Implement UIKitBackend to support iOS, tvOS and catalyst (#98)

* UIKitBackend rough draft (woefully incomplete but arguably functional)

* More progress, but something's still broken

* Minor cleanup

* A bunch of updates, mostly window fixes

* Add alerts and compile on tvOS

* final cleanup

* Fix a few more things, oops

* Fix buttons and toggles

Author: bbrk24

Package.swift

@@ -61,14 +61,15 @@ switch ProcessInfo.processInfo.environment["SCUI_LIBRARY_TYPE"] {
 
 let package = Package(
     name: "swift-cross-ui",
-    platforms: [.macOS(.v10_15)],
+    platforms: [.macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .macCatalyst(.v13)],
     products: [
         .library(name: "SwiftCrossUI", type: libraryType, targets: ["SwiftCrossUI"]),
         .library(name: "AppKitBackend", type: libraryType, targets: ["AppKitBackend"]),
         .library(name: "GtkBackend", type: libraryType, targets: ["GtkBackend"]),
         .library(name: "Gtk3Backend", type: libraryType, targets: ["Gtk3Backend"]),
         .library(name: "WinUIBackend", targets: ["WinUIBackend"]),
         .library(name: "DefaultBackend", type: libraryType, targets: ["DefaultBackend"]),
+        .library(name: "UIKitBackend", type: libraryType, targets: ["UIKitBackend"]),
         .library(name: "Gtk", type: libraryType, targets: ["Gtk"]),
         .executable(name: "GtkExample", targets: ["GtkExample"]),
         // .library(name: "CursesBackend", type: libraryType, targets: ["CursesBackend"]),
@@ -145,7 +146,11 @@ let package = Package(
         .target(
             name: "DefaultBackend",
             dependencies: [
-                .target(name: defaultBackend)
+                .target(name: defaultBackend, condition: .when(platforms: [.linux, .macOS, .windows])),
+                // Non-desktop platforms need to be handled separately:
+                // Only one backend is supported, and `#if` won't work because it's evaluated
+                // on the compiling desktop, not the target.
+                .target(name: "UIKitBackend", condition: .when(platforms: [.iOS, .tvOS, .macCatalyst])),
             ]
         ),
         .target(name: "AppKitBackend", dependencies: ["SwiftCrossUI"]),
@@ -219,6 +224,7 @@ let package = Package(
             ],
             swiftSettings: swiftSettings
         ),
+        .target(name: "UIKitBackend", dependencies: ["SwiftCrossUI"]),
         .target(
             name: "WinUIBackend",
             dependencies: [

README.md

@@ -30,10 +30,11 @@ SwiftCrossUI has a variety of backends tailored to different operating systems.
 
 If you use `DefaultBackend`, like the examples do, you can override the default when compiling your app by setting the `SCUI_DEFAULT_BACKEND` environment variable to the name of your desired backend. This can be quite useful when you e.g. want to test the Gtk version of your app while using a Mac.
 
-- `DefaultBackend`: Adapts to your target operating system. On macOS it uses `AppKitBackend`, on Windows it uses `WinUIBackend`, and on Linux it uses `GtkBackend`.
+- `DefaultBackend`: Adapts to your target operating system. On macOS it uses `AppKitBackend`, on Windows it uses `WinUIBackend`, on Linux it uses `GtkBackend`, and on iOS and tvOS it uses `UIKitBackend`.
 - `GtkBackend`: Works on Linux, macOS, and Windows. Requires gtk 4 to be installed. Supports all SwiftCrossUI features.
 - `AppKitBackend`: The native macOS backend. Supports all SwiftCrossUI features.
 - `WinUIBackend`: The native Windows backend. Supports most SwiftCrossUI features.
+- `UIKitBackend`: The native iOS & tvOS backend. Supports most SwiftCrossUI features.
 - `QtBackend`: ***Experimental***, requires `qt5` to be installed, and currently supports a very limited subset of SwiftCrossUI features.
 - `CursesBackend`: ***Experimental***, requires `curses` to be installed, and supports a *very very* limited subset of SwiftCrossUI features.
 

