassignments to on-selected-changed: functions have no equality semantics other than their identity, so on every change to this view (for example, when changing it's text), on-selected-changed will be replaced with another function with same behavior.

To mitigate these problems, cljfx allows to define event handlers as arbitrary maps, and provide a function to a renderer that performs actual handling of these map-events (with additional :fx/event key containing dispatched event):

;; Define view as just data

(defn todo-view [{:keys [text id done]}]
  {:fx/type :h-box
   :spacing 5
   :padding 5
   :children [{:fx/type :check-box
               :selected done
               :on-selected-changed {:event/type ::set-done :id id}}
              {:fx/type :label
               :style {:-fx-text-fill (if done :grey :black)}
               :text text}]})

;; Define single map-event-handler that does mutation

(defn map-event-handler [event]
  (case (:event/type event)
    ::set-done (swap! *state assoc-in [:by-id (:id event) :done] (:fx/event event))))

;; Provide map-event-handler to renderer as an option

    :middleware (fx/wrap-map-desc assoc :fx/type root)
    :opts {:fx.opt/map-event-handler map-event-handler}))

You can see full example at examples/e09_todo_app.clj.

Interactive development

Another useful aspect of renderer function that should be used during development is refresh functionality: you can call renderer function with zero args and it will recreate all the components with current description.

See walk-through in examples/e12_interactive_development.clj as an example of how to iterate on cljfx app in REPL.

Dev tools

Check out cljfx/dev for tools that might help you when developing cljfx applications. These tools include:

  • specs and validation, both for individual cljfx descriptions and running apps;
  • helper reference for existing types and their props;
  • cljfx component stack reporting in exceptions to help with debugging.


Iterating on styling is usually cumbersome: styles are defined in external files, they are not reloaded on change, they are opaque: you can't refer from the code to values defined in CSS. Cljfx has a complementary library that aims to help with all those problems: cljfx/css.

Special keys

Sometimes components accept specially treated keys. Main uses are:

  1. Reordering of nodes (instead of re-creating them) in parents that may have many children. Descriptions that have :fx/key during advancing get reordered instead of recreated if their position in child list is changed. Consider this example:

    (let [component-1 (fx/create-component
                        {:fx/type :v-box
                         :children [{:fx/type :label
                                     :fx/key 1
                                     :text "- buy milk"}
                                    {:fx/type :label
                                     :fx/key 2
                                     :text "- buy socks"}]})
          [milk-1 socks-1] (vec (.getChildren (fx/instance component-1)))
          component-2 (fx/advance-component
                        {:fx/type :v-box
                         :children [{:fx/type :label
                                     :fx/key 2
                                     :text "- buy socks"}
                                    {:fx/type :label
                                     :fx/key 1
                                     :text "- buy milk"}]})
          [socks-2 milk-2] (vec (.getChildren (fx/instance component-2)))]
      (and (identical? milk-1 milk-2)
           (identical? socks-1 socks-2)))
    => true

    With :fx/key-s specified, advancing of this component reordered children of VBox, and didn't change text of any labels, because their descriptions stayed the same.

  2. Providing extra props available in certain contexts. If node is placed inside a pane, pane can layout it differently by looking into properties map of a node. Nodes placed in ButtonBar can have OS-specific ordering depending on assigned ButtonData. These properties can be specified via keywords namespaced by container's fx-type. Example:

        {:fx/type :stage
         :showing true
         :scene {:fx/type :scene
                 :root {:fx/type :stack-pane
                        :children [{:fx/type :rectangle
                                    :width 200
                                    :height 200
                                    :fill :lightgray}
                                   {:fx/type :label
                                    :stack-pane/alignment :bottom-left
                                    :stack-pane/margin 5
                                    :text "bottom-left"}
                                   {:fx/type :label
                                    :stack-pane/alignment :top-right
                                    :stack-pane/margin 5
                                    :text "top-right"}]}}}))

    Evaluating code above produces this window:

