Merge pull request #56 from PureSwift/feature/geometry-reader

Add GeometryReader

Author: colemancda

AndroidSwiftUICore/Sources/AndroidSwiftUICore/Primitives/GeometryReader.swift

@@ -0,0 +1,70 @@
+//
+//  GeometryReader.swift
+//  AndroidSwiftUICore
+//
+//  Size feedback from layout back into Swift — the one place the data flows
+//  backwards. Swift resolves a tree *before* Compose lays it out, so the size
+//  can't be known on the first pass: the interpreter measures, reports the size
+//  through a callback, and the resulting store update re-evaluates with the real
+//  numbers. Rendering a GeometryReader therefore settles over two passes.
+//
+//  The reported size comes from the *constraints the parent offers*, never from
+//  the content, so measuring can't feed back into itself. The store also drops
+//  a report that matches what it already holds, so a steady layout stops.
+//
+
+import Foundation
+
+/// The geometry of the space a `GeometryReader` was offered.
+public struct GeometryProxy: Sendable {
+    public let size: CGSize
+}
+
+/// Holds the last size the interpreter reported for one GeometryReader,
+/// persisted at its identity path so it survives re-evaluation.
+internal final class GeometrySizeStore {
+
+    private(set) var size = CGSize(width: 0, height: 0)
+    var onChange: (() -> Void)?
+
+    /// Records a `"<width>,<height>"` report, re-evaluating only on a change.
+    func update(from payload: String) {
+        let parts = payload.split(separator: ",").compactMap { Double($0) }
+        guard parts.count == 2 else { return }
+        guard parts[0] != size.width || parts[1] != size.height else { return }
+        size = CGSize(width: parts[0], height: parts[1])
+        onChange?()
+    }
+}
+
+public struct GeometryReader<Content: View>: View {
+
+    internal let content: (GeometryProxy) -> Content
+
+    public init(@ViewBuilder content: @escaping (GeometryProxy) -> Content) {
+        self.content = content
+    }
+
+    public typealias Body = Never
+}
+
+extension GeometryReader: PrimitiveView {
+
+    public func _render(in context: ResolveContext) -> RenderNode {
+        let store = context.storage.persistentObject(at: context.path + ".geometry") {
+            GeometrySizeStore()
+        }
+        store.onChange = context.storage.onChange
+        let id = context.callbacks.register(.string { [store] payload in
+            store.update(from: payload)
+        })
+        // first pass resolves against .zero; the report brings the real size
+        let proxy = GeometryProxy(size: store.size)
+        return RenderNode(
+            type: "GeometryReader",
+            id: context.path,
+            props: ["onSize": .int(Int(id))],
+            children: Evaluator.resolveChildren(content(proxy), context.descending("content"))
+        )
+    }
+}

AndroidSwiftUICore/Tests/AndroidSwiftUICoreTests/EvaluatorTests.swift

@@ -520,3 +520,69 @@ struct AnimationTests {
         #expect(anim?.args["token"] == .string("3"))
     }
 }
+
+@Suite("Geometry")
+struct GeometryTests {
+
+    /// Finds a GeometryReader's size-report callback anywhere in a tree.
+    private func sizeCallback(_ node: RenderNode) -> Int64? {
+        if node.type == "GeometryReader", case .int(let id)? = node.props["onSize"] {
+            return Int64(id)
+        }
+        for child in node.children {
+            if let found = sizeCallback(child) { return found }
+        }
+        return nil
+    }
+
+    @Test("A GeometryReader resolves against zero until a size is reported")
+    func geometrySettlesOverTwoPasses() {
+        struct Screen: View {
+            var body: some View {
+                GeometryReader { geometry in
+                    Text("w=\(Int(geometry.size.width)) h=\(Int(geometry.size.height))")
+                }
+            }
+        }
+        let host = ViewHost(Screen())
+        var node = host.evaluate()
+        #expect(node.type == "GeometryReader")
+        // first pass: layout hasn't happened, so the proxy reads zero
+        #expect(firstTextString(node) == "w=0 h=0")
+
+        guard let report = sizeCallback(node) else {
+            Issue.record("missing size callback"); return
+        }
+        host.callbacks.invokeString(report, "320.0,240.0")
+        node = host.evaluate()
+        // second pass: the reported size reaches the content
+        #expect(firstTextString(node) == "w=320 h=240")
+    }
+
+    @Test("The size store re-evaluates on a change and ignores everything else")
+    func geometryStoreSettles() {
+        // Driving the store directly is the precise way to pin the settling
+        // rule: without it a steady layout would re-evaluate forever.
+        let store = GeometrySizeStore()
+        var changes = 0
+        store.onChange = { changes += 1 }
+
+        store.update(from: "100.0,50.0")
+        #expect(changes == 1)
+        #expect(store.size.width == 100)
+        #expect(store.size.height == 50)
+
+        store.update(from: "100.0,50.0")
+        #expect(changes == 1)          // same size — layout has settled
+
+        store.update(from: "180.0,50.0")
+        #expect(changes == 2)          // a real change reports again
+        #expect(store.size.width == 180)
+
+        store.update(from: "garbage")
+        store.update(from: "")
+        store.update(from: "1.0")      // too few components
+        #expect(changes == 2)          // malformed reports never re-evaluate
+        #expect(store.size.width == 180)
+    }
+}

