SKILL.md
MetricKit Diagnostics
Collect aggregated performance metrics and crash diagnostics from production
devices using MetricKit. The framework delivers daily metric payloads (CPU,
memory, launch time, hang rate, animation hitches, network usage) and
immediate diagnostic payloads (crashes, hangs, disk-write exceptions) with
full call-stack trees for triage.
Contents
- [Subscriber Setup](#subscriber-setup)
- [Receiving Metric Payloads](#receiving-metric-payloads)
- [Receiving Diagnostic Payloads](#receiving-diagnostic-payloads)
- [Key Metrics](#key-metrics)
- [Call Stack Trees](#call-stack-trees)
- [Custom Signpost Metrics](#custom-signpost-metrics)
- [Exporting and Uploading Payloads](#exporting-and-uploading-payloads)
- [Extended Launch Measurement](#extended-launch-measurement)
- [Xcode Organizer Integration](#xcode-organizer-integration)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)
Subscriber Setup
Register a subscriber as early as possible — ideally in
application(_:didFinishLaunchingWithOptions:) or App.init. MetricKit
starts accumulating reports after the first access to MXMetricManager.shared.
import MetricKit
final class MetricsSubscriber: NSObject, MXMetricManagerSubscriber {
static let shared = MetricsSubscriber()
func subscribe() {
MXMetricManager.shared.add(self)
}
func unsubscribe() {
MXMetricManager.shared.remove(self)
}
func didReceive(_ payloads: [MXMetricPayload]) {
// Handle daily metrics
}
func didReceive(_ payloads: [MXDiagnosticPayload]) {
// Handle diagnostics (crashes, hangs, disk writes)
}
}UIKit Registration
func application(
_ application: UIApplication,
didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
MetricsSubscriber.shared.subscribe()
return true
}SwiftUI Registration
@main
struct MyApp: App {
init() {
MetricsSubscriber.shared.subscribe()
}
var body: some Scene {
WindowGroup { ContentView() }
}
}Receiving Metric Payloads
MXMetricPayload arrives approximately once per 24 hours containing
aggregated metrics. The array may contain multiple payloads if prior
deliveries were missed.
func didReceive(_ payloads: [MXMetricPayload]) {
for payload in payloads {
let begin = payload.timeStampBegin
let end = payload.timeStampEnd
let version = payload.latestApplicationVersion
// Persist raw JSON before processing
let jsonData = payload.jsonRepresentation()
persistPayload(jsonData, from: begin, to: end)
processMetrics(payload)
}
}Availability: MXMetricPayload — iOS 13.0+, macOS 10.15+, visionOS 1.0+
Receiving Diagnostic Payloads
MXDiagnosticPayload delivers crash, hang, CPU exception, disk-write, and
app-launch diagnostics. On iOS 15+ and macOS 12+, diagnostics arrive
immediately rather than bundled with the daily report.
func didReceive(_ payloads: [MXDiagnosticPayload]) {
for payload in payloads {
let jsonData = payload.jsonRepresentation()
persistPayload(jsonData)
if let crashes = payload.crashDiagnostics {
for crash in crashes {
handleCrash(crash)
}
}
if let hangs = payload.hangDiagnostics {
for hang in hangs {
handleHang(hang)
}
}
if let diskWrites = payload.diskWriteExceptionDiagnostics {
for diskWrite in diskWrites {
handleDiskWrite(diskWrite)
}
}
if let cpuExceptions = payload.cpuExceptionDiagnostics {
for cpuException in cpuExceptions {
handleCPUException(cpuException)
}
}
if let launchDiags = payload.appLaunchDiagnostics {
for launchDiag in launchDiags {
handleSlowLaunch(launchDiag)
}
}
}
}Availability: MXDiagnosticPayload — iOS 14.0+, macOS 12.0+, visionOS 1.0+
Key Metrics
Launch Time — MXAppLaunchMetric
if let launch = payload.applicationLaunchMetrics {
let firstDraw = launch.histogrammedTimeToFirstDraw
let optimized = launch.histogrammedOptimizedTimeToFirstDraw
let resume = launch.histogrammedApplicationResumeTime
let extended = launch.histogrammedExtendedLaunch
}Run Time — MXAppRunTimeMetric
if let runTime = payload.applicationTimeMetrics {
let fg = runTime.cumulativeForegroundTime // Measurement<UnitDuration>
let bg = runTime.cumulativeBackgroundTime
let bgAudio = runTime.cumulativeBackgroundAudioTime
let bgLocation = runTime.cumulativeBackgroundLocationTime
}CPU, Memory, and Responsiveness
if let cpu = payload.cpuMetrics {
let cpuTime = cpu.cumulativeCPUTime // Measurement<UnitDuration>
}
if let memory = payload.memoryMetrics {
let peakMemory = memory.peakMemoryUsage // Measurement<UnitInformationStorage>
}
if let responsiveness = payload.applicationResponsivenessMetrics {
let hangTime = responsiveness.histogrammedApplicationHangTime
}
if let animation = payload.animationMetrics {
let scrollHitchRate = animation.scrollHitchTimeRatio // Measurement<Unit>
}Network and Cellular
if let network = payload.networkTransferMetrics {
let wifiUp = network.cumulativeWifiUpload // Measurement<UnitInformationStorage>
let wifiDown = network.cumulativeWifiDownload
let cellUp = network.cumulativeCellularUpload
let cellDown = network.cumulativeCellularDownload
}App Exit Metrics
if let exits = payload.applicationExitMetrics {
let fg = exits.foregroundExitData
let bg = exits.backgroundExitData
// Inspect normal, abnormal, watchdog, memory, etc.
