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let swift = UTType.swiftSource
print(swift) // => Swift Source Code
print(swift.rawValue) // public.swift-source
print(swift.conforms(to: .sourceCode)) // => true
print(swift.declaration?[.conformsTo] as Any) // [ "public.source-code" ]
print(swift.declaringBundleURL as Any) // => /System/Library/CoreServices/MobileCoreTypes.bundle
print(swift.isDeclared) // => true
print(swift.isDynamic) // => false
print(swift.preferredTag(for: .filenameExtension) as Any) // => "swift"
print(UTType(preferredTag: "swift", for: .filenameExtension, conformingTo: nil) == swift) // => "true"

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//
//  CustomFontMetrics.swift
//
//  Created by Zachary Waldowski on 6/6/17.
//  Licensed under MIT.
//

import UIKit

private extension UITraitCollection {

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import Foundation

#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)

    extension Data {

        init(referencing data: DispatchData) {
            self = (data as AnyObject) as! Data
        }

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class MyViewController: UIViewController {

    var d1: String?
    var d2: String? = "foo"

}

let vc = MyViewController()
Array(Mirror(reflecting: vc).children) /* => [
    (label: "d1" as String?, value: Optional.none as Any),

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defaults write com.apple.Safari WebKitMediaPlaybackAllowsInline -bool false
defaults write com.apple.SafariTechnologyPreview WebKitMediaPlaybackAllowsInline -bool false
defaults write com.apple.Safari com.apple.Safari.ContentPageGroupIdentifier.WebKit2AllowsInlineMediaPlayback -bool false
defaults write com.apple.SafariTechnologyPreview com.apple.Safari.ContentPageGroupIdentifier.WebKit2AllowsInlineMediaPlayback -bool false

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import Foundation

/// A type representing an concrete part of an application, such that it can be
/// identified in logs.
///
/// A typical use is as a nested type:
///
///     extension MyViewController {
///         enum Log: Error {
///             case user

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import UIKit

extension UIViewController {
    
    func afterCurrentTransition(_ handler: @escaping(Void) -> Void) {
        guard let coordinator = transitionCoordinator else {
            return completion()
        }
        
        coordinator.animate(alongsideTransition: nil, completion: { _ in

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import UIKit

extension UINavigationController {
    
    func pushViewController(_ viewController: UIViewController, animated: Bool = true, completion: @escaping(Void) -> Void) {
        pushViewController(viewController, animated: animated)
        
        guard animated, let coordinator = transitionCoordinator else {
            completion()
            return

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// This incantation works well for a fullscreen, paging collection view.
// Non-fullscreen works too, with a little finesse.
extension MyCollectionViewController {

    private func updateCollectionViewLayout(forBoundingSize size: CGSize) { ... }

    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
    
        updateCollectionViewLayout(forBoundingSize: self.view.bounds.size)

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private extension NSManagedObjectContext {

    func executeAsync<T: NSFetchRequestResult>(request: NSFetchRequest, resultsOf _: T.Type = T.self) -> Future<[T]> {
        let d = Deferred<[T]>()

        let afr = NSAsynchronousFetchRequest(fetchRequest: request) { results in
            d.fill(results.finalResult.map({ $0 as! [T] }) ?? [])
        }

        do {