    For a more complete example of available pane keys, see examples/e07_extra_props.clj

Factory props

There are some props in JavaFX that represent not a value, but a way to construct a value from some input:

  • :page-factory in pagination, you can use function receiving page index and returning any component description for this prop (see example in examples/e06_pagination.clj)
  • various versions of :cell-factory in controls designed to display multiples of items (table views, list views etc.) can be described using the following form:
    {:fx/cell-type :list-cell
     :describe (fn [item] {:text (my.ns/item-as-text item)})} 
    The lifecycle of cells is a bit different than lifecycle of other components: JavaFX pools a minimal amount of cells needed to be shown at the same time and updates them on scrolling. This is great for performance, but it imposes a restriction: cell type is "static". That's why cljfx uses :fx/cell-type that has to be a keyword (like :list-cell or :table-cell) and a separate :describe function that receives an item and returns a prop map for that cell type. There are various usage examples available in examples/e16_cell_factories.clj

Subscriptions and contexts

Once application becomes complex enough, you can find yourself passing very big chunks of state everywhere. Consider this example: you develop a task tracker for an organization. A typical task view on a dashboard displays a description of that task and an assignee. Required state for this view is plain and simple, just a simple data like that:

{:title "Fix NPE on logout during full moon"
 :state :todo
 :assignee {:id 42 :name "Fred"}}

Then one day comes a requirement: users of this task tracker should be able to change assignee from the dashboard. Now, we need a combo-box with all assignable users to render such a view, and required data becomes this:

{:title "Fix NPE on logout during full moon"
 :state :todo
 :assignee {:id 42 :name "Fred"}
 :users [{:id 42 :name "Fred"}
         {:id 43 :name "Alice"}
         {:id 44 :name "Rick"}]}

And you need to compute it once in one place and then pass it along multiple layers of ui to this view. This is undesirable:

  • it will lead to unnecessary re-renderings of views that just pass data further when it changes
  • it complects reasoning about what actually a view needs: is it just a task? or a task with some precomputed attributes?

To mitigate this problem, cljfx introduces optional abstraction called context, which is inspired by re-frame's subscriptions. Context is a black-box wrapper around application state (usually a map), with 2 functions to look inside the wrapped state:

  1. fx/sub-val that subscribes a function to the value wrapped in the context directly (usually it's used for data accessors like get or get-in);
  2. fx/sub-ctx that subscribes a function to the context itself, which is then used by the function to subscribe to some view of the wrapped value indirectly (can be used for slower computations like sorting).

Returned values from subscription functions are memoized in this context (so it actually is a memoization context), and subsequent sub-* calls will result in cache lookup. The best thing about context is that not only does it support updating wrapped values via swap-context and reset-context, it also reuses this memoization cache to minimize re-calculation of subscription functions in successors of this context. This is done via tracking of fx/sub-* calls inside subscription functions, and checking if their dependencies changed. Example:

(def context-1
    {:tasks [{:text "Buy milk" :done false}
             {:text "Buy socks" :done true}]}))

;; Simple subscription function that depends on :tasks key of wrapped map. Whenever value
;; of :tasks key "changes" (meaning whenever there will be created a new context with
;; different value on :tasks key), subscribing to this function will lead to a call to
;; this function instead of cache lookup
(defn task-count [context]
  (count (fx/sub-val context :tasks)))

;; Using subscription functions:
(fx/sub-ctx context-1 task-count) ; => 2

;; Another subscription function depends on :tasks key of wrapped map
(defn remaining-task-count [context]
  (count (remove :done (fx/sub-val context :tasks))))

(fx/sub-ctx context-1 remaining-task-count) ; => 1

;; Indirect subscription function that depends on 2 previously defined subscription
;; functions, which means that whenever value returned by `task-count` or
;; `remaining-task-count` changes, subscribing to this function will lead to a call
;; instead of cache lookup
(defn task-summary [context]
  (prn :task-summary)
  (format "Tasks: %d/%d"
          (fx/sub-ctx context remaining-task-count)
          (fx/sub-ctx context task-count)))

