diff --git a/canvasblocker.xpi b/canvasblocker.xpi
index eca7d16..20f48ae 100644
Binary files a/canvasblocker.xpi and b/canvasblocker.xpi differ
diff --git a/data/inject.js b/data/inject.js
index 4c80cfa..37c39d5 100644
--- a/data/inject.js
+++ b/data/inject.js
@@ -2,215 +2,94 @@
 (function(){
 	"use strict";
 	
-	var originalGetContext = unsafeWindow.HTMLCanvasElement.prototype.getContext;
-	var originalToDataURL = unsafeWindow.HTMLCanvasElement.prototype.toDataURL;
-	var originalGetImageData = unsafeWindow.CanvasRenderingContext2D.prototype.getImageData;
-	var askFunctionName = Math.random().toString(16);
+	var blockMode = {
+		getContext: {
+			status: "block",
+			askStatus: {
+				askOnce: false,
+				alreadyAsked: false,
+				answer: null
+			}
+		},
+		readAPI: {
+			status: "allow"
+		}
+	};
 	
-	function transformFunctionToUnsafeWindow(func, name){
-		var funcString = func.toString();
-		var parameter = funcString.match(/^function\s*\(([^\)]*)\)\s*\{/)[1];
-		funcString = funcString
-			.replace(/^function\s*\(([^\)]*)\)\s*\{|\}\s*$/g, "")
-			.replace(/(^|\n|\r)\t{3}/g, "$1")
-			.replace(/askFunctionName/g, JSON.stringify(askFunctionName))
-			.replace(/askForPermission/g, _("askForPermission"))
-			.replace(/askForInvisiblePermission/g, _("askForInvisiblePermission"));
-		var unsafeFunction = new unsafeWindow.Function(parameter, funcString);
-		unsafeFunction.toString =
-		unsafeFunction.toLocaleString =
-		unsafeFunction.toSource = new unsafeWindow.Function(
-			"return \"function " + name + "() {\\n    [native code]\\n}\";"
-		);
-		return unsafeFunction;
-	}
-	function setAPIFunctions(getContext, toDataURL, getImageData){
-		delete unsafeWindow.HTMLCanvasElement.prototype[askFunctionName];
-		if (getContext){
-			if (getContext === true){
-				getContext = originalGetContext;
-			}
-			else {
-				var secretObject = new unsafeWindow.Object();
-				Object.defineProperties(
-					secretObject,
-					{
-					  getContext: {value: originalGetContext},
-					  confirm: {value: unsafeWindow.confirm}
-					}
-				);
-				Object.defineProperty(
-					unsafeWindow.HTMLCanvasElement.prototype,
-					askFunctionName,
-					{
-						value: secretObject,
-						configurable: true,
-						enumerable: false
-					}
-				);
-				getContext = transformFunctionToUnsafeWindow(getContext, "getContext");
-			}
-		}
-		unsafeWindow.HTMLCanvasElement.prototype.getContext = getContext;
-		
-		if (toDataURL){
-			if (toDataURL === true){
-				toDataURL = originalToDataURL;
-			}
-			else {
-				toDataURL = transformFunctionToUnsafeWindow(toDataURL, "toDataURL");
-			}
-		}
-		unsafeWindow.HTMLCanvasElement.prototype.toDataURL = toDataURL;
-		
-		if (getImageData){
-			if (getImageData === true){
-				getImageData = originalGetImageData;
-			}
-			else {
-				getImageData = transformFunctionToUnsafeWindow(getImageData, "getImageData");
-			}
-		}
-		unsafeWindow.CanvasRenderingContext2D.prototype.getImageData = getImageData;
-	}
-
-	function checkPDF(blocking){
-		if (document.contentType.match(/\/pdf$/i)){
-			self.port.emit("isPDF", blocking);
-			return true;
-		}
-		return false;
-	}
-
-	function block(force){
-		if (force || !checkPDF("block")){
-			setAPIFunctions(null, null, null);
-		}
-	}
-
-	function askVisible(force, askOnlyOnce){
-		if (force || !checkPDF("askVisible")){
-			setAPIFunctions(
-				askOnlyOnce?
-				function(){
-					if (this.parentNode){
-						var oldBorder = this.style.border;
-						this.style.border = "2px dashed red";
-						var confirmText = "askForPermission";
-						// try {throw new Error();}
-						// catch (e){
-							// console.log(e.stack.split(/\s*(?:-?>|@)\s*/));
-						// }
-						var allow = this[askFunctionName].confirm.call(window, confirmText);
-						this.style.border = oldBorder;
-						if (allow){
-							HTMLCanvasElement.prototype.getContext = this[askFunctionName].getContext;
-							delete HTMLCanvasElement.prototype[askFunctionName];
-							return this.getContext.apply(this, arguments);
+	var undef;
+	var originalGetContext = unsafeWindow.HTMLCanvasElement.prototype.getContext;
+	Object.defineProperty(
+		unsafeWindow.HTMLCanvasElement.prototype,
+		"getContext",
+		{
+			enumerable: true,
+			configureable: false,
+			get: exportFunction(function(){
+				switch (blockMode.getContext.status){
