CoreMotion

Read device sensor data -- accelerometer, gyroscope, magnetometer, pedometer, and activity recognition -- on iOS and watchOS. CoreMotion fuses raw sensor inputs into processed device-motion data and provides pedometer/activity APIs for fitness and navigation use cases. Targets 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.

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

Authorization

CoreMotion uses CMAuthorizationStatus for pedometer and activity APIs. Sensor APIs (accelerometer, gyro) do not require explicit authorization but do require the usage description key.

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.

guard motionManager.isDeviceMotionAvailable else { return }

motionManager.deviceMotionUpdateInterval = 1.0 / 60.0

motionManager.startDeviceMotionUpdates(
    using: .xArbitraryZVertical,
    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         // 0-360 degrees (requires magnetometer)

    print("Pitch: \(attitude.pitch), Roll: \(attitude.roll)")
}

motionManager.stopDeviceMotionUpdates()

Attitude Reference Frames

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: