Changes

In the [[Dive into Mozilla Modifying Firefox Lab|previous lab]] we made a small change to the behaviour of Firefox by modifying the browser's source code. In this lab we explore how to achieve the same effect using an '''extension''' rather than modifying the tree. ''(Thanks to [http://www.starkravingfinkle.org/blog/ Mark Finkle] for the idea of redoing this as an extension.)''

The goal of this exercise is to expose you to Firefox extensions and to show you how to modify or extend the browser without changing it's its source code directly. Some thought will also be given to the two methods of doing this (i.e., in the tree vs. as an extension), comparing their advantages and disadvantages.

A logical alternative might be to try and get your change accepted into the tree. This involves filing a bug on https://bugzilla.mozilla.org and then creating and attaching a patch with your changes. The problem here is that even though you (and I) might think it is a good idea for tabs to be created in the way we've specified, the community may not--people have already put thought into the way things work now, users are accustomed to it, etc. In this case your bug will likely be marked '''WONTFIX''', which means that your patch won't make it into the tree.

Extensions allow third-party developers (and Mozilla, for that matter) to write add-on packages that users can install to extend or modify the standard browser. By rewriting our earlier code as an extension, we can give our users an extension to installa small add-on, which will have the same effect as our custom build. For all but the most universal of changes, extensions are the best way for developers to write code that targets the browser.

Instead of searching for existing code to change, we have to take the browser as a given and find ways to work with it. Here are some interesting pieces of To that end, let's look again at the code for [http://lxr.mozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml#1122 tabbrowser.xml's addTab] method:

After the new tab is created and appended to the end of the list, an '''event''' is '''[http://developer.mozilla.org/en/docs/DOM:element.dispatchEvent dispatched'''] to the new tab element. The developers of tabbrowser.xml wanted their code to be extensible; that is, they wanted to provide other a way for future developers to extend the code by inserting a hook on which . In this case, after a tab is created an opportunity is provided to hang their codedo ''something''. Their foresight makes our job much easier.

Now we have ===How to move a The name of the event is significant, as we'll have to write co'''TabOpen''', which is important for us.tab?===

Start by creating Now we know that there is a new extensionlogical time/place to run our code via the '''TabOpen''' event. The next thing to consider is what our code will do. Previously we replaced '''append''' with '''insertBefore''' and stopped the problem before it happened. However, either in this case by handthe time the '''TabOpen''' event is dispatched, or using Ted Mielczarek's wonderful wizard: the tab will already be created and positioned at the end of the list.

We need another solution. It's time to go hunting in tabbrowser's [http://developer.mozilla.org/en/docs/XUL:tabbrowser documentation] and [http://tedlxr.mielczarekmozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml code]. What we need is a way to reposition a tab after it has been created. A quick look through the docs for [http://developer.mozilla.org/en/docs/XUL:tabbrowser tabbrowser] reveals nothing. However, the [http://lxr.mozilla.org/seamonkey/source/toolkit/content/extensionwizwidgets/tabbrowser.xml code] is more helpful:

Directory structureThe '''moveTabTo''' method takes a tab to be moved, and a new position. This is exactly what we need. Now there are just two things left to figure out:1) how to get the newly inserted tab; and 2) the index of currently selected tab so we can go one to the right.

Here // In an extension, gBrowser is a global reference to the '''contenttabbrowser element var container = gBrowser.manifest''' file:tabContainer; var lastIndex = container.childNodes.length - 1; var newTab = container.childNodes[lastIndex];

content addtabbeside content/ overlay chromeHowever, looking back at the event dispatching code for '''TabOpen''' we see that the event is dispatched to t, where t is the newly created tab://browser/content/browser.xul chrome://addtabbeside/content/firefoxOverlay.xul

Here is the We'''installll discuss this code in more detail below.rdf''' file for the extension:

