rxappkit-bindings

RxAppKit Bindings

RxAppKit ships first-class Rx bindings for almost every AppKit control and for NSOutlineView / NSTableView / NSCollectionView data sources. Do not roll your own. Hand-wired PublishRelay/@objc plumbing and bespoke NSOutlineViewDataSource implementations are a code smell — the framework already provides the Rx surface.

Apply these rules whenever you touch an AppKit ViewController that uses RxSwift.

Rules

1. Reactive is @dynamicMemberLookup — use rx.<anyProperty> directly

RxSwift.Reactive<Base> is declared @dynamicMemberLookup. By default it synthesizes a Binder<Property> (write-only) for any writable reference key path on the base.

RxAppKit overrides this for every HasTargeAction conformer with two more subscripts (from HasTargeAction+Rx.swift):

extension Reactive where Base: HasTargeAction {
    public subscript<Property>(dynamicMember keyPath: ReferenceWritableKeyPath<Base, Property>) -> ControlProperty<Property>
    @_disfavoredOverload
    public subscript<Property>(dynamicMember keyPath: ReferenceWritableKeyPath<Base, Property>) -> ControlEvent<Property>
}

Conformers shipped by RxAppKit: NSControl (and every subclass: NSButton, NSTextField, NSPopUpButton, NSSlider, NSStepper, NSColorWell, NSSegmentedControl, …), NSMenuItem, NSToolbarItem, NSGestureRecognizer, NSColorPanel.

That means every writable property on these types is automatically reachable through rx.<name> as a ControlProperty — driven by the underlying target/action. You almost never need to write an explicit binder by hand:

// ✅ All of these work without RxAppKit defining an explicit accessor for the property
output.title.drive(button.rx.title)              // Driver<String> → button title
button.rx.state.asObservable()                    // emits NSControl.StateValue on each click
popUpButton.rx.indexOfSelectedItem.asSignal()     // emits new selection index
slider.rx.doubleValue.asSignal()                  // emits slider value while dragging
menuItem.rx.title.asSignal()                      // observe menu item title via menu action
toolbarItem.rx.label.asSignal()

ControlProperty is the default overload (ControlEvent is @_disfavoredOverload and only resolves when the call site explicitly demands it). ControlProperty already conforms to ObservableType and ObserverType, so you can .asSignal(), .asObservable(), .drive(...), etc.

Convenience accessors for the void-action case (you only care that an action fired, not which property changed):

• control.rx.click: ControlEvent<Void>
• control.rx.clickWithSelf: ControlEvent<Self> — emits the control instance
• control.rx.click(with: keyPath, isStartWithDefaultValue:) — emits the value at that keyPath each click

// ✅ GOOD — wire button actions through rx.click / rx.<property>
let input = ViewModel.Input(
    cancelAll: cancelAllButton.rx.click.asSignal(),
    openSettings: openSettingsButton.rx.click.asSignal(),
    isExpanded: disclosureButton.rx.state.map { $0 == .on }.asSignal(onErrorJustReturn: false)
)

// Local-only actions (no ViewModel involvement)
closeButton.rx.click.asSignal()
    .emit(onNext: { [weak self] in
        self?.dismiss(nil)
    })
    .disposed(by: disposeBag)

// ❌ BAD — relay + @objc plumbing duplicates what rx already provides
private let cancelAllRelay = PublishRelay<Void>()
private let openSettingsRelay = PublishRelay<Void>()

private func setupActions() {
    cancelAllButton.target = self
    cancelAllButton.action = #selector(cancelAllClicked)
    openSettingsButton.target = self
    openSettingsButton.action = #selector(openSettingsClicked)
}

@objc private func cancelAllClicked()    { cancelAllRelay.accept(()) }
@objc private func openSettingsClicked() { openSettingsRelay.accept(()) }

let input = ViewModel.Input(
    cancelAll: cancelAllRelay.asSignal(),
    openSettings: openSettingsRelay.asSignal()
)

rx.click returns ControlEvent<Void>. Call .asSignal() before chaining emit(onNext:) / before passing to a Signal-typed Input field.

Heuristic: before defining a PublishRelay, a Binder, or a manual target/action, type control.rx. and let auto-complete show you what's already synthesized — for any HasTargeAction conformer the answer is "all of its writable properties".

2. Bind list/tree views via RxAppKit data sources — never write NSXxxDataSource/Delegate by hand

For every list/tree view, RxAppKit ships an Rx data-source binding that owns the underlying NSXxxDataSource + NSXxxDelegate proxy and the diffing pipeline. Pick the right one by view type:

View	Primary binding	Element protocol	Provider signature
NSTableView	tableView.rx.items(_:) / .reorderableItems(_:) / .items(adapter:)	Differentiable	(NSTableView, NSTableColumn?, Int, Item) -> NSView?
NSOutlineView	outlineView.rx.nodes / .rootNode / .reorderableNodes / .nodes(adapter:)	OutlineNodeType & Hashable & Differentiable	(NSOutlineView, NSTableColumn?, Node) -> NSView?
NSCollectionView	collectionView.rx.items(_:) / .items(cellIdentifier:cellType:) / .items(dataSource:)	Differentiable	(NSCollectionView, IndexPath, Item) -> NSCollectionViewItem
NSBrowser	browser.rx.rootNode(cellClass:) / .rootNode(adapter:)	BrowserNodeType	(Node, Cell, row: Int, column: Int) -> Void

Differentiable is satisfied automatically for Hashable & Equatable types — override differenceIdentifier only when content mutations should keep the same row identity (e.g. progress updates inside an enum case).

