cs193p_2021笔记[6]_Persistence

作者: walkerwzy | 来源:发表于2021-11-08 01:08 被阅读0次

cs193p_2021_笔记_1
cs193p_2021_笔记_2
cs193p_2021_笔记_3_Animation_Transition
cs193p_2021_笔记_4_Color_Image_Gesture
cs193p_2021_笔记_5_Property Wrapper
cs193p_2021_笔记_6_Persistence
cs193p_2021_笔记_7_Document Architecture
cs193p_2021_笔记_8

Persistence

持久化数据的方式有

File system（FileManager）
Sqlite/CoreData
iCloud: 根据上面两种格式存储
CloutKit: a database in the cloud (network)
UserDefaults
Codable/JSON
UIDocument (UIKit feature)(与Files App集成)
3rd-party

UserDefaults

只能存储Property List
Property List支持String, Int, Bool, floating point, Date, Data, Array or Dictionary
- 任何其它类型需要转成Property List
- Codable converts structs into Data objects (and Data is a Property List).

let defaults = UserDefaults.standard
defaults.set(object, forKey: “SomeKey”) // object must be a Property List
defaults.setDouble(37.5, forKey: “MyDouble”)

// retrive

let i: Int = defaults.integer(forKey: “MyInteger”)
let b: Data = defaults.data(forKey: “MyData”)
let u: URL = defaults.url(forKey: “MyURL”)
let strings: [String] = defaults.stringArray(forKey: “MyString”) 
// etc.
// Retrieving Arrays of anything but String is more complicated ...
let a = defaults.array(forKey: “MyArray”) // will return Array<Any>
// 最好用Codable的data(forKey:)替代

Core Data

SwiftUI进行的集成:

创建的对象是ObservableObjects
一个property wrapper @FetchRequest
管理对象(context)是NSManagedObjectContext
context通过@Environment传入

demo:

@Environnment(\.managedObjectContext) var context
let flight = Flight(context: context)
flight.aircraft = “B737” // etc.

let ksjc = Airport(context: context)
ksjc.icao = “KSJC” // etc.

flight.origin = ksjc // this would add flight to ksjc.flightsFrom too try? context.save()

let request = NSFetchRequest<Flight>(entityName: “Flight”) request.predicate =
NSPredicate(format: “arrival < %@ and origin = %@“, Date(), ksjc) 
request.sortDescriptors = [NSSortDescriptor(key: “ident”, ascending: true)] 

let flights = try? context.fetch(request) // past KSJC flights sorted by ident
// flights is nil if fetch failed, [] if no such flights, otherwise [Flight]

以上是core data部分，还是浓浓的OC的痕迹，看看Swift UI的版本。

首先，上述的Flights, Airports都是ViewModel。它自然拥有它的Property Wrapper:

@FetchRequest(entity:sortDescriptors:predicate:) var flights: FetchedResults<Flight>
@FetchRequest(fetchRequest:) var airports: FetchedResults<Airport>

// flights and airports will continuously update as the database changes. 
ForEach(flights) { flight in
    // UI for a flight built using flight 
}

// bi-binding
_flights = FetchRequest(...)

Cloud Kit

上个demo吧

let db = CKContainer.default.public/shared/privateCloudDatabase 
// Record理解为Table
let tweet = CKRecord(“Tweet”)
// 索引理解为Field
tweet[“text”] = “140 characters of pure joy”
let tweeter = CKRecord(“TwitterUser”)
tweet[“tweeter”] = CKReference(record: tweeter, action: .deleteSelf)
db.save(tweet) { (savedRecord: CKRecord?, error: NSError?) -> Void in
    if error == nil {
    // hooray!
    } else if error?.errorCode == CKErrorCode. NotAuthenticated.rawValue {
        // tell user he or she has to be logged in to iCloud for this to work!
    } else {
        // report other errors (there are 29 different CKErrorCodes!) 
    }
}

// Query
// 类似core data, 构造predict, request(就是query)即可

let predicate = NSPredicate(format: “text contains %@“, searchString)
let query = CKQuery(recordType: “Tweet”, predicate: predicate)
db.perform(query) { (records: [CKRecord]?, error: NSError?) in
    if error == nil {
        // records will be an array of matching CKRecords
    } else if error?.errorCode == CKErrorCode.NotAuthenticated.rawValue {
        // tell user he or she has to be logged in to iCloud for this to work!
    } else {
        // report other errors (there are 29 different CKErrorCodes!) 
    }
}

One of the coolest features of Cloud Kit is its ability to send push notifications on changes. All you do is register an NSPredicate and whenever the database changes to match it,

File System

Sandbox包含：

Application directory — Your executable, .jpgs, etc.; not writeable.
Documents directory — Permanent storage created by and always visible to the user.
Application Support directory — Permanent storage not seen directly by the user.
Caches directory — Store temporary files here (this is not backed up).
Other directories (see documentation)
...

let url: URL = FileManager.default.url(
    for directory: FileManager.SearchPathDirectory.documentDirectory, // for example 
    in domainMask: .userDomainMask // always .userDomainMask on iOS
    appropriateFor: nil, // only meaningful for “replace” file operations
    create: true // whether to create the system directory if it doesn’t already exist
 )

Examples of SearchPathDirectory values :

.documentDirectory, 
.applicationSupportDirectory, 
.cachesDirectory, 
etc.

