Alamofire源码解读系列(二)之错误处理(AFError)
本篇主要讲解Alamofire中错误的处理机制
前言
在开发中，往往最容易被忽略的内容就是对错误的处理。有经验的开发者，能够对自己写的每行代码负责，而且非常清楚自己写的代码在什么时候会出现异常，这样就能提前做好错误处理。
Alamofire的错误封装很经典，是使用swift中enum的一个典型案例。读完这篇文章，一定能让大家对swift的枚举有一个更深的理解，同时增加一些枚举的高级使用技巧。
那么有一个很重要的问题，我们应该在什么情况下考虑使用枚举呢？只要结果可能是有限的集合的情况下，我们就尽量考虑使用枚举。 其实枚举本身还是数据的一种载体，swift中，枚举有着很丰富的使用方法，在下边的内容中，我们会介绍到枚举的主流用法。
开胃菜
先总结一下swfit中enum中的用法：
1.正常用法
enum Movement {
case Left
case Right
case Top
case Bottom
}
let aMovement = Movement.Left
switch aMovement {
case .Left:
print("left")
default:
print("Unknow")
}
if case .Left = aMovement {
print("Left")
}
if .Left == aMovement {
print("Left")
}
2.声明为整型
enum Season: Int {
case Spring = 0
case Summer = 1
case Autumn = 2
case Winter = 3
}
3.声明为字符串类型
enum House: String {
case ZhangSan = "I am zhangsan"
case LiSi = "I am lisi"
}
let zs = House.ZhangSan
print(zs.rawValue)
enum CompassPoint: String {
case North, South, East, West
}
let n = CompassPoint.North
print(n.rawValue)
let s = CompassPoint(rawValue: "South");
4.声明为浮点类型
enum Constants: Double {
case π = 3.14159
case e = 2.71828
case φ = 1.61803398874
case λ = 1.30357
}
let pai = Constants.π
print(pai.rawValue)
5.其他类型
enum VNodeFlags : UInt32 {
case Delete = 0x00000001
case Write = 0x00000002
case Extended = 0x00000004
case Attrib = 0x00000008
case Link = 0x00000010
case Rename = 0x00000020
case Revoke = 0x00000040
case None = 0x00000080
}
6.enum包含enum
enum Character {
enum Weapon {
case Bow
case Sword
case Lance
case Dagger
}
enum Helmet {
case Wooden
case Iron
case Diamond
}
case Thief
case Warrior
case Knight
}
let character = Character.Thief
let weapon = Character.Weapon.Bow
let helmet = Character.Helmet.Iron
7.结构体和枚举
struct Scharacter {
enum CharacterType {
case Thief
case Warrior
case Knight
}
enum Weapon {
case Bow
case Sword
case Lance
case Dagger
}
let type: CharacterType
let weapon: Weapon
}
let sc = Scharacter(type: .Thief, weapon: .Bow)
print(sc.type)
8.值关联
enum Trade {
case Buy(stock: String, amount: Int)
case Sell(stock: String, amount: Int)
}
let trade = Trade.Buy(stock: "Car", amount: 100)
if case let Trade.Buy(stock, amount) = trade {
print("buy \(amount) of \(stock)")
}
enum Trade0 {
case Buy(String, Int)
case Sell(String, Int)
}
let trade0 = Trade0.Buy("Car0", 100)
if case let Trade0.Buy(stock, amount) = trade0 {
print("buy \(amount) of \(stock)")
}
9.枚举中的函数
enum Wearable {
enum Weight: Int {
case Light = 2
}
enum Armor: Int {
case Light = 2
}
case Helmet(weight: Weight, armor: Armor)
func attributes() -> (weight: Int, armor: Int) {
switch self {
case .Helmet(let w, let a):
return (weight: w.rawValue * 2, armor: a.rawValue * 4)
}
}
}
let test = Wearable.Helmet(weight: .Light, armor: .Light).attributes()
print(test)
enum Device {
case iPad, iPhone, AppleTV, AppleWatch
func introduced() -> String {
switch self {
case .AppleTV: return "\(self) was introduced 2006"
case .iPhone: return "\(self) was introduced 2007"
case .iPad: return "\(self) was introduced 2010"
case .AppleWatch: return "\(self) was introduced 2014"
}
}
}
print (Device.iPhone.introduced())
10.枚举中的属性
enum Device1 {
case iPad, iPhone
var year: Int {
switch self {
case .iPad:
return 2010
case .iPhone:
return 2007
}
}
}
let iPhone = Device1.iPhone
print(iPhone.year)
ParameterEncodingFailureReason
通过ParameterEncodingFailureReason我们能够很清楚的看出来这是一个参数编码的错误原因。大家注意reason这个词，在命名中，有或者没有这个词，表达的意境完全不同，因此，Alamofire牛逼就体现在这些细节之中。
public enum AFError: Error {
/// The underlying reason the parameter encoding error occurred.
