Because the project chooses QtWebKit as its rendering engine, I think some Kitten may have an interest. Actually the browser is making some interesting design decision, which is worth covering here.
HTML as Browser UI
You must have heard about Gecko, the rendering engine of Firefox. Gecko invented an HTML-like XML vocabulary called XUL for building its whole UI. Not only Firefox but also most of Mozilla originated desktop products are built on top of XUL.
Gecko (including XUL) by itself is a platform for running desktop applications. This approach is totally different from WebKit-based browsers, which generally employ OS-native toolkits. For example, Safari is using Cocoa and Windows Chromium is using Win32. WebKit is just an HTML renderer for these browsers, even though that is a significant construct.
However, Isis browser — which is also based on WebKit — chooses another approach: Building its UI using HTML. It’s kinda like XUL, but without any custom-made markup vocabulary.
BrowserAdaptor and BrowserServer
Then the question is how it gets stuff work. How can we navigate web sites inside HTML?
Are they use
<iframe>? It might be possible. But it won’t be sweet
because WebKit doesn’t support any process isolation for
<iframe>, which is one of the essential requirements of modern browsers.
And here an interesting idea arrives: Isis addresses this requirement by implementing a browser plugin, which renders HTML using WebKit!
They call it BrowserAdaptor. BrowserAdaptor is a plugin, which accepts NPAPI to run inside WebKit. But it can render HTML by itself, using (again) WebKit. With this plugin, Isis can render its UI in the topmost WebKit, while rendering navigating Web pages through WebKit inside BrowserAdaptor.
The plugin browser supports process isolation. This process-isolated renderer is called BrowserServer. It’s hosted in a separate git repo. BrowserAdaptor delegates page rendering to BrowserServer.
“Process isolated WebKit!” You kitten might get excited and ask, “So do they adopt WebKit2?” No. They don’t. They have their own IPC mechanism, their own serializer, code generator and their own lifecycle model.
Talking about lifecyle, BrowserServer is more like “real” server than
WebProcess of WebKit2 or
renderer of Chromium.
It is designed for running as a linux daemon. It has a boot script for Upstart.
Its diagnose message goes through
syslog. Its resource is managed under cgroups.
This design might make it hard to port Isis to non-Linux platforms.
It could be a affordable price though, if it can fully utilize Linux kernel ability.
What clear is that Isis is taking a step into a frontier no other Kitten has explored, where the boundary between UI and page is blurring. Honestly I thought that such approach is possible only with platformish architecture like Gecko or WPF. But Isis demonstrates it being possible even for librarish WebKittens, if you hole the page somehow.
Coincidentally, B2G — a Gecko-based mobile platform developed by Mozilla — is following a similar path as Isis: They are building mobile applications using HTML. Their built-in applications include phonecall, address book and Web browser. Yes, B2G is building its Web browser on top of HTML! They don’t use XUL for some reason.
Then same question arises. That is, how do they achieve process isolation?
Implementing Gecko plugin? Or they just give it up and use
Neither. They use
<iframe> and they have process isolation.
As a part of their Electrolysis effort,
they’ve implemented process isolation mechanism for
<browser> element in XUL.
<iframe> for XUL. But once you give
@remote attribute to it, Gecko creates a dedicated process for the element to render its content.
I can imagine that it was a tough work. But that is Mozilla. Their hardcore hackers managed to do it.
Even though they are suspending the effort
for reasons, they finished underlying plumbing.
And this time, they bring it to
explains that each
<iframe> with special
@mozbrowser attribute is going to have dedicated process.
The role of mozbrowser-ed iframe is almost same as Isis’s BrowserAdaptor plugin.
But it comes with more Web-looking face.
I’m wondering why these two mobile platforms adopt HTML as their GUI toolkit, regardless no desktop browser has done it. Is it really coincidence?
My humble guess is that’s because desktop applications need windows, chromes or frames, and lack of ability to control it from HTML prevented that choice. For each web page, its windows is given by the browser. And every modern browser does hard work to create unique looking, slick feeling frames. Only few browser adopt platform’s default frames.
Mobile is different. Basically every application runs fullscreen. That means it doesn’t need any cool frames, and that’s why tools like PhoneGap are welcomed. Isis and B2G are also in same party; They live in the fullscreen world where cutting-edge HTML shines.
Of course, there must have been more general lack of ability other than frame stuff. HTML didn’t have
flexbox for example.
So its overall improvement would’ve also helped to revert the dated decision.
By the way, is the out-of-process
<iframe> or plugin browser applicable only for building browsers? I don’t think so.
Any web page which is mashing up widgets using
<iframe> possibly could take advantage of it.
It is true that
<iframe> is adopted broadly. But that doesn’t mean there is no shortcoming of that.
In reality, it’s slow, it drains memory and its isolation wall is fragile.
This is because
<iframe> shares many structure with its host page.
If it lives in a separate process, this situation will drastically change. Each host page will be no longer hurt by its rogue embedees anymore because their rendering will happen in separate processes. They don’t block its parent rendering. Memory will be easily reclaimed for each of them closed. You no longer need to worry about memory leaks nor crashes of child frames.
With out-of-process iframe, mashup ability of browsers will advance to the next level. I hope someday it gets standardized and becomes available not only for Web “browser” developers, but also (non-browser) Web developers. It will be tough job to implement it in WebKit. but wizard kittens will make it happen if people want.
P.S. My friends who loves Microsoft stack told me that it was called OLE in their world and it supported both out-of-process hosting and IPC. Well, there are always something worth learning from ancient literature. Agreed … and ordered.