+					case "allow":
+						// console.log("allow");
+						return originalGetContext;
+					case "ask":
+						// console.log("ask");
+						var status = blockMode.getContext.askStatus;
+						var allow;
+						if (status.askOnce && status.alreadyAsked){
+							// console.log("already asked");
+							allow = status.answer;
 						}
 						else {
-							HTMLCanvasElement.prototype.getContext = null;
-							delete HTMLCanvasElement.prototype[askFunctionName];
-							return null;
+							// console.log("asking");
+							allow = window.confirm(_("askForPermission"));
+							status.alreadyAsked = true;
+							status.answer = allow;
 						}
-					}
-					else {
-						return null;
-					}
-				}:
-				function(){
-					if (this.parentNode){
-						var oldBorder = this.style.border;
-						this.style.border = "2px dashed red";
-						var confirmText = "askForPermission";
-						// try {throw new Error();}
-						// catch (e){
-							// console.log(e.stack.split(/\s*(?:-?>|@)\s*/));
-						// }
-						var allow = this[askFunctionName].confirm.call(window, confirmText);
-						this.style.border = oldBorder;
-						if (allow){
-							this.getContext = this[askFunctionName].getContext;
-							return this.getContext.apply(this, arguments);
-						}
-						else {
-							this.getContext = null;
-							return null;
-						}
-					}
-					else {
-						return null;
-					}
-				},
-				true,
-				true
-			);
-		}
-	}
-	function askInvisible(force, askOnlyOnce){
-		if (force || !checkPDF("askInvisible")){
-			setAPIFunctions(
-				askOnlyOnce?
-				function(){
-					var oldBorder = this.style.border;
-					this.style.border = "2px dashed red";
-					var confirmText =
-						this.parentNode?
-						"askForPermission":
-						"askForInvisiblePermission";
-					var allow = this[askFunctionName].confirm.call(window, confirmText);
-					this.style.border = oldBorder;
-					if (allow){
-						HTMLCanvasElement.prototype.getContext = this[askFunctionName].getContext;
-						delete HTMLCanvasElement.prototype[askFunctionName];
-						return this.getContext.apply(this, arguments);
-					}
-					else {
-						HTMLCanvasElement.prototype.getContext = null;
-						delete HTMLCanvasElement.prototype[askFunctionName];
-						return null;
-					}
-				}:
-				function(){
-					var oldBorder = this.style.border;
-					this.style.border = "2px dashed red";
-					var confirmText =
-						this.parentNode?
-						"askForPermission":
-						"askForInvisiblePermission";
-					var allow = this[askFunctionName].confirm.call(window, confirmText);
-					this.style.border = oldBorder;
-					if (allow){
-						this.getContext = this[askFunctionName].getContext;
-						return this.getContext.apply(this, arguments);
-					}
-					else {
-						this.getContext = null;
-						return null;
-					}
-				},
-				true,
-				true
-			);
-		}
-	}
-	function blockReadout(force){
-		if (force || !checkPDF("blockReadout")){
-			setAPIFunctions(
-				true,
-				function(){
-					return "data:image/png;base64";
-				},
-				function(sx, sy, sw, sh){
-					var imageData = this.createImageData(sw, sh);
-					return imageData;
+						return allow? originalGetContext: undef;
+					case "block":
+					default:
+						// console.log("block");
+						return undef;
 				}
-			);
+			}, unsafeWindow)
 		}
-	}
-	function unblock(){
-		setAPIFunctions(true, true, true);
-	}
+	);
+	
+	var originalToDataURL = unsafeWindow.HTMLCanvasElement.prototype.toDataURL;
+	Object.defineProperty(
+		unsafeWindow.HTMLCanvasElement.prototype,
+		"toDataURL",
+		{
+			enumerable: true,
+			configureable: false,
+			get: exportFunction(function(){
+				switch (blockMode.readAPI.status){
+					case "allow":
+						return originalToDataURL;
+					case "block":
+					default:
+						return undef;
+				}
+			}, unsafeWindow)
+		}
+	);
+	
+	var originalGetImageData = unsafeWindow.CanvasRenderingContext2D.prototype.getImageData;
+	Object.defineProperty(
+		unsafeWindow.CanvasRenderingContext2D.prototype,
+		"getImageData",
+		{
+			enumerable: true,
+			configureable: false,
+			get: exportFunction(function(){
+				switch (blockMode.readAPI.status){
+					case "allow":
+						return originalGetImageData;
+					case "block":
+					default:
+						return undef;
+				}
+			}, unsafeWindow)
+		}
+	);
 