(fx/sub-ctx context-1 task-summary) ; (prints :task-summary) => "Tasks: 1/2"

;; Creating derived context that reuses cache from `context-1`
(def context-2
  (fx/swap-context context-1 assoc-in [:tasks 0 :text] "Buy bread"))

;; Validating that cache entry is reused. Even though we updated :tasks key, there is no
;; reason to call `task-summary` again, because it's dependencies, even though
;; recalculated, return the same values
(fx/sub-ctx context-2 task-summary) ; (does not print anything) => "Tasks: 1/2"

This tracking imposes a restriction on subscription functions: they should not call fx/sub-* after they return (which is possible if they return lazy sequence that calls fx/sub-* during element calculation).

Note that all functions subscribed with fx/sub-val are always invalidated for derived contexts, so they should be reasonably fast (like get). Their upside is that they are decoupled from context completely: since they receive wrapped value as first argument, any function can be used. Functions subscribed with fx/sub-ctx, on the other hand, are invalidated only when their dependencies change, so they can be slower (like sort). Their downside is coupling to cljfx — they receive context as first argument.

Using context in cljfx application requires 2 things:

  • passing context to all lifecycles in a component graph, which is done by using fx/wrap-context-desc middleware
  • using special lifecycle (fx/fn->lifecycle-with-context) for function fx-types that uses this context

Minimal app example using contexts:

;; you will need core.cache dependency if you are going to use contexts!
(require '[clojure.core.cache :as cache])

;; Define application state as context

(def *state
  (atom (fx/create-context {:title "Hello world"} cache/lru-cache-factory)))

;; Every description function receives context at `:fx/context` key

(defn root [{:keys [fx/context]}]
  {:fx/type :stage
   :showing true
   :scene {:fx/type :scene
           :root {:fx/type :h-box
                  :children [{:fx/type :label
                              :text (fx/sub context :title)}]}}})

(def renderer
    :middleware (comp
                  ;; Pass context to every lifecycle as part of option map
                  (fx/wrap-map-desc (fn [_] {:fx/type root})))
    :opts {:fx.opt/type->lifecycle #(or (fx/keyword->lifecycle %)
                                        ;; For functions in `:fx/type` values, pass
                                        ;; context from option map to these functions
                                        (fx/fn->lifecycle-with-context %))}))

(fx/mount-renderer *state renderer)

Using contexts effectively makes every fx-type function a subscription function, so no-lazy-fx-subs-in-returns restriction applies to them too. On a plus side, it makes re-rendering more efficient: fx-type components get re-rendered only when their subscription values change.

For a bigger example see examples/e15_task_tracker.clj.

Preventing cache from growing forever

Another point of concern for context is cache size. By default it will grow forever, which at certain point might become problematic, and we may want to trade some cpu cycles for recalculations to decrease memory consumption. There is a perfect library for it: core.cache. fx/create-context supports cache factory (a function taking initial cache map and returning cache) as a second argument. What kind of cache to use is a question with no easy answer, you probably should try different caches and see what is a better fit for your app.

Cljfx has a runtime optional dependency on core.cache: you need to add it yourself if you are going to use contexts.

Event handling on steroids

While using maps to describe events is a good step towards mostly pure applications, there is still a room for improvement:

  • many event handlers dereference app state, which makes them coupled with an atom: mutable place
  • almost every event handler still mutates app state, which also makes them coupled

Cljfx borrows solutions to these problems from re-frame, providing map event handler wrappers that allow having co-effects (pure inputs) and effects (pure outputs). Lets walk through this example event handler and see how we can make it pure:

(def *state
  (atom {:todos []}))

(defn handle [event]
  (let [state @*state
        {:keys [event/type text]} event]
    (case type
      ::add-todo (reset! *state (update state :todos conj {:text text :done false})))))

