refactor: default to file scheme

[erisu]

Motivation and Context

Use file as default scheme to avoid break in current apps/

Description

1. defaults to file if preference scheme is not set.
2. continue to uses file if preference scheme is set to file.
3. use what ever value the user defines in as preference scheme as a custom scheme handler, excluding file which was defined in the above case 2.
4. preference hostname is only used when scheme is not file.
5. CDV_ASSETS_URL is set to <scheme>://<hostname> -> if file, hostname = “”. -> if !file, hostname = "localhost" or user define value for preference hostname.
6. Extension of above case 3, if scheme is set and is not file, it will also validate against WKWebView handlesURLScheme, if it is not valid, it will default to app

Testing

• cordova build
• run in simulator

Checklist

• [x] I've run the tests to see all new and existing tests pass

[codecov-commenter]

Codecov Report

Merging #866 into master will not change coverage. The diff coverage is n/a.

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  Coverage   74.40%   74.40%           
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  Files          13       13           
  Lines        1676     1676           
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  Hits         1247     1247           
  Misses        429      429           

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[ghost]

I have a hard time understanding how to use custom scheme with cordova-ios 6, specifically the behaviour introduced here :

In the description of the behaviour by erisu, it says :

it will also validate against WKWebView handlesURLScheme, if it is not valid, it will default to app

But in the code, the actual condition is :

if(scheme == nil || [WKWebView handlesURLScheme:scheme]){
     scheme = @"app";
}

which, to my understanding of Objective-C, means that if it IS valid, it will default to app, cf. WKWebView handlesURLScheme documentation here

Could someone clarify this to me ?

[breautek]

During the course of development, defaulting to custom scheme was changed to default to the file url.

The reasoning is because changing protocols will cause users to be missing their web storage data. So we wanted to make schemes an opt-in feature instead.

I don't think this feature is documented yet, but an example config to use schemes can be found at https://github.com/apache/cordova-ios/pull/781#pullrequestreview-357408875

[jcesarmobile]

handlesURLScheme returns true if the webview already handles the scheme, if the webview already handles that scheme, it means we can't use that scheme for serving the app content, so if you set the scheme to http, https, file or a few others, handlesURLScheme is true and we default to app, if it returns false, it means whatever you configured is safe to use.

[lovelyelfpop]

Cordova-ios@6.1.0 says 'refactor: default to file scheme', why the url of my app is still myscheme:// not 'file://'? How to open file:// url while still remain <preference name="scheme" value="myscheme" />?