Gtk3Backend: Re-scale images when window changes DPI (changing monitors)

This should complete HiDPI display support for Gtk3Backend

Author: stackotter

Sources/AppKitBackend/AppKitBackend.swift

@@ -20,6 +20,7 @@ public final class AppKitBackend: AppBackend {
     public let defaultPaddingAmount = 10
     public let requiresToggleSwitchSpacer = false
     public let defaultToggleStyle = ToggleStyle.button
+    public let requiresImageUpdateOnScaleFactorChange = false
 
     public var scrollBarWidth: Int {
         // We assume that all scrollers have their controlSize set to `.regular` by default.
@@ -270,6 +271,21 @@ public final class AppKitBackend: AppBackend {
         }
     }
 
+    public func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues {
+        // TODO: Record window scale factor in here
+        rootEnvironment
+    }
+
+    public func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    ) {
+        // TODO: Notify when window scale factor changes
+    }
+
     public func setIncomingURLHandler(to action: @escaping (URL) -> Void) {
         appDelegate.onOpenURLs = { urls in
             for url in urls {

Sources/Gtk3/Widgets/ApplicationWindow.swift

@@ -9,7 +9,26 @@ public class ApplicationWindow: Window {
         self.init(
             gtk_application_window_new(application.applicationPointer)
         )
+
+        let handler2:
+            @convention(c) (
+                UnsafeMutableRawPointer,
+                gint,
+                UnsafeMutableRawPointer
+            ) -> Void = { _, value1, data in
+                SignalBox1<gint>.run(data, value1)
+            }
+
+        addSignal(
+            name: "notify::scale-factor",
+            handler: gCallback(handler2)
+        ) { [weak self] (scaleFactor: gint) in
+            guard let self = self else { return }
+            self.notifyScaleFactor?(Int(scaleFactor))
+        }
     }
 
+    public var notifyScaleFactor: ((Int) -> Void)?
+
     @GObjectProperty(named: "show-menubar") public var showMenuBar: Bool
 }

Sources/Gtk3Backend/Gtk3Backend.swift

@@ -29,6 +29,7 @@ public final class Gtk3Backend: AppBackend {
     public let requiresToggleSwitchSpacer = false
     public let scrollBarWidth = 0
     public let defaultToggleStyle = ToggleStyle.button
+    public let requiresImageUpdateOnScaleFactorChange = true
 
     var gtkApp: Application
 
@@ -272,6 +273,23 @@ public final class Gtk3Backend: AppBackend {
         // TODO: React to theme changes
     }
 
+    public func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues {
+        let windowScaleFactor = Int(gtk_widget_get_scale_factor(window.widgetPointer))
+        return rootEnvironment.with(\.windowScaleFactor, windowScaleFactor)
+    }
+
+    public func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    ) {
+        window.notifyScaleFactor = { _ in
+            action()
+        }
+    }
+
     public func setIncomingURLHandler(to action: @escaping (URL) -> Void) {
         gtkApp.onOpen = { urls in
             for url in urls {

Sources/GtkBackend/GtkBackend.swift

@@ -29,6 +29,7 @@ public final class GtkBackend: AppBackend {
     public let scrollBarWidth = 0
     public let requiresToggleSwitchSpacer = false
     public let defaultToggleStyle = ToggleStyle.button
+    public let requiresImageUpdateOnScaleFactorChange = false
 
     var gtkApp: Application
 
@@ -262,6 +263,21 @@ public final class GtkBackend: AppBackend {
         // TODO: React to theme changes
     }
 
+    public func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues {
+        // TODO: Record window scale factor in here
+        rootEnvironment
+    }
+
+    public func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    ) {
+        // TODO: Notify when window scale factor changes
+    }
+
     public func setIncomingURLHandler(to action: @escaping (URL) -> Void) {
         gtkApp.onOpen = { urls in
             for url in urls {

Sources/SwiftCrossUI/Backend/AppBackend.swift

@@ -75,6 +75,15 @@ public protocol AppBackend {
     var requiresToggleSwitchSpacer: Bool { get }
     /// The default style for toggles.
     var defaultToggleStyle: ToggleStyle { get }
+    /// If `true`, all images in a window will get updated when the window's
+    /// scale factor changes (``EnvironmentValues/windowScaleFactor``).
+    ///
+    /// Backends based on modern UI frameworks can usually get away with setting
+    /// this to `false`, but backends such as `Gtk3Backend` have to set this to
+    /// `true` to properly support HiDPI (aka Retina) displays because they
+    /// manually rescale the image meaning that it must get rescaled when the
+    /// scale factor changes.
+    var requiresImageUpdateOnScaleFactorChange: Bool { 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
@@ -164,6 +173,23 @@ public protocol AppBackend {
     /// may or may not override the previous handler.
     func setRootEnvironmentChangeHandler(to action: @escaping () -> Void)
 
+    /// Computes a window's environment based off the root environment. This may involve
+    /// updating ``EnvironmentValues/windowScaleFactor`` etc.
+    func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues
+    /// Sets the handler to be notified when the window's contribution to the
+    /// environment may have to be recomputed. Use this for things such as
+    /// updating a window's scale factor in the environment when the window
+    /// changes displays.
+    ///
+    /// In future this may be useful for color space handling.
+    func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    )
+
     /// Sets the handler for URLs directed to the application (e.g. URLs
     /// associated with a custom URL scheme).
     func setIncomingURLHandler(to action: @escaping (URL) -> Void)

Sources/SwiftCrossUI/Environment/EnvironmentValues.swift

@@ -35,6 +35,9 @@ public struct EnvironmentValues {
     /// pressing Enter/Return).
     public var onSubmit: (() -> Void)?
 