;; usage:
(handle {:event/type ::add-todo :text "Buy milk"})
  1. Co-effects: wrap-co-effects

    It would be nice to not have to deref state atom and instead receive it as an argument, and that is what co-effects are for. Co-effect is a term taken from re-frame, and it means current state as data, as presented to event handler. In cljfx you describe co-effects as a map from arbitrary key to function that produces some data that is then passed to handler:

    (defn handle [event]
      ;; receive state as part of an event
      (let [{:keys [event/type text state]} event]
        (case type
          ::add-todo (reset! *state (update state :todos conj {:text text :done false})))))
    (def actual-handler 
      (-> handle
          (fx/wrap-co-effects {:state #(deref *state)})))
    ;; usage:
    (actual-handler {:event/type ::add-todo :text "Buy milk"})
  2. Effects: wrap-effects

    Instead of performing side-effecting operations from handlers, we can return data that describes how to perform these side-effecting operations. fx/wrap-effects uses that data to perform side effects. You describe effects as a map from arbitrary keys to side-effecting function. A wrapped handler in turn should return a seqable of 2-element vectors. First element is a key used to find side-effecting function, and second is an argument to it:

    (defn handle [event]
      (let [{:keys [event/type text state]} event]
        (case type
          ;; Now handlers not only receive just data, they also return just data
          ;; Returning map is a convenience option that can be used as a return
          ;; value, and sequences like [[:state ...] [:state ...]] are fine too 
          ::add-todo {:state (update state :todos conj {:text text :done false})})))
    (def actual-handler
      (-> handle
          (fx/wrap-co-effects {:state #(deref *state)})
          (fx/wrap-effects {:state (fn [state _] (reset! *state state))})))

    In addition to value provided by wrapped handler, side-effecting function receives a function they can call to dispatch new events. While it's useless for resetting state, it can be useful in other circumstances. One is you can create a :dispatch effect that dispatches other events, and another is you can describe asynchronous operations such as http requests as just data. Since effect handlers are run on the UI thread, you should delegate the execution of potentially blocking effects to a different thread using this approach. Examples of both can be found at examples/e18_pure_event_handling.clj. This approach allows to specify side effects in a few places, and then have easily testable handlers:

    (handle {:event/type ::add-todo
             :text "Buy milk"
             :state {:todos []}})
    => {:state {:todos [{:text "Buy milk", :done false}]}}
    ;; data in, data out, no mocks necessary! 

How does it actually work

There are 3 main building blocks of cljfx: components, lifecycles and mutators. Each are represented by protocols, here they are:

(defprotocol Component
  :extend-via-metadata true
  (instance [this]))

(defprotocol Lifecycle
  :extend-via-metadata true
  (create [this desc opts])
  (advance [this component desc opts])
  (delete [this component opts]))

(defprotocol Mutator
  :extend-via-metadata true
  (assign! [this instance coerce value])
  (replace! [this instance coerce old-value new-value])
  (retract! [this instance coerce value]))

Component is an immutable value representing some object in some state (that object may be mutable — usually it's a javafx object), that also has a reference to said object instance.

Lifecycle is well, a lifecycle of a component. Component gets created from a description once, advanced to new description zero or more times, and then deleted. Cljfx is a composition of multiple different lifecycles, each useful in their own place. opts is a map that contains some data used by different lifecycles. 2 opt keys that are used by default in cljfx are:

  • :fx.opt/type->lifecycle — used in dynamic lifecycle to select what lifecycle will be actually used for description based by value in :fx/type key.
  • :fx.opt/map-event-handler — used in event-handler lifecycle that checks if event handler is a map, and if it is, call function provided by this key when event happens. It should be noted, that such event handlers receive additional key in a map (:fx/event) that contains event object, which may be context dependent: for JavaFX change listeners it's a new value, for JavaFX event handlers it's an event, for runnables it's nil, etc.

