metrickit

MetricKit (iOS production telemetry)

Field-collected power/performance metrics and crash/hang/exception diagnostics, aggregated on-device by the system and delivered to your app — no third-party SDK, no per-event network cost. Grounded in Apple's MetricKit framework.

• On-device & privacy-preserving: the OS aggregates over a 24h window and hands you a payload; nothing leaves the device until you upload it. No PII, no user identifiers — safe for a fintech privacy posture. Still declare any upload/collection in the privacy manifest (see app-store-review).
• Delivery cadence: subscriber callbacks fire at most once per day, on a background queue, and only when the app has been foregrounded since the last delivery. Diagnostics can also arrive right after the next launch following a crash/hang.
• Two APIs: the stable delegate-based MXMetricManager (iOS 13+, crashes/diagnostics iOS 14+) — the default for the iOS 26 era — and the new async MetricManager (iOS 27, WWDC26) covered in references/new-metricmanager.md.
• Not a profiler. MetricKit tells you what regressed in the field, aggregated; to root-cause locally with call trees and allocations use debugging-instruments.

Contents

• Subscribe
• MXMetricPayload — the metrics
• Custom metrics with mxSignpost
• MXDiagnosticPayload — crashes/hangs/exceptions
• Extended launch diagnostics
• Durable export & upload
• Testing / simulating payloads
• KMP boundary
• New async MetricManager (iOS 27)
• Correctness checklist
• References: references/payload-fields.md, references/new-metricmanager.md

Subscribe

Register one long-lived subscriber at launch — a payload delivered before you subscribe is lost. A singleton owned by the app delegate / App is the right scope.

import MetricKit

final class TelemetrySubscriber: NSObject, MXMetricManagerSubscriber {
    static let shared = TelemetrySubscriber()

    func start() { MXMetricManager.shared.add(self) }   // idempotent-safe to guard yourself
    func stop()  { MXMetricManager.shared.remove(self) }

    // Called on a BACKGROUND queue, ~once/day. Both methods are optional.
    func didReceive(_ payloads: [MXMetricPayload]) {
        for payload in payloads { PayloadStore.persist(payload.jsonRepresentation()) }
    }

    func didReceive(_ payloads: [MXDiagnosticPayload]) {   // iOS 14+
        for payload in payloads { PayloadStore.persist(payload.jsonRepresentation()) }
    }
}