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Component Lifecycleλ︎

Clojure has several library to manage the lifecycle of components that make up the application, especially those components with state. Components can be started and stopped in a specific order.

mount integrant component system

Reloade Workflowλ︎

Set up a project to work with component lifecycle service, for example by adding components to the dev.clj file under a dev directory.

To load the service code automatically, add require and in-ns expressions to dev/user.clj and use the -M:env/dev alias when starting the REPL from Spacemacs.

Example project with component lifecycleλ︎

Using mount, its common to define a dev.clj file with go, stop and restart functions that manage the lifecycle of mount components. A start function contains the list of components with optional state.

(defn start []
  (mount/start #'app.conf/environment

The go function calls start and marks all components as ready.

(defn go
  "Start all states defined by defstate"

The stop function stops all components, removing all non-persistent state.

(defn stop [] (mount/stop))

The reset function that calls stop, refreshes the namespaces so that stale definitions are removed and starts all components (loading in any new code).

(defn reset
  "Stop all states defined by defstate.
  Reload modified source files and restart all states"
  (namespace/refresh :after 'dev/go))

Example dev.clj file for mount

Define a system which contains one or more component definitions.

A component definition may include references to other components and signal handlers that specify behavior.

Example: a :printer and :stack as service components. When the system receives the :donut.system/start signal, the :printer starts to process the data on the :stack

(ns donut.examples.printer
  (:require [donut.system :as system]))

(def printer-system
     {:stack #::system{:start  (fn [{{:keys [items]} ::system/config}]
                                 (atom (vec (range items))))
                       :stop   (fn [{::system/keys[instance]}]
                                 (reset! instance []))
                       :config {:items 10}}}
    {:printer #::system{:start  (fn [{{:keys [stack]} ::system/config}]
                                  (doto (Thread.
                                         (fn [] (prn "peek:" (peek @stack))
                                           (swap! stack pop)
                                           (Thread/sleep 1000)
                        :stop   (fn [{::system/keys [instance]}]
                                  (.interrupt instance))
                        :config {:stack (system/ref [:services :stack])}}}}})

;; start the system, let it run for 5 seconsystem, then stop it
  (let [system-in-flight (system/signal printer-system ::system/start)]
    (Thread/sleep 5000)
    (system/signal system-in-flight ::system/stop)))

Lifecycle with cider-refreshλ︎

Create a .dir-locals.el file that calls the relevant mount functions when using cider-refresh.

Space p e to open a .dirs.locals.el in the current project, creating the file if it does not exist.

During development call the reset function to stop components, clean the namespace and start all components again.

((clojure-mode . ((cider-refresh-before-fn . ""))))

Space f s to save the file. Refresh a buffer from the project or open a new file to trigger the reading of the .dir-locals.el configuration by Emacs.

Alternatively, if a namespace refresh is not required, configure the .dirs-local.el file to call stop then start.

((clojure-mode . ((cider-refresh-before-fn . "")
         (cider-refresh-after-fn . ""))))

This code calls the stop function from the component lifecycle library at the start of the cider-refresh function. At the end of cider-refresh, the start function is called to restart all the components in the defined order in the project.

Prevent cider-ns-refresh calling component lifecycle functions

Ctrl+- , e n or Space u - 1 , en calls cider-ns-refresh but prevents the refresh functions defined in cider-ns-refresh-before-fn and cider-ns-refresh-after-fn from being invoked.


Last update: December 22, 2022