 	var _ = function(name){
 		return _[name] || name;
@@ -218,12 +97,40 @@
 	self.port.on("setTranslation", function(name, translation){
 		_[name] = translation;
 	});
-
-	block();
-	self.port.on("block", block);
-	self.port.on("askVisible", askVisible);
-	self.port.on("askInvisible", askInvisible);
-	self.port.on("blockReadout", blockReadout);
-	self.port.on("unblock", unblock);
-	//self.port.on("detach", unblock); // produces memory leak due to the reference to unsafeWindow
+	
+	function checkPDF(blocking){
+		if (document.contentType.match(/\/pdf$/i)){
+			self.port.emit("isPDF", blocking);
+			return true;
+		}
+		return false;
+	}
+	
+	self.port.on("block", function(force){
+		if (force || !checkPDF("block")){
+			blockMode.getContext.status = "block";
+			blockMode.readAPI.status = "allow";
+		}
+	});
+	self.port.on("ask", function(force, askOnce){
+		if (force || !checkPDF("askVisible")){
+			blockMode.getContext.status = "ask";
+			blockMode.getContext.askStatus.askOnce = askOnce;
+			blockMode.readAPI.status = "allow";
+		}
+	});
+	self.port.on("blockReadout", function(force){
+		if (force || !checkPDF("blockReadout")){
+			blockMode.getContext.status = "allow";
+			blockMode.readAPI.status = "block";
+		}
+	});
+	self.port.on("unblock", function(){
+		blockMode.getContext.status = "allow";
+		blockMode.readAPI.status = "allow";
+	});
+	self.port.on("detach", function(force){
+		blockMode.getContext.status = "allow";
+		blockMode.readAPI.status = "allow";
+	});
 }());
diff --git a/lib/main.js b/lib/main.js
index 622907e..8da65a6 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -42,12 +42,18 @@
 
 	var self = require("sdk/self");
 	var pageMod = require("sdk/page-mod");
+	var array = require("sdk/util/array");
 	var preferences = require("sdk/simple-prefs");
 	var prefs = preferences.prefs;
 	var {URL} = require("sdk/url");
 	var _ = require("sdk/l10n").get;
 
 	// preferences
+	Object.keys(prefs).forEach(function(pref){
+		preferences.on(pref, function(){
+			workers.forEach(checkWorker);
+		});
+	});
 	var whiteList;
 	function updateWhiteList(){
 		whiteList = getDomainRegExpList(prefs.whiteList);
@@ -55,7 +61,6 @@
 	updateWhiteList();
 	preferences.on("whiteList", function(){
 		updateWhiteList();
-		// workers.forEach(checkWorker);
 	});
 
 	var blackList;
@@ -65,100 +70,84 @@
 	updateBlackList();
 	preferences.on("blackList", function(){
 		updateBlackList();
-		// workers.forEach(checkWorker);
 	});
-	
-	
-	// preferences.on("blockMode", function(){
-		// workers.forEach(checkWorker);
-	// });
-	// preferences.on("allowPDFCanvas", function(){
-		// workers.forEach(checkWorker);
-	// });
 
