Make undo deleting feeds/folders work.

Commands/DeleteFromSidebarCommand.swift

@@ -54,7 +54,7 @@ final class DeleteFromSidebarCommand: UndoableCommand {
 	func undo() {
 
 		BatchUpdate.shared.perform {
-            
+			itemSpecifiers.forEach { $0.restore() }
 		}
 		registerRedo()
 	}
@@ -145,6 +145,49 @@ private struct SidebarItemSpecifier {
 			container.deleteFolder(folder)
 		}
 	}
+
+	func restore() {
+
+		if let feed = feed {
+			restoreFeed()
+		}
+		else if let folder = folder {
+			restoreFolder()
+		}
+	}
+
+	private func restoreFeed() {
+
+		guard let account = account, let feed = feed else {
+			return
+		}
+
+		let feedToUse = uniquedFeed(feed)
+		account.addFeed(feedToUse, to: resolvedFolder())
+	}
+
+	private func restoreFolder() {
+
+		guard let account = account, let folder = folder else {
+			return
+		}
+		account.addFolder(folder, to: nil)
+	}
+
+	private func uniquedFeed(_ feed: Feed) -> Feed {
+
+		// A Feed may appear in multiple places in a given account,
+		// but it’s best if they’re the same Feed instance.
+		// Usually this will return the same Feed that was passed-in,
+		// but not necessarily always.
+
+		return account?.existingFeed(with: feed.feedID) ?? feed
+	}
+
+	private func resolvedFolder() -> Folder? {
+
+		return path.resolveContainer() as? Folder
+	}
 }
 
 private extension Node {

Evergreen/MainWindow/Sidebar/SidebarViewController.swift

@@ -20,6 +20,7 @@ import RSCore
 		TreeController(delegate: treeControllerDelegate)
 	}()
     var undoableCommands = [UndoableCommand]()
+	private var animatingChanges = false
 
 	//MARK: NSViewController
 
@@ -79,11 +80,13 @@ import RSCore
         
 		let selectedRows = outlineView.selectedRowIndexes
 
+		animatingChanges = true
 		outlineView.beginUpdates()
 		outlineView.removeItems(at: selectedRows, inParent: nil, withAnimation: [.slideDown])
 		outlineView.endUpdates()
 
         runCommand(deleteCommand)
+		animatingChanges = false
 	}
 
 	// MARK: Navigation
@@ -170,7 +173,7 @@ private extension SidebarViewController {
 	
 	func rebuildTreeAndReloadDataIfNeeded() {
 		
-		if !BatchUpdate.shared.isPerforming {
+		if !animatingChanges && !BatchUpdate.shared.isPerforming {
 			treeController.rebuild()
 			outlineView.reloadData()
 		}

Frameworks/Account/Account.swift

@@ -276,7 +276,15 @@ public final class Account: DisplayNameProvider, UnreadCountProvider, Container,
 	@discardableResult
 	public func addFolder(_ folder: Folder, to parentFolder: Folder?) -> Bool {
 
-		return false // TODO
+		// TODO: support subfolders, maybe, some day, if one of the sync systems
+		// supports subfolders. But, for now, parentFolder is ignored.
+
+		if objectIsChild(folder) {
+			return true
+		}
+		children += [folder]
+		postChildrenDidChangeNotification()
+		return true
 	}
 
  	public func importOPML(_ opmlDocument: RSOPMLDocument) {