<pre> <?xml version="1.0" encoding="UTF-8"?> <RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:em="http://www.mozilla.org/2004/em-rdf#"> <Description about="urn:mozilla:install-manifest"> <em:id>addtabbeside@senecac.on.ca</em:id> <em:name>Add Tab Beside</em:name> <em:version>0.1</em:version> <em:creator>David Humphrey</em:creator> <em:description>New tabs are created beside How to get the current tab instead of at the end index of the currently selected tab list.</em:description> <em:targetApplication> <Description> <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id> <!-- firefox --> <em:minVersion>2.0</em:minVersion> <em:maxVersion>3.0a3pre</em:maxVersion> <!-- trunk build Feb 27, 2007 --> </Description> </em:targetApplication> </Description> </RDF></pre>?===

Here is firefoxOverlay var currentIndex = this.xul:mTabContainer.selectedIndex;

<pre> <?xml version="1.0" encoding="UTF-8"?> <overlay id="addtabbeside-overlay" xmlns="httpObviously we can't use '''this''' outside the context of tabbrowser, so in an extension we use '''gBrowser''' instead to get a reference to tabbrowser://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"> <script src="overlay.js"/> </overlay></pre>

Load Listener codeIf we want to get the tab to the right of the current tab, we simply do this:

http://developer var positionPlusOne = gBrowser.mozillatabContainer.org/en/docs/Extension_Frequently_Asked_Questions#Why_doesn.27t_my_script_run_properly.3FselectedIndex + 1;

Reading through the rest of the '''moveTab''' method, we see that when a ===Saving tab is successfully created, the '''TabOpen''' event is dispatched position state===

(see It's clear that we need data from the past--the position of the tab we were on before this new tab was created. We can't go back in time, so we'll have to store this data in a variable for inspection later..)

// Dispatch a new tab notification. We do In order to accomplish this once , we're // entirely done, so need another event that things are in a consistent state // even if the event listener opens or closes tabs. var evt = document.createEvent("Events"); evt.initEvent("TabOpen", true, false); t.dispatchEvent(evt); return t; This is useful information, because it tells us that if we want to know when a tab has been created, become the active tab so we have to add a listener can store the position state. A quick search through tabbrowser for this eventother events (e.  moveTabTo http://lxrg.mozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml#1958  Create a file named: , search on '''addtabbeside@senecac.on.ca"dispatchEvent"''' This file should contain the full path to your extension, for example:  C) shows these possibilities:\temp\addtabbeside

The first event looks interesting. C:\Documents and Settings\All we need to do now is have a variable that gets updated with the value of ''Username'tabContainer.selectedIndex'\Application Data\Mozilla\Firefox\Profiles\''dev-profileevery time ''\extensions'TabSelect''' occurs. Then, we can use this variable's value to calculate the desired position in '''moveTabTo'''.

To keep things clean we'll write all this code in a custom object called ''overlay'AddTabBeside'''. Here is '''addtabbeside.js''':

// when When a different tab is selected, remember which one. This is

ItWe need to create 's one thing to say you'd like to change the browser's behaviour, but quite another to actually do itaddtabbdeside/chrome/content/overlay. The change you have in mind might be quite simple, in the end (ours is). But you still have to figure out where that simple code needs to go. That can be difficult. However, difficult isnxul'''t the same as impossible.

How do you begin<pre><? First, let's start at the top and find some UI notation we can search for in the codexml version="1. In our case, we can focus on the various methods for creating a new tab0" encoding="UTF-8"?> <overlay id="addtabbeside-overlay" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"> <script type="application/x-javascript" src="addtabbeside.js"/> </overlay></pre>

The second Having added our script and third methods are useful, as they provide us with a unique string we can search for in the code. Before we can change anythingoverlay files, we have now need to search and read existing code in order add a couple of metadata files to help Firefox understand where to begin--this is the standard pattern for open source and Mozilla developmentinstall/load our extension.

