CS193P - Lecture 11: Error Handling Persistence

0:44 error handling

• 請自行翻書 swift-book/ErrorHandling

9:59 persistence

1. FileManager (filesystem)
2. CoreData (SQL database)
3. CloudKit (database in the cloud)
4. UserDefault (lightweight data)

12:23 File System

• file system like a normal Unix filesystem, it starts at /
• /application: executable, .jpgs, not writable
• /documents: permanent storage, and always visiable to the user
• /support: permanent storage not seen directly by the user
• /caches: store temporary file

let manager = FileManager.default
let url = manager.urls(for: .documentDirectory, in: .userDomainMask).first

17:30 URL & Data

// IMG0011.jpg
func appendingPathComponent(String) -> URL // "IMG0011"
func appendingPathExtension(Stirng) -> URL // "jpg"

// URL
var isFileURL: Bool 
func resourceValues(for keys: [URLResourceKey]) throws -> [URLResourceKey:Any]? // 檔案建立日期等資訊

// Data
init(contentsOf: URL, options: Data.ReadingOptions) throws
func write(to url: URL, options: Data.WritingOptions) throws -> Bool

19:28 Archiving

• 使用 Codable 把物件轉換成 Data 後存擋
• 只要 struct 內的資料都有遵守 codable，那麼該 struct 即有遵守 codable (Swift幫我們做了)
• enum 如果帶有 associate value 的話，要手動處理
• CodingKeys

手動處理：

struct MyStruct: Codable {
    var someDate: Date
    var someString: String
    var other: SomeOtherType // 非典型型別
    
    init(form decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self) 
        someDate = try container.decode(Date.self, forKey: .someDate) 
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(someDate, forKey: .someDate)
    }
}

29:00 UserDefault

• String, Int, Bool, Float, Date, Data, Array or Dictionary

let defaults = UserDefault.standard
defaults.set(object, forKey: "SomeKey") // object must be a Property List
defaults.setDouble(37.5, forKey: "MyDouble")
let i: Int = defaults.intefer(forKey: "MyInteger")

34:53 Demo

概要

• JSON/Codable
• Dealing with thrown errors
• saving out EmojiArt document to the File System
• Implementing the ViewModel for our next MVVM in EmojiArt: PaletteStore
• UserDefault

點出問題

• 拖拉圖片、拖拉 emoji 之後
• 但 xcode rebuild 又要重頭開始
• 所以我們開始做保存

寫 func save(to url: URL)

• EmojiArtDocument 內的 emojiArt 是我們要保存的資料
• EmojiArtModel 弄出一個 func json() -> Data，但會發現沒有遵守 Codable
• 原來是 EmojiArtModel.Background 這個 enum 沒有遵守 Codable
• 針對這點，開始實作 init(form decoder: Decode) & func encode(to encoder: Encoder)

// EmojiArtModel
enum Background: Equatable, Codable {
    case blank
    case url(URL)
    case imageData(Data)
    
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if let url = try? container.decoder(URL.self, forKey: .url) {
            self = .url(url)
        } else if let imageData = try? container.decoder(Data.self, forKey: .imageData) {
            self = .imageData(imageData)
        } else {
            self = .blank
        }
    }
    
    enum CodingKeys: String, CodingKey {
        case url = "theURL"
        case imageData
    }
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        switch self {
            case .url(let url): try container.encode(url, forKey: .url)
            case .imageData(let data): try container.encoder(data, forKey: .imageData)
            case .blank: break
        }
    }
}

func json() throws -> Data {
    return try JSONEncoder().encoder(self)
}

// EmojiArtDocument
func save(to url: URL) {
    do {
        let data: Data = try emojiArt.json()
        try data.write(to: url)
    } catch {
        print("error = \(error)")
    }
}
func autoSave() {
    if let url = AutoSave.url {
        save(to: url)
    }
}
private struct AutoSave {
    static let filename = "Autosaved.emojiart"
    static var url: URL? {
        let documentDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first
        return documentDirectory?.appendingPathComponent(filename)
    }
}

現在關掉重開後還是會發現東西要重來，因為還沒做載入

// EmojiArtModel
init(json: Data) throws {
    self = try JSONDecoder().decode(self)
}
init(url: URL) throws {
    let data = try Data(contentsOf: url)
    self = try EmojiArtModel(json: data)
}

// EmojiArtDocument
init() {
    if let Autosave.url, let autosavedEmojiArt = try? EmojiArtModel(url: url) {
        emojiArt = autosavedEmojiArt
        fetchBackgroundImageDataIfNecessary()
    } else {
        emojiArt = EmojiArtModel()
    }
}

在 model 的 didSet 方法內新增 scheduleAutosave()

func scheduleAutosave() {
    autosaveTimer?.invalidate()
    autosaveTimer = Timer.scheduelTimer(withTimeInterval: Autosave.coalescingInterval, repeats: false) { _ in
        self.autosave()
    }
}

1:23:29 Palette

• 有自己的 MVVM，跟畫布用的不一樣
• store in userDefault -> HW: iCloud

struct EmojiApp: App {
    let document = EmojiArtDocument()
    let paletteStore = PaletteStore(named: "Default")
    var body: some View {
        WindowGroup {
            EmojiArtDocumentView(document: document)
        }
    }
}