///
/// - missingURL: The URL request did not have a URL to encode.
/// - jsonEncodingFailed: JSON serialization failed with an underlying system error during the
/// encoding process.
/// - propertyListEncodingFailed: Property list serialization failed with an underlying system error during
/// encoding process.
public enum ParameterEncodingFailureReason {
case missingURL
case jsonEncodingFailed(error: Error)
case propertyListEncodingFailed(error: Error)
}
}
ParameterEncodingFailureReason本身是一个enum，同时，它又被包含在AFError之中，这说明枚举之中可以有另一个枚举。那么像这种情况我们怎么使用呢？看下边的代码：
let parameterErrorReason = AFError.ParameterEncodingFailureReason.missingURL
枚举的访问是一级一级进行的。我们再看这行代码：case jsonEncodingFailed(error: Error)。jsonEncodingFailed(error: Error)并不是函数，就是枚举的一个普通的子选项。(error: Error)是它的一个关联值，相对于任何一个子选项，我们都可以关联任何值，它的意义就在于，把这些值与子选项进行绑定，方便在需要的时候调用。我们会在下边讲解如何获取关联值。
参数编码有一下几种方式：
- 把参数编码到URL中
- 把参数编码到httpBody中
Alamofire中是如何进行参数编码的，这方面的内容会在后续的ParameterEncoding.swift这一篇文章中给出详细的解释。那么编码失败的原因可能为：
missingURL给定的urlRequest.url为nil的情况抛出错误jsonEncodingFailed(error: Error)当选择把参数编码成JSON格式的情况下，参数JSON化抛出的错误propertyListEncodingFailed(error: Error)这个同上
综上所述，ParameterEncodingFailureReason封装了参数编码的错误，可能出现的错误类型为Error，说明这些所谓一般是调用系统Api产生的错误。
MultipartEncodingFailureReason
public enum MultipartEncodingFailureReason {
case bodyPartURLInvalid(url: URL)
case bodyPartFilenameInvalid(in: URL)
case bodyPartFileNotReachable(at: URL)
case bodyPartFileNotReachableWithError(atURL: URL, error: Error)
case bodyPartFileIsDirectory(at: URL)
case bodyPartFileSizeNotAvailable(at: URL)
case bodyPartFileSizeQueryFailedWithError(forURL: URL, error: Error)
case bodyPartInputStreamCreationFailed(for: URL)
case outputStreamCreationFailed(for: URL)
case outputStreamFileAlreadyExists(at: URL)
case outputStreamURLInvalid(url: URL)
case outputStreamWriteFailed(error: Error)
case inputStreamReadFailed(error: Error)
}
多部分编码错误一般发生在上传或下载请求中对数据的处理过程中，这里边最重要的是对上传数据的处理过程，会在后续的MultipartFormData.swift这一篇文章中给出详细的解释，我们就简单的分析下MultipartEncodingFailureReason子选项错误出现的原因：
bodyPartURLInvalid(url: URL)上传数据时，可以通过fileURL的方式，读取本地文件数据，如果fileURL不可用，就会抛出这个错误bodyPartFilenameInvalid(in: URL)如果使用fileURL的lastPathComponent或者pathExtension获取filename为空抛出的错误bodyPartFileNotReachable(at: URL)通过fileURL不能访问数据，也就是不可达的bodyPartFileNotReachableWithError(atURL: URL, error: Error)这个不同于bodyPartFileNotReachable(at: URL),当尝试检测fileURL是不是可达的情况下抛出的错误bodyPartFileIsDirectory(at: URL)当fileURL是一个文件夹时抛出错误bodyPartFileSizeNotAvailable(at: URL)当使用系统Api获取fileURL指定文件的size出现错误bodyPartFileSizeQueryFailedWithError(forURL: URL, error: Error)查询fileURL指定文件size出现错误bodyPartInputStreamCreationFailed(for: URL)通过fileURL创建inputStream出现错误outputStreamCreationFailed(for: URL)当尝试把编码后的数据写入到硬盘时，创建outputStream出现错误outputStreamFileAlreadyExists(at: URL)数据不能被写入，因为指定的fileURL已经存在outputStreamURLInvalid(url: URL)fileURL不是一个file URLoutputStreamWriteFailed(error: Error)数据流写入错误inputStreamReadFailed(error: Error)数据流读入错误
综上所述，这些错误基本上都跟数据的操作相关，这个在后续会做出很详细的说明。
ResponseValidationFailureReason
public enum ResponseValidationFailureReason {
case dataFileNil
case dataFileReadFailed(at: URL)
case missingContentType(acceptableContentTypes: [String])