-	// var workers = [];
-	// function detachWorker(worker, workerArray) {
-		// var index = workerArray.indexOf(worker);
-		// if (index !== -1){
-			// workerArray.splice(index, 1);
-		// }
-	// }
 	function checkWorker(worker){
-		var url = new URL(worker.url);
-		var mode = "block";
-		switch (prefs.blockMode){
-			case "blockEverything":
-				mode = "block";
-				break;
-			case "allowOnlyWhiteList":
-				if (whiteList.match(url)){
-					mode = "unblock";
-				}
-				else {
+		try {
+			var url = new URL(worker.url);
+			var mode = "block";
+			switch (prefs.blockMode){
+				case "blockEverything":
 					mode = "block";
-				}
-				break;
-			case "askVisible":
-				if (whiteList.match(url)){
+					break;
+				case "allowOnlyWhiteList":
+					if (whiteList.match(url)){
+						mode = "unblock";
+					}
+					else {
+						mode = "block";
+					}
+					break;
+				case "ask":
+					if (whiteList.match(url)){
+						mode = "unblock";
+					}
+					else if (blackList.match(url)){
+						mode = "block";
+					}
+					else {
+						mode = "ask";
+					}
+					break;
+				case "blockReadout":
+					if (whiteList.match(url)){
+						mode = "unblock";
+					}
+					else if (blackList.match(url)){
+						mode = "block";
+					}
+					else {
+						mode = "blockReadout";
+					}
+					break;
+				case "blockOnlyBlackList":
+					if (blackList.match(url)){
+						mode = "block";
+					}
+					else {
+						mode = "unblock";
+					}
+					break;
+				case "allowEverything":
 					mode = "unblock";
-				}
-				else if (blackList.match(url)){
-					mode = "block";
-				}
-				else {
-					mode = "askVisible";
-				}
-				break;
-			case "askInvisible":
-				if (whiteList.match(url)){
-					mode = "unblock";
-				}
-				else if (blackList.match(url)){
-					mode = "block";
-				}
-				else {
-					mode = "askInvisible";
-				}
-				break;
-			case "blockReadout":
-				if (whiteList.match(url)){
-					mode = "unblock";
-				}
-				else if (blackList.match(url)){
-					mode = "block";
-				}
-				else {
-					mode = "blockReadout";
-				}
-				break;
-			case "blockOnlyBlackList":
-				if (blackList.match(url)){
-					mode = "block";
-				}
-				else {
-					mode = "unblock";
-				}
-				break;
-			case "allowEverything":
-				mode = "unblock";
-				break;
-			default:
-				console.log("Unknown blocking mode. Default to block everything.");
+					break;
+				default:
+					console.log("Unknown blocking mode. Default to block everything.");
+			}
+			worker.port.emit(mode, false, prefs.askOnlyOnce);
+		}
+		catch (e){
+			console.log("Error updating " + worker.url + ": " + e.message);
 		}
-		worker.port.emit(mode, false, prefs.askOnlyOnce);
 	}
 
-
+	var workers = [];
 	pageMod.PageMod({
 		include: "*",
 		contentScriptWhen: "start",
 		contentScriptFile: self.data.url("inject.js"),
 		onAttach: function(worker){
 			
