Allowing specification of dependencies of a state

README.md

@@ -26,6 +26,7 @@ _**Alan J. Perlis** from [Structure and Interpretation of Computer Programs](htt
   - [Using State](#using-state)
 - [Dependencies](#dependencies)
   - [Talking States](#talking-states)
+  - [Automatically Starting Dependencies](#automatically-starting-dependencies)
 - [Value of Values](#value-of-values)
 - [The Importance of Being Reloadable](#the-importance-of-being-reloadable)
 - [Start and Stop Order](#start-and-stop-order)
@@ -163,6 +164,20 @@ this `config`, being top level, can be used in other namespaces, including the o
 [here](dev/clj/app/www.clj#L32)
 is an example of a web server that "depends" on a similar `config`.
 
+### Automatically Starting Dependencies
+
+It is also possible to start the dependencies of a state, when a particular state is started. We can modify the example above as follows
+
+```clojure
+(ns app.database
+  (:require [mount.core :refer [defstate]]
+            [app.config :refer [config]]))
+
+(defstate conn :deps [#'config] :start (create-connection config))
+
+(mount/start #'conn) ;; => {:started ["#'app.config/config" "#'app.database/conn"]}
+```
+
 ## Value of values
 
 Lifecycle functions start/stop can take both functions and values. This is "valuable" and also works:

src/mount/core.cljc

@@ -135,14 +135,15 @@
 #?(:clj
     (defmacro defstate [state & body]
       (let [[state params] (macro/name-with-attributes state body)
-            {:keys [start stop] :as lifecycle} (apply hash-map params)
+            {:keys [start stop deps] :as lifecycle} (apply hash-map params)
             state-name (with-ns *ns* state)
             order (make-state-seq state-name)]
         (validate lifecycle)
         (let [s-meta (cond-> {:order order
                               :start `(fn [] ~start)
                               :status #{:stopped}}
-                       stop (assoc :stop `(fn [] ~stop)))]
+                       stop (assoc :stop `(fn [] ~stop))
+                       deps (assoc :deps deps))]
           `(do
              ;; (log (str "|| mounting... " ~state-name))
              (~'defonce ~state (DerefableState. ~state-name))
@@ -241,13 +242,22 @@
 (defn- all-without-subs []
   (remove (comp :sub? @meta-state) (find-all-states)))
 
+(defn- deps-chain [states]
+  (when (seq states)
+    (into (set states)
+          (->> states
+               (map var-to-str)
+               (map @meta-state)
+               (mapcat :deps)
+               deps-chain))))
+
 (defn start [& states]
   (let [fs (-> states first)]
     (if (coll? fs)
       (if-not (empty? fs)                      ;; (mount/start) vs. (mount/start #{}) vs. (mount/start #{1 2 3})
         (apply start fs)
         {:started #{}})
-      (let [states (or (seq states)
+      (let [states (or (deps-chain states)
                        (all-without-subs))]
         {:started (bring states up <)}))))
 

test/core/mount/test/start_dependencies.cljc

@@ -0,0 +1,32 @@
+(ns core.mount.test.start-dependencies
+  (:require
+    #?@(:cljs [[cljs.test :as t :refer-macros [is deftest use-fixtures]]
+               [mount.core :as mount :refer-macros [defstate]]]
+        :clj  [[clojure.test :as t :refer [is deftest use-fixtures]]
+               [mount.core :as mount :refer [defstate]]])
+    [mount.test.helper :refer [dval]]))
+
+#?(:clj (alter-meta! *ns* assoc ::load false))
+
+(defstate dependency-1 :start 1)
+(defstate dependency-2 :start 2)
+(defstate thing-to-start
+  :deps [#'dependency-1 #'dependency-2]
+  :start 3)
+(defstate should-not-start :start 4)
+
+(defn- start-states []
+  (mount/start #'dependency-1))
+
+(use-fixtures :once
+              #?(:cljs {:before start-states
+                        :after mount/stop}
+                 :clj #((start-states) (%) (mount/stop))))
+
+(deftest dependencies-test
+  (is (= {:started ["#'core.mount.test.start-dependencies/dependency-2" "#'core.mount.test.start-dependencies/thing-to-start"]}
+         (mount/start #'thing-to-start)))
+  (is (= 3 (dval thing-to-start)))
+  (is (= 2 (dval dependency-2)))
+  (is (= 1 (dval dependency-1)))
+  (is (instance? mount.core.NotStartedState (dval should-not-start))))