Saves screenshots in a Cocoa application (wrapper around screencapture).

ScreenshotManager.swift

import AppKit

struct ScreenshotManager {

    /// Matches macOS' screenshot filename format.
    private static let filenameFormatter: DateFormatter = {
        let formatter = DateFormatter()
        formatter.dateFormat = "'Screen Shot' yyyy-MM-dd 'at' HH.mm.ss"
        return formatter
    }()

    /// The paths to save the screenshot(s) at.
    private static var paths: [String] {
        let date = Date()
        let filename = filenameFormatter.string(from: date)

        return (1...NSScreen.screens.count).map { screenNumber -> String in
            let suffix = screenNumber > 1 ? " (\(screenNumber))" : ""
            return "/tmp/\(filename)\(suffix).png"
        }
    }

    /// Takes and saves screenshots of the user's displays.
    static func capture(completionHandler: @escaping ([String]) -> Void) {
        let outputPaths = paths
        let process = Process()
        process.launchPath = "/usr/sbin/screencapture"
        process.arguments = ["-x"] + outputPaths
        process.standardOutput = Pipe()
        process.terminationHandler = { task in
            guard task.terminationStatus == 0 else {
                return
            }

            completionHandler(outputPaths)
        }
        process.launch()
    }
}