我是编程新手。我已经创建了一个 Codable 类型，但我不知道必须在函数 viewWillDisappear 中放入什么，其中结构应保存到 UserDefaults > 以及我必须在函数 viewDidAppear 中放入的内容，其中保存的结构应该再次呈现。
这是我到目前为止的代码:
import UIKit
class QuotesViewController: UIViewController {
var randomItems: RandomItems?
var quotes: RandomItems! = RandomItems([
"Andre",
"James",
"Max",
"Stephen",
"Sarah",
"Jeff",
"Paul",
"Mary",
])
}
struct RandomItems: Codable
{
var items : [String]
var seen = 0
init(items:[String], seen: Int)
{
self.items = items
self.seen = seen
}
init(_ items:[String])
{ self.init(items: items, seen: 0) }
mutating func next() -> String
{
let index = Int(arc4random_uniform(UInt32(items.count - seen)))
let item = items.remove(at:index)
items.append(item)
seen = (seen + 1) % items.count
return item
}
func toPropertyList() -> [String: Any] {
return [
"items": items,
"seen": seen
]
}
}
override func viewDidLoad() {
let defaults = UserDefaults.standard
defaults.set(codable: quotes, forKey: "quotes")
}
override func viewDidAppear(_ animated: Bool) {
// Code to load the struct again after the view appears.
let defaults = UserDefaults.standard
quotes = defaults.codable(RandomItems.self, forKey: "quotes") ??
RandomItems([])
}
override func viewWillDisappear(_ animated: Bool) {
// Code to save struct before the view disappears.
let defaults = UserDefaults.standard
if let quotes = quotes {
defaults.set(codable: quotes, forKey: "quotes")
}
}
}
extension UserDefaults {
func set<T: Encodable>(codable: T, forKey key: String) {
let encoder = JSONEncoder()
do {
let data = try encoder.encode(codable)
let jsonString = String(data: data, encoding: .utf8)!
print("Saving \"\(key)\": \(jsonString)")
self.set(jsonString, forKey: key)
} catch {
print("Saving \"\(key)\" failed: \(error)")
}
}
func codable<T: Decodable>(_ codable: T.Type, forKey key: String) -> T? {
guard let jsonString = self.string(forKey: key) else { return nil }
guard let data = jsonString.data(using: .utf8) else { return nil }
let decoder = JSONDecoder()
print("Loading \"\(key)\": \(jsonString)")
return try? decoder.decode(codable, from: data)
}
}
最佳答案
一个很好的设计方法是使用一对辅助方法来扩展 UserDefaults:
extension UserDefaults {
func set<T: Encodable>(codable: T, forKey key: String) {
let encoder = JSONEncoder()
do {
let data = try encoder.encode(codable)
let jsonString = String(data: data, encoding: .utf8)!
print("Saving \"\(key)\": \(jsonString)")
self.set(jsonString, forKey: key)
} catch {
print("Saving \"\(key)\" failed: \(error)")
}
}
func codable<T: Decodable>(_ codable: T.Type, forKey key: String) -> T? {
guard let jsonString = self.string(forKey: key) else { return nil }
guard let data = jsonString.data(using: .utf8) else { return nil }
let decoder = JSONDecoder()
print("Loading \"\(key)\": \(jsonString)")
return try? decoder.decode(codable, from: data)
}
}
然后在QuotesViewController中像这样使用它:
class QuotesViewController: UIViewController {
struct RandomItems: Codable {
var items : [String]
var seen = 0
init(items:[String], seen: Int) {
self.items = items
self.seen = seen
}
init(_ items: [String]) {
self.init(items: items, seen: 0)
}
}
var quotes: RandomItems! = RandomItems([
"James",
"John",
"William",
])
override func viewDidAppear(_ animated: Bool) {
// Code to load the struct again after the view appears.
let defaults = UserDefaults.standard
quotes = defaults.codable(RandomItems.self, forKey: "quotes")
}
override func viewWillDisappear(_ animated: Bool) {
// Code to save struct before the view disappears.
let defaults = UserDefaults.standard
if let quotes = quotes {
defaults.set(codable: quotes, forKey: "quotes")
}
}
}
进入(和退出)上述 View Controller 会打印:
Saving "quotes": {"items":["James","John","William"],"seen":0}
Loading "quotes": {"items":["James","John","William"],"seen":0}
更新。就其值(value)而言，下面是我用来在 Xcode 9 Playground 中测试此代码的完整代码:
import Foundation
extension UserDefaults {
func set<T: Encodable>(codable: T, forKey key: String) {
let encoder = JSONEncoder()
do {
let data = try encoder.encode(codable)
let jsonString = String(data: data, encoding: .utf8)!
print("Saving \"\(key)\": \(jsonString)")
self.set(jsonString, forKey: key)
} catch {
print("Saving \"\(key)\" failed: \(error)")
}
}
func codable<T: Decodable>(_ codable: T.Type, forKey key: String) -> T? {
guard let jsonString = self.string(forKey: key) else { return nil }
guard let data = jsonString.data(using: .utf8) else { return nil }
let decoder = JSONDecoder()
print("Loading \"\(key)\": \(jsonString)")
return try? decoder.decode(codable, from: data)
}
}
class UIViewController: NSObject {
func viewDidAppear(_ animated: Bool) { }
func viewWillDisappear(_ animated: Bool) { }
}
class QuotesViewController: UIViewController {
struct RandomItems: Codable {
var items : [String]
var seen = 0
init(items:[String], seen: Int) {
self.items = items
self.seen = seen
}
init(_ items: [String]) {
self.init(items: items, seen: 0)
}
}
var quotes: RandomItems! = RandomItems([
"James",
"John",
"William",
"Mary",
"Sarah",
"Michael",
"Steve",
"Lisa",
"Jeff",
"Chris",
])
override func viewDidAppear(_ animated: Bool) {
// Code to load the struct again after the view appears.
let defaults = UserDefaults.standard
quotes = defaults.codable(RandomItems.self, forKey: "quotes")
}
override func viewWillDisappear(_ animated: Bool) {
// Code to save struct before the view disappears.
let defaults = UserDefaults.standard
if let quotes = quotes {
defaults.set(codable: quotes, forKey: "quotes")
}
}
}
let vc = QuotesViewController()
vc.viewWillDisappear(false)
vc.quotes = nil
vc.viewDidAppear(true)
print(vc.quotes)
关于swift - 我如何使用 UserDefaults 来保存和加载我的结构？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/46873214/