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tiensonqintiensonqin
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Prepare virtual list destinations before distant status bar returns
1 parent 903b1cd commit 9a8de61

3 files changed

Lines changed: 189 additions & 0 deletions

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‎platform/apple/Sources/LUIAppleBackend/LUISwiftUIRoot.swift‎

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Original file line numberDiff line numberDiff line change
@@ -7039,6 +7039,16 @@ private struct LUIVirtualListView: View {
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}
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}
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.coordinateSpace(name: contentSpace)
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.background {
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#if os(iOS)
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if let first = rows.segments.first(where: { $0.window != nil }), let window = first.window, window.count > 0 {
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LUIVirtualScrollTopBridge(prepared: window.start == 0) {
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backend.performVisibleRange(node: first.source, first: 0, last: min(31, window.count - 1))
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}
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.frame(width: 0, height: 0)
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}
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#endif
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}
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.overlay(alignment: .topLeading) {
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if let editor {
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let row = rows[editor.position]

‎platform/apple/Sources/LUIAppleBackend/LUIVirtualScrollTopBridge.swift‎

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@@ -0,0 +1,107 @@
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#if os(iOS)
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import SwiftUI
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import UIKit
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/// A distant status-bar return needs its destination window before UIKit moves.
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struct LUIVirtualScrollTopBridge: UIViewRepresentable {
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let prepared: Bool
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let requestTop: () -> Void
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func makeUIView(context: Context) -> Probe { Probe() }
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func updateUIView(_ view: Probe, context: Context) {
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view.prepared = prepared
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view.requestTop = requestTop
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view.install()
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view.completeReturn()
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}
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static func dismantleUIView(_ view: Probe, coordinator: ()) { view.uninstall() }
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final class Probe: UIView {
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var prepared = false
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var requestTop: (() -> Void)?
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private weak var scroll: UIScrollView?
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private let proxy = DelegateProxy()
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private var returning = false
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init() {
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super.init(frame: .zero)
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isUserInteractionEnabled = false
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proxy.shouldReturn = { [weak self] scroll in
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guard let self, scroll.contentOffset.y + scroll.adjustedContentInset.top > scroll.bounds.height else { return true }
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returning = true
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// Stop momentum without replacing the visible window while the destination is prepared.
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scroll.setContentOffset(scroll.contentOffset, animated: false)
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if prepared { completeReturn() } else { requestTop?() }
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return false
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}
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proxy.cancelReturn = { [weak self] in self?.returning = false }
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}
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required init?(coder: NSCoder) { fatalError("init(coder:) has not been implemented") }
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override func didMoveToWindow() {
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super.didMoveToWindow()
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if window != nil { install() }
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}
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override func layoutSubviews() {
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super.layoutSubviews()
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install()
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}
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func install() {
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var ancestor = superview
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while let view = ancestor {
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if let scroll = view as? UIScrollView {
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if self.scroll !== scroll { uninstall(); self.scroll = scroll }
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if scroll.delegate !== proxy {
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proxy.original = scroll.delegate
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scroll.delegate = proxy
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}
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return
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}
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ancestor = view.superview
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}
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}
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func uninstall() {
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if let scroll, scroll.delegate === proxy { scroll.delegate = proxy.original }
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scroll = nil
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}
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func completeReturn() {
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guard returning, prepared, let scroll else { return }
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returning = false
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scroll.setContentOffset(CGPoint(x: scroll.contentOffset.x, y: -scroll.adjustedContentInset.top), animated: false)
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}
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}
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private final class DelegateProxy: NSObject, UIScrollViewDelegate {
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weak var original: UIScrollViewDelegate?
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var shouldReturn: (@MainActor (UIScrollView) -> Bool)?
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var cancelReturn: (@MainActor () -> Void)?
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override func responds(to selector: Selector!) -> Bool {
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if super.responds(to: selector) { return true }
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return MainActor.assumeIsolated { original?.responds(to: selector) == true }
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}
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override func forwardingTarget(for selector: Selector!) -> Any? {
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let target = MainActor.assumeIsolated { original }
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return target?.responds(to: selector) == true ? target : super.forwardingTarget(for: selector)
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}
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func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool {
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guard original?.scrollViewShouldScrollToTop?(scrollView) != false else { return false }
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return shouldReturn?(scrollView) ?? true
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}
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func scrollViewWillBeginDragging(_ scrollView: UIScrollView) {
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cancelReturn?()
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original?.scrollViewWillBeginDragging?(scrollView)
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}
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}
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}
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#endif

