core-motion

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npx skills add dpearson2699/swift-ios-skills --skill core-motion

Documentation

SKILL.md

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Documentation

SKILL.md

CoreMotion

Read device motion, pedometer/activity, altitude, headphone, batched-workout, and submersion sensors with Core Motion. Scope: Swift 6.3, iOS 26+.

Setup
CMMotionManager: Sensor Data
Processed Device Motion
CMPedometer: Step and Distance Data
CMMotionActivityManager: Activity Recognition
CMAltimeter: Altitude Data
Update Intervals and Battery
Common Mistakes
Review Checklist
References

Setup

Info.plist

Add NSMotionUsageDescription to Info.plist with a user-facing string explaining why your app needs motion data. Without this key, the app crashes on first access.

<key>NSMotionUsageDescription</key>
<string>This app uses motion data to track your activity.</string>

Authorization

Use the matching manager's authorizationStatus() or authorizationStatus property when an API exposes one (CMPedometer, CMMotionActivityManager, CMAltimeter, headphone motion, batched sensors, and submersion). Raw CMMotionManager accelerometer/gyro/device-motion streams have no explicit authorization request API; still ship the usage string and handle errors from start/update callbacks.

import CoreMotion
 
let status = CMMotionActivityManager.authorizationStatus()
switch status {
case .notDetermined:
    // Will prompt on first use
    break
case .authorized:
    break
case .restricted, .denied:
    // Direct user to Settings
    break
@unknown default:
    break
}

CMMotionManager: Sensor Data

Create exactly one CMMotionManager per app. Multiple instances degrade sensor update rates.

import CoreMotion
 
let motionManager = CMMotionManager()

Accelerometer Updates

guard motionManager.isAccelerometerAvailable else { return }
 
motionManager.accelerometerUpdateInterval = 1.0 / 60.0  // 60 Hz
 
motionManager.startAccelerometerUpdates(to: .main) { data, error in
    guard let acceleration = data?.acceleration else { return }
    print("x: \(acceleration.x), y: \(acceleration.y), z: \(acceleration.z)")
}
 
// When done:
motionManager.stopAccelerometerUpdates()

Gyroscope Updates

guard motionManager.isGyroAvailable else { return }
 
motionManager.gyroUpdateInterval = 1.0 / 60.0
 
motionManager.startGyroUpdates(to: .main) { data, error in
    guard let rotationRate = data?.rotationRate else { return }
    print("x: \(rotationRate.x), y: \(rotationRate.y), z: \(rotationRate.z)")
}
 
motionManager.stopGyroUpdates()

Polling Pattern (Games)

For games, start updates without a handler and poll the latest sample each frame:

motionManager.startAccelerometerUpdates()
 
// In your game loop / display link:
if let data = motionManager.accelerometerData {
    let tilt = data.acceleration.x
    // Move player based on tilt
}

Processed Device Motion

Device motion fuses accelerometer, gyroscope, and magnetometer into a single CMDeviceMotion object with attitude, user acceleration (gravity removed), rotation rate, and calibrated magnetic field.

When giving device-motion guidance, show the runtime frame check in the snippet instead of hard-coding a corrected, magnetic-north, or true-north frame. Fall back to .xArbitraryZVertical when the preferred frame is unavailable.

guard motionManager.isDeviceMotionAvailable else { return }
 
let availableFrames = CMMotionManager.availableAttitudeReferenceFrames()
let frame: CMAttitudeReferenceFrame = availableFrames.contains(.xArbitraryCorrectedZVertical)
    ? .xArbitraryCorrectedZVertical
    : .xArbitraryZVertical
 
motionManager.deviceMotionUpdateInterval = 1.0 / 60.0
 
motionManager.startDeviceMotionUpdates(
    using: frame,
    to: .main
) { motion, error in
    guard let motion else { return }
 
    let attitude = motion.attitude       // roll, pitch, yaw
    let userAccel = motion.userAcceleration
    let gravity = motion.gravity
    let heading = motion.heading         // degrees relative to the current frame
 
    print("Pitch: \(attitude.pitch), Roll: \(attitude.roll)")
}
 
motionManager.stopDeviceMotionUpdates()

Attitude Reference Frames

For simple tilt controls, use .xArbitraryZVertical or .xArbitraryCorrectedZVertical; they avoid magnetometer/location dependencies. Before requesting corrected, magnetic-north, or true-north frames, call CMMotionManager.availableAttitudeReferenceFrames() and fall back to an available frame.

