Swift Codable 精華——手動decoder需要掌握知識點(diǎn)

注：代碼基于Swift4.0

導(dǎo)讀：Swift 4 現(xiàn)在可以支持很方便的轉(zhuǎn)模型了。例：

Book結(jié)構(gòu)體遵守Decodable協(xié)議

struct Book: Decodable {
    var title: String
    var author: String
    var rating: Float
}

自動解碼

override func viewDidLoad() {
        super.viewDidLoad()
        
        
        let jsonString = """
{   "title": "War and Peace: A protocol oriented approach to diplomacy",
    "author": "A. Keed Decoder",
    "rating": 5.0
}
"""
        if let data = jsonString.data(using: .utf8) {
            let decoder = JSONDecoder()
            if let book = try? decoder.decode(Book.self, from: data) {
                print(book.title) //War and Peace: A protocol oriented approach to diplomacy
            } else {
                print("decode failed") 
            }
            
        }
    }

正題：老代碼可能出現(xiàn)手動Decoder怔鳖，那就探索一下吧吗铐。

一：可選明垢，解碼用decodeIfPresent

struct Book: Decodable {
    var title: String
    var author: String
    var rating: Float?

    init(from decoder: Decoder) throws {
        let keyedContainer = try decoder.container(keyedBy: CodingKeys.self)
        title = try keyedContainer.decode(String.self, forKey: .title)
        author = try keyedContainer.decode(String.self, forKey: .author)
        rating =  try keyedContainer.decodeIfPresent(Float.self, forKey: CodingKeys.rating)
    }

    enum CodingKeys: String, CodingKey {
        case title
        case author
        case rating
    }
}

二：非可選员辩，但json數(shù)據(jù)為nil時解碼失敗怎么辦（不想用可選撞鹉，就設(shè)置個默認(rèn)值吧）

struct Book: Decodable {
    var title: String
    var author: String
    var rating: Float

    init(from decoder: Decoder) throws {
        let keyedContainer = try decoder.container(keyedBy: CodingKeys.self)
        title = try keyedContainer.decode(String.self, forKey: .title)
        author = try keyedContainer.decode(String.self, forKey: .author)
        if let ratingValue = try keyedContainer.decodeIfPresent(Float.self, forKey: CodingKeys.rating) {
            rating = ratingValue
        } else {
            rating = 0
        }
    }

    enum CodingKeys: String, CodingKey {
        case title
        case author
        case rating
    }
}

三：decode失敗, rating為非可選痛悯，但服務(wù)給的json偏偏就不返這個字段余黎，或者返的這個字段為nil

客戶端這么寫rating

struct Book: Decodable {
    var title: String
    var author: String
    var rating: Float
    
    init(from decoder: Decoder) throws {
        let keyedContainer = try decoder.container(keyedBy: CodingKeys.self)
        title = try keyedContainer.decode(String.self, forKey: .title)
        author = try keyedContainer.decode(String.self, forKey: .author)
        rating = try keyedContainer.decode(Float.self, forKey: .rating)
    }

    enum CodingKeys: String, CodingKey {
        case title
        case author
        case rating
    }
}

服務(wù)端就想這么給數(shù)據(jù)

        let jsonString = """
{   "title": "War and Peace: A protocol oriented approach to diplomacy",
    "author": "A. Keed Decoder",
    "rating":
}
"""

或者這么返

        let jsonString = """
{   "title": "War and Peace: A protocol oriented approach to diplomacy",
    "author": "A. Keed Decoder",
}
"""

好的。 print("decode failed")
失敗倒也沒啥载萌，反正不是崩潰惧财，但是Fabric上面捕獲到N多條Non-Fatals log（就是研究這個log才有了這篇記錄）。當(dāng)然還是要處理啦扭仁。所以盡量用可選垮衷，或者不想給可選至少也給個默認(rèn)值吧。