Sources/AppKitBackend/AppKitBackend.swift

@@ -18,6 +18,8 @@ public final class AppKitBackend: AppBackend {
     public let defaultTableRowContentHeight = 20
     public let defaultTableCellVerticalPadding = 4
     public let defaultPaddingAmount = 10
+    public let requiresToggleSwitchSpacer = false
+    public let defaultToggleStyle = ToggleStyle.button
 
     public var scrollBarWidth: Int {
         // We assume that all scrollers have their controlSize set to `.regular` by default.
@@ -71,6 +73,10 @@ public final class AppKitBackend: AppBackend {
         )
     }
 
+    public func isFixedSizeWindow(_ window: Window) -> Bool {
+        !window.styleMask.contains(.fullScreen)
+    }
+
     public func setSize(ofWindow window: Window, to newSize: SIMD2<Int>) {
         window.setContentSize(NSSize(width: newSize.x, height: newSize.y))
     }
@@ -175,18 +181,27 @@ public final class AppKitBackend: AppBackend {
     public static func createDefaultAboutMenu() -> NSMenu {
         let appName = ProcessInfo.processInfo.processName
         let appMenu = NSMenu(title: appName)
-        appMenu.addItem(withTitle: "About \(appName)", action: #selector(NSApp.orderFrontStandardAboutPanel(_:)), keyEquivalent: "")
+        appMenu.addItem(
+            withTitle: "About \(appName)",
+            action: #selector(NSApp.orderFrontStandardAboutPanel(_:)), keyEquivalent: "")
         appMenu.addItem(NSMenuItem.separator())
 
-        let hideMenu = appMenu.addItem(withTitle: "Hide \(appName)", action: #selector(NSApp.hide(_:)), keyEquivalent: "h")
+        let hideMenu = appMenu.addItem(
+            withTitle: "Hide \(appName)", action: #selector(NSApp.hide(_:)), keyEquivalent: "h")
         hideMenu.keyEquivalentModifierMask = .command
 
-        let hideOthers = appMenu.addItem(withTitle: "Hide Others", action: #selector(NSApp.hideOtherApplications(_:)), keyEquivalent: "h")
+        let hideOthers = appMenu.addItem(
+            withTitle: "Hide Others", action: #selector(NSApp.hideOtherApplications(_:)),
+            keyEquivalent: "h")
         hideOthers.keyEquivalentModifierMask = [.option, .command]
 
-        appMenu.addItem(withTitle: "Show All", action: #selector(NSApp.unhideAllApplications(_:)), keyEquivalent: "")
+        appMenu.addItem(
+            withTitle: "Show All", action: #selector(NSApp.unhideAllApplications(_:)),
+            keyEquivalent: "")
 
-        let quitMenu = appMenu.addItem(withTitle: "Quit \(appName)", action: #selector(NSApp.terminate(_:)), keyEquivalent: "q")
+        let quitMenu = appMenu.addItem(
+            withTitle: "Quit \(appName)", action: #selector(NSApp.terminate(_:)), keyEquivalent: "q"
+        )
         quitMenu.keyEquivalentModifierMask = .command
 
         return appMenu

Sources/DefaultBackend/DefaultBackend.swift

@@ -16,6 +16,9 @@
 #elseif canImport(CursesBackend)
     import CursesBackend
     public typealias DefaultBackend = CursesBackend
+#elseif canImport(UIKitBackend)
+    import UIKitBackend
+    public typealias DefaultBackend = UIKitBackend
 #else
     #error("Unknown backend selected")
 #endif

Sources/GtkBackend/GtkBackend.swift

@@ -27,6 +27,8 @@ public final class GtkBackend: AppBackend {
     public let defaultTableCellVerticalPadding = 4
     public let defaultPaddingAmount = 10
     public let scrollBarWidth = 0
+    public let requiresToggleSwitchSpacer = false
+    public let defaultToggleStyle = ToggleStyle.button
 
     var gtkApp: Application
 
@@ -117,6 +119,11 @@ public final class GtkBackend: AppBackend {
         return SIMD2(size.width, size.height)
     }
 
