Add basic ExtensionPoint support.

Shared/Extensions/AddWebFeedDefaultContainer.swift

@@ -0,0 +1,51 @@
+//
+//  AddWebFeedDefaultContainer.swift
+//  NetNewsWire-iOS
+//
+//  Created by Maurice Parker on 11/16/19.
+//  Copyright © 2019 Ranchero Software. All rights reserved.
+//
+
+import Foundation
+import Account
+
+struct AddWebFeedDefaultContainer {
+	
+	static var defaultContainer: Container? {
+		
+		if let accountID = AppDefaults.addWebFeedAccountID, let account = AccountManager.shared.activeAccounts.first(where: { $0.accountID == accountID }) {
+			if let folderName = AppDefaults.addWebFeedFolderName, let folder = account.existingFolder(withDisplayName: folderName) {
+				return folder
+			} else {
+				return substituteContainerIfNeeded(account: account)
+			}
+		} else if let account = AccountManager.shared.sortedActiveAccounts.first {
+			return substituteContainerIfNeeded(account: account)
+		} else {
+			return nil
+		}
+		
+	}
+	
+	static func saveDefaultContainer(_ container: Container) {
+		AppDefaults.addWebFeedAccountID = container.account?.accountID
+		if let folder = container as? Folder {
+			AppDefaults.addWebFeedFolderName = folder.nameForDisplay
+		} else {
+			AppDefaults.addWebFeedFolderName = nil
+		}
+	}
+	
+	private static func substituteContainerIfNeeded(account: Account) -> Container? {
+		if !account.behaviors.contains(.disallowFeedInRootFolder) {
+			return account
+		} else {
+			if let folder = account.sortedFolders?.first {
+				return folder
+			} else {
+				return nil
+			}
+		}
+	}
+	
+}