再列些常用api：

// URL

func appendingPathComponent(String) -> URL
func appendingPathExtension(String) -> URL // e.g. “jpg”
var isFileURL: Bool // is this a file URL (whether file exists or not) or something else? 
func resourceValues(for keys: [URLResourceKey]) throws -> [URLResourceKey:Any]? 
// Example keys: .creationDateKey, .isDirectoryKey, .fileSizeKey

// Data

// retrive binary data
// option almost always []
init(contentsOf: URL, options: Data.ReadingOptions) throws 
// write
// The options can be things like .atomic (write to tmp file, then swap) or .withoutOverwriting.
func write(to url: URL, options: Data.WritingOptions) throws -> Bool

// FileManager
fileExists(atPath: String) -> Bool
// Can also create and enumerate directories; move, copy, delete files; etc.

Codable

保留一个对象所有的var（变量）的机制
如果一个Struct它的成员变是Codable的，那么Swift会帮你把这个Struct实现Codable，比如没有associated data的Enum。
帮你实现不代表不要显式声明
基础类型基本上都实现了Codable

let object: MyType = ...
// encode
let jsonData: Data? = try? JSONEncoder().encode(object)

// write file
try jsonData.write(to: url)

// deocde as string
let jsonString = String(data: jsonData!, encoding: .utf8)

// decode as object
let myObject: MyType = try? JSONDecoder().decode(MyType.self, from: jsonData!)
// 从字符串到对象没有一步到位的办法，只能先string->Data
let data = jsstring.data(using: .utf8) // 再把data传到上术方法里

encode, decode是会throw的，注意try_catch相应的Error，比如.keyNotFound, .dataCorrupted...

CodingKeys

json与对象相互进行转化有一个通用的需求，就是键的映射，这更常用在外部API与本地类的映射中，比如userId，别人叫guestId，等等，Swift中，用一个叫CodingKeys的枚举来实现这个映射：

private enum CodingKeys: String, CodingKey {
    case uid = "user_id"
    case someDate = "some_date"
    case pname = "panme" // 表示在JSON中也叫这个名字 
    case sku // 如果名字一样的话，可以这么简写 
    // 但是不写的话，序列化的时候就不会序列这个字段了
    // 解码时会有 KeyNotFound 类的错误
}

// 结合起来，用在init中
init(from decoder: Decoder) throws {
    // container是切入点，要弄清楚
    // 如果没有手写键的映射表，那么keydBy就是自己
    let container = try decoder.container(keyedBy: CodingKeys.self)
    someDate = try container.decode(Date.self, forKey: .someDate) 
    // 从json中加载.someDate对应的键的值，尝试解码成Date
    // other vars (每种case必须全部都有)
}

Enum

序列化枚举有点复杂：

简单枚举应该怎么序列化？其实是序列化成case对应的名字和表示空JSON的{}组成的键值对，比如{"math":{}}
有关联数据的枚举呢？那就得自己提供encoder:
- case url: try container.encode(url, forKey: .url) 即对相应的枚举值进行相应的encode
并且自行decode，但是与struct（为每一个key填值）不同，因为枚举变量只是一个值，所以是依次尝试，解码成功就认定是那一个枚举值

if let url = try? container.decode(URL.self, forKey: .url) {
    self = .url(url)
} else if ... // 别的尝试

// 此句的作用是根据.url对应的键名，取出值，反射成URL对象，如果成功，那么这个枚举值是.url无疑
// 而且关联数据就是反射的结果
// 如果失败，继续换一个键名，将对应的值转成对应的类型，依次类推

那么如何手动decode一个原始的枚举呢？
- 我们知道上述实践是为了反射出关联数据，并且根据能够成功反射关联数据来判断枚举类型
- 原始枚举需要encode哪个值呢？-> 目前我只能做一个空struct来实现序列化成{}的目的 -> 为了跟默认形态保持一致
  - 事实上你是可以encode成任意值的（比如100，"hello"，因为我们只关心有没有这个键，有的话，就是这个枚举类型，只是{}拥有可读性
  - 你encode成什么值，decode的时候对对应的键尝试去反射回这个值就行了

最后，思考题：

上面说了，原生枚举序列化成： {"math":{}}，也说了，如果，键对应的值对原生枚举序列化是没意义的，可以是任何值，那么对于{"math":100}，能否顺序序列化回其枚举形态.math呢？

答案：

值为100报错了
于是我改为""或"other“等字符串或空字符串，解码的结果是nil

也就是说，默认的decode只认{}

image.png

而前面我们知道了，如果是自己手写，它可以是任何值，它的意义仅仅是个标识，并不会取它的值。验证：

enum NormEnum: Codable {
    case history, math, geometry
    
    private enum keyMap: String, CodingKey{
        case history  = "HIST"
        case math     = "MATH"
        case geometry = "GEOM"
    }

    func encode(to encoder: Encoder) throws {
        var container = try encoder.container(keyedBy: keyMap.self)
        switch self{
        case .history: try container.encode("", forKey: .history)
        case .math: try container.encode("", forKey: .math)
        case .geometry: try container.encode("", forKey: .geometry)
        }
    }
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: keyMap.self)
        if let s = try? container.decode(String.self, forKey: .history) {
            self = .history
        } else if let s = try? container.decode(String.self, forKey: .math) {
            self = .math
        } else {
            self = .geometry
        }
    }
}

上面的代码中，我将三个字段全部用空字符串编码，并且给了三个不同的键名，现在，我真入任意值，比如"HAHA"，解码看看：

let js2 = "{\"MATH\":\"HAHA\"}"
let js2d = js2.data(using: .utf8)
let myobj2 = try? JSONDecoder().decode(NormEnum.self, from: js2d!)

结果成功认出myobj2是一个.math。原理当然是我的代码里在尝试转成一个字符串，而没有限定是什么字符串。

cs193p_2021笔记[6]_Persistence

Persistence

UserDefaults

Core Data

Cloud Kit

File System

Codable

CodingKeys

Enum

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