Demo/App.swiftpm/Sources/Catalog.swift

@@ -36,6 +36,7 @@ struct CatalogEntry: Identifiable {
         CatalogEntry(id: "morecontrols", title: "More Controls", screen: AnyCatalogScreen(MoreControlsPlayground())),
         CatalogEntry(id: "controlstyle", title: "Control Styles", screen: AnyCatalogScreen(ControlStylePlayground())),
         CatalogEntry(id: "stack", title: "Stacks", screen: AnyCatalogScreen(StackPlayground())),
+        CatalogEntry(id: "geometry", title: "GeometryReader", screen: AnyCatalogScreen(GeometryPlayground())),
         CatalogEntry(id: "frame", title: "Frames", screen: AnyCatalogScreen(FramePlayground())),
         CatalogEntry(id: "spacer", title: "Spacer & Divider", screen: AnyCatalogScreen(SpacerDividerPlayground())),
         CatalogEntry(id: "color", title: "Color", screen: AnyCatalogScreen(ColorPlayground())),

Demo/App.swiftpm/Sources/GeometryPlaygrounds.swift

@@ -0,0 +1,36 @@
+#if canImport(AndroidSwiftUI)
+import AndroidSwiftUI
+#else
+import SwiftUI
+#endif
+
+struct GeometryPlayground: View {
+
+    @State private var tall = false
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 16) {
+            Text("A GeometryReader reports the space its parent offers.")
+
+            GeometryReader { geometry in
+                VStack(alignment: .leading, spacing: 10) {
+                    Text("Offered: \(Int(geometry.size.width)) × \(Int(geometry.size.height))")
+                    // bars sized from the measurement — proof the number is live
+                    Text("half width")
+                    Rectangle()
+                        .fill(.blue)
+                        .frame(width: geometry.size.width / 2, height: 22)
+                    Text("one quarter")
+                    Rectangle()
+                        .fill(.green)
+                        .frame(width: geometry.size.width / 4, height: 22)
+                }
+            }
+            .frame(height: tall ? 320 : 200)
+
+            Button(tall ? "Shrink the reader" : "Grow the reader") { tall.toggle() }
+        }
+        .padding()
+        .navigationTitle("GeometryReader")
+    }
+}

Demo/swiftui/src/commonMain/kotlin/com/pureswift/swiftui/Render.kt

@@ -13,6 +13,7 @@ import androidx.compose.foundation.horizontalScroll
 import androidx.compose.foundation.verticalScroll
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.BoxWithConstraints
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.ColumnScope
 import androidx.compose.foundation.layout.Row
@@ -331,6 +332,8 @@ private fun RenderResolved(node: ViewNode) {
                 }
             }
 
+            "GeometryReader" -> RenderGeometryReader(node)
+
             "LazyVStack" -> RenderLazyStack(node, vertical = true)
             "LazyHStack" -> RenderLazyStack(node, vertical = false)
 
@@ -546,6 +549,24 @@ private fun RenderToggle(node: ViewNode) {
     }
 }
 
+// Reports the size the PARENT offers, never the content's own size — that is
+// what keeps measurement from feeding back into itself. Swift ignores a report
+// equal to what it already holds, so a settled layout reports once and stops.
+@Composable
+private fun RenderGeometryReader(node: ViewNode) {
+    val onSize = node.long("onSize")
+    BoxWithConstraints(modifier = node.composeModifiers().fillMaxWidth()) {
+        val width = maxWidth.value.toDouble()
+        // an unbounded height (inside a scrolling parent) isn't a real offer;
+        // report 0 so the content can fall back rather than see infinity
+        val height = if (constraints.hasBoundedHeight) maxHeight.value.toDouble() else 0.0
+        LaunchedEffect(width, height) {
+            onSize?.let { SwiftBridge.sink.invokeString(it, "$width,$height") }
+        }
+        RenderChildren(node)
+    }
+}
+
 private fun ViewNode.isPresentation(): Boolean =
     type == "Sheet" || type == "Alert" || type == "ConfirmationDialog" || type == "ToolbarItem"