Add custom item offset scrolling

CHANGELOG.md

@@ -4,6 +4,18 @@
 
 ### Added
 
+- Added item-aware content offset adjustment APIs, declarative auto-scroll support, and scroll-in-progress state for custom scrolling behaviors.
+  ```swift
+  list.autoScrollAction = .pin(
+      .item(targetIdentifier),
+      itemPosition: .verticalContentOffsetAdjustment { info in
+          max(0.0, info.itemFrame.maxY - info.visibleContentFrame.maxY)
+      },
+      scrollInterruptionPolicy: .deferDuringUserScrolling
+  )
+  ```
+  Use `.skipDuringUserScrolling` instead when the auto-scroll should be dropped rather than retried after the user scroll ends.
+
 ### Removed
 
 ### Changed

Development/Sources/Demos/Demo Screens/CustomAutoScrollingViewController.swift

@@ -0,0 +1,254 @@
+//
+//  CustomAutoScrollingViewController.swift
+//  Demo
+//
+//  Created by Square on 5/22/26.
+//
+
+import BlueprintUI
+import BlueprintUICommonControls
+import BlueprintUILists
+import ListableUI
+import UIKit
+
+
+final class CustomAutoScrollingViewController : UIViewController
+{
+    private let list = ListView()
+    private let footer = UIView()
+    private let footerTitle = UILabel()
+
+    private var selectedRow = 24
+    private var expandedRows = Set<Int>()
+    private var hasPerformedInitialLayoutUpdate = false
+
+    override func loadView()
+    {
+        self.view = UIView()
+        self.view.backgroundColor = .white
+
+        self.list.translatesAutoresizingMaskIntoConstraints = false
+        self.footer.translatesAutoresizingMaskIntoConstraints = false
+
+        self.view.addSubview(self.list)
+        self.view.addSubview(self.footer)
+
+        NSLayoutConstraint.activate([
+            self.list.topAnchor.constraint(equalTo: self.view.topAnchor),
+            self.list.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
+            self.list.trailingAnchor.constraint(equalTo: self.view.trailingAnchor),
+            self.list.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
+
+            self.footer.leadingAnchor.constraint(equalTo: self.view.leadingAnchor),
+            self.footer.trailingAnchor.constraint(equalTo: self.view.trailingAnchor),
+            self.footer.bottomAnchor.constraint(equalTo: self.view.bottomAnchor),
+            self.footer.heightAnchor.constraint(equalToConstant: 112.0),
+        ])
+
+        self.configureFooter()
+    }
+
+    override func viewDidLoad()
+    {
+        super.viewDidLoad()
+
+        self.title = "Custom Auto Scrolling"
+        self.updateList()
+    }
+
+    override func viewDidLayoutSubviews()
+    {
+        super.viewDidLayoutSubviews()
+
+        guard self.hasPerformedInitialLayoutUpdate == false else {
+            return
+        }
+
+        self.hasPerformedInitialLayoutUpdate = true
+        self.updateList()
+    }
+
+    private func configureFooter()
+    {
+        self.footer.backgroundColor = .systemBackground
+        self.footer.layer.shadowColor = UIColor.black.cgColor
+        self.footer.layer.shadowOpacity = 0.18
+        self.footer.layer.shadowRadius = 8.0
+        self.footer.layer.shadowOffset = CGSize(width: 0.0, height: -2.0)
+
+        let previous = UIButton(type: .system)
+        previous.setTitle("Previous", for: .normal)
+        previous.addTarget(self, action: #selector(selectPreviousRow), for: .touchUpInside)
+
+        let next = UIButton(type: .system)
+        next.setTitle("Next", for: .normal)
+        next.addTarget(self, action: #selector(selectNextRow), for: .touchUpInside)
+
+        let toggleHeight = UIButton(type: .system)
+        toggleHeight.setTitle("Toggle Height", for: .normal)
+        toggleHeight.addTarget(self, action: #selector(toggleSelectedRowHeight), for: .touchUpInside)
+
+        self.footerTitle.font = .systemFont(ofSize: 16.0, weight: .semibold)
+        self.footerTitle.textAlignment = .center
+
+        let buttons = UIStackView(arrangedSubviews: [previous, next, toggleHeight])
+        buttons.axis = .horizontal
+        buttons.alignment = .center
+        buttons.distribution = .equalSpacing
+        buttons.spacing = 16.0
+
+        let stack = UIStackView(arrangedSubviews: [self.footerTitle, buttons])
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        stack.axis = .vertical
+        stack.alignment = .fill
+        stack.spacing = 10.0
+
+        self.footer.addSubview(stack)
+
+        NSLayoutConstraint.activate([
+            stack.topAnchor.constraint(equalTo: self.footer.topAnchor, constant: 12.0),
+            stack.leadingAnchor.constraint(equalTo: self.footer.leadingAnchor, constant: 20.0),
+            stack.trailingAnchor.constraint(equalTo: self.footer.trailingAnchor, constant: -20.0),
+        ])
+    }
+
+    @objc private func selectPreviousRow()
+    {
+        self.selectedRow = max(0, self.selectedRow - 1)
+        self.updateList()
+    }
+
+    @objc private func selectNextRow()
