本篇带来Alamofire中关于Timeline的一些思路
前言
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：
-
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 -
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 -
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 -
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：
Response.swiftTimeline作为Response的一个属性，肯定会出现在这里Timeline.swift这是它自身的实现-
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
代码
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: ", ") + " }"
}
}
总结
通过解读源码学到了很多，除了学到了一些平时不了解的技术外，最大的收获就是学会了从设计的角度去开发程序。也许若干年后，你依然会记得当初某个程序的设计思想。
有时间会写一个如何管理时间复杂度的文章。
由于知识水平有限，如有错误，还望指出
链接
Alamofire源码解读系列(一)之概述和使用 简书-----博客园
Alamofire源码解读系列(二)之错误处理(AFError) 简书-----博客园
Alamofire源码解读系列(三)之通知处理(Notification) 简书-----博客园
Alamofire源码解读系列(四)之参数编码(ParameterEncoding) 简书-----博客园
Alamofire源码解读系列(五)之结果封装(Result) 简书-----博客园
Alamofire源码解读系列(六)之Task代理(TaskDelegate) 简书-----博客园
Alamofire源码解读系列(七)之网络监控(NetworkReachabilityManager) 简书-----博客园
Alamofire源码解读系列(八)之安全策略(ServerTrustPolicy) 简书-----博客园
Alamofire源码解读系列(九)之响应封装(Response) 简书-----博客园