+    public func isFixedSizeWindow(_ window: Window) -> Bool {
+        // TODO: Detect whether window is fullscreen
+        return false
+    }
+
     public func setSize(ofWindow window: Window, to newSize: SIMD2<Int>) {
         let child = window.getChild() as! CustomRootWidget
         window.size = Size(

Sources/SwiftCrossUI/Backend/AppBackend.swift

@@ -11,8 +11,9 @@ import Foundation
 /// ``AppBackend/setTitle(ofWindow:to:)``, ``AppBackend/setResizability(ofWindow:to:)``,
 /// ``AppBackend/setChild(ofWindow:to:)``, ``AppBackend/show(window:)``,
 /// ``AppBackend/runMainLoop()``, ``AppBackend/runInMainThread(action:)``,
-/// ``AppBackend/show(widget:)``. Many of these can simply be given dummy
-/// implementations until you're ready to implement them properly.
+/// ``AppBackend/isFixedSizeWindow(_:)``, ``AppBackend/show(widget:)``.
+/// Many of these can simply be given dummy implementations until you're ready
+/// to implement them properly.
 ///
 /// If you need to modify the children of a widget after creation but there
 /// aren't update methods available, this is an intentional limitation to
@@ -66,6 +67,10 @@ public protocol AppBackend {
     /// ensure that the configured root environment change handler gets called so
     /// that SwiftCrossUI can update the app's layout accordingly.
     var scrollBarWidth: Int { get }
+    /// If `true`, a toggle in the ``ToggleStyle/switch`` style grows to fill its parent container.
+    var requiresToggleSwitchSpacer: Bool { get }
+    /// The default style for toggles.
+    var defaultToggleStyle: ToggleStyle { get }
 
     /// Often in UI frameworks (such as Gtk), code is run in a callback
     /// after starting the app, and hence this generic root window creation
@@ -112,6 +117,9 @@ public protocol AppBackend {
     func setChild(ofWindow window: Window, to child: Widget)
     /// Gets the size of the given window in pixels.
     func size(ofWindow window: Window) -> SIMD2<Int>
+    /// Check whether a window is programmatically resizable. This value does not necessarily
+    /// reflect whether the window is resizable by the user.
+    func isFixedSizeWindow(_ window: Window) -> Bool
     /// Sets the size of the given window in pixels.
     func setSize(ofWindow window: Window, to newSize: SIMD2<Int>)
     /// Sets the minimum width and height of the window. Prevents the user from making the

Sources/SwiftCrossUI/Scenes/WindowGroupNode.swift

@@ -66,13 +66,16 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
             fatalError("Scene updated with a backend incompatible with the window it was given")
         }
 
+        let isFixedSize = backend.isFixedSizeWindow(window)
+
         _ = update(
             newScene,
-            proposedWindowSize: isFirstUpdate
+            proposedWindowSize: isFirstUpdate && !isFixedSize
                 ? (newScene ?? scene).defaultSize
                 : backend.size(ofWindow: window),
             backend: backend,
-            environment: environment
+            environment: environment,
+            windowSizeIsFinal: isFixedSize
         )
     }
 
@@ -147,17 +150,12 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
             }
         }
 
-        let finalContentResult: ViewUpdateResult
-        if windowSizeIsFinal {
-            finalContentResult = contentResult
-        } else {
-            finalContentResult = viewGraph.update(
-                with: newScene?.body,
-                proposedSize: proposedWindowSize,
-                environment: environment,
-                dryRun: false
-            )
-        }
+        let finalContentResult = viewGraph.update(
+            with: newScene?.body,
+            proposedSize: proposedWindowSize,
+            environment: environment,
+            dryRun: false
+        )
 
         // The Gtk 3 backend has some broken sizing code that can't really be
         // fixed due to the design of Gtk 3. Our layout system underestimates

Sources/SwiftCrossUI/Views/Toggle.swift

@@ -1,4 +1,6 @@
 public struct Toggle: View {
+    @Environment(\.backend) var backend
+
     /// The style of toggle shown.
     var selectedToggleStyle: ToggleStyle
     /// The label to be shown on or beside the toggle.
@@ -8,7 +10,7 @@ public struct Toggle: View {
 