+    {
+        self.selectedRow = min(Self.rowCount - 1, self.selectedRow + 1)
+        self.updateList()
+    }
+
+    @objc private func toggleSelectedRowHeight()
+    {
+        if self.expandedRows.contains(self.selectedRow) {
+            self.expandedRows.remove(self.selectedRow)
+        } else {
+            self.expandedRows.insert(self.selectedRow)
+        }
+
+        self.updateList()
+    }
+
+    private func updateList()
+    {
+        self.footerTitle.text = "Target row \(self.selectedRow + 1) stays above the fixed footer"
+
+        let selectedRow = self.selectedRow
+        let targetIdentifier = FooterAwarePinnedItem.identifier(with: selectedRow)
+
+        self.list.configure { list in
+            list.appearance = .demoAppearance
+            list.layout = .demoLayout
+            list.animation = .fast
+            list.scrollIndicatorInsets.bottom = 112.0
+
+            list.autoScrollAction = .pin(
+                .item(targetIdentifier),
+                itemPosition: .verticalContentOffsetAdjustment { [weak self] info in
+                    self?.footerAwareScrollDelta(for: info) ?? 0.0
+                },
+                animated: false,
+                scrollInterruptionPolicy: .deferDuringUserScrolling,
+                shouldPerform: { _ in true }
+            )
+
+            list += Section("rows") {
+                for row in 0..<Self.rowCount {
+                    FooterAwarePinnedItem(
+                        row: row,
+                        isSelected: row == selectedRow,
+                        isExpanded: self.expandedRows.contains(row)
+                    )
+                }
+            }
+        }
+    }
+
+    private func footerAwareScrollDelta(for info: ListItemScrollPositionInfo) -> CGFloat
+    {
+        let topGap : CGFloat = 16.0
+        let footerGap : CGFloat = 16.0
+        let footerHeight = self.footer.bounds.height
+
+        let idealTop = info.visibleContentFrame.minY + topGap
+        let idealBottom = info.visibleContentFrame.maxY - footerHeight - footerGap
+
+        if info.itemFrame.height > idealBottom - idealTop {
+            return info.itemFrame.minY - idealTop
+        }
+
+        if info.itemFrame.minY < idealTop {
+            return info.itemFrame.minY - idealTop
+        }
+
+        if info.itemFrame.maxY > idealBottom {
+            return info.itemFrame.maxY - idealBottom
+        }
+
+        return 0.0
+    }
+
+    private static let rowCount = 50
+}
+
+
+private struct FooterAwarePinnedItem : BlueprintItemContent, Equatable
+{
+    var row : Int
+    var isSelected : Bool
+    var isExpanded : Bool
+
+    var identifierValue : Int {
+        self.row
+    }
+
+    func element(with info : ApplyItemContentInfo) -> Element
+    {
+        let title = Label(text: "Row \(self.row + 1)") {
+            $0.font = .systemFont(ofSize: 17.0, weight: self.isSelected ? .semibold : .regular)
+            $0.color = self.isSelected ? .systemBlue : .label
+        }
+
+        let detail = Label(text: self.detailText) {
+            $0.font = .systemFont(ofSize: 14.0, weight: .regular)
+            $0.color = .secondaryLabel
+        }
+
+        let content = Column(alignment: .fill, minimumSpacing: 6.0) {
+            title
+            detail
+        }
+
+        var box = Box(
+            backgroundColor: self.isSelected ? UIColor.systemBlue.withAlphaComponent(0.08) : .white,
+            cornerStyle: .rounded(radius: 6.0),
+            wrapping: Inset(
+                uniformInset: 14.0,
+                wrapping: content
+            )
+        )
+
+        box.borderStyle = .solid(
+            color: self.isSelected ? .systemBlue : .white(0.9),
+            width: 2.0
+        )
+
+        return box
+    }
+
+    private var detailText : String {
+        if self.isExpanded {
+            return "Expanded row content demonstrates a layout update that re-runs declarative custom auto-scroll."
+        } else if self.isSelected {
+            return "Selected target row"
+        } else {
+            return "Regular row"
+        }
+    }
+}

Development/Sources/Demos/DemosRootViewController.swift

@@ -81,6 +81,14 @@ public final class DemosRootViewController : ListViewController
                         self?.push(PinAutoscrollingViewController())
                     }
                 )
+
+                Item(
+                    DemoItem(text: "Custom Auto Scrolling (Footer-Aware Pin)"),
+                    selectionStyle: .selectable(),
+                    onSelect : { _ in
+                        self?.push(CustomAutoScrollingViewController())
+                    }
+                )
 
                 Item(
                     DemoItem(text: "scrollTo(item: ...) completion handler"),
@@ -426,4 +434,3 @@ public final class DemosRootViewController : ListViewController
         }
     }
 }
-