==Search 1 - finding The first is '''addtabbeside/chrome.manifest''', which is a UI string==[http://developer.mozilla.org/en/docs/Chrome_Manifest Chrome Manifest]. This file helps Firefox translate between chrome:// URIs and actual files on disk. It also allows us to specify where our overlay will be merged into the browser:

We # Chrome package addtabbeside has it're looking for a unique string--"New Tab"==, so we'll use [http:s content in ./chrome/content content addtabbeside chrome/content/lxr # Overlay the browser.mozillaxul file with overlay.org LXR's] '''Text Search''' featurexul overlay chrome://browser/content/browser. Here are the results you get when you search for "New Tab"xul chrome://addtabbeside/content/overlay.xul

<blockquote>httpThe first line registers the location for our content (i.e., .xul, .js). The second line registers our overlay, and says that overlay.xul will be merged with browser.xul. Mozilla uses chrome://lxr.mozilla.orgURIs to refer to aspects of the interface, and chrome://seamonkeybrowser/search?string=New+Tab<content/blockquote>browser.xul ''is'' the browser (try typing it into the address bar).

Lots of results, many of which point The second metadata file we need to comments in the codecreate is '''addtabbeside/install.rdf'''. HoweverThis is an [http://developer.mozilla.org/en/docs/Install_Manifests install manifest] that tells the Firefox add-on manager how to install the extension, with information like who wrote it, the first result looks interesting:version number, compatibility information, etc.

<blockquotepre><?xml version="1.0" encoding="UTF-8"?><RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:em="http://lxrwww.mozilla.org/seamonkey2004/sourceem-rdf#"> <Description about="urn:mozilla:install-manifest"> <em:id>addtabbeside@senecac.on.ca</toolkitem:id> <em:name>Add Tab Beside</localesem:name> <em:version>0.1</enem:version> <em:creator>David Humphrey</em:creator> <em:description>New tabs are created beside the current tab instead of at the end of the tab list.</em:description> <em:targetApplication> <Description> <em:id>{ec8030f7-USc20a-464f-9b0e-13a3a9e97384}</chromeem:id> <!-- firefox --> <em:minVersion>2.0</globalem:minVersion> <!--<em:maxVersion>3.0a9pre</em:maxVersion>--> <!-- trunk build Oct 14, 2007 --> <em:maxVersion>3.0+</tabbrowserem:maxVersion> <!-- work for v3.dtd#20 and above --> </Description> </em:targetApplication> </Description></RDF></blockquotepre>

Here we see ===Testing the DTD file describing the key/value pairs for the en-US localized strings. Mozilla uses this technique to allow localizers to translate strings in an application into many different languages without having to change hard-coded strings in the code (you can read more about localization, DTDs, and Entities [http://developer.mozilla.org/en/docs/XUL_Tutorial:Localization here]) extension===

Looking closely at Eventually we'''tabbrowserll package our extension properly into a redistributable .xpi.dtd' However, while we're testing it' s nice to be able to use it in an expanded form so we see that our English string, "New Tab", has the following entity: can make changes.

<!ENTITY newTabTo this end, create a file named '''addtabbeside@senecac.label "New Tab">on.ca''' and put it in your development profile's extensions directory (NOTE: replace ''Username'' with your username and ''dev-profile'' with your development profile name):

This is good information, because it allows us to repeat our search with an entity instead of a string, which should help us get closer to the code we C:\Documents and Settings\'re after'Username''\Application Data\Mozilla\Firefox\Profiles\''dev-profile''\extensions\addtabbeside@senecac.on.ca

<blockquote>httpThis file should contain a single line of text--the full path to your extension, for example://lxr.mozilla.org/seamonkey/search?string=newTab.label</blockquote>