Frame	Use Case
`.xArbitraryZVertical`	Default. Z is vertical, X arbitrary at start. Most games.
`.xArbitraryCorrectedZVertical`	Same as above, corrected for gyro drift over time.
`.xMagneticNorthZVertical`	X points to magnetic north. Requires magnetometer.
`.xTrueNorthZVertical`	X points to true north. Requires magnetometer + location.

Check available frames before use:

let available = CMMotionManager.availableAttitudeReferenceFrames()
if available.contains(.xTrueNorthZVertical) {
    // Safe to use true north
}

CMPedometer: Step and Distance Data

CMPedometer provides step counts, distance, pace, cadence, and floor counts.

let pedometer = CMPedometer()
 
guard CMPedometer.isStepCountingAvailable() else { return }
 
// Historical query
pedometer.queryPedometerData(
    from: Calendar.current.startOfDay(for: Date()),
    to: Date()
) { data, error in
    guard let data else { return }
    print("Steps today: \(data.numberOfSteps)")
    print("Distance: \(data.distance?.doubleValue ?? 0) meters")
    print("Floors up: \(data.floorsAscended?.intValue ?? 0)")
}
 
// Live updates
pedometer.startUpdates(from: Date()) { data, error in
    guard let data else { return }
    print("Steps: \(data.numberOfSteps)")
}
 
// Stop when done
pedometer.stopUpdates()

Availability Checks

Method	What It Checks
`isStepCountingAvailable()`	Step counter hardware
`isDistanceAvailable()`	Distance estimation
`isFloorCountingAvailable()`	Barometric altimeter for floors
`isPaceAvailable()`	Pace data
`isCadenceAvailable()`	Cadence data

CMMotionActivityManager: Activity Recognition

Detects whether the user is stationary, walking, running, cycling, or in a vehicle.

let activityManager = CMMotionActivityManager()
 
guard CMMotionActivityManager.isActivityAvailable() else { return }
 
// Live activity updates
activityManager.startActivityUpdates(to: .main) { activity in
    guard let activity else { return }
 
    if activity.walking {
        print("Walking (confidence: \(activity.confidence.rawValue))")
    } else if activity.running {
        print("Running")
    } else if activity.automotive {
        print("In vehicle")
    } else if activity.cycling {
        print("Cycling")
    } else if activity.stationary {
        print("Stationary")
    }
}
 
activityManager.stopActivityUpdates()

Historical Activity Query

let yesterday = Calendar.current.date(byAdding: .day, value: -1, to: Date())!
 
activityManager.queryActivityStarting(
    from: yesterday,
    to: Date(),
    to: .main
) { activities, error in
    guard let activities else { return }
    for activity in activities {
        print("\(activity.startDate): walking=\(activity.walking)")
    }
}

CMAltimeter: Altitude Data

Altimeter access is covered by NSMotionUsageDescription; handle denied motion access through unavailable data and update-handler errors.

let altimeter = CMAltimeter()
 
guard CMAltimeter.isRelativeAltitudeAvailable() else { return }
 
altimeter.startRelativeAltitudeUpdates(to: .main) { data, error in
    guard let data else { return }
    print("Relative altitude: \(data.relativeAltitude) meters")
    print("Pressure: \(data.pressure) kPa")
}
 
altimeter.stopRelativeAltitudeUpdates()