NSTableView

// ✅ GOOD — single-column or multi-column table
output.items
    .drive(tableView.rx.items) { (tableView: NSTableView, column: NSTableColumn?, row: Int, item: Item) -> NSView? in
        let cell = (tableView.makeView(withIdentifier: .nameCell, owner: nil) as? NameCellView) ?? NameCellView()
        cell.configure(item: item)
        return cell
    }
    .disposed(by: disposeBag)

// Add a row-view provider when needed (selection styling, separators, etc.)
output.items
    .drive(tableView.rx.items) ({ tableView, _, _, item in … },
                                { tableView, row, items in MyRowView() })
    .disposed(by: disposeBag)

Reorderable variant: tableView.rx.reorderableItems(_:) registers internal drag-and-drop. Sync the upstream relay via tableView.rx.modelMoved() (emits the new array) or rx.itemMoved() (emits source/destination indexes).

Other table events: rx.itemSelected(), rx.itemClicked(), rx.itemDoubleClicked(), rx.modelSelected(), rx.didAddRow(), rx.didRemoveRow(), rx.didClickColumn(), rx.didDragColumn(), rx.didScrollEnd().

NSOutlineView

The element type must conform to OutlineNodeType & Hashable & Differentiable. OutlineNodeType only requires var children: [Self] { get }.

// ✅ GOOD — model conforms to OutlineNodeType + Differentiable
enum MyNode: Hashable {
    case section(Section, items: [MyNode])
    case item(Item)
}

extension MyNode: OutlineNodeType {
    var children: [MyNode] {
        switch self {
        case .section(_, let items): return items
        case .item: return []
        }
    }
}

extension MyNode: Differentiable {
    enum Identifier: Hashable {
        case section(Section.ID)
        case item(Item.ID)
    }
    var differenceIdentifier: Identifier {
        switch self {
        case .section(let s, _): return .section(s.id)
        case .item(let i):       return .item(i.id)
        }
    }
}

output.nodes
    .drive(outlineView.rx.nodes) { (outlineView: NSOutlineView, _: NSTableColumn?, node: MyNode) -> NSView? in
        switch node {
        case .section(let section, _):
            let cell = (outlineView.makeView(withIdentifier: .sectionCell, owner: nil) as? SectionCellView) ?? SectionCellView()
            cell.configure(section: section)
            return cell
        case .item(let item):
            let cell = (outlineView.makeView(withIdentifier: .itemCell, owner: nil) as? ItemCellView) ?? ItemCellView()
            cell.configure(item: item)
            return cell
        }
    }
    .disposed(by: disposeBag)

Use rx.rootNode(source:) when the source emits a single root node, rx.reorderableNodes for drag-and-drop trees. Selection events: rx.modelSelected(), rx.modelClicked(), rx.modelDoubleClicked(). Reorder events: rx.nodeMoved() (full OutlineMove), rx.modelMoved() (new root array).

Drive form: output.nodes.drive(outlineView.rx.nodes) { … } works through RxSwift's drive(_:curriedArgument:) overload — outlineView.rx.nodes is a function reference of type (Observable) -> (CellViewProvider) -> Disposable, the trailing closure is the cell view provider.

Do not subclass OutlineViewAdapter "to force reloadData". RxNSOutlineViewAdapter.outlineView(_:observedEvent:) already passes interrupt: { _ in true } to reload(using:), which falls back to outlineView.reloadData() on every event. The default binding is a full reload.

If you need to expand all rows after each update, attach a separate subscription to the same Driver:

output.nodes.drive(onNext: { [weak self] _ in
    self?.outlineView.expandItem(nil, expandChildren: true)
})
.disposed(by: disposeBag)

NSCollectionView

Three binding flavours, in order of convenience:

// ✅ GOOD — `cellIdentifier:cellType:` form: dequeue + configure in one closure
output.items
    .bind(to: collectionView.rx.items(cellIdentifier: .photoCell, cellType: PhotoCollectionViewItem.self)) { (indexPath, item, cell) in
        cell.configure(photo: item)
    }
    .disposed(by: disposeBag)

// Or supply your own item provider when you need full control over dequeue
output.items
    .bind(to: collectionView.rx.items) { (collectionView, indexPath, item) -> NSCollectionViewItem in
        let cell = collectionView.makeItem(withIdentifier: .photoCell, for: indexPath) as! PhotoCollectionViewItem
        cell.configure(photo: item)
        return cell
    }
    .disposed(by: disposeBag)

// Or, for sectioned data, plug a custom data source through `rx.items(dataSource:)`.

