fetch following contacts if we are missing any

Signed-off-by: William Casarin <jb55@jb55.com>

damus/Models/FollowingModel.swift

@@ -0,0 +1,70 @@
+//
+//  FollowingModel.swift
+//  damus
+//
+//  Created by William Casarin on 2022-05-24.
+//
+
+import Foundation
+
+class FollowingModel: ObservableObject {
+    let damus_state: DamusState
+    var needs_sub: Bool = true
+    
+    var has_contact: Set<String> = Set()
+    let contacts: [String]
+    
+    let sub_id: String = UUID().description
+    
+    init(damus_state: DamusState, contacts: [String]) {
+        self.damus_state = damus_state
+        self.contacts = contacts
+    }
+    
+    func get_filter() -> NostrFilter {
+        var f = NostrFilter.filter_kinds([0])
+        f.authors = self.contacts.reduce(into: Array<String>()) { acc, pk in
+            // don't fetch profiles we already have
+            if damus_state.profiles.lookup(id: pk) != nil {
+                return
+            }
+            acc.append(pk)
+        }
+        return f
+    }
+    
+    func subscribe() {
+        let filter = get_filter()
+        if (filter.authors?.count ?? 0) == 0 {
+            needs_sub = false
+            return
+        }
+        let filters = [filter]
+        print_filters(relay_id: "following", filters: [filters])
+        self.damus_state.pool.subscribe(sub_id: sub_id, filters: filters, handler: handle_event)
+    }
+    
+    func unsubscribe() {
+        if !needs_sub {
+            return
+        }
+        print("unsubscribing from following \(sub_id)")
+        self.damus_state.pool.unsubscribe(sub_id: sub_id)
+    }
+    
+    func handle_event(relay_id: String, ev: NostrConnectionEvent) {
+        switch ev {
+        case .ws_event:
+            break
+        case .nostr_event(let nev):
+            switch nev {
+            case .event(_, let ev):
+                if ev.kind == 0 {
+                    process_metadata_event(profiles: damus_state.profiles, ev: ev)
+                }
+            case .notice(let msg):
+                print("followingmodel notice: \(msg)")
+            }
+        }
+    }
+}