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Skillscore-motion

core-motion

Coding & development

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Updated 10-16 05:52

About this skill

CoreMotion

Access accelerometer, gyroscope, attitude/device motion, pedometer, altitude, and specialized sensors using Core Motion. Targets Swift 6.3 / iOS 26+.

Contents

  • Setup & Authorization
  • Motion Manager & Raw Sensors
  • Device Motion (Attitude & Gravity)
  • Pedometer & Activity Recognition
  • Altimeter & Specialized Sensors
  • Battery & Lifecycle
  • Common Mistakes
  • Review Checklist
  • References

Setup & Authorization

Add NSMotionUsageDescription to Info.plist explaining why the app accesses motion sensors.

Check authorization status for managers exposing privacy controls (CMPedometer, CMMotionActivityManager, CMAltimeter):

import CoreMotion

guard CMMotionActivityManager.authorizationStatus() != .denied else {
    // Prompt user to enable motion permissions in Settings
    return
}

Raw accelerometer/gyro streams under CMMotionManager do not prompt for permission directly but require the usage description and throw errors on unauthorized access.

Motion Manager & Raw Sensors

Maintain exactly one shared CMMotionManager instance per application to avoid sensor contention:

final class MotionProvider {
    static let shared = MotionProvider()
    let manager = CMMotionManager()

    func startAccelerometer() {
        guard manager.isAccelerometerAvailable else { return }
        manager.accelerometerUpdateInterval = 1.0 / 60.0 // 60 Hz
        manager.startAccelerometerUpdates(to: .main) { data, error in
            guard let accel = data?.acceleration else { return }
            // Process acceleration: x, y, z
        }
    }

    func stop() {
        manager.stopAccelerometerUpdates()
    }
}

Device Motion (Attitude & Gravity)

Prefer CMDeviceMotion over raw sensor data. It fuses accelerometer, gyro, and magnetometer data into clean attitude, user acceleration, and gravity vectors:

manager.deviceMotionUpdateInterval = 1.0 / 60.0
manager.startDeviceMotionUpdates(using: .xArbitraryZVertical, to: .main) { motion, error in
    guard let motion else { return }
    let roll = motion.attitude.roll
    let pitch = motion.attitude.pitch
    let yaw = motion.attitude.yaw
}

Pedometer & Activity Recognition

Track steps, distance, and user movement state:

let pedometer = CMPedometer()
if CMPedometer.isStepCountingAvailable() {
    pedometer.startUpdates(from: .now) { data, error in
        guard let data else { return }
        print("Steps: \(data.numberOfSteps)")
    }
}

let activityManager = CMMotionActivityManager()
if CMMotionActivityManager.isActivityAvailable() {
    activityManager.startActivityUpdates(to: .main) { activity in
        guard let activity else { return }
        if activity.walking { print("Walking") }
        if activity.running { print("Running") }
    }
}

Altimeter & Specialized Sensors

  • CMAltimeter: Track relative altitude changes or absolute barometric pressure (startRelativeAltitudeUpdates).
  • CMHeadphoneMotionManager: Track head pose from AirPods for spatial audio interactions.
  • CMBatchedSensorManager (iOS 17+): Receive batched high-frequency workout motion data with low CPU wake overhead.
  • CMWaterSubmersionManager: Track depth, temperature, and submersion state on supported Apple Watch hardware.

Battery & Lifecycle

  • Stop updates (stopDeviceMotionUpdates()) when views disappear or when the app transitions to the background.
  • Select the lowest sensor update frequency adequate for your feature (e.g. 10–20 Hz for tilt detection; 60 Hz only for real-time physics/gaming).

Common Mistakes

  • Instantiating multiple CMMotionManager objects: Degrades system update rates and increases battery drain. Use a single shared manager.
  • Missing NSMotionUsageDescription: The app crashes immediately upon accessing activity, pedometer, or altimeter services.
  • Not stopping updates on backgrounding: Running motion updates indefinitely drains battery and causes system termination.
  • Using raw accelerometer for orientation: Raw data includes gravity and user movement noise. Use CMDeviceMotion.attitude instead.
  • Omitting availability checks: Always check isAvailable flags before calling startUpdates.

Review Checklist

  • NSMotionUsageDescription present and descriptive in Info.plist
  • Single shared CMMotionManager instance used app-wide
  • Sensor availability checked before initiating streams
  • Updates explicitly halted when views dismiss or background transitions occur
  • CMDeviceMotion used for orientation and tilt calculations instead of raw accelerometer
  • Update intervals chosen conservatively to preserve battery life

References