Another notable lifecycle is cljfx.composite/lifecycle: it manages mutable JavaFX objects: creates instance in create, advances any changes to props (each individual prop may be seen as lifecycle + mutator), and has some useful macros to simplify generating composite lifecycles for concrete classes.

Finally, mutator is a part of prop in composite lifecycles that performs actual mutation on instance when values change. It also receives coerce function which is called on value before applying it. Most common mutator is setter, but there are some other, for example, property-change-listener, which uses addListener and removeListener.

Extending cljfx

Cljfx might have some missing parts that you'll want to fill. Not everything can be configured with lifecycle opts and renderer middleware, and in that case you are encouraged to create and use extension lifecycles. Fx-types in descriptions can be implementations of Lifecycle protocol, and with this escape hatch you get a lot more freedom. Since these lifecycles can introduce different meanings for what descriptions mean in their context, they should stand out from other keyword or function lifecycles, and convention is to have ext- prefix in their names.

Included extension lifecycles

  1. fx/ext-instance-factory

    Using this extension lifecycle you can simply create a component using 0-argument factory function:

        {:fx/type fx/ext-instance-factory
         :create #(Duration/valueOf "10ms")}))
    => #object[javafx.util.Duration 0x2f5eb358 "10.0 ms"]
  2. fx/ext-on-instance-lifecycle

    You can use this lifecycle to additionally setup/tear down instance of otherwise declaratively created value:

        {:fx/type fx/ext-on-instance-lifecycle
         :on-created #(prn "created" %)
         :desc {:fx/type fx/ext-instance-factory
                :create #(Duration/valueOf "10ms")}}))
    ;; prints "created" #object[javafx.util.Duration 0x284cdce9 "10.0 ms"]
    => #object[javafx.util.Duration 0x284cdce9 "10.0 ms"]
  3. fx/ext-let-refs and fx/ext-get-ref

    You can create managed components outside of component tree using fx/ext-let-refs, and then use instances of them, possibly in multiple places, using fx/ext-get-ref:

    {:fx/type fx/ext-let-refs
     :refs {::button-a {:fx/type :button
                        :text "Press Alt+A to focus on me"}}
     :desc {:fx/type :v-box
            :children [{:fx/type :label
                        :text "Mnemonic _A"
                        :mnemonic-parsing true
                        :label-for {:fx/type fx/ext-get-ref
                                    :ref ::button-a}}
                       {:fx/type fx/ext-get-ref
                        :ref ::button-a}]}}

    One use case is for using references in props that expect nodes in a scene graph (such as label's :label-for), and another is having dialogs defined close to usage places, you can find an example of such dialog at examples/e22_button_with_confirmation_dialog.clj

  4. fx/ext-set-env and fx/ext-get-env

    You can put any values into component tree environment with fx/ext-set-env, and then retrieve values from this environment with fx/ext-get-env:

    {:fx/type fx/ext-set-env
     :env {::global-text-style {:-fx-text-fill :red}}
     :desc {:fx/type :v-box
            :children [{:fx/type fx/ext-get-env
                        :env {::global-text-style :style}
                        :desc {:fx/type :label 
                               ;; will receive :style prop that makes text red
                               :text "Hello world"}}]}}
  5. fx/ext-many

    Usually props that expect collections of elements already ask for a collection of descriptions, but there might be cases where you want to manage a coll even though you are asked for a single element. In this case you can use fx/ext-many to describe multiple of components, for example, to show multiple windows at once:

        {:fx/type fx/ext-many
         :desc [{:fx/type :stage
                 :showing true}
                {:fx/type :stage
                 :showing true}]}))

    See examples/e10_multiple_windows.clj and examples/e17_dialogs.clj

  6. fx/make-ext-with-props

    Using this function you can create extension lifecycles that handle whatever additional props you need. These props will be applied after props of original lifecycle. There are some predefined lifecycles providing extra props:

Examples of included extension lifecycles are available at examples/e21_extension_lifecycles.clj.

Writing extension lifecycles

If that's not enough, you can write your own, but this requires more thorough knowledge of cljfx: take a look at cljfx.lifecycle namespace to see how other lifecycles are implemented.

Wrapping other java-based JavaFX components

There is cljfx.composite/props macro to create a prop-map for arbitrary Java class. Also there is a cljfx.composite/describe macro that allows to construct a lifecycle from a class and a prop map, and plenty of examples in cljfx.fx.* namespaces that can help you make custom java components for JavaFX cljfx-friendly.

Combining it all together

Now that every piece is laid out, it's time to combine them into application. What suits your needs is up to you, but if you plan to build something non-trivial, you'll probably want to combine all of the pieces, and easiest way to start is using create-app function. It accepts app atom, event handler and function producing view description and wires them all together:

(def app
  (fx/create-app *context
    :event-handler handle-event
    :desc-fn (fn [_]
               {:fx/type root-view})))

Using that as a starting point, you can build your application using pure functions for everything: views, subscriptions, events. create-app also allows some optional settings, such as :effects, :co-effects and :async-agent-options for configuring event handling and :renderer-middleware for configuring renderer. An example of such application can be found at examples/e20_markdown_editor.clj.


:fx/key should be put on descriptions in a list, not inside these descriptions

For example:

;; Don't do it, this won't work:

(defn item-view [{:keys [item]}]
  {:fx/type :label
   ;; Do not specify `:fx/key` here!
   :fx/key (:id item)
   :text (:title item)})

(defn item-list-view [items]
  {:fx/type :v-box
   :children (for [i items]
               {:fx/type item-view
                :item i})})

Lifecycle that manages lists of things (dynamics) can't see how it's elements will unfold, so it needs to have :fx/key-s where it can see them — in the element descriptions that it gets:

;; Do this to specify `:fx/key`-s:

(defn item-view [{:keys [item]}]
  {:fx/type :label
   :text (:title item)})

(defn item-list-view [items]
  {:fx/type :v-box
   :children (for [i items]
               {:fx/type item-view
                ;; Put `:fx/key` to description that is a part of a list
                :fx/key (:id i)
                :item i})})

:fx/type is for mutable objects only

Lifecycles describe how things change, and some things in JavaFX don't change. For example, Insets class represents an immutable value, so when describing padding you don't need a map with :fx/type key:

{:fx/type :region
 :padding {:top 10 :bottom 10 :left 10 :right 10}}

It doesn't have to be a map at all:

{:fx/type :region
 :padding 10}

How does it work? Instead of using lifecycle there is a coercion mechanism that transforms values before assigning them to a model, most of them are in cljfx.coerce namespace.


Some notable coercion examples and approaches:

  • all enums and enum-like things can be expressed as kebab-cased keywords, for example :red for colors, :crosshair for cursors
  • you still can use actual instances of target classes, for example Cursor/CROSSHAIR for cursors
  • for classes with 1-arg constructors you can supply just that, for example url string for images
  • for classes with multi-arg constructors you can supply args as a map, for example map with :url and :background-loading for images
  • styles can be specified as maps, for example {:-fx-background-color :lightgray}
  • durations can be specified as vector like [10 :ms] or [2 :h]
  • key combinations can be vectors. There are 2 flavors of key combinations in JavaFX: KeyCodeCombination, created if last element of that vector is keyword, for example, [:ctrl :period], and KeyCharacterCombination, created if last element of that vector is string, for example [:ctrl "."]