四：如何通過Crashlytics捕獲異常乖坠。

在book結(jié)構(gòu)體中加一個出版日期屬性

struct Book: Decodable {
    var title: String
    var author: String
    var rating: Float
    let publishedAt: Date
    
    private static func dateDecode(_ container: KeyedDecodingContainer<CodingKeys>, key: CodingKeys) throws -> Date {
        let date: Date
        do {
            date = try container.decode(Date.self, forKey: key)
        }
        catch {
            date = Date(timeIntervalSince1970: 0)
            let dateAtString = try container.decode(String.self, forKey: key)
            let bookTitle = try container.decode(String.self, forKey: CodingKeys.title)
            
            let error = CrashlyticsError.bookDateParsingFailed(codingPath: key,
                                                               bookTitle: bookTitle,
                                                                dateValue: dateAtString)
            print(error)
//            Crashlytics.sharedInstance().recordError(error) //Fabric 可以直接記錄
        }
        
        return date
    }
    
    init(from decoder: Decoder) throws {
        let keyedContainer = try decoder.container(keyedBy: CodingKeys.self)
        title = try keyedContainer.decode(String.self, forKey: .title)
        author = try keyedContainer.decode(String.self, forKey: .author)
        rating = try keyedContainer.decode(Float.self, forKey: .rating)
        publishedAt = try Book.dateDecode(keyedContainer, key: CodingKeys.publishedAt)
    }

    enum CodingKeys: String, CodingKey {
        case title
        case author
        case rating
        case publishedAt
    }
    
//string 轉(zhuǎn)date會用到
    static func dateFormatter() -> DateFormatter {
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"
        dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
        return dateFormatter
    }
}

加一個捕獲異常CrashlyticsError文件

import UIKit

enum CrashlyticsError: CustomNSError {
    case bookParsingFailed(codingPath: [CodingKey], debugDescription: String)
    case bookDateParsingFailed(codingPath: CodingKey, bookTitle: String, dateValue: String)
    
    static var errorDomain: String {
        return "XXDecoderDemo"
    }
    
    var errorCode: Int {
        switch self {
        case .bookParsingFailed(_, _):
            return 7780
        case .bookDateParsingFailed(_,_,_):
            return 7781
        }
    }
    
    var errorUserInfo: [String : Any] {
        switch self {
        case .bookParsingFailed(let codingPath, let debugDescription):
            var userInfo = [NSLocalizedDescriptionKey :  "Book Parsing",
                            NSLocalizedFailureReasonErrorKey : "Can't parse",
                            "Description" : debugDescription]
            for (index, element) in codingPath.enumerated() {
                userInfo["Coding Key \(index)"] = element.stringValue
            }
            return userInfo
            
        case .bookDateParsingFailed(let codingPath, let bookTitle, let dateValue):
            return [NSLocalizedDescriptionKey :  "Book Date Parsing",
                    NSLocalizedFailureReasonErrorKey : "Can't parse \(codingPath.stringValue)",
                "Book Title"   : bookTitle,
                "Date value" : dateValue]
        }
    }
}

viewDidLoad代碼搀突。這里publishedAt的jsonString故意寫成錯的格式，然后進(jìn)入到捕獲異常程序熊泵。正常格式應(yīng)是：2018-01-01T00:00:00.000Z（和dateFormatter保持一致）

override func viewDidLoad() {
        super.viewDidLoad()
        
        
        let jsonString = """
{   "title": "War and Peace: A protocol oriented approach to diplomacy",
    "author": "A. Keed Decoder",
    "rating": 5.0,
    "publishedAt": "019-04-16T9:24:37TPM.000Z"

}
"""
        if let data = jsonString.data(using: .utf8) {
            let decoder = JSONDecoder()
            decoder.dateDecodingStrategy = .formatted(Book.dateFormatter())
            if let book = try? decoder.decode(Book.self, from: data) {
                print(book.title) //War and Peace: A protocol oriented approach to diplomacy
            } else {
                print("decode failed")
            }
            
        }
    }

可想而知仰迁，最終解碼不會失敗，還是會打印出書名顽分。

總結(jié)：雖然獲取到的publishedAt是錯的格式轩勘，轉(zhuǎn)化成date會不成功，但是進(jìn)行了異常捕獲怯邪，在catch代碼塊中，將date設(shè)置成date = Date(timeIntervalSince1970: 0)花墩，也相當(dāng)于是設(shè)置默認(rèn)值了悬秉。解碼不失敗澄步，同時還記錄了為什么沒有轉(zhuǎn)化成功的log。可以快速定位到出問題的那條信息和泌。完美村缸！

捕獲到的error信息如下（Fabric記錄的話，會更加清晰）：

print(error)  //bookDateParsingFailed(codingPath: CodingKeys(stringValue: "publishedAt", intValue: nil), bookTitle: "War and Peace: A protocol oriented approach to diplomacy", dateValue: "019-04-16T9:24:37TPM.000Z")

空了會將demo上傳武氓。

每天學(xué)習(xí)一點(diǎn)點(diǎn)梯皿，加油??！