-			// workers.push(worker);
-			// worker.on("detach", function(){
-				// detachWorker(this, workers);
-			// });
+			array.add(workers, worker);
+			worker.on("pageshow", function(){
+				array.add(workers, this);
+			});
+			worker.on("pagehide", function(){
+				array.remove(workers, this);
+			});
+			worker.on("detach", function(){
+				array.remove(workers, this);
+			});
 			worker.port.on("isPDF", function(blocking){
 				if (prefs.allowPDFCanvas){
 					this.emit("unblock");
@@ -168,7 +157,6 @@
 				}
 			});
 			worker.port.emit("setTranslation", "askForPermission", _("askForPermission"));
-			worker.port.emit("setTranslation", "askForInvisiblePermission", _("askForInvisiblePermission"));
 			checkWorker(worker);
 		},
 	});
diff --git a/locale/de-DE.properties b/locale/de-DE.properties
index 2f1dd5c..6d0ff55 100644
--- a/locale/de-DE.properties
+++ b/locale/de-DE.properties
@@ -9,8 +9,7 @@ blockMode_description=
 
 blockMode_options.block everything= alles blockieren
 blockMode_options.allow only white list= nur Einträge der Whitelist erlauben
-blockMode_options.ask for permission for visible <canvas>= bei sichtbaren <canvas> um Erlaubnis fragen
-blockMode_options.ask for permision for invisible <canvas>= bei unsichtbaren <canvas> um Erlaubnis fragen
+blockMode_options.ask for permission= um Erlaubnis fragen
 blockMode_options.block readout API= Auslese-API blockieren
 blockMode_options.block only black list= nur Einträge der Blacklist blockieren
 blockMode_options.allow everything= alles erlauben
@@ -21,5 +20,4 @@ askOnlyOnce_description= Wenn eine Seite öfters versucht, die <canvas>-API abzu
 allowPDFCanvas_title= <canvas> in PDFs erlauben
 allowPDFCanvas_description= Die native pdf.js verwendet <canvas> um den Inhalt von PDFs anzuzeigen. Wenn dies nicht markiert ist, werden viele Nachfragedialoge erscheinen oder die PDF Ansicht nicht funktionieren.
 
-askForPermission= Wollen Sie das rot umrandete <canvas> erlauben?
-askForInvisiblePermission= Wollen Sie ein verstecktes <canvas> erlauben?
\ No newline at end of file
+askForPermission= Wollen Sie <canvas> erlauben?
\ No newline at end of file
diff --git a/locale/en-US.properties b/locale/en-US.properties
index 2b7bb73..8ba708e 100644
--- a/locale/en-US.properties
+++ b/locale/en-US.properties
@@ -9,8 +9,7 @@ blockMode_description=
 
 blockMode_options.block everything= block everything
 blockMode_options.allow only white list= allow only white list
-blockMode_options.ask for permission for visible <canvas>= ask for permission for visible <canvas>
-blockMode_options.ask for permision for invisible <canvas>= ask for permision for invisible <canvas>
+blockMode_options.ask for permission= ask for permission
 blockMode_options.block readout API= block readout API
 blockMode_options.block only black list= block only black list
 blockMode_options.allow everything= allow everything
@@ -21,5 +20,4 @@ askOnlyOnce_description= If a page tries to access the <canvas>-API several time
 allowPDFCanvas_title= Allow canvas in PDFs
 allowPDFCanvas_description= The native pdf.js uses <canvas> to display the PDF content. If this is unchecked there will lots of annoying ask dialogs or the PDF display will not work.
 
-askForPermission= Do you want to allow the red bordered <canvas>?
-askForInvisiblePermission= Do you want to allow an invisibe <canvas>?
\ No newline at end of file
+askForPermission= Do you want to allow <canvas>?
\ No newline at end of file
diff --git a/package.json b/package.json
index 33ff05b..497dbd1 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,7 @@
 			"name": "blockMode",
 			"title": "block mode",
 			"type": "menulist",
-			"value": "askVisible",
+			"value": "ask",
 			"options": [
 				{
 					"value": "blockEverything",
@@ -31,12 +31,8 @@
 					"label": "allow only white list"
 				},
 				{
-					"value": "askVisible",
-					"label": "ask for permission for visible <canvas>"
-				},
-				{
-					"value": "askInvisible",
-					"label": "ask for permision for invisible <canvas>"
+					"value": "ask",
+					"label": "ask for permission"
 				},
 				{
 					"value": "blockReadout",
@@ -56,7 +52,7 @@
 			"name": "askOnlyOnce",
 			"title": "Ask only once",
 			"type": "bool",
-			"value": false
+			"value": true
 		},
 		{
 			"name": "allowPDFCanvas",