}Call Stack Trees
MXCallStackTree is attached to each diagnostic (crash, hang, CPU exception,
disk write, app launch). Use jsonRepresentation() to extract and symbolicate.
func handleCrash(_ crash: MXCrashDiagnostic) {
let tree = crash.callStackTree
let treeJSON = tree.jsonRepresentation()
let exceptionType = crash.exceptionType
let signal = crash.signal
let reason = crash.terminationReason
uploadDiagnostic(
type: "crash",
exceptionType: exceptionType,
signal: signal,
reason: reason,
callStack: treeJSON
)
}
func handleHang(_ hang: MXHangDiagnostic) {
let tree = hang.callStackTree
let duration = hang.hangDuration // Measurement<UnitDuration>
uploadDiagnostic(type: "hang", duration: duration, callStack: tree.jsonRepresentation())
}The JSON structure contains an array of call stack frames with binary name,
offset, and address. Symbolicate using atos or upload dSYMs to your
analytics service.
Availability: MXCallStackTree — iOS 14.0+, macOS 12.0+, visionOS 1.0+
Custom Signpost Metrics
Use mxSignpost with a MetricKit log handle to capture custom performance
intervals. These appear in the daily MXMetricPayload under signpostMetrics.
Creating a Log Handle
let metricLog = MXMetricManager.makeLogHandle(category: "Networking")Emitting Signposts
import os
func fetchData() async throws -> Data {
let signpostID = MXSignpostIntervalData.makeSignpostID(log: metricLog)
mxSignpost(.begin, log: metricLog, name: "DataFetch", signpostID: signpostID)
let data = try await URLSession.shared.data(from: url).0
mxSignpost(.end, log: metricLog, name: "DataFetch", signpostID: signpostID)
return data
}Reading Custom Metrics from Payload
if let signposts = payload.signpostMetrics {
for metric in signposts {
let name = metric.signpostName // "DataFetch"
let category = metric.signpostCategory // "Networking"
let count = metric.totalCount
if let intervalData = metric.signpostIntervalData {
let avgMemory = intervalData.averageMemory
let cumulativeCPUTime = intervalData.cumulativeCPUTime
}
}
}The system limits the number of custom signpost metrics per log to reduce
on-device overhead. Reserve custom metrics for critical code paths.
Exporting and Uploading Payloads
Both payload types conform to NSSecureCoding and provide
jsonRepresentation() for easy serialization.
func persistPayload(_ jsonData: Data, from: Date? = nil, to: Date? = nil) {
let fileName = "metrics_\(ISO8601DateFormatter().string(from: Date())).json"
let url = FileManager.default.temporaryDirectory.appending(path: fileName)
try? jsonData.write(to: url)
}
func uploadPayloads(_ jsonData: Data) {
Task.detached(priority: .utility) {
var request = URLRequest(url: URL(string: "https://api.example.com/metrics")!)
request.httpMethod = "POST"
request.setValue("application/json", forHTTPHeaderField: "Content-Type")
request.httpBody = jsonData
_ = try? await URLSession.shared.data(for: request)
}
}Retrieving Past Payloads
If the subscriber was not registered when payloads arrived, retrieve them
using pastPayloads and pastDiagnosticPayloads. These return reports
generated since the last allocation of the shared manager.
let pastMetrics = MXMetricManager.shared.pastPayloads
let pastDiags = MXMetricManager.shared.pastDiagnosticPayloadsExtended Launch Measurement
Track post-first-draw setup work (loading databases, restoring state) as
part of the launch metric using extended launch measurement.
let taskID = MXLaunchTaskID("com.example.app.loadDatabase")
MXMetricManager.shared.extendLaunchMeasurement(forTaskID: taskID)
// Perform extended launch work...
await database.load()
MXMetricManager.shared.finishExtendedLaunchMeasurement(forTaskID: taskID)Extended launch times appear under histogrammedExtendedLaunch in
MXAppLaunchMetric.
