Timeline

Timeline翻译后的意思是时间轴，可以表示一个事件从开始到结束的时间节点。时间轴的概念能够应用在很多地方，比如说微博的主页就是一个时间轴。

Alamofire中Timeline的代码很少，非常简单。因此本篇文章中，我们不会把重点放到代码的解读上，我们通过追踪Timeline的身影，来讲讲关于代码设计方面的东东。

为什么要设计Timeline

很简单，我需要知道一个请求过程中，每个关键时间点的值或者时间点与时间点之间的距离。这样的一个需求不仅能够用于程序的调试，而且能为别的设计提供必要的参数支持。

我们通过下边的代码进行打印：

print(response.timeline)

显示的结果是：

Timeline: { "Latency": 0.092 secs, "Request Duration": 0.092 secs, "Serialization Duration": 0.458 secs, "Total Duration": 0.551 secs }

上边的代码提供的信息有：

(1)Latency: 0.092 secs 延迟，它表示从请求开始到收到或者发送第一个字节的时间长度，这里把它理解成建立连接花费的时间

(2)Request Duration: 0.092 secs 请求时间，它表示从请求开始到结束的时间长度。这里跟Latency: 0.092 secs都是0.092，原因是我用的POST请求

(3)Serialization Duration: 0.458 secs 序列化用时，这里用了0.458秒，说明在当前的这个请求中，最耗时的操作是数据的序列化，因此，程序可以在这方面进行优化

(4)Total Duration: 0.551 secs 总耗时 用序列化完成的时间点减去请求开始的时间点

print(response.timeline)之所以能够打印出上边这些信息，是因为它重写了CustomStringConvertible协议的var description: String。当然，如果要打印更详细的信息，可以重写CustomDebugStringConvertible的var debugDescription: String。我们通过代码打印出来：

print(response.timeline.debugDescription)

打印结果是：

Timeline: { "Request Start Time": 513055266.217, "Initial Response Time": 513055266.241, "Request Completed Time": 513055266.241, "Serialization Completed Time": 513055266.752, "Latency": 0.024 secs, "Request Duration": 0.024 secs, "Serialization Duration": 0.511 secs, "Total Duration": 0.535 secs }

如何设计Timeline

Alamofire中，不管Request请求成功还是失败都会返回response。因此Timeline只有跟response绑定才合理。所以应该把他设为response的一个属性，在之前的文章中，我们也详细的介绍了response，他是一个struct类型的数据存储属性，因此在初始化的时候给Timeline赋值。

这样我们就解决了取出Timeline的问题，那么Timeline又是如何赋值的呢？我们在Alamofire中追踪Timeline的身影，最终发现只有三个文件中出现了它的身影：

(1)Response.swift Timeline作为Response的一个属性，肯定会出现在这里

(2)Timeline.swift 这是它自身的实现

(3)ResponseSerialization.swift 在这个文件中，为Request做了一个扩展，代码如下：

extension Request {
      var timeline: Timeline {
          let requestCompletedTime = self.endTime ?? CFAbsoluteTimeGetCurrent()
          let initialResponseTime = self.delegate.initialResponseTime ?? requestCompletedTime

          return Timeline(
              requestStartTime: self.startTime ?? CFAbsoluteTimeGetCurrent(),
              initialResponseTime: initialResponseTime,
              requestCompletedTime: requestCompletedTime,
              serializationCompletedTime: CFAbsoluteTimeGetCurrent()
          )
      }
  }

这个扩展说明Timeline的值最终是通过getter方法获取的，获取的是计算后的值。
对于这样的设计，能够给我们一些启示，我们在设计某一个功能的时候，尽量保持这个功能不去污染其他的程序。我们应该避免这样的设计：创建一个对象后，在程序的很多地方给它的属性赋值
只是把代码弄上来：

/// Responsible for computing the timing metrics for the complete lifecycle of a `Request`.
public struct Timeline {
    /// The time the request was initialized.
    public let requestStartTime: CFAbsoluteTime

    /// The time the first bytes were received from or sent to the server.
    public let initialResponseTime: CFAbsoluteTime

    /// The time when the request was completed.
    public let requestCompletedTime: CFAbsoluteTime

    /// The time when the response serialization was completed.
    public let serializationCompletedTime: CFAbsoluteTime

    /// The time interval in seconds from the time the request started to the initial response from the server.
    public let latency: TimeInterval

    /// The time interval in seconds from the time the request started to the time the request completed.
    public let requestDuration: TimeInterval