Differences with JavaFX

There are some "synthetic" properties that provide needed functionality usually used through some other API:

  • Canvas has a :draw prop that is a function that receives Canvas as an argument and should use it to draw on it (example)
  • MediaPlayer has :state prop that can be either :playing, :paused or :stopped, and will call play/pause/stop methods on media player when this prop is changed
  • :url prop of WebView will call load method on this view's web engine

AOT-compilation is complicated

Requiring cljfx starts a JavaFX application thread, which makes sense for repl and running application, but problematic for AOT compilation. To turn off this behavior for compilation, you should set cljfx.skip-javafx-initialization java property to true for your compilation task. This can be done in lein or clj by specifying the following jvm opts:

:jvm-opts ["-Dcljfx.skip-javafx-initialization=true"] 

Please note that while this will help in most cases, you still might have compilation related issues if your code imports JavaFX classes from javafx.scene.control package: classes defined there require JavaFX runtime to be running by accessing it in Control's static initializer. If you need to do that in your application code, you should not skip JavaFX initialization, and instead make your build tool call (javafx.application.Platform/exit) when it finished compiling.

No local mutable state

One thing that is easy to do in react/reagent, but actually complects things, is local mutable state: every component can have its own mutable state that lives independently of overall app state. This makes reasoning about state of the app harder: you need to take lots of small pieces into account. Another problem is this state is unreliable, because it is only here when a component is here. If it gets recreated, for example, after closing some panel it resides in and reopening it back, this state will be lost. Sometimes we want this behavior, sometimes we don't, and it's possible to choose whether this state will be retained or not only if it's a part of a global app state.

No controlled props

In react, setting value prop on text input makes it controlled, meaning it can't be changed unless there is also a change listener updating this value on typing. This is much harder to do in JavaFX, so there is no such thing. But you still can keep typed text in sync with internal state by having both :text and :on-text-changed props (see example in examples/e09_todo_app.clj)

More examples

There are various examples available in examples folder. To try them out:

  1. Clone this repo and cd into it:
    git clone
    cd cljfx 
  2. Ensure you have java 11 installed.
  3. Launch repl with :examples alias and require examples:
    clj -A:examples
    # Clojure 1.10
    # user=> (require 'e15-task-tracker)
    # nil ;; window appears

Full project examples

Full project examples are in the example-projects directory. Consult the example project's for usage.

More information

If you want to learn more about JavaFX, its documentation is available here.

If you want to learn more about React programming model cljfx is based on, there is an in-depth guide to it: React as UI Runtime. Feel free to skip sections about hooks since cljfx does not have them.

I also gave a talk about cljfx — it goes from basic building blocks to how you build reactive applications and provides some context to why I created it. Slides are here.

API's stability, public and internal code

Newer versions of cljfx should never introduce breaking changes, so if an update broke something, please file a bug report. Growth of cljfx should happen only by accretion (providing more), relaxation (requiring less) and fixation (bashing bugs).

This applies to public API of cljfx. cljfx.api namespace and all behaviors that can be observed by using it are a public API. Other namespaces have a docstring stating what is and is not a public API.

Current shapes of values implementing Lifecycle, Component and Mutator protocols are internal and subject to change: treat them as a protocol implementations only. Context is not a protocol, but it's shape is internal too.

Keywords with fx namespace in component descriptions are reserved: new ones may be introduced.

Getting help

Feel free to ask questions on Slack or create an issue. Have a look at previously asked questions.

Food for thought

Internal list of ideas to explore:

  • missing observable maps: Scene's getMnemonics
  • :row-factory in tree-view/tree-table-view should be similar to cell factories
  • are controlled props possible? (controls, also stage's :showing)
  • wrap-factory may use some memoizing and advancing
  • add tests for various lifecycles and re-calculations
  • update to same desc should be identical (component-vec)
  • expand on props and composite lifecycle. What's known about them:
    • ctor:
      • scene requires root, root can be replaced afterwards
      • xy-chart requires axis, they can't be replaced afterwards
    • prop in composite lifecycle may be a map or a function taking instance and returning prop!
    • changing media should re-create media player
  • big app with everything in it to check if/how it works (generative tests maybe?)
  • if animation is to be implemented, it probably should be done as in
  • declarative timers? problem is to figure out start/loop semantics. Examples:
    • caret in custom text input may have timer that restarts on typing
    • flipbook animation player needs to restart timer on FPS settings change