问为什么是“字符串？”不符合Equatable

EN

import UIKit

final class Observable<T: Equatable> {
  typealias ObserverHandler = ((T) -> Void)
  typealias ObserverEntry = (owner: AnyObject, didChange: ObserverHandler)

  var value: T {
    didSet {
      if self.value == oldValue { return }
      self.observers.forEach({ (entry: ObserverEntry) in entry.didChange(self.value) })
    }
  }
  private var observers: [ObserverEntry] = []

  init(_ value: T) {
    self.value = value
  }

  func subscribe(observer: AnyObject, didChange block: @escaping ObserverHandler) {
    let entry: ObserverEntry = (owner: observer, didChange: block)
    observers.append(entry)
  }

  func unsubscribe(observer: AnyObject) {
    let filteredEntries: [ObserverEntry] = observers.filter({ (entry: ObserverEntry) -> Bool in
      return (entry.owner !== observer)
    })
    self.observers = filteredEntries
  }

}

// [!] Type 'String?' does not conform to protocol 'Equatable'
var foo: Observable<String?> = Observable(nil)