So WKWebView's dynamically generated code is running safely firewalled in a separate address space controlled by Apple and not accessible to your app, while older UIWebViews run in the address space of your application, and aren't allowed to write to code pages, so their JIT compiler is disabled.
Since it's running in another process, WkWebView's JavaScriptEngine lacks the ability to expose your own Objective C classes to JavaScript so they can be called directly [2], but it does include a less efficient way of adding script message handlers that call back to Objective C code via IPC [3].
[1] https://developer.apple.com/reference/iosurface
[2] https://developer.apple.com/reference/javascriptcore/jsexpor...
[3] https://developer.apple.com/reference/webkit/wkusercontentco...
Is that true? I understand the security argument against JITs, but I don't see why it would be objectionable to run a copy of e.g. Gecko if you excluded the JS JIT component. (Of course you would never actually do that. But I'm trying to tease apart the separate arguments and see if they're actually connected.)
It seems to me that the root cause for this is all the JIT. If the JIT in iOS has to be exposed via IPC for security (as claimed in a sibling comment) it's simply not going to be a win to try to use it in combination with a separate rendering engine, if it's even possible at all.
So you can maybe ship Gecko + SpiderMonkey, as long as you only run JS that's part of your app. But if you want to run JS from a website (or indeed any code from anywhere), you're not allowed to do that on iOS.
I haven't checked whether it would be OK to ship a browser engine that doesn't support JS at all, but that's a fairly academic question anyway.
I guess what's bothering me is that there is an unstated assumption underlying this entire discussion which people never bring up: for performance reasons, the rendering engine has to be tied closely to the JS implementation. Specifically, calling over some IPC interface at every entrypoint into the JS interpreter would be a performance nightmare. This is the reason why these can't be disentangled. The connection to the interpreter is really an indirect one.
That is not true. Many apps do this. Games have been doing this for a decade. It is not a problem to do this. Apple has never made a big deal of that. Only if you are trying to build a shadow app store.
Apple does not care. As long as you are operating in the spirit of their App Store.
You know when they care? When you start charging money for those downloads. Via an alternative payment channel. Or when you run an alternative app store from within your app.
If you download assets on demand, and some of those turn out to be executable* code, nobody cares.
* Where executable means 'interpreted' because you cannot run unsigned code. But for games that is fine, they have been doing this with JS, Lua and Python for a decade.
(You can downvote me - but this is based on a decade of experience)
The subject we're talking about here would not fly under their radar.
Pebble used this quite effectively to add phone-side apps to their product.
It is. Mozilla has an iOS port of Gecko. It runs well, even without the JIT, but we are not allowed to use it.
https://hg.mozilla.org/mozilla-central/file/tip/embedding/io...
Sorry I dont know where we keep the build instructions.
> 2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript.
On iOS browsers like Chrome and Firefox are basically unable to compete in terms of performance, web standards or features like browser plugins. So people that use them do so just to get sync. Of course Safari Mobile is decent nowadays, but that's not the issue.
iOS 9 eliminated that performance penalty-- it introduced a new kind of web view backed by an out-of-process renderer and JS engine, which, since it's Apple-blessed, doesn't have the JIT restriction. Third-party browsers updated to use the new framework can get the same performance as Safari.
But that framework is missing various things, such as content blocking. (That goes a long way to explain why there's both "Focus" and "Firefox" on iOS)
Not really - SFSafariViewController uses content blockers, autofill, etc. There's also WKWebView for those that want to implement their own content blockers and autofill engines.
Curious why you'd want safari's content blockers to run inside chrome or firefox though? That's not how browser plugins work anywhere else, and it seems like if content blockers were forced in to 3rd party browsers there'd be a lot more angst over that decision.
[1] https://developer.apple.com/library/ios/documentation/Safari...
Because WKWebView has such a limited API that it currently is impossible to implement content blocking.
WKWebView is both a blessing and a curse for browsers like Chrome and Firefox.
A blessing because tt works really well inside that square area where content is rendered. It is as fast as Safari, as responsive to touch gestures, supports web APIs pretty well.
A curse because the API that it exposes is completely lacking. No way to intercept network requests, no way to interact with the DOM, no way to interact with JavaScript, no way to customize it fully, no way to do proper session store/restore, no way to interact with its internal caches, no way to set the cookie policy, no way to customize the context menus that it exposes, no way to handle errors better, no way to set headers on requests, no way to use content filters ..
I can go on for a while. You can look at the Chrome and Firefox source code to understand the crazy workaround both browsers had to do to get some pretty basic functionality.
Search for WKWebView on https://bugz.webkit.org to see what people have been asking for, for the past three years.
Because there's currently no way for those browsers to get any content blocking without switching to the slower WebView.
I'm fairly confident that both the Google Chrome team and Mozilla's Firefox for iOS team are better informed here than you are.
Apple has decided that Content Blocking is a Safari only feature. Apple has made the decision to not expose the private APIs in WKWebView that would allow browsers like Chrome and Firefox to use the same speedy content blocking.
The old WebView, UIWebView, also does not support content blocking. But since it runs completely in the same process as your app, you can replace the HTTP handling with your own and patch content blocking on top of that.
That is what Firefox Focus and Brave do for example. It works, but it is far from ideal. Also, it is likely an API on the way to be deprecated.
SFSafariViewController is useles for apps like Chrome and Firefox. It is an embedded Safari controller. Not an embedded generic webview controller.
SFSafariViewController has zero API. All it can do is display a webpage. Applications that use it have zero ability to even modify the UI.
That is intentional. It is supposed to be a Safari view. And for security reasons that is probabl good, otherwise it would be a gateway to your browsing data.
Mozilla also made a statement about it: https://motherboard.vice.com/read/heres-why-firefox-for-ios-...
It is not something that WKWebView automagically does.
While it is technically possible to use content blockers in WKWebView, it can only be done via private APIs, and Apple capriciously bans apps from their appstore that use private APIs.
See section 2.5.6 of the Appstore guidelines
"2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit Javascript."
And regarding your quesiton: I'm sure you cannot replace their webkit usage with one switch to Blink... I'm sure the iOS internal webview is deeply integrated/
You can't "locally compile" code on iOS. That's against App Store guidelines.