Pre post cleanup callbacks

README.md

@@ -97,6 +97,18 @@ committed). It can be modified with the `storage-delay` atom or the
 ```
 
 
+It's possible to modify the contents of the atom on the way into storage
+and back out again using the optional arguments `pre-clean-fn` and
+`post-clean-fn`.
+
+ * `pre-clean-fn` takes a single argument which is the value on the way
+ to being saved.
+ * `post-clean-fn` takes two arguments. The first is the value as it
+ comes out of storage. The second is the previous value (if any). You
+ can use this to merge the existing state with the state coming out of
+ storage.
+
+
 [enduro](https://github.com/alandipert/enduro) is a Clojure library
 that provides similar functionality by using files or a database for
 storage.

src/alandipert/storage_atom.cljs

@@ -19,14 +19,14 @@
   (-get [this not-found])
   (-commit! [this value] "Commit value to storage at location."))
 
-(deftype StorageBackend [store key]
+(deftype StorageBackend [store key pre-clean-fn post-clean-fn]
   IStorageBackend
-  (-get [this not-found]
+  (-get [_this not-found]
     (if-let [existing (.getItem store (clj->json key))]
-      (json->clj existing)
+      ((or post-clean-fn identity) (json->clj existing) nil)
       not-found))
-  (-commit! [this value]
-    (.setItem store (clj->json key) (clj->json value))))
+  (-commit! [_this value]
+    (.setItem store (clj->json key) (clj->json ((or pre-clean-fn identity) value)))))
 
 
 (defn debounce-factory
@@ -66,7 +66,7 @@ discarded an only the new one is committed."
                                   @storage-delay)))))))
 
 (defn maybe-update-backend
-  [atom storage k default e]
+  [atom storage k default post-clean-fn e]
   (when (identical? storage (.-storageArea e))
     (if (empty? (.-key e)) ;; is all storage is being cleared?
       (binding [*watch-active* false]
@@ -77,15 +77,15 @@ discarded an only the new one is committed."
             (binding [*watch-active* false]
               (reset! atom (let [value (.-newValue e)] ;; new value, or is key being removed?
                              (if-not (string/blank? value)
-                               (json->clj value)
+                               ((or post-clean-fn identity) (json->clj value) @atom)
                                default))))))
-        (catch :default e)))))
+        (catch :default _e)))))
 
 (defn link-storage
-  [atom storage k]
+  [atom storage k post-clean-fn]
   (let [default @atom]
     (.addEventListener js/window "storage"
-                       #(maybe-update-backend atom storage k default %))))
+                       #(maybe-update-backend atom storage k default post-clean-fn %))))
 
 (defn dispatch-remove-event!
   "Create and dispatch a synthetic StorageEvent. Expects key to be a string.
@@ -102,7 +102,7 @@ discarded an only the new one is committed."
 
 (defn load-html-storage
   [storage k]
-  (-get (StorageBackend. storage k) nil))
+  (-get (StorageBackend. storage k nil nil) nil))
 
 (defn load-local-storage [k]
   (load-html-storage js/localStorage k))
@@ -113,17 +113,17 @@ discarded an only the new one is committed."
 ;;; main API
 
 (defn html-storage
-  [atom storage k]
-  (link-storage atom storage k)
-  (store atom (StorageBackend. storage k)))
+  [atom storage k pre-clean-fn post-clean-fn]
+  (link-storage atom storage k post-clean-fn)
+  (store atom (StorageBackend. storage k pre-clean-fn post-clean-fn)))
 
 (defn local-storage
-  [atom k]
-  (html-storage atom js/localStorage k))
+  [atom k & [pre-clean-fn post-clean-fn]]
+  (html-storage atom js/localStorage k pre-clean-fn post-clean-fn))
 
 (defn session-storage
-  [atom k]
-  (html-storage atom js/sessionStorage k))
+  [atom k & [pre-clean-fn post-clean-fn]]
+  (html-storage atom js/sessionStorage k pre-clean-fn post-clean-fn))
 
 ;; Methods to safely remove items from storage or clear storage entirely.