To build a high-performance content blocker, first choose where its rules will run: inside your app’s WKWebView, in a Safari Content Blocker extension, or in a Safari web extension. These use related declarative ideas, but have distinct APIs, rule formats, and deployment paths. For an app-owned web view, compile the JSON rule list with WKContentRuleListStore, then attach the compiled list to that web view’s WKUserContentController. For Safari itself, use the extension mechanism that matches your product.
Choose the API that matches the browser you are targeting
“Safari content blocker” can refer to different implementations. The choice determines how rules are expressed, delivered, and applied; do not mix their rule formats.
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| Target | Mechanism | What to keep in mind |
|---|---|---|
| An app-owned web view | WKContentRuleListStore and WKContentRuleList |
Compile content-blocker-format JSON, then add the resulting list to the content controller for the specific web view. See Apple’s WKContentRuleList and WKContentRuleListStore documentation. |
| Safari through a Safari Content Blocker extension | Safari Content Blocker JSON rules | Safari receives the declarative rules. Apple’s guide describes the trigger-and-action format: Creating a content blocker. |
| Safari through a Safari web extension | Declarative Net Request API | This has its own rule format, action support, and permission requirements. Consult Apple’s Declarative Net Request documentation. |
These paths do not have comparable benchmark results in the cited Apple documentation. Choose by deployment target and required behavior, not by assuming one is universally faster.
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A compiled rule list is not automatically active in every web view. Compilation, storage, and attachment are separate parts of the lifecycle.
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- Prepare the rule JSON. Use the content-blocker rule format, with a trigger and action for each rule.
- Compile it. Call
compileContentRuleList(forIdentifier:encodedContentRuleList:completionHandler:)onWKContentRuleListStore. Apple describes compilation as producing an efficient byte format. - Retain or retrieve the compiled list. The store persists existing lists; retrieve or otherwise retain the list your app needs.
- Attach it to the target web view. Add the list to the
WKUserContentControllerused by that web view’s configuration. A stored list does not apply until attached.
For an app that updates rules, a sensible lifecycle is to compile when rule data changes, reuse stored compiled lists, and attach only the lists needed by each web view. These are engineering practices derived from Apple’s compile/store/apply model, not published performance measurements.
Build rules around triggers and intended actions
Safari Content Blocker rules are a JSON array. Each rule has a required url-filter in its trigger and an action. Triggers can be narrowed by conditions such as included or excluded domains, resource types, load types, top-level URL, or frame URL. Scope patterns as specifically as the intended behavior requires: a rule that matches too broadly can affect legitimate page resources.
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Understand the action before choosing it
blockprevents a matching resource from loading.block-cookiesconcerns cookie handling, which remains subject to Safari’s privacy policy.css-display-nonehides matching page elements; it is not the same as preventing their resources from loading.ignore-previous-ruleschanges how earlier matching rules affect subsequent handling, so use it only when its override behavior is intended.make-httpsupgrades eligible HTTP URLs; it does not mean every possible request can be upgraded.
Apple documents this action set in its Safari Content Blocker guide. Treat action semantics and compatibility as part of the rule design rather than interchangeable ways to hide content.
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Safari’s declarative model lets the browser receive rules in advance rather than consulting an extension for each page load. Apple says the rules are compiled and explicitly advises: “Group the rules with similar actions together to improve performance.” Its guide explains that when a domain matches a trigger, Safari skips later rules specifying the same action; grouping like actions reduces unnecessary rule evaluation.
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- Group rules that share an action, while preserving the intended order where rule interactions matter.
- For
WKWebView, avoid recompiling unchanged data and avoid attaching irrelevant rule lists to a web view. - Test representative pages and rules on the iOS and Safari versions you support, checking both intended blocking and unintended breakage.
- Measure compilation time, memory use, page behavior, and load outcomes on representative devices before claiming an improvement.
Apple’s cited documentation gives no reproducible benchmark, numeric speedup, or established current rule-count limit for this workflow. Do not translate its qualitative efficiency guidance into a percentage or device-independent promise.
Keep privacy claims within the content-blocker boundary
Apple states that Safari Content Blockers have no knowledge of users’ browsing history or the websites they visit. That describes the content-blocker mechanism, not necessarily every feature in an app that contains one. An archived Apple extension guide also discusses reduced requests, loading time, and Safari memory use, but it is historical context and does not establish current quantitative performance guarantees.
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Check Safari web extension support and permissions
A Safari web extension’s Declarative Net Request API is distinct from Safari Content Blocker JSON. Apple documents block, allow, upgradeScheme, and allowAllRequests for main-frame use. Support for other actions depends on Safari version:
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| Action | Documented version information | Permission note |
|---|---|---|
redirect |
Supported from Safari 15.4 | Host-access and user-permission conditions apply. |
modifyHeaders |
Supported from Safari 16.4 | Host-access and user-permission conditions apply. |
Check Apple’s Declarative Net Request documentation for the applicable permission and action requirements. Do not copy Safari Content Blocker trigger/action JSON into this API or assume an action is available on every Safari release.
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Validate the implementation on the target platforms
Before shipping, verify that the chosen mechanism, rule syntax, and expected behavior line up with the supported platform versions. A compact test plan should include:
Quick Recap
- Whether every intended resource type and domain is matched—and unrelated resources remain unaffected.
- Whether each action has the expected effect, especially the distinction between blocking a load and hiding an element.
- For
WKWebView, whether the list is compiled, retrieved, and attached to the correct configured web view. - For a Safari web extension, whether the Safari version and granted permissions support each requested action.
- Performance and memory observations across representative devices and pages, without treating one test as a universal result.
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