case unacceptableContentType(acceptableContentTypes: [String], responseContentType: String)
case unacceptableStatusCode(code: Int)
}
Alamofire不管请求是否成功，都会返回response。它提供了验证ContentType和StatusCode的功能，关于验证，再后续的文章中会有详细的解答，我们先看看这些原因：
dataFileNil保存数据的URL不存在，这种情况一般出现在下载任务中，指的是下载代理中的fileURL缺失dataFileReadFailed(at: URL)保存数据的URL无法读取数据，同上missingContentType(acceptableContentTypes: [String])服务器返回的response不包含ContentType且提供的acceptableContentTypes不包含通配符(通配符表示可以接受任何类型)unacceptableContentType(acceptableContentTypes: [String], responseContentType: String)ContentTypes不匹配unacceptableStatusCode(code: Int)StatusCode不匹配
ResponseSerializationFailureReason
public enum ResponseSerializationFailureReason {
case inputDataNil
case inputDataNilOrZeroLength
case inputFileNil
case inputFileReadFailed(at: URL)
case stringSerializationFailed(encoding: String.Encoding)
case jsonSerializationFailed(error: Error)
case propertyListSerializationFailed(error: Error)
}
我们在Alamofire源码解读系列(一)之概述和使用中已经提到，Alamofire支持把服务器的response序列成几种数据格式。
- response 直接返回HTTPResponse，未序列化
- responseData 序列化为Data
- responseJSON 序列化为Json
- responseString 序列化为字符串
- responsePropertyList 序列化为Any
那么在序列化的过程中，很可能会发生下边的错误：
inputDataNil服务器返回的response没有数据inputDataNilOrZeroLength服务器返回的response没有数据或者数据的长度是0inputFileNil指向数据的URL不存在inputFileReadFailed(at: URL)指向数据的URL无法读取数据stringSerializationFailed(encoding: String.Encoding)当使用指定的String.Encoding序列化数据为字符串时，抛出的错误jsonSerializationFailed(error: Error)JSON序列化错误propertyListSerializationFailed(error: Error)plist序列化错误
AFError
上边内容中介绍的ParameterEncodingFailureReason MultipartEncodingFailureReason ResponseValidationFailureReason和 ResponseSerializationFailureReason，他们是定义在AFError中独立的枚举，他们之间是包含和被包含的关系，理解这一点很重要，因为有了这种包含的管理，在使用中就需要通过AFError.ParameterEncodingFailureReason这种方式进行操作。
那么最重要的问题就是，如何把上边4个独立的枚举进行串联呢？Alamofire巧妙的地方就在这里，有4个独立的枚举，分别代表4大错误。也就是说这个网络框架肯定有这4大错误模块，我们只需要给AFError设计4个子选项，每个子选项关联上上边4个独立枚举的值就ok了。
这个设计真的很巧妙，试想，如果把所有的错误都放到AFError中，就显得非常冗余。那么下边的代码就呼之欲出了，大家好好体会体会在swift下这么设计的妙用：
case invalidURL(url: URLConvertible)
case parameterEncodingFailed(reason: ParameterEncodingFailureReason)
case multipartEncodingFailed(reason: MultipartEncodingFailureReason)
case responseValidationFailed(reason: ResponseValidationFailureReason)
case responseSerializationFailed(reason: ResponseSerializationFailureReason)
AFError的扩展
也许在开发中，我们完成了上边的代码就认为够用了，但对于一个开源框架而言，远远是不够的。我们一点点进行剖析：
现在给定一条数据：
func findErrorType(error: AFError) {
}
我只需要知道这个error是不是参数编码错误，应该怎么办？因此为AFError提供5个布尔类型的属性，专门用来获取当前的错误是不是某个指定的类型。这个功能的实现比较简单，代码如下：
extension AFError {
/// Returns whether the AFError is an invalid URL error.
public var isInvalidURLError: Bool {
if case .invalidURL = self { return true }
return false
}
/// Returns whether the AFError is a parameter encoding error. When `true`, the `underlyingError` property will
/// contain the associated value.
public var isParameterEncodingError: Bool {
if case .parameterEncodingFailed = self { return true }
return false
}
/// Returns whether the AFError is a multipart encoding error. When `true`, the `url` and `underlyingError` properties
/// will contain the associated values.
