Add and update lastViewedDate to Tab model

DuckDuckGo/Tab.swift

@@ -36,12 +36,20 @@ public class Tab: NSObject, NSCoding {
         static let link = "link"
         static let viewed = "viewed"
         static let desktop = "desktop"
+        static let lastViewedDate = "lastViewedDate"
     }
 
     private var observersHolder = [WeaklyHeldTabObserver]()
 
     let uid: String
-
+
+    /// The date last time this tab was displayed.
+    ///
+    /// - Warning: This value **must not** be used for any other purpose than for inactive tabs buckets aggregation
+    /// into a daily pixel in `TabSwitcherOpenDailyPixel`. If you plan to do something else,
+    /// read through https://app.asana.com/0/69071770703008/1208795393823862/f and reopen if necessary.
+    private(set) var lastViewedDate: Date?
+
     var isDesktop: Bool = false {
         didSet {
             notifyObservers()
@@ -56,33 +64,40 @@ public class Tab: NSObject, NSCoding {
 
     var viewed: Bool = false {
         didSet {
+            if viewed {
+                lastViewedDate = Date()
+            }
             notifyObservers()
         }
     }
 
     public init(uid: String? = nil,
                 link: Link? = nil,
                 viewed: Bool = false,
-                desktop: Bool = AppWidthObserver.shared.isLargeWidth) {
+                desktop: Bool = AppWidthObserver.shared.isLargeWidth,
+                lastViewedDate: Date? = nil) {
         self.uid = uid ?? UUID().uuidString
         self.link = link
         self.viewed = viewed
         self.isDesktop = desktop
+        self.lastViewedDate = lastViewedDate
     }
 
     public convenience required init?(coder decoder: NSCoder) {
         let uid = decoder.decodeObject(forKey: NSCodingKeys.uid) as? String
         let link = decoder.decodeObject(forKey: NSCodingKeys.link) as? Link
         let viewed = decoder.containsValue(forKey: NSCodingKeys.viewed) ? decoder.decodeBool(forKey: NSCodingKeys.viewed) : true
         let desktop = decoder.containsValue(forKey: NSCodingKeys.desktop) ? decoder.decodeBool(forKey: NSCodingKeys.desktop) : false
-        self.init(uid: uid, link: link, viewed: viewed, desktop: desktop)
+        let lastViewedDate = decoder.containsValue(forKey: NSCodingKeys.lastViewedDate) ? decoder.decodeObject(forKey: NSCodingKeys.lastViewedDate) as? Date : nil
+        self.init(uid: uid, link: link, viewed: viewed, desktop: desktop, lastViewedDate: lastViewedDate)
     }
 
     public func encode(with coder: NSCoder) {
         coder.encode(uid, forKey: NSCodingKeys.uid)
         coder.encode(link, forKey: NSCodingKeys.link)
         coder.encode(viewed, forKey: NSCodingKeys.viewed)
         coder.encode(isDesktop, forKey: NSCodingKeys.desktop)
+        coder.encode(lastViewedDate, forKey: NSCodingKeys.lastViewedDate)
     }
 
     public override func isEqual(_ other: Any?) -> Bool {