loading polymer app into an iOS WKWebView fails

[Jim Trainor]

Polymer applications fail to load in an iOS WKWebView, as supported by Cordova. The root cause of the problem is that WKWebView does not permit loading of content via file:.  Cordova's support for WKWebView bypasses this restriction by implementing a in-app web server to host the static content. Polymer fails because the polymer code tries to load a local, static, html file, using a file:// url via an XHR request. The file:// XHR request is rejected by WKWebView.

This has turned into a blocking issue for publishing Polymer applications, using Cordova, on iOS.

Does this sound familiar to anybody?

Is there anyway to cleanly define the URL where Polymer loads elements (or whatever is loaded via the file:// XHR)?

Here is the Apache bug: Allow WKWebView to proxy file:// url loading in XmlHttpRequest.open

[Karl Tiedt]

Nothing in polymer appears to reference file:// anything... this sounds like it may be a Cordova issue?

-Karl Tiedt

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[Jim Trainor]

 am about to revisit this in depth… but… the last time I dug in there was definitely an XHR load of an html file originating from polymer (from the web component.min.js, actually, as I recall, which I am classifying generically as “polymer”).

As I recall the XHR request was loading html web components from its host (I think it was loading the entire vulcanized html file). If that host url happens to be file:// then that is what it used for the XHR request.  The problem is… WKWebView rejects that access… so no-go for the Polymer app.

If the page is loaded via an ordinary http request then all is well. The exact same code loading via an Android WebView is fine also.  The problem is limited to security restrictions in WKWebView. See the bug link that I posted for more info.

[Eric Bidelman]

On IOS (a polyfilled browser), the web components polyfill loads html imports using XHR. This means that from a webview, they're loaded from file:///, which is typically blocked by browsers/environments for security reasons.

Unsure if there's an equivalent in ios, but in android webview, you can override this setting:

https://www.polymer-project.org/0.5/articles/webview.html#tweaking-the-webview-settings

The other option is to vulcanize your entire app (vulcanize index.html -o index.v.html) which removes all import requests.

To view this discussion on the web visit https://groups.google.com/d/msgid/polymer-dev/CACDv4tDu6JrtF1TQcv_KtB1V_W%3Dfouhtbrv_AV6SaMGeLMTH9Q%40mail.gmail.com.

[Jim Trainor]

I am not aware of any override on WKWebView that permits file:// access.

I’m the second person to have posted this as a bug on the  Apache Cordova bug tracker. It is not hard to file other reports of people stumbling over the lack of file:// support in WkWebView.  The embedded web server approach seems to be the usual solution.  But that doesn’t capture the file:// XHR requests.

… so… hmm…. head scratch… seems to be a blocker at the moment.

[Eric Bidelman]

Or the other suggestion:

> The other option is to vulcanize your entire app (vulcanize index.html -o index.v.html) which removes all import requests.

To view this discussion on the web visit https://groups.google.com/d/msgid/polymer-dev/CACDv4tDUgatzLXBSXGkjJn7xWomi6%2By-uQCGF0JN1i_mwJ%2B8aQ%40mail.gmail.com.

[jim.t...@wellbalancedmeal.com]

Thanks for reiterating. I will try that suggestion.