[英]How to use KVO for UserDefaults in Swift?
我正在重寫應用程序的一部分,並找到了以下代碼:
fileprivate let defaults = UserDefaults.standard
func storeValue(_ value: AnyObject, forKey key:String) {
defaults.set(value, forKey: key)
defaults.synchronize()
NotificationCenter.default.post(name: Notification.Name(rawValue: "persistanceServiceValueChangedNotification"), object: key)
}
func getValueForKey(_ key:String, defaultValue:AnyObject? = nil) -> AnyObject? {
return defaults.object(forKey: key) as AnyObject? ?? defaultValue
}
當 CMD 單擊行defaults.synchronize()
我看到synchronize
計划已棄用。 代碼中是這樣寫的:
/*!
-synchronize is deprecated and will be marked with the NS_DEPRECATED macro in a future release.
-synchronize blocks the calling thread until all in-progress set operations have completed. This is no longer necessary. Replacements for previous uses of -synchronize depend on what the intent of calling synchronize was. If you synchronized...
- ...before reading in order to fetch updated values: remove the synchronize call
- ...after writing in order to notify another program to read: the other program can use KVO to observe the default without needing to notify
- ...before exiting in a non-app (command line tool, agent, or daemon) process: call CFPreferencesAppSynchronize(kCFPreferencesCurrentApplication)
- ...for any other reason: remove the synchronize call
*/
據我所知,我的情況下的用法符合第二種描述:寫后同步,以便通知其他人。
它建議使用 KVO 進行 ovserve,但是如何呢? 當我搜索這個時,我發現了一堆稍微舊的 Objective-C 示例。 觀察 UserDefaults 的最佳做法是什么?
從 iOS 11 + Swift 4 開始,推薦的方式(根據SwiftLint )是使用基於塊的 KVO API。
例子:
假設我有一個整數值存儲在我的用戶默認值中,它被稱為greetingsCount
。
首先我需要擴展UserDefaults
:
extension UserDefaults {
@objc dynamic var greetingsCount: Int {
return integer(forKey: "greetingsCount")
}
}
這允許我們稍后定義觀察的關鍵路徑,如下所示:
var observer: NSKeyValueObservation?
init() {
observer = UserDefaults.standard.observe(\.greetingsCount, options: [.initial, .new], changeHandler: { (defaults, change) in
// your change logic here
})
}
永遠不要忘記清理:
deinit {
observer?.invalidate()
}
來自 David Smith 的博客http://dscoder.com/defaults.html https://twitter.com/catfish_man/status/674727133017587712
如果一個進程設置了共享默認值,然后通知另一個進程讀取它,那么您可能處於在以下情況下調用 -synchronize 方法很有用的少數幾種情況之一: -synchronize 充當“屏障”,因為它保證一旦它返回,讀取該默認值的任何其他進程將看到新值而不是舊值。
對於在 iOS 9.3 及更高版本/macOS Sierra 及更高版本上運行的應用程序,即使在這種情況下也不需要(或推薦)-synchronize,因為默認值的鍵值觀察現在在進程之間起作用,因此讀取過程可以直接觀察值來改變。 因此,在這些操作系統上運行的應用程序通常不應調用同步。
因此,在很可能的情況下,您不需要設置調用同步。 它由 KVO 自動處理。
為此,您需要在處理persistanceServiceValueChangedNotification
通知的類中添加觀察者。 假設您正在設置一個名為“myKey”的密鑰
在您的班級中添加觀察者可能是viewDidLoad
等
UserDefaults.standard.addObserver(self, forKeyPath: "myKey", options: NSKeyValueObservingOptions.new, context: nil)
處理觀察者
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
//do your changes with for key
}
還要在deinit
刪除您的觀察者
對於將來尋找答案的任何人,只有在對同一進程進行更改時才會發布didChangeNotification
,如果您想接收所有更新,而不管進程使用 KVO。
當在當前進程之外進行更改或無處不在的默認值更改時,不會發布此通知。 您可以使用鍵值觀察來為感興趣的特定鍵注冊觀察者,以便收到所有更新的通知,無論是在當前進程內部還是外部進行更改。
這是演示 Xcode 項目的鏈接,它顯示了如何在 UserDefaults 上設置基於塊的 KVO。
使用可重用類型制作的 Swift 4 版本:
文件: KeyValueObserver.swift - 通用可重用 KVO 觀察器(適用於無法使用純 Swift 可觀察值的情況)。
public final class KeyValueObserver<ValueType: Any>: NSObject, Observable {
public typealias ChangeCallback = (KeyValueObserverResult<ValueType>) -> Void
private var context = 0 // Value don't reaaly matter. Only address is important.
