I tried to create a content blocker for Safari and did it through Safari App Extension and Safari Web Extension.
The XCode project for Safari Extension creates 2 applications: the “Main Application” (which is compiled into ".app") and the “Safari Extension” itself (which is compiled into ".appex").
If you want to create a content blocker, you must add “Content Blocker Extension” from the “File → New → Target” menu. This will be the third application in the group, which is also compiled into "*.appex".
And now about differences...
The first difference is that Web Extension does not have the right to work on any domains by default, unlike App Extension. For the App Extension, in the "info.plist" in the "SFSafariWebsiteAccess" section, we can remove "Allowed Domains" and in "Level" replace "Some" to "All". This is important because the content blocker should work on any domain, and not be turned on manually for each individual. This will not work in Web Extension and will have to be activated for each domain separately.
The second significant difference between Web Extension and App Extension is that for security reasons it is more limited in page management.
Web Extensions are supposedly focused on a common API for all browsers (Firefox, Chrome, Opera, Edge, Safari), but if I use Safari and want maximum integration with it, then I don't care about all those browsers. In this case, I don't need Web Extension.
App Extension functions as a client-server even within the “Main Application”. This is quite expensive and can be fixed. You can remove the “webView” component from storyBoard and remove this files: “script.js”, “main.html”, “style.css”. After this you will get the pure storyBoard without any tinsel and communication between these layers. That’s will work perfectly because these are native Swift applications.
p.s. To show the "Safari Extension" window after clicking on the icon on the toolbar, you need to change key from "Command" to "Popover" in the "info.plist" in the section "SFSafariToolbarItem" in the "Action".
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