refactor: move endpoint host resolution to Swift

[nickolas-dimitrakas]

The stack

1. refactor: move request signing to Swift #872 — request signing
2. refactor: move URL request header assembly to Swift #873 — URL request headers
3. refactor: move endpoint host resolution to Swift #874 — endpoint host resolution
4. test: pin the exact endpoint URL strings before converting them #875 — endpoint URL characterization tests
5. refactor: move endpoint path-style decisions to Swift #876 — endpoint path-style decisions

Review bottom-up; merge top-down. This is #874.

---

Third of five, stacked on the previous.

What moves

MPEndpointHostResolver owns two things:

• defaultHost(subdomain:apiKey:) parses the pod prefix out of the API key. A key with
  no hyphen, an empty key and a nil key all route to us1, matching
  componentsSeparatedByString: on nil returning nil and the original count <= 1 test.
• resolvedHost(...) is the one precedence ladder behind all five endpoint families:
  customBaseURL host, then the tracking host when ATT is authorized, then the configured
  host, then the default. Config and audience pass a nil tracking host, which is what the
  ObjC did by having no tracking branch at all.

The .m keeps every read of MPStateMachine, MPNetworkOptions and MPUploadSettings,
and every MPILogWarning, at the same call sites with the same text. A new private
-attAuthorized collapses the four copies of the ATT comparison; it is not declared in any
header.

Two behaviours worth naming, both preserved

• An empty-string host overrides the default rather than falling through, because
  ObjC's ?: treats @"" as a live object. Covered by a test so a future Swift rewrite
  cannot quietly "fix" it.
• The audience endpoint ignores eventsTrackingHost entirely, unlike events — it only
  picks up the tracking subdomain indirectly through defaultEventHost. Reproduced, not
  corrected.

self.defaultEventHost / defaultIdentityHost are now evaluated eagerly as the
defaultHost: argument instead of behind ?:. Both are pure functions of the ATT status
and API key, and both were already called unconditionally on the following line to build
defaultURL, so nothing observable changes.

The alias endpoint's eventsOnly fallback stays a plain ?: — a two-way default with no
tracking or custom host, where the resolver would only add noise.

Gate

Item	Result
1. Zero public-ABI diff	pass — abi-guard.sh check clean, Include/ diff empty
2. pod lib lint x3 podspecs	iOS pass; tvOS not runnable locally
3. Build iOS + tvOS	iOS pass, no new warnings; tvOS not runnable locally
4. trunk check	pass — no new issues
5. ObjC + Swift suites	pass — ObjC 982/982, Swift 405/405 (13 new here)

The first build of this slice left two unused variable 'stateMachine' warnings behind in
identityURL: and modifyURL; both locals are gone and the build is warning-clean.

tvOS and test-flakiness caveats as in the first PR of the stack.

🤖 Generated with Claude Code

[github-actions]

🐦 Swift Migration Progress

Production implementation code at 14996c65bf2f compared with edb9962d4228.

Area	Goal	Progress	Base	This PR	Swift SLOC	Objective-C remaining	Change
Core SDK	Short term — in scope	████▎░░░░░	42.85%	42.99%	6,294	8,346	🚀 +0.14 pp
Core SDK	Long term — all Objective-C	███▏░░░░░░	30.60%	30.71%	6,294	14,201	🚀 +0.11 pp
SDK kit infrastructure	Short term — in scope	░░░░░░░░░░	0.00%	0.00%	0	3,340	➖ 0.00 pp
SDK kit infrastructure	Long term — all Objective-C	░░░░░░░░░░	0.00%	0.00%	0	3,725	➖ 0.00 pp
Standalone kits	Short term — in scope	▏░░░░░░░░░	1.06%	1.06%	152	14,231	➖ 0.00 pp
Standalone kits	Long term — all Objective-C	▏░░░░░░░░░	1.06%	1.06%	152	14,231	➖ 0.00 pp

Objective-C retained by design: Core SDK 5,855 · SDK kit infrastructure 385 · Standalone kits 0.

This PR's code movement

Area	Swift lines added	Objective-C lines removed
Core SDK	29	60
SDK kit infrastructure	0	0
Standalone kits	0	0

How this is measured

• Current composition uses production source lines of code (SLOC) from cloc; comments and blank lines are excluded.
• Short term — in scope excludes the Objective-C the migration will not delete, so 100% is the end of this project: every in-scope implementation gone.
• Long term — all Objective-C keeps the full denominator. Reaching 100% there means the public API itself becomes Swift, which is a breaking change reserved for a future major release.
• The gap between the two rows is the retained public/kit contract, runtime-identity, and boundary-glue surface listed in Tools/swift-migration-retained-objc.txt.
• Retained wrappers keep their Objective-C interface but still shed logic to Swift. That thinning moves the long-term row and the retained figure, not the short-term row.
• Both revisions are measured with the manifest from the head revision, so a manifest edit does not by itself move the reported change. A retained file this pull request renamed or deleted still counts as retained at the base.
• Pull request movement uses physical additions/deletions from git diff base...head --numstat; it counts retained files too and is intentionally separate from SLOC totals.
• Core excludes SDK kit infrastructure and vendored libraries. Standalone kits include only files below Kits/**/Sources.
• Tests, examples, headers, build outputs, vendored libraries, and the MParticle/Sources Swift overlay are excluded.
• Objective-C++ (.mm) is included in the Objective-C figures and removed counts.

Generated with cloc 2.10. This report is informational and does not gate migration direction.

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	2.18 MB	2.18 MB	+N/A
Executable Impact	848 bytes	848 bytes	+N/A
XCFramework Size	6.69 MB	6.69 MB	+N/A

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/swift-migration):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2320,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2236,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6852}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2320,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2236,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6852}

[cursor]

PR Summary

Medium Risk
Changes which backend hostnames the SDK uses for config, events, identity, and related uploads; mistakes would misroute traffic or break ATT-gated tracking endpoints, though the PR is intended as behavior-preserving with broad test coverage.

Overview
Endpoint host selection is centralized in a new Swift MPEndpointHostResolver, and MPNetworkCommunication delegates to it instead of duplicating inline precedence logic.

defaultHost(subdomain:apiKey:) builds pod-based default hostnames from the API key prefix (falling back to us1). resolvedHost(...) applies one shared ladder everywhere it’s wired: custom base URL host, then tracking host when ATT is authorized, then configured host, then default. Config and audience pass a nil tracking host, matching prior ObjC behavior.

ObjC still owns warnings, network options, and upload settings at each call site; a private -attAuthorized replaces repeated ATT checks for default event/identity subdomains and resolver calls. New Swift unit tests lock in pod routing, precedence, and legacy quirks such as empty-string hosts not falling through to the default.

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