Not surprisingly, the first result is the same DTD file (i.e., tabbrowser.dtd) we already found. The second result looks interesting, though: <blockquote>http://lxr.mozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml#80</blockquote> Here we see the code to generate the pop-up context menu for a tab (i.e., what you get when you right-click on a tab in the browser):  <pre> <xul:menuitem label="&newTab.label;" accesskey="&newTab.accesskey;" xbl:inherits="oncommand=onnewtab"/> </pre> Having found the appropriate entity value, we also notice the use of a function name, '''onnewtab'''. This line of code says that the xul:menuitem will inherit the '''oncommand''' value from its parent (you can read more about XBL attribute inheritance [http://developer.mozilla.org/en/docs/XUL_Tutorial:XBL_Attribute_Inheritance here]). In other words, when this menu item is clicked, call the '''onnewtab''' function. ==Search 3 - finding a Function== Armed with this new information, we are even closer to finding the right spot to begin working. We've gone from UI string to XML ENTITY to function. All we have to do now is find that function: <blockquote>http://lxr.mozilla.org/seamonkey/search?string=onnewtab</blockquote> This returns many results for things we aren't interested in, including files rooted in /suite, /db, etc. Since we are interested in finding this behaviour in Firefox, we need to focus on the files rooted in '''/browser'''. One looks particularly interesting: <blockquote>http://lxr.mozilla.org/seamonkey/source/browser/base/content/browser.xul#503</blockquote> In this case, the tabbrowser widget has the onnewtab property set to another function, '''BrowserOpenTab();''' (i.e., Firefox seems to handle tab creation in a non-standard way, providing its own method instead of using the default). Since we want to find the definition of this function, we search for '''"function BrowserOpenTab("''', which returns two results: <blockquote>http://lxr.mozilla.org/seamonkey/search?string=function+browseropentab%28</blockquote> Again, we're interested in Firefox (i.e., browser) instead of SeaMonkey (i.e., suite), so we skip to the second result: <blockquote>http://lxr.mozilla.org/seamonkey/source/browser/base/content/browser.js#1802</blockquote> This shows us that we need to be looking for yet another function, '''loadOneTab()'''. Another search: <blockquote>http://lxr.mozilla.org/seamonkey/search?string=loadonetab</blockquote> The first result is not surprising, and we're back to the tabbrowser widget. The '''loadOneTab''' method calls another method to actually create and insert the new tab:  var tab = this.addTab(aURI, aReferrerURI, aCharset, aPostData, owner, aAllowThirdPartyFixup); Since '''addTab''' is a method of '''this''' we can search within the current document (CTRL+F) to find the '''addTab''' method. Finally we've found the right spot! <blockquote>http://lxr.mozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml#1160</blockquote>  this.mTabContainer.appendChild(t); Now all that we have to do is modify it to insert rather than append. =The 'How': the necessary changes to the code= There are different ways you could go about making this change, and someone with more experience using tabbrowser might recommend a different strategy or outcome. I decided to work on something that I knew nothing about in order to highlight the process one goes through, or at least the process I went through, when working with someone else's code. Since my goal is to show you how to do this, I also discuss my errors and mistakes below--they are an important part of the process too. ==First Attempt== The goal is to make as small a change as possible, since the existing code works well--I just want it to work slightly different. I'm also not interested in reading all of the code in order to make such a small change. I want to leverage as much of what is already there as I can. I assume that the '''appendChild()''' method is responsible for the behaviour I don't like (i.e., adding new tabs to the end of the list). I'm not sure what to replace it with, so I do another search inside tabbrowser.xml (i.e., using CTRL+F) looking for other methods/attributes of '''mTabContainer'''. I come-up with some interesting options:  index = this.mTabContainer.selectedIndex; ... this.mTabContainer.insertBefore(aTab, this.mTabContainer.childNodes.item(aIndex)); ... var position = this.mTabContainer.childNodes.length-1; I decide that I can probably accomplish my goal using these alone, and so start working on a solution. Here is my first attempt, showing the changes to '''mozilla/toolkit/content/widgets/tabbrowser.xml''' and the '''addTab''' method:  // Insert tab after current tab, not at end. if (this.mTabContainer.childNodes.length == 0) { this.mTabContainer.appendChild(t); } else { var currentTabIndex = this.mTabContainer.selectedIndex; this.mTabContainer.insertBefore(t, currentTabIndex + 1); } I then repackage the toolkit.jar file (change ''objdir'' to your objdir name)C: \temp\addtabbeside

