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To build a content blocker for an app’s own WKWebView, describe rules as JSON, compile them with WKContentRuleListStore, then attach the compiled list to that web view’s WKUserContentController. To block content in Safari itself, build a Safari Content Blocker extension instead. The APIs and deployment paths differ; choosing the right one is the first performance decision.
Choose the WebKit path that matches where blocking must happen
“Safari content blocker” can refer to distinct mechanisms. An app-owned web view and the Safari browser do not share one deployment recipe.
| Target | Mechanism | What to know |
|---|---|---|
| An app’s own embedded web content | WKContentRuleListStore and WKContentRuleList |
Compile and store a rule list, then explicitly add it to the content controller for the target web view. Stored rules are not automatically applied to every web view. |
| Safari browser content | Safari Content Blocker app extension | Provide Safari a declarative JSON rule list in the content-blocker format. Apple says this mechanism does not give the blocker knowledge of users’ browsing history or the websites they visit. |
| Safari browser functionality built as a web extension | Safari web extension declarative net request API | This is a separate API and rule format, with capabilities and permission requirements that vary by Safari version. |
Apple documents the WKContentRuleList and WKContentRuleListStore APIs for app-owned web views, and separate guidance for creating a Safari Content Blocker and Safari web extension declarative net request.
Compile and apply rules to an app-owned WKWebView
The WebKit workflow has two separate jobs: compile or retrieve the rules, then attach them to the specific web view. Compilation produces an efficient byte format; storing a list does not, by itself, change page loading in a web view.
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- Prepare the rule JSON. Use the content-blocker rule syntax supported by WebKit. A rule contains a
triggerdescribing matching requests and anactiondescribing what to do. - Compile the JSON. Call
WKContentRuleListStore.compileContentRuleList(forIdentifier:encodedContentRuleList:completionHandler:)with an identifier and the encoded JSON. Handle completion by checking for an error before using the result. - Retain or retrieve the compiled list. The store persists existing lists, so an app can reuse a compiled list rather than recompiling unchanged rules each time it creates a web view.
- Attach the list to the target web view. Add the compiled
WKContentRuleListto theWKUserContentControllerassociated with that web view’s configuration. Apply only the lists needed for that web view.
Compile when rule data changes, not as a substitute for applying the list to each relevant web view. This follows from Apple’s compile/store/apply lifecycle; Apple’s documentation does not publish a universal performance gain or a numeric rule-count limit for this workflow.
Design triggers and actions for the intended behavior
Safari Content Blocker rules are a JSON array. Each rule has a required url-filter within its trigger; additional trigger conditions can narrow matching by domain, resource type, load type, top-level URL, or frame URL. Scope matches deliberately so a rule affects only the requests or page contexts it is meant to cover.
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Do not assume that Safari Content Blocker JSON can be copied unchanged into a Safari web extension declarative net request implementation. The web extension API has its own format and version-dependent behavior.
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| Safari Content Blocker action | Effect | Design consideration |
|---|---|---|
block |
Prevents a matching resource from loading. | Use when the desired outcome is to stop the request, rather than merely hide something visually. |
css-display-none |
Hides matching page elements through CSS. | Hiding an element is not the same as blocking the network resource that supplied it. |
block-cookies |
Applies cookie-blocking behavior subject to Safari’s privacy policy. | Do not treat this as interchangeable with blocking a resource; cookie behavior is constrained by Safari’s policy. |
make-https |
Upgrades eligible HTTP URLs to HTTPS. | It changes eligible URL schemes; it is not a general-purpose block action. |
ignore-previous-rules |
Stops earlier matching rules from determining the result in the applicable case. | Position and matching conditions matter; use it to express an intentional exception, not as a substitute for precise triggers. |
Apple’s Content Blocker authoring guide documents these actions and trigger conditions. Keep each rule’s intended effect clear: preventing a request, hiding a rendered element, handling cookies, and upgrading a URL solve different problems.
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Organize rules to support efficient evaluation
Apple’s explicit authoring advice is to “Group the rules with similar actions together to improve performance.” The reason Apple gives is that after a domain matches a trigger, Safari skips later rules specifying the same action. Arrange rules by action rather than interleaving actions without a reason.
The declarative design also means Safari receives rules in advance instead of consulting the extension during each page load. Apple describes the rules as compiled to bytecode. For an app-owned WKWebView, the corresponding practical approach is to reuse stored compiled lists, compile only when rule data changes, and attach only the lists required by each web view.
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There is no official numeric speedup or current maximum rule count established for this workflow in Apple’s cited documentation. Measure compile time, memory use, page behavior, and load outcomes on representative devices and operating-system versions before making performance claims. The results depend on your rules and workload; do not convert Apple’s qualitative efficiency guidance into a universal percentage or latency promise.
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Apple says a Safari Content Blocker has no knowledge of users’ history or the websites they visit. That is a statement about the content-blocker mechanism, not a guarantee about every other feature or data flow in the containing app.
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Safari web extensions use a different declarative net request format. Apple documents block, allow, upgradeScheme, and allowAllRequests for main-frame use. The documented version caveats are important when choosing actions:
redirectsupport is documented from Safari 15.4.modifyHeaderssupport is documented from Safari 16.4.- Those actions have host-access permission and user-permission conditions. Check Apple’s declarative net request documentation for the relevant requirements rather than assuming a rule works without granted access.
Apple’s archived Content Blocker extension guide describes historical architectural benefits such as fewer requests and reduced loading time or Safari memory use. It is archived guidance, not a current quantitative performance guarantee.
Validate performance without overstating it
- Test rules against representative pages and request types, including intended exceptions.
- For
WKWebView, verify both compilation and attachment to the correct content controller. - Check that CSS hiding rules do not get mistaken for network blocking and that HTTPS upgrades apply only to eligible URLs.
- Measure on the devices and OS versions your app supports; record compile time, memory, and page-load outcomes for the actual rule set.
- For Safari web extensions, test the deployed Safari versions and required host/user permissions for each action used.
Apple’s sources provide no comparable benchmark across app-owned web views, Safari Content Blockers, and Safari web extensions. A credible performance claim therefore needs measurements for the specific implementation and test conditions.
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