如何将 Swift 4 编码的 JSON 写入文件？

Question

如何将通过 Swift 4 Codable 协议编码的 JSON 对象写入文件？在 Swift 4 之前，我在创建的数据（或字符串）上使用 JSONSerialization.writeJSONObject，但现在 JSONSerialization.isValidJSONObject returns false。一个例子：

import Foundation

class Shark : Codable
{
    var name:String = ""
    var carnivorous:Bool = true
    var numOfTeeth:Int = 0
    var hobbies:[String] = []
}

class JSON
{
    class func encode<T:Encodable>(_ obj:T) -> String?
    {
        if let encodedData = try? JSONEncoder().encode(obj)
        {
            return String(data: encodedData, encoding: .utf8)
        }
        return nil
    }

    class func writeToStream(data:Any, path:String) -> Bool
    {
        var success = false
        if JSONSerialization.isValidJSONObject(data)
        {
            if let stream = OutputStream(toFileAtPath: "\(path)", append: false)
            {
                stream.open()
                var error:NSError?
                JSONSerialization.writeJSONObject(data, to: stream, options: [], error: &error)
                stream.close()
                if let error = error
                {
                    print("Failed to write JSON data: \(error.localizedDescription)")
                    success = false
                }
            }
            else
            {
                print("Could not open JSON file stream at \(path).")
                success = false
            }
        }
        else
        {
            print("Data is not a valid format for JSON serialization: \(data)")
            success = false
        }
        return success
    }
}


let shark = Shark()
shark.name = "Nancy"
shark.carnivorous = true
shark.numOfTeeth = 48
shark.hobbies = ["Dancing", "Swiming", "Eating people"]

if let jsonString = JSON.encode(shark)
{
    let success = JSON.writeToStream(data: jsonString.data(using: .utf8), path: "\(NSHomeDirectory())/Documents")
}

这两种格式都对 JSONSerialization.isValidJSONObject() 无效：

JSON.writeToStream(data: jsonString, path: "\(NSHomeDirectory())/Documents")
JSON.writeToStream(data: jsonString.data(using: .utf8), path: "\(NSHomeDirectory())/Documents")

Data is not a valid format for JSON serialization:
{"numOfTeeth":48,"hobbies":["Dancing","Swiming","Eating people"],"name":"Nancy","carnivorous":true}
Data is not a valid format for JSON serialization: Optional(99 bytes)

如何让它通过 JSON 验证，然后将其写入文件？

Answer 1

JSON序列化。您的 JSONSerialization.isValidJSONObject 用法有误。作为文档 states:

A Foundation object that may be converted to JSON must have the following properties:
• The top level object is an NSArray or NSDictionary.
• All objects are instances of NSString, NSNumber, NSArray, NSDictionary, or NSNull.
• All dictionary keys are instances of NSString.

因此，Data 或 String 类型根本无效 ;)

写入编码数据。要实际编写生成的 Data，请改用相应的 Data.write(to: URL) 方法。例如：

if let encodedData = try? JSONEncoder().encode(obj) {
    let path = "/path/to/obj.json"
    let pathAsURL = URL(fileURLWithPath: path)
    do {
        try encodedData.write(to: pathAsURL)
    } 
    catch {
        print("Failed to write JSON data: \(error.localizedDescription)")
    }
}

只要 JSON 数据是由标准 JSONEncoder 生成的，就不需要额外的验证 ;)

Answer 2

要添加到@Paulo 答案，也可以使用 JSONEncoder 以与 JSONSerialization 类似的方式指定输出格式。为此，您需要创建一个 JSONEncoder 实例并将其 outputFormatting 属性设置为您想要的值（它需要是 JSONEncoder.OutputFormatting 的实例）。

示例：

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted

if let encodedData = try? encoder.encode(obj) {
    let path = "/path/to/obj.json"
    let pathAsURL = URL(fileURLWithPath: path)
    do {
        try encodedData.write(to: pathAsURL)
    } 
    catch {
        print("Failed to write JSON data: \(error.localizedDescription)")
    }
}

现在 encodedData 将使用适当的缩进和换行符编写。

如何将 Swift 4 编码的 JSON 写入文件？

How to write a Swift 4 encoded JSON to a file?

json

swift

serialization

swift4

codable