Selection / display events: rx.itemSelected(), rx.itemDeselected(), rx.modelSelected(MyModel.self), rx.modelDeselected(_:), rx.itemHighlightState(), rx.willDisplayItem(), rx.didEndDisplayingItem(), rx.didScrollEnd(inSection:), plus the supplementary-view variants.

NSBrowser

NSBrowser uses its own BrowserNodeType protocol (note: distinct from OutlineNodeType) — requires var title: String and var children: [NodeType].

struct FileNode: BrowserNodeType {
    let title: String
    var children: [FileNode]
}

// rx.rootNode is doubly curried: (cellClass:) → (source) → (configureCell)
output.fileTree
    .drive(browser.rx.rootNode(cellClass: NSBrowserCell.self)) { (node: FileNode, cell: NSBrowserCell, row: Int, column: Int) in
        cell.title = node.title
        cell.isLeaf = node.isLeaf
    }
    .disposed(by: disposeBag)

Click / selection events: rx.clickedIndex / rx.doubleClicked (return (row: Int, column: Int)), rx.selectedIndexPath. Path control: rx.path: ControlProperty<String> — both observable and bindable.

Anti-pattern: hand-rolled data source for any of the above

// ❌ BAD — re-implementing what rx.nodes / rx.items already does
private var renderedNodes: [MyNode] = []

extension MyViewController: NSOutlineViewDataSource, NSOutlineViewDelegate {
    func outlineView(_ outlineView: NSOutlineView, numberOfChildrenOfItem item: Any?) -> Int { ... }
    func outlineView(_ outlineView: NSOutlineView, child index: Int, ofItem item: Any?) -> Any { ... }
    func outlineView(_ outlineView: NSOutlineView, isItemExpandable item: Any) -> Bool { ... }
    func outlineView(_ outlineView: NSOutlineView, viewFor tableColumn: NSTableColumn?, item: Any) -> NSView? { ... }
}

output.nodes.drive(onNext: { [weak self] nodes in
    self?.renderedNodes = nodes
    self?.outlineView.reloadData()
})
.disposed(by: disposeBag)

The same anti-pattern applies to writing NSTableViewDataSource, NSCollectionViewDataSource, or NSBrowserDelegate by hand when the data already lives in an Rx stream.

3. Drive view state through rx.isHidden / rx.stringValue / etc. — never assign in a closure

// ✅ GOOD — declarative bindings
output.isEnabled.drive(view.rx.isHidden).disposed(by: disposeBag)
output.hasFailure.map(!).drive(button.rx.isHidden).disposed(by: disposeBag)
output.subtitle.drive(subtitleLabel.rx.stringValue).disposed(by: disposeBag)

Driver.combineLatest(output.isEnabled, output.hasItems) { enabled, hasItems in
    !enabled || !hasItems
}
.drive(scrollView.rx.isHidden)
.disposed(by: disposeBag)

// ❌ BAD — imperative assignment buried inside a closure
output.isEnabled.drive(onNext: { [weak self] enabled in
    self?.view.isHidden = enabled
    self?.button.isHidden = enabled
})
.disposed(by: disposeBag)

Observable.combineLatest(output.isEnabled.asObservable(), output.hasItems.asObservable())
    .subscribe(onNext: { [weak self] enabled, hasItems in
        self?.scrollView.isHidden = !enabled || !hasItems
    })
    .disposed(by: disposeBag)

Driver.combineLatest(_:_:resultSelector:) is the right tool for combined visibility logic. Negate a Driver<Bool> with .map(!).

Imports

import RxSwift
import RxCocoa
import RxAppKit  // also re-exports DifferenceKit, so `Differentiable` is accessible

RxAppKit does not re-export RxSwift or RxCocoa — import them explicitly.

Common Mistakes

Mistake	Fix
let xxxRelay = PublishRelay<Void>() + @objc func xxxClicked() for a button	button.rx.click.asSignal() (Rule 1)
Defining an explicit Binder<Property> / ControlProperty for a property already on a HasTargeAction type	Just call control.rx.<propertyName> — @dynamicMemberLookup already synthesizes it (Rule 1)
closeButton.rx.click.emit(onNext:) (compile error: ControlEvent has no emit)	rx.click is ControlEvent<Void> — call .asSignal() first
Hand-rolled NSTableViewDataSource / NSOutlineViewDataSource / NSCollectionViewDataSource / NSBrowserDelegate for data already in an Rx stream	Bind through tableView.rx.items / outlineView.rx.nodes / collectionView.rx.items / browser.rx.rootNode (Rule 2)
Subclassing OutlineViewAdapter "so we get reloadData"	The default adapter's interrupt closure already triggers reloadData() (Rule 2)
Keeping a private var renderedNodes: [Node] / private var rows: [Item] on the ViewController	Let the adapter own it; the Driver is the source of truth
Confusing OutlineNodeType with BrowserNodeType	They are separate protocols — BrowserNodeType requires var title: String plus children
view.isHidden = … inside a drive(onNext:) closure	output.isEnabled.drive(view.rx.isHidden) (Rule 3)
Manual target = self / action = #selector(...) for any control with an Rx wrapper	Use the rx.* extension instead