Bug 1354682 Add transform to XPCOMUtils.defineLazyPreferenceGetter r=kmag

MozReview-Commit-ID: Lm59IHMNcy9

--HG--
extra : rebase_source : 796d9b0b7a6bf94782ebeeaf35d237176687c0ac

js/xpconnect/loader/XPCOMUtils.jsm

@@ -299,11 +299,16 @@ this.XPCOMUtils = {
    * @param aOnUpdate
    *        A function to call upon update. Receives as arguments
    *         `(aPreference, previousValue, newValue)`
+   * @param aTransform
+   *        An optional function to transform the value.  If provided,
+   *        this function receives the new preference value as an argument
+   *        and its return value is used by the getter.
    */
   defineLazyPreferenceGetter: function XPCU_defineLazyPreferenceGetter(
                                    aObject, aName, aPreference,
                                    aDefaultValue = null,
-                                   aOnUpdate = null)
+                                   aOnUpdate = null,
+                                   aTransform = val => val)
   {
     // Note: We need to keep a reference to this observer alive as long
     // as aObject is alive. This means that all of our getters need to
@@ -323,7 +328,7 @@ this.XPCOMUtils = {
             // Fetch and cache value.
             this.value = undefined;
             let latest = lazyGetter();
-            aOnUpdate(data, previous, latest);
+            aOnUpdate.apply(aObject, data, previous, latest);
           } else {
 
             // Empty cache, next call to the getter will cause refetch.
@@ -343,7 +348,7 @@ this.XPCOMUtils = {
 
     function lazyGetter() {
       if (observer.value === undefined) {
-        observer.value = Preferences.get(aPreference, aDefaultValue);
+        observer.value = aTransform(Preferences.get(aPreference, aDefaultValue));
       }
       return observer.value;
     }

js/xpconnect/tests/unit/test_xpcomutils.js

@@ -151,6 +151,18 @@ add_test(function test_defineLazyPreferenceGetter()
 
     equal(obj.pref, "defaultValue", "Should return default value after pref is reset");
 
+    obj = {};
+    XPCOMUtils.defineLazyPreferenceGetter(obj, "pref", PREF, "a,b",
+                                          null, value => value.split(","));
+
+    deepEqual(obj.pref, ["a", "b"], "transform is applied to default value");
+
+    Preferences.set(PREF, "x,y,z");
+    deepEqual(obj.pref, ["x", "y", "z"], "transform is applied to updated value");
+
+    Preferences.reset(PREF);
+    deepEqual(obj.pref, ["a", "b"], "transform is applied to reset default");
+
     run_next_test();
 });