Xcode Organizer Integration
Xcode Organizer shows the same MetricKit data aggregated across all users
who have opted in to share diagnostics. Use Organizer for trend analysis:
- Metrics tab: Battery, performance, and disk-write metrics over time
- Regressions tab: Automatic detection of metric regressions per version
- Crashes tab: Crash logs with symbolicated stack traces
MetricKit on-device collection complements Organizer by letting you route
raw data to your own backend for custom dashboards, alerting, and filtering
by user cohort.
Common Mistakes
DON'T: Subscribe to MXMetricManager too late
The system may deliver pending payloads shortly after launch. Subscribing
late (e.g., in a view controller) risks missing them entirely.
// WRONG — subscribing in a view controller
override func viewDidLoad() {
super.viewDidLoad()
MXMetricManager.shared.add(self)
}
// CORRECT — subscribe in application(_:didFinishLaunchingWithOptions:)
func application(
_ application: UIApplication,
didFinishLaunchingWithOptions opts: [UIApplication.LaunchOptionsKey: Any]?
) -> Bool {
MXMetricManager.shared.add(metricsSubscriber)
return true
}DON'T: Ignore MXDiagnosticPayload
Only handling MXMetricPayload means you miss crash, hang, and disk-write
diagnostics — the most actionable data MetricKit provides.
// WRONG — only implementing metric callback
func didReceive(_ payloads: [MXMetricPayload]) { /* ... */ }
// CORRECT — implement both callbacks
func didReceive(_ payloads: [MXMetricPayload]) { /* ... */ }
func didReceive(_ payloads: [MXDiagnosticPayload]) { /* ... */ }DON'T: Process payloads without persisting first
The system delivers each payload once. If your subscriber crashes during
processing, the data is lost permanently.
// WRONG — process inline, crash loses data
func didReceive(_ payloads: [MXDiagnosticPayload]) {
for p in payloads {
riskyProcessing(p) // If this crashes, payload is gone
}
}
// CORRECT — persist raw JSON first, then process
func didReceive(_ payloads: [MXDiagnosticPayload]) {
for p in payloads {
let json = p.jsonRepresentation()
try? json.write(to: localCacheURL()) // Safe on disk
Task.detached { self.processAsync(json) }
}
}DON'T: Do heavy work synchronously in didReceive
The callback runs on an arbitrary thread. Blocking it with heavy processing
or synchronous network calls delays delivery of subsequent payloads.
// WRONG — synchronous upload in callback
func didReceive(_ payloads: [MXMetricPayload]) {
for p in payloads {
let data = p.jsonRepresentation()
URLSession.shared.uploadTask(with: request, from: data).resume() // sync wait
}
}
// CORRECT — persist and dispatch async
func didReceive(_ payloads: [MXMetricPayload]) {
for p in payloads {
let json = p.jsonRepresentation()
persistLocally(json)
Task.detached(priority: .utility) {
await self.uploadToBackend(json)
}
}
}DON'T: Expect immediate data in development
MetricKit aggregates data over 24-hour windows. Payloads do not arrive
immediately after instrumenting. Use Xcode Organizer or simulated payloads
for faster iteration during development.
Review Checklist
MXMetricManager.shared.add(subscriber)called inapplication(_:didFinishLaunchingWithOptions:)orApp.init
- Subscriber conforms to
MXMetricManagerSubscriberand inheritsNSObject
- Both
didReceive(_: [MXMetricPayload])anddidReceive(_: [MXDiagnosticPayload])implemented
- Raw
jsonRepresentation()persisted to disk before processing
- Heavy processing dispatched asynchronously off the callback thread
MXCallStackTreeJSON uploaded with dSYMs for symbolication
- Custom signpost metrics limited to critical code paths
pastPayloadsandpastDiagnosticPayloadschecked on launch for missed deliveries
- Extended launch tasks call both
extendLaunchMeasurementandfinishExtendedLaunchMeasurement
- Analytics backend accepts and stores MetricKit JSON format
- Xcode Organizer reviewed for regression trends alongside on-device data