private var object: NSObject
private var keyPath: String
private var callback: ChangeCallback
public var isSuspended = false
public init(object: NSObject, keyPath: String, options: NSKeyValueObservingOptions = .new,
callback: @escaping ChangeCallback) {
self.object = object
self.keyPath = keyPath
self.callback = callback
super.init()
object.addObserver(self, forKeyPath: keyPath, options: options, context: &context)
}
deinit {
dispose()
}
public func dispose() {
object.removeObserver(self, forKeyPath: keyPath, context: &context)
}
public static func observeNew<T>(object: NSObject, keyPath: String,
callback: @escaping (T) -> Void) -> Observable {
let observer = KeyValueObserver<T>(object: object, keyPath: keyPath, options: .new) { result in
if let value = result.valueNew {
callback(value)
}
}
return observer
}
public override func observeValue(forKeyPath keyPath: String?, of object: Any?,
change: [NSKeyValueChangeKey: Any]?, context: UnsafeMutableRawPointer?) {
if context == &self.context && keyPath == self.keyPath {
if !isSuspended, let change = change, let result = KeyValueObserverResult<ValueType>(change: change) {
callback(result)
}
} else {
super.observeValue(forKeyPath: keyPath, of: object, change: change, context: context)
}
}
}
文件: KeyValueObserverResult.swift – 保存 KVO 觀測數據的輔助類型。
public struct KeyValueObserverResult<T: Any> {
public private(set) var change: [NSKeyValueChangeKey: Any]
public private(set) var kind: NSKeyValueChange
init?(change: [NSKeyValueChangeKey: Any]) {
self.change = change
guard
let changeKindNumberValue = change[.kindKey] as? NSNumber,
let changeKindEnumValue = NSKeyValueChange(rawValue: changeKindNumberValue.uintValue) else {
return nil
}
kind = changeKindEnumValue
}
// MARK: -
public var valueNew: T? {
return change[.newKey] as? T
}
public var valueOld: T? {
return change[.oldKey] as? T
}
var isPrior: Bool {
return (change[.notificationIsPriorKey] as? NSNumber)?.boolValue ?? false
}
var indexes: NSIndexSet? {
return change[.indexesKey] as? NSIndexSet
}
}
文件: Observable.swift - 暫停/恢復和處置觀察者的協議。
public protocol Observable {
var isSuspended: Bool { get set }
func dispose()
}
extension Array where Element == Observable {
public func suspend() {
forEach {
var observer = $0
observer.isSuspended = true
}
}
public func resume() {
forEach {
var observer = $0
observer.isSuspended = false
}
}
}
文件: UserDefaults.swift - 用戶默認設置的便捷擴展。
extension UserDefaults {
public func observe<T: Any>(key: String, callback: @escaping (T) -> Void) -> Observable {
let result = KeyValueObserver<T>.observeNew(object: self, keyPath: key) {
callback($0)
}
return result
}
public func observeString(key: String, callback: @escaping (String) -> Void) -> Observable {
return observe(key: key, callback: callback)
}
}
用法:
class MyClass {
private var observables: [Observable] = []
// IMPORTANT: DON'T use DOT `.` in key.
// DOT `.` used to define `KeyPath` and this is what we don't need here.
private let key = "app-some:test_key"
func setupHandlers() {
observables.append(UserDefaults.standard.observeString(key: key) {
print($0) // Will print `AAA` and then `BBB`.
})
}
func doSomething() {
UserDefaults.standard.set("AAA", forKey: key)
UserDefaults.standard.set("BBB", forKey: key)
}
}
從命令行更新默認值:
# Running shell command below while sample code above is running will print `CCC`
defaults write com.my.bundleID app-some:test_key CCC
從 iOS 13 開始,現在有一種更酷的方法來做到這一點,使用結合:
import Foundation
import Combine
extension UserDefaults {
/// Observe UserDefaults for changes at the supplied KeyPath.
///
/// Note: first, extend UserDefaults with an `@objc dynamic` variable
/// to create a KeyPath.
///
/// - Parameters:
/// - keyPath: the KeyPath to observe for changes.
/// - handler: closure to run when/if the value changes.
public func observe<T>(
_ keyPath: KeyPath<UserDefaults, T>,
handler: @escaping (T) -> Void)
{
let subscriber = Subscribers.Sink<T, Never> { _ in }
receiveValue: { newValue in
handler(newValue)
}
self.publisher(for: keyPath, options: [.initial, .new])
.subscribe(subscriber)
}
}
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