From 0a624ce4ec8fe57138371083de2068d86065a93e Mon Sep 17 00:00:00 2001
From: Luke Zhao <me@lkzhao.com>
Date: Wed, 31 Jul 2024 14:57:31 -0700
Subject: [PATCH] Add ItemOffset component

---
 .../Components/Layout/Other/Offset.swift      | 19 ++++---
 .../Layout/Stack/StackItemOffset.swift        | 55 +++++++++++++++++++
 .../Core/Model/RenderNode/RenderNode.swift    |  3 +-
 3 files changed, 68 insertions(+), 9 deletions(-)
 create mode 100644 Sources/UIComponent/Components/Layout/Stack/StackItemOffset.swift

diff --git a/Sources/UIComponent/Components/Layout/Other/Offset.swift b/Sources/UIComponent/Components/Layout/Other/Offset.swift
index 0b3c18c..2b23faa 100644
--- a/Sources/UIComponent/Components/Layout/Other/Offset.swift
+++ b/Sources/UIComponent/Components/Layout/Other/Offset.swift
@@ -38,24 +38,29 @@ public struct DynamicOffset: Component {
     }
 }
 
-struct OffsetRenderNode: RenderNode {
-    typealias View = UIView
+public struct OffsetRenderNode: RenderNode {
+    public typealias View = UIView
 
-    let content: any RenderNode
-    let offset: CGPoint
+    public let content: any RenderNode
+    public let offset: CGPoint
+
+    public init(content: any RenderNode, offset: CGPoint) {
+        self.content = content
+        self.offset = offset
+    }
 
     /// The size of the render node, adjusted for the insets.
-    var size: CGSize {
+    public var size: CGSize {
         content.size
     }
 
     /// The content render nodes of this render node.
-    var children: [any RenderNode] {
+    public var children: [any RenderNode] {
         [content]
     }
 
     /// The positions of the content render nodes within this render node.
-    var positions: [CGPoint] {
+    public var positions: [CGPoint] {
         [offset]
     }
 }
diff --git a/Sources/UIComponent/Components/Layout/Stack/StackItemOffset.swift b/Sources/UIComponent/Components/Layout/Stack/StackItemOffset.swift
new file mode 100644
index 0000000..06b4372
--- /dev/null
+++ b/Sources/UIComponent/Components/Layout/Stack/StackItemOffset.swift
@@ -0,0 +1,55 @@
+//  Created by Luke Zhao on 7/29/24.
+
+import UIKit
+import simd
+
+public struct VStackItemOffset: Component {
+    public let offset: CGFloat
+    public let stack: VStack
+    public init(offset: CGFloat, stack: VStack) {
+        self.offset = offset
+        self.stack = stack
+    }
+    public func layout(_ constraint: Constraint) -> some RenderNode {
+        let renderNode = stack.layout(constraint)
+        guard !renderNode.children.isEmpty else { return OffsetRenderNode(content: renderNode, offset: .zero) }
+        let previousIndex = Int(offset).clamp(0, stack.children.count - 1)
+        let nextIndex = min(previousIndex + 1, stack.children.count - 1)
+        let previousPosition = renderNode.positions[previousIndex].y
+        let nextPosition = nextIndex == previousIndex ? renderNode.positions[previousIndex].y + renderNode.children[previousIndex].size.height + stack.spacing : renderNode.positions[nextIndex].y
+        let interpolatedOffset = simd_mix(previousPosition, nextPosition, offset - CGFloat(previousIndex))
+        return OffsetRenderNode(content: renderNode, offset: CGPoint(x: 0, y: -interpolatedOffset))
+    }
+}
+
+public extension VStack {
+    func offsetByItem(_ offset: CGFloat) -> some Component {
+        VStackItemOffset(offset: offset, stack: self)
+    }
+}
+
+public struct HStackItemOffset: Component {
+    public let offset: CGFloat
+    public let stack: HStack
+    public init(offset: CGFloat, stack: HStack) {
+        self.offset = offset
+        self.stack = stack
+    }
+    public func layout(_ constraint: Constraint) -> some RenderNode {
+        let renderNode = stack.layout(constraint)
+        guard !renderNode.children.isEmpty else { return OffsetRenderNode(content: renderNode, offset: .zero) }
+        let previousIndex = Int(offset).clamp(0, stack.children.count - 1)
+        let nextIndex = min(previousIndex + 1, stack.children.count - 1)
+        let previousPosition = renderNode.positions[previousIndex].x
+        let nextPosition = nextIndex == previousIndex ? renderNode.positions[previousIndex].x + renderNode.children[previousIndex].size.width + stack.spacing : renderNode.positions[nextIndex].x
+        let interpolatedOffset = simd_mix(previousPosition, nextPosition, offset - CGFloat(previousIndex))
+        return OffsetRenderNode(content: renderNode, offset: CGPoint(x: -interpolatedOffset, y: 0))
+    }
+}
+
+public extension HStack {
+    func offsetByItem(_ offset: CGFloat) -> some Component {
+        HStackItemOffset(offset: offset, stack: self)
+    }
+}
+
diff --git a/Sources/UIComponent/Core/Model/RenderNode/RenderNode.swift b/Sources/UIComponent/Core/Model/RenderNode/RenderNode.swift
index 9d1305d..741ee69 100644
--- a/Sources/UIComponent/Core/Model/RenderNode/RenderNode.swift
+++ b/Sources/UIComponent/Core/Model/RenderNode/RenderNode.swift
@@ -156,8 +156,7 @@ extension RenderNode {
         let frame = adjustVisibleFrame(frame: frame)
         let children = visibleChildren(in: frame)
         for child in children {
-            let childFrame = CGRect(origin: child.position, size: child.renderNode.size)
-            let childVisibleFrame = frame.intersection(childFrame) - child.position
+            let childVisibleFrame = frame - child.position
             let childRenderables = child.renderNode._visibleRenderables(in: childVisibleFrame).map {
                 Renderable(id: $0.renderNode.id ?? "item-\(child.index)-\($0.id)", frame: $0.frame + child.position, renderNode: $0.renderNode)
             }