+    /// The scale factor of the current window.
+    public var windowScaleFactor: Int
+
     /// Called by view graph nodes when they resize due to an internal state
     /// change and end up changing size. Each view graph node sets its own
     /// handler when passing the environment on to its children, setting up
@@ -116,6 +119,7 @@ public struct EnvironmentValues {
         font = .system(size: 12)
         multilineTextAlignment = .leading
         colorScheme = .light
+        windowScaleFactor = 1
         window = nil
     }
 

Sources/SwiftCrossUI/Scenes/WindowGroupNode.swift

@@ -52,7 +52,21 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
                 self.scene,
                 proposedWindowSize: newSize,
                 backend: backend,
-                environment: parentEnvironment,
+                environment: self.parentEnvironment,
+                windowSizeIsFinal:
+                    !backend.isWindowProgrammaticallyResizable(window)
+            )
+        }
+
+        backend.setWindowEnvironmentChangeHandler(of: window) { [weak self] in
+            guard let self else {
+                return
+            }
+            _ = self.update(
+                self.scene,
+                proposedWindowSize: backend.size(ofWindow: window),
+                backend: backend,
+                environment: self.parentEnvironment,
                 windowSizeIsFinal:
                     !backend.isWindowProgrammaticallyResizable(window)
             )
@@ -107,7 +121,7 @@ public final class WindowGroupNode<Content: View>: SceneGraphNode {
         }
 
         let environment =
-            environment
+            backend.computeWindowEnvironment(window: window, rootEnvironment: environment)
             .with(\.onResize) { [weak self] _ in
                 guard let self = self else { return }
                 // TODO: Figure out whether this would still work if we didn't recompute the

Sources/SwiftCrossUI/Views/Image.swift

@@ -114,7 +114,12 @@ public struct Image: TypeSafeView, View {
         }
 
         let hasResized = children.cachedImageDisplaySize != size.size
-        if !dryRun && (children.imageChanged || hasResized) {
+        if !dryRun
+            && (children.imageChanged
+                || hasResized
+                || (backend.requiresImageUpdateOnScaleFactorChange
+                    && children.lastScaleFactor != environment.windowScaleFactor))
+        {
             if let image {
                 backend.updateImageView(
                     children.imageWidget.into(),
@@ -135,6 +140,7 @@ public struct Image: TypeSafeView, View {
                 children.isContainerEmpty = true
             }
             children.imageChanged = false
+            children.lastScaleFactor = environment.windowScaleFactor
         }
 
         children.cachedImageDisplaySize = size.size
@@ -156,6 +162,7 @@ class _ImageChildren: ViewGraphNodeChildren {
     var imageWidget: AnyWidget
     var imageChanged = false
     var isContainerEmpty = true
+    var lastScaleFactor = 1
 
     init<Backend: AppBackend>(backend: Backend) {
         container = AnyWidget(backend.createContainer())

Sources/UIKitBackend/UIKitBackend.swift

@@ -19,6 +19,8 @@ public final class UIKitBackend: AppBackend {
     public let defaultTableRowContentHeight = -1
     public let defaultTableCellVerticalPadding = -1
 
+    public let requiresImageUpdateOnScaleFactorChange = false
+
     var onTraitCollectionChange: (() -> Void)?
 
     private let appDelegateClass: ApplicationDelegate.Type
@@ -87,6 +89,21 @@ public final class UIKitBackend: AppBackend {
         onTraitCollectionChange = action
     }
 
+    public func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues {
+        // TODO: Record window scale factor in here
+        rootEnvironment
+    }
+
+    public func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    ) {
+        // TODO: Notify when window scale factor changes
+    }
+
     public func runInMainThread(action: @escaping () -> Void) {
         DispatchQueue.main.async(execute: action)
     }

Sources/WinUIBackend/WinUIBackend.swift

@@ -37,6 +37,7 @@ public final class WinUIBackend: AppBackend {
     public let defaultPaddingAmount = 10
     public let requiresToggleSwitchSpacer = false
     public let defaultToggleStyle = ToggleStyle.button
+    public let requiresImageUpdateOnScaleFactorChange = false
 
     public var scrollBarWidth: Int {
         12
@@ -280,6 +281,21 @@ public final class WinUIBackend: AppBackend {
         internalState.themeChangeAction = action
     }
 
+    public func computeWindowEnvironment(
+        window: Window,
+        rootEnvironment: EnvironmentValues
+    ) -> EnvironmentValues {
+        // TODO: Record window scale factor in here
+        rootEnvironment
+    }
+
+    public func setWindowEnvironmentChangeHandler(
+        of window: Window,
+        to action: @escaping () -> Void
+    ) {
+        // TODO: Notify when window scale factor changes
+    }
+
     public func setIncomingURLHandler(to action: @escaping (URL) -> Void) {
         print("Implement set incoming url handler")
         // TODO