Alamofire源码解读系列(二)之错误处理(AFError)

    xiaoxiao2021-03-26  18

    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 URL outputStreamWriteFailed(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0 inputFileNil 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

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