    /// The time interval in seconds from the time the request completed to the time response serialization completed.
    public let serializationDuration: TimeInterval

    /// The time interval in seconds from the time the request started to the time response serialization completed.
    public let totalDuration: TimeInterval

    /// Creates a new `Timeline` instance with the specified request times.
    ///
    /// - parameter requestStartTime:           The time the request was initialized. Defaults to `0.0`.
    /// - parameter initialResponseTime:        The time the first bytes were received from or sent to the server.
    ///                                         Defaults to `0.0`.
    /// - parameter requestCompletedTime:       The time when the request was completed. Defaults to `0.0`.
    /// - parameter serializationCompletedTime: The time when the response serialization was completed. Defaults
    ///                                         to `0.0`.
    ///
    /// - returns: The new `Timeline` instance.
    public init(
        requestStartTime: CFAbsoluteTime = 0.0,
        initialResponseTime: CFAbsoluteTime = 0.0,
        requestCompletedTime: CFAbsoluteTime = 0.0,
        serializationCompletedTime: CFAbsoluteTime = 0.0)
    {
        self.requestStartTime = requestStartTime
        self.initialResponseTime = initialResponseTime
        self.requestCompletedTime = requestCompletedTime
        self.serializationCompletedTime = serializationCompletedTime

        self.latency = initialResponseTime - requestStartTime
        self.requestDuration = requestCompletedTime - requestStartTime
        self.serializationDuration = serializationCompletedTime - requestCompletedTime
        self.totalDuration = serializationCompletedTime - requestStartTime
    }
}

// MARK: - CustomStringConvertible

extension Timeline: CustomStringConvertible {
    /// The textual representation used when written to an output stream, which includes the latency, the request
    /// duration and the total duration.
    public var description: String {
        let latency = String(format: "%.3f", self.latency)
        let requestDuration = String(format: "%.3f", self.requestDuration)
        let serializationDuration = String(format: "%.3f", self.serializationDuration)
        let totalDuration = String(format: "%.3f", self.totalDuration)

        // NOTE: Had to move to string concatenation due to memory leak filed as rdar://26761490. Once memory leak is
        // fixed, we should move back to string interpolation by reverting commit 7d4a43b1.
        let timings = [
            "\"Latency\": " + latency + " secs",
            "\"Request Duration\": " + requestDuration + " secs",
            "\"Serialization Duration\": " + serializationDuration + " secs",
            "\"Total Duration\": " + totalDuration + " secs"
        ]

        return "Timeline: { " + timings.joined(separator: ", ") + " }"
    }
}
/// 使用timeline 可以让我们很清楚的查看某个网络请求过程的耗时，可以借此分析服务器端是不是有问题，同时也可以简介的得出当前的网络情况
// MARK: - CustomDebugStringConvertible

extension Timeline: CustomDebugStringConvertible {
    /// The textual representation used when written to an output stream, which includes the request start time, the
    /// initial response time, the request completed time, the serialization completed time, the latency, the request
    /// duration and the total duration.
    public var debugDescription: String {
        let requestStartTime = String(format: "%.3f", self.requestStartTime)
        let initialResponseTime = String(format: "%.3f", self.initialResponseTime)
        let requestCompletedTime = String(format: "%.3f", self.requestCompletedTime)
        let serializationCompletedTime = String(format: "%.3f", self.serializationCompletedTime)
        let latency = String(format: "%.3f", self.latency)
        let requestDuration = String(format: "%.3f", self.requestDuration)
        let serializationDuration = String(format: "%.3f", self.serializationDuration)
        let totalDuration = String(format: "%.3f", self.totalDuration)

        // NOTE: Had to move to string concatenation due to memory leak filed as rdar://26761490. Once memory leak is
        // fixed, we should move back to string interpolation by reverting commit 7d4a43b1.
        let timings = [
            "\"Request Start Time\": " + requestStartTime,
            "\"Initial Response Time\": " + initialResponseTime,
            "\"Request Completed Time\": " + requestCompletedTime,
            "\"Serialization Completed Time\": " + serializationCompletedTime,
            "\"Latency\": " + latency + " secs",
            "\"Request Duration\": " + requestDuration + " secs",
            "\"Serialization Duration\": " + serializationDuration + " secs",
            "\"Total Duration\": " + totalDuration + " secs"
        ]

        return "Timeline: { " + timings.joined(separator: ", ") + " }"
    }
}