Start your browser and make sure your extension is loaded (check '''Tools > Add-ons''') and working properly by creating some tabs to see where they get positioned. $ cd mozilla/If you're having problems, check the '''Error Console'objdir''/toolkit/content $ make('''Tools > Error Console''') for hints.

then run ===Packaging the browser to test (NOTE: ''minefield'' is my testing profile):extension===

$ ..Once debugging and testing is complete, you'll want to [http://developer.mozilla.org/disten/bindocs/firefoxExtension_Packaging create an installable .xpi file] to give to your users. As was previously mentioned, .xpi files are just .zip files with a different extension. All .xpi files must contain the '''install.rdf''' file in the root (i.e., when you unzip it, install.rdf should be in the top directory).exe -p minefield --no-remote

I try Follow these steps to create a new tab using '''File > New Tab''' and nothing happensan installable .xpi:

Clearly my code has some problems, since I've completely broken addTab. I decide to look for clues You can try installing your extension in the '''Error Console''' (Tools > Error Console) and notice the following exception whenever I try to add a new tab: <code>Error: uncaught exception: [Exception... "Could not convert JavaScript argument" nsresult: "0x80570009 (NS_ERROR_XPC_BAD_CONVERT_JS)" location: "JS frame :: chrome://global/content/bindings/tabbrowser.xml :: addTab :: line 1161" data: no]</code> I make a guess browser that childNodes.length is not zero, but 1 by default (i.e., there is always at least one tab, even if it isndoesn't visible). A quick modification to the code, and I test again:  if (this.mTabContainer.childNodes.length have it by simply dragging '''== 1addtabbeside.xpi''') { into it. ... ==Third Attempt== This works, but only should trigger the first time I create a new tab. Clearly I still have some misconceptions about how '''mTabContainer.selectedIndex''' add-on manager and '''mTabContainer.insertBefore()''' really work. I can't yet see how my code is wrong, but give you the exception I'm getting clearly indicates that I've got some sort of type conversion problem. I decide option to look again at the code examples in tabbrowser.xml that I'm using as a guide, specifically '''insertChild()'''. After a few seconds the error is obvious: I've used an Integer where a Tab was required. Here is install your extension and restart the corrected code:  // Insert tab after current tab, not at end. if (this.mTabContainer.childNodes.length == 1) { this.mTabContainer.appendChild(t); } else { var currentTabIndex = this.mTabContainer.selectedIndex; this.mTabContainerbrowser.insertBefore(t, '''this.mTabContainer.childNodes.item(currentTabIndex + 1)'''); }

After repackaging the toolkit.jar file and running the browser, I'm able The rewrite to confirm that this last change an extension has been successful. Opening a new tab now works in the way I originally describedsuccess. I make a few more tests In both cases we've managed to insure that I haven't broken anything else, for example, what happens if I am on achieve the last tab and not in the middlesame goal using almost completely different methods. This works, which makes me realize that using Using an extension we'''append()''' is probably not necessary at all, and I can safely shorten my code down ve made it possible to share our changes with any Firefox user worldwide without having to the following:ship a custom build.

// Insert tab after current tabHowever, as was the case in our previous attempt, not at endour code has a bug. var currentTabIndex = this.mTabContainer.selectedIndexMoving existing tabs doesn't update our position state, since we only modify '''mPreviousIndex''' when a new tab is selected; thismoved tabs remain selected, but change their order (i.mTabContainere.insertBefore(, TabSelect won't, thisget dispatched on a move).mTabContainer.childNodes.item(currentTabIndex + 1));

This means that six lines Luckily we've already stumbled upon the solution to this problem--the '''TabMove''' event. Here is the updated version of code become two'''addtabbeside.js''', and with that reduction in number of lines, hopefully a reduction the changes in new bugs I've added (NOTEbold: within reason, favour fewer rather than more lines of code).

