네 이론 말고 기기를 읽어

device=Example-iPhone              # a name, UDID or identifier from: xcrun devicectl list devices
out=build/device-read; mkdir -p "$out"

# Is the extension still alive long after its sheet closed? Then it never finished: a hang.
xcrun devicectl device info processes --device "$device" | grep -i spark

# Did anything crash? (--source "" is required: a subpath is refused with "File paths cannot contain '..'")
xcrun devicectl device copy from --device "$device" --domain-type systemCrashLogs \
  --source "" --destination "$out/crashes"

# What did the app and its extension leave in the shared container?
xcrun devicectl device copy from --device "$device" --domain-type appGroupDataContainer \
  --domain-identifier group.com.example.spark --source "" --destination "$out/group"
tail -20 "$out/group/share-trail.log"

# Caution: the copy silently OMITS files it cannot open. A file written with complete
# protection may be missing from the copy while it exists on the phone.

import Foundation

/// One line per step, in a file a cabled Mac and the app's own Settings can both read.
/// Facts only (kinds, type identifiers, byte counts, ids, errors), never the user's content.
struct StepTrail: Sendable {
    let url: URL
    var cap = 16 * 1024   // keep the tail; a trail that grows forever becomes the next problem

    init(directory: URL, name: String = "share-trail.log") {
        url = directory.appending(path: name)
    }

    func record(_ step: String, now: Date = .now) {
        let line = "\(now.formatted(.iso8601)) \(step)\n"
        var data = (try? Data(contentsOf: url)) ?? Data()
        data.append(Data(line.utf8))
        if data.count > cap {
            let tail = data.suffix(cap)
            // Start the kept tail at a line boundary so the first line is never half a line.
            data = tail.firstIndex(of: UInt8(ascii: "\n")).map { Data(tail[tail.index(after: $0)...]) } ?? Data(tail)
        }
        #if os(iOS)
        try? data.write(to: url, options: [.atomic, .completeFileProtection])
        #else
        try? data.write(to: url, options: .atomic)
        #endif
    }

    func tail(lines: Int = 20) -> [String] {
        guard let text = try? String(contentsOf: url, encoding: .utf8) else { return [] }
        return Array(text.split(separator: "\n").suffix(lines).map(String.init))
    }
}