public var isMultipartEncodingError: Bool {
if case .multipartEncodingFailed = self { return true }
return false
}
/// Returns whether the `AFError` is a response validation error. When `true`, the `acceptableContentTypes`,
/// `responseContentType`, and `responseCode` properties will contain the associated values.
public var isResponseValidationError: Bool {
if case .responseValidationFailed = self { return true }
return false
}
/// Returns whether the `AFError` is a response serialization error. When `true`, the `failedStringEncoding` and
/// `underlyingError` properties will contain the associated values.
public var isResponseSerializationError: Bool {
if case .responseSerializationFailed = self { return true }
return false
}
}
总而言之，这些都是给AFError这个枚举扩展的属性，还包含下边这些属性：
-
urlConvertible: URLConvertible?获取某个属性，这个属性实现了URLConvertible协议，在AFError中只有case invalidURL(url: URLConvertible)这个选项符合要求/// The `URLConvertible` associated with the error. public var urlConvertible: URLConvertible? { switch self { case .invalidURL(let url): return url default: return nil } } -
url: URL?获取AFError中的URL，当然这个URL只跟MultipartEncodingFailureReason这个子选项有关/// The `URL` associated with the error. public var url: URL? { switch self { case .multipartEncodingFailed(let reason): return reason.url default: return nil } } -
underlyingError: Error?AFError中封装的所有的可能出现的错误中，并不是每种可能都会返回Error这个错误信息，因此这个属性是可选的/// The `Error` returned by a system framework associated with a `.parameterEncodingFailed`, /// `.multipartEncodingFailed` or `.responseSerializationFailed` error. public var underlyingError: Error? { switch self { case .parameterEncodingFailed(let reason): return reason.underlyingError case .multipartEncodingFailed(let reason): return reason.underlyingError case .responseSerializationFailed(let reason): return reason.underlyingError default: return nil } } -
acceptableContentTypes: [String]?可接受的ContentType/// The response `Content-Type` of a `.responseValidationFailed` error. public var responseContentType: String? { switch self { case .responseValidationFailed(let reason): return reason.responseContentType default: return nil } } -
responseCode: Int?响应码/// The response code of a `.responseValidationFailed` error. public var responseCode: Int? { switch self { case .responseValidationFailed(let reason): return reason.responseCode default: return nil } } -
failedStringEncoding: String.Encoding?错误的字符串编码/// The `String.Encoding` associated with a failed `.stringResponse()` call. public var failedStringEncoding: String.Encoding? { switch self { case .responseSerializationFailed(let reason): return reason.failedStringEncoding default: return nil } }
这里是一个小的分割线，在上边属性的获取中，也是用到了下边代码中的扩展功能：
extension AFError.ParameterEncodingFailureReason {
var underlyingError: Error? {
switch self {
case .jsonEncodingFailed(let error), .propertyListEncodingFailed(let error):
return error
default:
return nil
}
}
}
extension AFError.MultipartEncodingFailureReason {
var url: URL? {
switch self {
case .bodyPartURLInvalid(let url), .bodyPartFilenameInvalid(let url), .bodyPartFileNotReachable(let url),
.bodyPartFileIsDirectory(let url), .bodyPartFileSizeNotAvailable(let url),
.bodyPartInputStreamCreationFailed(let url), .outputStreamCreationFailed(let url),
.outputStreamFileAlreadyExists(let url), .outputStreamURLInvalid(let url),
.bodyPartFileNotReachableWithError(let url, _), .bodyPartFileSizeQueryFailedWithError(let url, _):
return url
default:
return nil
}
}
var underlyingError: Error? {
switch self {
case .bodyPartFileNotReachableWithError(_, let error), .bodyPartFileSizeQueryFailedWithError(_, let error),
.outputStreamWriteFailed(let error), .inputStreamReadFailed(let error):
return error
default:
return nil
}
}
}
extension AFError.ResponseValidationFailureReason {
var acceptableContentTypes: [String]? {
switch self {
case .missingContentType(let types), .unacceptableContentType(let types, _):
return types
default:
return nil
}
}
var responseContentType: String? {
switch self {
case .unacceptableContentType(_, let responseType):
return responseType
default:
return nil
}
}
var responseCode: Int? {
switch self {
case .unacceptableStatusCode(let code):
return code
default:
return nil
}
}
}
extension AFError.ResponseSerializationFailureReason {
var failedStringEncoding: String.Encoding? {
switch self {
case .stringSerializationFailed(let encoding):
return encoding
default:
return nil
}
}
var underlyingError: Error? {
switch self {