     /// Creates a toggle that displays a custom label.
     public init(_ label: String, active: Binding<Bool>) {
-        self.selectedToggleStyle = .button
+        self.selectedToggleStyle = backend.defaultToggleStyle
         self.label = label
         self.active = active
     }
@@ -18,6 +20,11 @@ public struct Toggle: View {
             case .switch:
                 HStack {
                     Text(label)
+
+                    if backend.requiresToggleSwitchSpacer {
+                        Spacer()
+                    }
+
                     ToggleSwitch(active: active)
                 }
             case .button:

Sources/UIKitBackend/BaseWidget.swift

@@ -0,0 +1,124 @@
+//
+//  BaseWidget.swift
+//  swift-cross-ui
+//
+//  Created by William Baker on 1/10/25.
+//
+
+import UIKit
+
+public class BaseWidget: UIView {
+    private var leftConstraint: NSLayoutConstraint?
+    private var topConstraint: NSLayoutConstraint?
+    private var widthConstraint: NSLayoutConstraint?
+    private var heightConstraint: NSLayoutConstraint?
+
+    internal var x = 0 {
+        didSet {
+            if x != oldValue {
+                updateLeftConstraint()
+            }
+        }
+    }
+
+    internal var y = 0 {
+        didSet {
+            if y != oldValue {
+                updateTopConstraint()
+            }
+        }
+    }
+
+    internal var width = 0 {
+        didSet {
+            if width != oldValue {
+                updateWidthConstraint()
+            }
+        }
+    }
+
+    internal var height = 0 {
+        didSet {
+            if height != oldValue {
+                updateHeightConstraint()
+            }
+        }
+    }
+
+    internal init() {
+        super.init(frame: .zero)
+
+        self.translatesAutoresizingMaskIntoConstraints = false
+    }
+
+    @available(*, unavailable)
+    public required init?(coder: NSCoder) {
+        fatalError("init(coder:) is not used for this view")
+    }
+
+    private func updateLeftConstraint() {
+        leftConstraint?.isActive = false
+        guard let superview else { return }
+        leftConstraint = self.leftAnchor.constraint(
+            equalTo: superview.safeAreaLayoutGuide.leftAnchor, constant: CGFloat(x))
+        leftConstraint!.isActive = true
+    }
+
+    private func updateTopConstraint() {
+        topConstraint?.isActive = false
+        guard let superview else { return }
+        topConstraint = self.topAnchor.constraint(
+            equalTo: superview.safeAreaLayoutGuide.topAnchor, constant: CGFloat(y))
+        topConstraint!.isActive = true
+    }
+
+    private func updateWidthConstraint() {
+        widthConstraint?.isActive = false
+        widthConstraint = self.widthAnchor.constraint(equalToConstant: CGFloat(width))
+        widthConstraint!.isActive = true
+    }
+
+    private func updateHeightConstraint() {
+        heightConstraint?.isActive = false
+        heightConstraint = self.heightAnchor.constraint(equalToConstant: CGFloat(height))
+        heightConstraint!.isActive = true
+    }
+
+    public override func didMoveToSuperview() {
+        super.didMoveToSuperview()
+
+        updateLeftConstraint()
+        updateTopConstraint()
+    }
+}
+
+extension UIKitBackend {
+    public typealias Widget = BaseWidget
+}
+
+internal class WrapperWidget<View: UIView>: BaseWidget {
+    init(child: View) {
+        super.init()
+
+        self.addSubview(child)
+        child.translatesAutoresizingMaskIntoConstraints = false
+        NSLayoutConstraint.activate([
+            child.topAnchor.constraint(equalTo: self.topAnchor),
+            child.leadingAnchor.constraint(equalTo: self.leadingAnchor),
+            child.bottomAnchor.constraint(equalTo: self.bottomAnchor),
+            child.trailingAnchor.constraint(equalTo: self.trailingAnchor),
+        ])
+    }
+
+    override convenience init() {
+        self.init(child: View(frame: .zero))
+    }
+
+    var child: View {
+        subviews[0] as! View
+    }
+
+    override var intrinsicContentSize: CGSize {
+        child.intrinsicContentSize
+    }
+}