Speaking of bugs, a closer read of '''addTab''' (see [http: var AddTabBeside = { //lxrState info on the last tab to be selected.mozilla.org mPreviousIndex: 0, onLoad: function() { /seamonkey/sourceAdd a listener for the TabOpen event, which gets called as /toolkit/content/widgets/part of addTab in tabbrowser.xml#1219 line 1219] var container = gBrowser.tabContainer; container.addEventListener("TabOpen", this.onTabOpen, false) would indicate that ; // Also add a listener for TabSelect so we've introduced know when focus changes to a few with our new positioning code:tab container.addEventListener("TabSelect", this.onTabSelect, false); '''// wire up And a progress listener for the new browser object.TabMove to fix an edge case''' var position = '''container.addEventListener("TabMove", this.mTabContainer.childNodes.length-1onTabSelect, false);''' var tabListener = this // Finally, add a listener for shutdown window.mTabProgressListeneraddEventListener(t"unload", bthis.onUnload, blankfalse); }, onUnload: function() { // Remove our listeners var container = gBrowser.tabContainer; container.removeEventListener("TabOpen", this.onTabOpen, false); container.removeEventListener("TabSelect", this.mTabListeners[position] = tabListeneronTabSelect, false); '''container.removeEventListener("TabMove", this.mTabFilters[position] = filteronTabSelect, false);''' }, onTabSelect: function (e) { // When a different tab is selected, remember which one. This is // necessary because when a new tab is created, it will get pushed // to the end of the list, but we need to know where to put it. this.mPreviousIndex = gBrowser. ttabContainer._tPos = positionselectedIndex; }, Where the assumption before was that onTabOpen: function (e) { // Get the newly created tab was at , which will be last in the list var newTab = e.target; // Move this new tab to the end right of the listpreviously selected tab, // checking to see how many tabs there are currently. By default // there is 1 tab, and the new code breaks thatfirst time onTabOpen is called, there will // be 2 (the default plus the newly created tab). ThereforeIn this case, we also need to update the value of '''position''don't // move the new tab, since it is already in the right spot. In all // wire up a progress listener for other cases, move the tab to the right of the new browser objectcurrent tab. if (gBrowser.tabContainer.childNodes.length > 2) { var position = currentTabIndex gBrowser.moveTabTo(newTab, this.mPreviousIndex + 1); } }, };

No other obvious defects are visible from // Insure that our changescode gets loaded at start-up window.addEventListener("load", function(e) { AddTabBeside.onLoad(e); }, false);

The change I was making was simple enough that I didnMany of the steps we did in order to create the extension't bother looking at any documentation or using s installation and registration files will be the JavaScript debuggersame each time. I found out afterward that tabbrowser has good Rather than doing it by hand, you can use [http://developerted.mozillamielczarek.org/encode/docsmozilla/XUL:tabbrowser documentation extensionwiz/ an on MDC-line wizard]. Another trick worth trying when you're making lots of JavaScript changes like this is to add the following line to your build a basic extension.mozconfig file:  ac_add_options --enable-chrome-format=flat

This will cause The process of developing extensions is greatly improved with the .jar files to be expanded so that you can edit the .xml/.js/.xul files in place and skip the repackaging step above (see addition of [http://wwwkb.mozillamozillazine.org/build/jar-packaging.html). If you also use Setting_up_extension_development_environment some preferences] to the browser, as well as the [http://ted.mielczarek.org/code/mozilla/extensiondev/index.html Extension Developer's extension] . This extension will automate many of the steps described here. It will also allow you can to reload the browser's chrome at runtime. This is a great way to test changes to your XUL/JS files without restarting having to restart the browser.