Core Motion

CoreMotion

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

Contents

• 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.

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<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.

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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.

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import CoreMotion

let motionManager = CMMotionManager()

Accelerometer Updates

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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

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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:

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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.

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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.

Check available frames before use:

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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.

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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

CMMotionActivityManager: Activity Recognition

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

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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

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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.

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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.

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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

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

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// 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

• NSMotionUsageDescription present in Info.plist with a clear explanation
• Single CMMotionManager instance shared across the app
• Sensor availability checked before starting updates (isAccelerometerAvailable, etc.)
• Authorization status checked before pedometer/activity APIs
• Update interval set to the lowest acceptable frequency
• All start*Updates calls have matching stop*Updates in lifecycle counterparts
• Handlers dispatched to appropriate queues (not blocking main for heavy processing)
• CMMotionActivity.confidence checked before acting on activity type
• Error parameters checked in update handlers
• Device-motion snippets call CMMotionManager.availableAttitudeReferenceFrames() before requesting a specific attitude frame
• Attitude reference frame chosen based on actual need (not defaulting to true north unnecessarily)

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