Bug 867875 - Add the pref to toggle reader mode parsing on load. r=mfinkle

mobile/android/app/mobile.js

@@ -649,8 +649,12 @@ pref("network.manage-offline-status", true);
 // increase the timeout clamp for background tabs to 15 minutes
 pref("dom.min_background_timeout_value", 900000);
 
+// The default state of reader mode works on loaded a page.
+pref("reader.parse-on-load.enabled", true);
+
+// Force to enable reader mode to parse on loaded a page.
 // Allow reader mode even on low-memory platforms
-pref("reader.force_allow", false);
+pref("reader.parse-on-load.force-enabled", false);
 
 // The default of font size in reader (1-7)
 pref("reader.font_size", 4);

mobile/android/chrome/content/browser.js

@@ -3277,9 +3277,7 @@ Tab.prototype = {
           tabID: this.id
         });
 
-        // For low-memory devices, don't allow reader mode since it takes up a lot of memory.
-        // See https://bugzilla.mozilla.org/show_bug.cgi?id=792603 for details.
-        if (BrowserApp.isOnLowMemoryPlatform && !Services.prefs.getBoolPref("reader.force_allow"))
+        if (!Reader.isEnabledForParseOnLoad)
           return;
 
         // Once document is fully loaded, parse it
@@ -6600,12 +6598,18 @@ let Reader = {
   // performance reasons)
   MAX_ELEMS_TO_PARSE: 3000,
 
+  isEnabledForParseOnLoad: false,
+
   init: function Reader_init() {
     this.log("Init()");
     this._requests = {};
 
+    this.isEnabledForParseOnLoad = this.getStateForParseOnLoad();
+
     Services.obs.addObserver(this, "Reader:Add", false);
     Services.obs.addObserver(this, "Reader:Remove", false);
+
+    Services.prefs.addObserver("reader.parse-on-load.", this, false);
   },
 
   observe: function(aMessage, aTopic, aData) {
@@ -6681,9 +6685,24 @@ let Reader = {
         }.bind(this));
         break;
       }
+
+      case "nsPref:changed": {
+        if (aData.startsWith("reader.parse-on-load.")) {
+          this.isEnabledForParseOnLoad = this.getStateForParseOnLoad();
+        }
+        break;
+      }
     }
   },
 
+  getStateForParseOnLoad: function Reader_getStateForParseOnLoad() {
+    let isEnabled = Services.prefs.getBoolPref("reader.parse-on-load.enabled");
+    let isForceEnabled = Services.prefs.getBoolPref("reader.parse-on-load.force-enabled");
+    // For low-memory devices, don't allow reader mode since it takes up a lot of memory.
+    // See https://bugzilla.mozilla.org/show_bug.cgi?id=792603 for details.
+    return isForceEnabled || (isEnabled && !BrowserApp.isOnLowMemoryPlatform);
+  },
+
   parseDocumentFromURL: function Reader_parseDocumentFromURL(url, callback) {
     // If there's an on-going request for the same URL, simply append one
     // more callback to it to be called when the request is done.
@@ -6845,6 +6864,8 @@ let Reader = {
   },
 
   uninit: function Reader_uninit() {
+    Services.prefs.removeObserver("reader.parse-on-load.", this);
+
     Services.obs.removeObserver(this, "Reader:Add");
     Services.obs.removeObserver(this, "Reader:Remove");