Absolute altitude is altitude relative to sea level, not GPS-based altitude. First check availability. Absolute altitude is available only on supported hardware such as iPhone 12 or later and Apple Watch Series 6, Apple Watch SE, or later.

guard CMAltimeter.isAbsoluteAltitudeAvailable() else { return }
 
altimeter.startAbsoluteAltitudeUpdates(to: .main) { data, error in
    guard let data else { return }
    print("Altitude: \(data.altitude)m, accuracy: \(data.accuracy)m")
}
 
altimeter.stopAbsoluteAltitudeUpdates()

Update Intervals and Battery

Interval	Hz	Use Case	Battery Impact
`1.0 / 10.0`	10	UI orientation	Low
`1.0 / 30.0`	30	Casual games	Moderate
`1.0 / 60.0`	60	Action games	High
`1.0 / 100.0`	100	Max rate (iPhone)	Very High

Use the lowest frequency that meets your needs. Do not assume a fixed maximum sample rate across devices. For high-frequency workout motion, use CMBatchedSensorManager where supported and read its reported accelerometerDataFrequency or deviceMotionDataFrequency instead of assigning those read-only properties.

Common Mistakes

DON'T: Create multiple CMMotionManager instances

Retain one app-level CMMotionManager; competing instances can reduce update rates.

DON'T: Skip sensor availability checks

Apply the matching is...Available gate immediately before starting each sensor stream.

DON'T: Forget to stop updates

Pair every start with the matching stop in the counterpart lifecycle or task cancellation path.

DON'T: Use unnecessarily high update rates

Choose the lowest rate that meets the interaction and use the Update Intervals and Battery table as a starting point.

DON'T: Assume all CMMotionActivity properties are mutually exclusive

// WRONG -- checking only one property
if activity.walking { handleWalking() }
 
// CORRECT -- multiple can be true simultaneously; check confidence
if activity.walking && activity.confidence == .high {
    handleWalking()
} else if activity.automotive && activity.confidence != .low {
    handleDriving()
}

Review Checklist

References

Extended patterns (SwiftUI integration, batched sensor manager, headphone motion, water submersion): references/motion-patterns.md
CoreMotion framework
CMMotionManager
CMPedometer
CMMotionActivityManager
CMDeviceMotion
CMAltimeter
CMAbsoluteAltitudeData
CMBatchedSensorManager
CMHeadphoneMotionManager
CMWaterSubmersionManager
Accessing submersion data
Getting processed device-motion data

core-motion

Documentation

Documentation

core-motion

Documentation

CoreMotion

Contents

Setup

Info.plist

Authorization

CMMotionManager: Sensor Data

Accelerometer Updates

Gyroscope Updates

Polling Pattern (Games)

Processed Device Motion

Attitude Reference Frames

CMPedometer: Step and Distance Data

Availability Checks

CMMotionActivityManager: Activity Recognition

Historical Activity Query

CMAltimeter: Altitude Data

Update Intervals and Battery

Common Mistakes

DON'T: Create multiple CMMotionManager instances

DON'T: Skip sensor availability checks

DON'T: Forget to stop updates

DON'T: Use unnecessarily high update rates

DON'T: Assume all CMMotionActivity properties are mutually exclusive

Review Checklist

References

Documentation

Documentation

CoreMotion

Contents

Setup

Info.plist

Authorization

CMMotionManager: Sensor Data

Accelerometer Updates

Gyroscope Updates

Polling Pattern (Games)

Processed Device Motion

Attitude Reference Frames

CMPedometer: Step and Distance Data

Availability Checks

CMMotionActivityManager: Activity Recognition

Historical Activity Query

CMAltimeter: Altitude Data

Update Intervals and Battery

Common Mistakes

DON'T: Create multiple CMMotionManager instances

DON'T: Skip sensor availability checks

DON'T: Forget to stop updates

DON'T: Use unnecessarily high update rates

DON'T: Assume all CMMotionActivity properties are mutually exclusive

Review Checklist

References