case .jsonSerializationFailed(let error), .propertyListSerializationFailed(let error):
return error
default:
return nil
}
}
}
错误描述
在开发中，如果程序遇到错误，我们往往会给用户展示更加直观的信息，这就要求我们把错误信息转换成易于理解的内容。因此我们只要实现LocalizedError协议就好了。这里边的内容很简单，在这里就直接把代码写上了，不做分析：
extension AFError: LocalizedError {
public var errorDescription: String? {
switch self {
case .invalidURL(let url):
return "URL is not valid: \(url)"
case .parameterEncodingFailed(let reason):
return reason.localizedDescription
case .multipartEncodingFailed(let reason):
return reason.localizedDescription
case .responseValidationFailed(let reason):
return reason.localizedDescription
case .responseSerializationFailed(let reason):
return reason.localizedDescription
}
}
}
extension AFError.ParameterEncodingFailureReason {
var localizedDescription: String {
switch self {
case .missingURL:
return "URL request to encode was missing a URL"
case .jsonEncodingFailed(let error):
return "JSON could not be encoded because of error:\n\(error.localizedDescription)"
case .propertyListEncodingFailed(let error):
return "PropertyList could not be encoded because of error:\n\(error.localizedDescription)"
}
}
}
extension AFError.MultipartEncodingFailureReason {
var localizedDescription: String {
switch self {
case .bodyPartURLInvalid(let url):
return "The URL provided is not a file URL: \(url)"
case .bodyPartFilenameInvalid(let url):
return "The URL provided does not have a valid filename: \(url)"
case .bodyPartFileNotReachable(let url):
return "The URL provided is not reachable: \(url)"
case .bodyPartFileNotReachableWithError(let url, let error):
return (
"The system returned an error while checking the provided URL for " +
"reachability.\nURL: \(url)\nError: \(error)"
)
case .bodyPartFileIsDirectory(let url):
return "The URL provided is a directory: \(url)"
case .bodyPartFileSizeNotAvailable(let url):
return "Could not fetch the file size from the provided URL: \(url)"
case .bodyPartFileSizeQueryFailedWithError(let url, let error):
return (
"The system returned an error while attempting to fetch the file size from the " +
"provided URL.\nURL: \(url)\nError: \(error)"
)
case .bodyPartInputStreamCreationFailed(let url):
return "Failed to create an InputStream for the provided URL: \(url)"
case .outputStreamCreationFailed(let url):
return "Failed to create an OutputStream for URL: \(url)"
case .outputStreamFileAlreadyExists(let url):
return "A file already exists at the provided URL: \(url)"
case .outputStreamURLInvalid(let url):
return "The provided OutputStream URL is invalid: \(url)"
case .outputStreamWriteFailed(let error):
return "OutputStream write failed with error: \(error)"
case .inputStreamReadFailed(let error):
return "InputStream read failed with error: \(error)"
}
}
}
extension AFError.ResponseSerializationFailureReason {
var localizedDescription: String {
switch self {
case .inputDataNil:
return "Response could not be serialized, input data was nil."
case .inputDataNilOrZeroLength:
return "Response could not be serialized, input data was nil or zero length."
case .inputFileNil:
return "Response could not be serialized, input file was nil."
case .inputFileReadFailed(let url):
return "Response could not be serialized, input file could not be read: \(url)."
case .stringSerializationFailed(let encoding):
return "String could not be serialized with encoding: \(encoding)."
case .jsonSerializationFailed(let error):
return "JSON could not be serialized because of error:\n\(error.localizedDescription)"
case .propertyListSerializationFailed(let error):
return "PropertyList could not be serialized because of error:\n\(error.localizedDescription)"
}
}
}
extension AFError.ResponseValidationFailureReason {
var localizedDescription: String {
switch self {
case .dataFileNil:
return "Response could not be validated, data file was nil."
case .dataFileReadFailed(let url):
return "Response could not be validated, data file could not be read: \(url)."
case .missingContentType(let types):
return (
"Response Content-Type was missing and acceptable content types " +
"(\(types.joined(separator: ","))) do not match \"*/*\"."
)
case .unacceptableContentType(let acceptableTypes, let responseType):
return (
"Response Content-Type \"\(responseType)\" does not match any acceptable types: " +
"\(acceptableTypes.joined(separator: ","))."
)
case .unacceptableStatusCode(let code):
return "Response status code was unacceptable: \(code)."
}
}
}
总结
通过阅读AFError这篇代码，给了我很大的震撼，在代码的设计上，可以参考这种设计方式。
容老衲休息一天，再带来下一篇Notifications.swift的源码解读。
由于知识水平有限，如有错误，还望指出
