Cancel in-progress reorders when the list is torn down

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### Fixed
 
+- Fixed a crash when a list left the window (or was deallocated) while an interactive reorder was still in progress — for example, navigating away while a drag was held. The native interactive-movement session outlived the content it was started against, so a later content update or layout pass read a now-stale index and trapped in the layout. `ListView` now cancels any in-progress reorder on `didMoveToWindow` (when leaving the window) and in `deinit`, while the data source and layout are still in sync.
+
 ### Added
 
 ### Removed

ListableUI/Sources/ListView/ListView.swift

@@ -144,6 +144,15 @@ public final class ListView : UIView
         // because the display link driving it is retained by the main runloop.
         self.cancelScrollAnimation()
 
+        // If the list is deallocated while a reorder gesture is still in flight, the native
+        // interactive-movement session would otherwise outlive the data source and layout.
+        // UIKit would then try to resolve the move against content that no longer matches the
+        // drag's index paths, reading a stale index and crashing in the layout. Cancel it here,
+        // while everything is still in sync.
+        if self.hasInProgressReorders {
+            self.cancelAllInProgressReorders()
+        }
+
         /**
          Even though these are zeroing weak references in UIKIt as of iOS 9.0,
 
@@ -1567,9 +1576,15 @@ public final class ListView : UIView
     public override func didMoveToWindow()
     {
         super.didMoveToWindow()
-        
+
         if self.window != nil {
             self.updateScrollViewInsets()
+        } else if self.hasInProgressReorders {
+            // Leaving the window — for example, navigating away while a drag is still held — ends
+            // any chance of the reorder gesture completing normally. Cancel it now, while the
+            // content and the drag's index paths still agree, so a later content update or layout
+            // pass cannot read a stale index and crash.
+            self.cancelAllInProgressReorders()
         }
     }
 
@@ -2467,7 +2482,7 @@ extension ListView : ReorderingActionsDelegate
         self.collectionView.cancelInteractiveMovement()
     }
 
-    private var hasInProgressReorders : Bool {
+    var hasInProgressReorders : Bool {
 
         for section in self.storage.presentationState.sections {
             for item in section.items {

ListableUI/Tests/ListView/ListView.ReorderTeardownTests.swift

@@ -0,0 +1,128 @@
+//
+//  ListView.ReorderTeardownTests.swift
+//  ListableUI-Unit-Tests
+//
+
+@testable import ListableUI
+import XCTest
+
+
+class ListView_ReorderTeardownTests: XCTestCase {
+
+    /// A reorder gesture that is still in flight when the list leaves the window — for example,
+    /// navigating away while a drag is held — must be cancelled during teardown. Otherwise the
+    /// native interactive-movement session outlives the content it was started against, and a
+    /// later content update or layout pass reads a now-stale index and crashes.
+    func test_reorder_is_cancelled_when_list_leaves_window() {
+
+        let viewController = ReorderTestViewController()
+
+        show(vc: viewController) { viewController in
+            let listView = viewController.list
+
+            let indexPath = IndexPath(item: 0, section: 0)
+            let item = listView.storage.presentationState.item(at: indexPath)
+
+            // Simulate an in-progress reorder without depending on the flakier native
+            // `beginInteractiveMovementForItem` gesture plumbing.
+            item.beginReorder(from: indexPath, with: listView.environment)
+
+            XCTAssertTrue(listView.hasInProgressReorders)
+
+            // Leaving the window is what navigating away mid-drag does to the list.
+            listView.removeFromSuperview()
+
+            XCTAssertFalse(
+                listView.hasInProgressReorders,
+                "Leaving the window should cancel any in-progress reorder."
+            )
+        }
+    }
+
+    /// A list deallocated with a reorder still in progress must not crash: teardown cancels the
+    /// interactive movement while the data source and layout are still valid.
+    func test_reorder_in_progress_does_not_crash_on_deinit() {
+
+        weak var weakList: ListView?
+
+        autoreleasepool {
+            let listView = ListView(frame: CGRect(x: 0, y: 0, width: 400, height: 600))
+            weakList = listView
+
+            listView.configure { list in
+                list.animatesChanges = false
+                list("section") { section in
+                    for number in 1...10 {
+                        section += Item(
+                            ReorderTestContent(title: "Item \(number)"),
+                            reordering: ItemReordering(sections: .all)
+                        )
+                    }
+                }
+            }
+
+            listView.collectionView.layoutIfNeeded()
+
+            let indexPath = IndexPath(item: 0, section: 0)
+            let item = listView.storage.presentationState.item(at: indexPath)
+            item.beginReorder(from: indexPath, with: listView.environment)
+
+            XCTAssertTrue(listView.hasInProgressReorders)
+        }
+
+        // Reaching here without a crash — and with the list deallocated — is the assertion.
+        XCTAssertNil(weakList)
+    }
+}
+
+
+fileprivate final class ReorderTestViewController: UIViewController {
+
+    let list = ListView()
+
+    override func loadView() {
+        view = UIView()
+        view.addSubview(list)
+        list.frame = CGRect(x: 0, y: 0, width: 400, height: 600)
+
+        list.configure { list in
+            list.animatesChanges = false
+            list("section") { section in
+                for number in 1...10 {
+                    section += Item(
+                        ReorderTestContent(title: "Item \(number)"),
+                        reordering: ItemReordering(sections: .all)
+                    )
+                }
+            }
+        }
+    }
+}
+
+
+fileprivate struct ReorderTestContent: ItemContent, Equatable {
+
+    var title: String
+
+    var identifierValue: String { title }
+
+    func apply(
+        to views: ItemContentViews<Self>,
+        for reason: ApplyReason,
+        with info: ApplyItemContentInfo
+    ) {
+        views.content.backgroundColor = .red
+    }
+
+    typealias ContentView = UIView
+
+    static func createReusableContentView(frame: CGRect) -> UIView {
+        UIView(frame: frame)
+    }
+
+    var defaultItemProperties: DefaultProperties {
+        .defaults { defaults in
+            defaults.sizing = .fixed(height: 50)
+        }
+    }
+}