这里列出了Alamofire里面的一些我平时没用到的代码细节技巧，有些东西我目前也没搞清楚为什么要这样写，如果你知道，麻烦告知，不甚感激！

1. 单例的写法

    // 基本写法
    open static let shared: SessionManager =  SessionManager()

    // 加强写法，用背包附加一些初始化
    open static let shared: SessionManager ={
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

        return SessionManager(configuration: configuration)
    }()

2. 常量的写法

摘Alamofire里面Notifications.swift的代码

import Foundation

extension Notification.Name {
    /// Used as a namespace for all `URLSessionTask` related notifications.
    public struct Task {
        /// Posted when a `URLSessionTask` is resumed. The notification `object` contains the resumed `URLSessionTask`.
        public static let DidResume = Notification.Name(rawValue: "org.alamofire.notification.name.task.didResume")

        /// Posted when a `URLSessionTask` is suspended. The notification `object` contains the suspended `URLSessionTask`.
        public static let DidSuspend = Notification.Name(rawValue: "org.alamofire.notification.name.task.didSuspend")

        /// Posted when a `URLSessionTask` is cancelled. The notification `object` contains the cancelled `URLSessionTask`.
        public static let DidCancel = Notification.Name(rawValue: "org.alamofire.notification.name.task.didCancel")

        /// Posted when a `URLSessionTask` is completed. The notification `object` contains the completed `URLSessionTask`.
        public static let DidComplete = Notification.Name(rawValue: "org.alamofire.notification.name.task.didComplete")
    }
}

// MARK: -

extension Notification {
    /// Used as a namespace for all `Notification` user info dictionary keys.
    public struct Key {
        /// User info dictionary key representing the `URLSessionTask` associated with the notification.
        public static let Task = "org.alamofire.notification.key.task"
    }
}

这篇文章里讲了一部分为什么

3. @discardableResult

@discardableResult的作用是方法返回的结果赋值给常量或者变量，并且在后面没有使用编译器不会warring

4. defer

defer的作用是将后面{}中的代码延迟到return之前调用

同样摘一段代码：

public func writeEncodedData(to fileURL: URL) throws {
        if let bodyPartError = bodyPartError {
            throw bodyPartError
        }

        if FileManager.default.fileExists(atPath: fileURL.path) {
            throw AFError.multipartEncodingFailed(reason: .outputStreamFileAlreadyExists(at: fileURL))
        } else if !fileURL.isFileURL {
            throw AFError.multipartEncodingFailed(reason: .outputStreamURLInvalid(url: fileURL))
        }

        guard let outputStream = OutputStream(url: fileURL, append: false) else {
            throw AFError.multipartEncodingFailed(reason: .outputStreamCreationFailed(for: fileURL))
        }

        outputStream.open()
        /// 注意这里
        defer { outputStream.close() }

        self.bodyParts.first?.hasInitialBoundary = true
        self.bodyParts.last?.hasFinalBoundary = true

        for bodyPart in self.bodyParts {
            try write(bodyPart, to: outputStream)
        }
    }

5. 锁的写法

    private var taskDelegate: TaskDelegate
    private var taskDelegateLock = NSLock()

    /// The delegate for the underlying task.
    open internal(set) var delegate: TaskDelegate {
        get {
            taskDelegateLock.lock() ; defer { taskDelegateLock.unlock() }
            return taskDelegate
        }
        set {
            taskDelegateLock.lock() ; defer { taskDelegateLock.unlock() }
            taskDelegate = newValue
        }
    }

6. 可以通过加 '' 来使用关键字保留字

    /// A default instance of `SessionManager`, used by top-level Alamofire request methods, and suitable for use
    /// directly for any ad hoc requests.
    open static let `default`: SessionManager = {
        let configuration = URLSessionConfiguration.default
        configuration.httpAdditionalHeaders = SessionManager.defaultHTTPHeaders

        return SessionManager(configuration: configuration)
    }()