Improve preference migration logic

lib/main.js

@@ -18,7 +18,7 @@
  */
 
 var webextension = require('sdk/webextension');
-var preferences = require('sdk/simple-prefs').prefs;
+var preferences = require('sdk/simple-prefs');
 
 var self = require('sdk/self');
 var tabs = require("sdk/tabs");
@@ -51,42 +51,56 @@ var featurelessVersions = {
 // Executed as soon as the add-on is loaded.
 exports.main = function (options) {
 
+    // Initialize add-on state.
+    interceptor.register();
+    loadWatcher.register();
+
+    // Display the release notes if desired.
+    if (preferences.prefs.showReleaseNotes) {
+
+        if (options.loadReason === 'install' || (options.loadReason === 'upgrade' && !featurelessVersions[self.version])) {
+
+            if (preferences.prefs['sdk.baseURI']) {
+                tabs.open(preferences.prefs['sdk.baseURI'] + 'static/release-notes.html');
+            }
+        }
+    }
+
     // Initialize the embedded WebExtension.
     webextension.startup().then(({ browser }) => {
 
         browser.runtime.onConnect.addListener(port => {
 
-            webextensionPort = port;
-
             if (port.name === 'webextension') {
 
+                webextensionPort = port;
+
+                preferences.on('', function (preferenceName) {
+
+                    if (preferenceName === 'amountInjected') {
+                        return;
+                    }
+
+                    port.postMessage({
+                        'subject': 'update-preferences',
+                        'content': {
+                            [preferenceName]: preferences.prefs[preferenceName]
+                        }
+                    });
+                });
+
                 port.postMessage({
                     'subject': 'migrate-preferences',
                     'content': {
-                        'amountInjected': preferences['amountInjected'],
-                        'blockMissing': preferences['blockMissing'],
-                        'domainWhitelist': preferences['domainWhitelist'],
-                        'showReleaseNotes': preferences['showReleaseNotes']
+                        'amountInjected': preferences.prefs['amountInjected'],
+                        'blockMissing': preferences.prefs['blockMissing'],
+                        'domainWhitelist': preferences.prefs['domainWhitelist'],
+                        'showReleaseNotes': preferences.prefs['showReleaseNotes']
                     }
                 });
             }
         });
     });
-
-    // Initialize add-on state.
-    interceptor.register();
-    loadWatcher.register();
-
-    // Display the release notes if desired.
-    if (preferences.showReleaseNotes) {
-
-        if (options.loadReason === 'install' || (options.loadReason === 'upgrade' && !featurelessVersions[self.version])) {
-
-            if (preferences['sdk.baseURI']) {
-                tabs.open(preferences['sdk.baseURI'] + 'static/release-notes.html');
-            }
-        }
-    }
 };
 
 // Executed as soon as the add-on is unloaded.

webextension/background.js

@@ -18,7 +18,9 @@
  */
 
 var webextensionPort = {};
-var amountInjected = 0;
+
+var amountInjected = null;
+var pendingCount = 0;
 
 /**
  * Initializations
@@ -30,31 +32,25 @@ webextensionPort = browser.runtime.connect({name: 'webextension'});
  * Event Handlers
  */
 
-// browser.storage.local.remove('amountInjected');
-
 webextensionPort.onMessage.addListener((message) => {
 
     if (message.subject === 'migrate-preferences') {
 
         browser.storage.local.get(function (items) {
 
-            for (let preference of Object.keys(message.content)) {
-
-                if (items.hasOwnProperty(preference)) {
+            // Covers storage API failures.
+            if (items === null) {
+                return;
+            }
 
-                    if (preference === 'amountInjected') {
-                        amountInjected = items.amountInjected;
-                    }
+            for (let preference of Object.keys(message.content)) {
 
-                } else {
+                // Makes sure no existing preferences are overwritten.
+                if (!items.hasOwnProperty(preference)) {
 
                     browser.storage.local.set({
                         [preference]: message.content[preference]
                     });
-
-                    if (preference === 'amountInjected') {
-                        amountInjected = message.content[preference];
-                    }
                 }
             }
         });
@@ -62,22 +58,33 @@ webextensionPort.onMessage.addListener((message) => {
 
     if (message.subject === 'register-injection') {
 
-        if (isNaN(amountInjected)) {
+        if (amountInjected !== null  && !isNaN(amountInjected)) {
+
+            ++amountInjected;
+            browser.storage.local.set({amountInjected});
+        }
+
+        ++pendingCount;
 
-            chrome.storage.local.get('amountInjected', function (items) {
+        if (pendingCount > 1) {
+            return;
+        }
 
-                amountInjected = items.amountInjected;
+        chrome.storage.local.get({
 
-                chrome.storage.local.set({
-                    'amountInjected': ++amountInjected
-                });
-            });
+            // The stored amount, or zero.
+            amountInjected: 0
 
-        } else {
+        }, function (items) {
 
-            chrome.storage.local.set({
-                'amountInjected': ++amountInjected
-            });
-        }
+            // Accounts for the fact that the storage API is asynchronous.
+            amountInjected = (items && items.amountInjected || 0) + pendingCount;
+            browser.storage.local.set({amountInjected});
+        });
+
+    }
+
+    if (message.subject === 'update-preferences') {
+        chrome.storage.local.set(message.content);
     }
 });