‎platform/apple/Tests/LUIAppleBackendTests/LUIVirtualScrollTopTests.swift‎

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#if os(iOS)
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import XCTest
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import UIKit
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@testable import LUIAppleBackend
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@MainActor final class LUIVirtualScrollTopTests: XCTestCase {
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private final class OriginalDelegate: NSObject, UIScrollViewDelegate {
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var allowsReturn = true
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var scrollEvents = 0
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func scrollViewShouldScrollToTop(_ scrollView: UIScrollView) -> Bool { allowsReturn }
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func scrollViewDidScroll(_ scrollView: UIScrollView) { scrollEvents += 1 }
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}
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func testDistantReturnWaitsForDestinationAndRestoresDelegate() {
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let scroll = UIScrollView(frame: CGRect(x: 0, y: 0, width: 320, height: 600))
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scroll.contentSize = CGSize(width: 320, height: 10_000)
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let original = OriginalDelegate()
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scroll.delegate = original
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let probe = LUIVirtualScrollTopBridge.Probe()
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scroll.addSubview(probe)
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probe.install()
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var requests = 0
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probe.requestTop = { requests += 1 }
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scroll.setContentOffset(CGPoint(x: 0, y: 5000), animated: false)
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XCTAssertEqual(scroll.delegate?.scrollViewShouldScrollToTop?(scroll), false)
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XCTAssertEqual(requests, 1)
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XCTAssertEqual(scroll.contentOffset.y, 5000)
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probe.prepared = true
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probe.completeReturn()
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XCTAssertEqual(scroll.contentOffset.y, 0)
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probe.uninstall()
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XCTAssertTrue(scroll.delegate === original)
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}
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func testNearbyReturnAndOriginalDelegateDecisionsArePreserved() {
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let scroll = UIScrollView(frame: CGRect(x: 0, y: 0, width: 320, height: 600))
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scroll.contentSize = CGSize(width: 320, height: 10_000)
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let original = OriginalDelegate()
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scroll.delegate = original
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let probe = LUIVirtualScrollTopBridge.Probe()
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scroll.addSubview(probe)
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probe.install()
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defer { probe.uninstall() }
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var requests = 0
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probe.requestTop = { requests += 1 }
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scroll.setContentOffset(CGPoint(x: 0, y: 50), animated: false)
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XCTAssertEqual(scroll.delegate?.scrollViewShouldScrollToTop?(scroll), true)
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scroll.setContentOffset(CGPoint(x: 0, y: 5000), animated: false)
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original.allowsReturn = false
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XCTAssertEqual(scroll.delegate?.scrollViewShouldScrollToTop?(scroll), false)
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XCTAssertEqual(requests, 0)
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let count = original.scrollEvents
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scroll.delegate?.scrollViewDidScroll?(scroll)
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XCTAssertEqual(original.scrollEvents, count + 1)
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}
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func testNewDragCancelsPendingReturn() {
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let scroll = UIScrollView(frame: CGRect(x: 0, y: 0, width: 320, height: 600))
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scroll.contentSize = CGSize(width: 320, height: 10_000)
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let probe = LUIVirtualScrollTopBridge.Probe()
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scroll.addSubview(probe)
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probe.install()
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defer { probe.uninstall() }
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scroll.setContentOffset(CGPoint(x: 0, y: 5000), animated: false)
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XCTAssertEqual(scroll.delegate?.scrollViewShouldScrollToTop?(scroll), false)
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scroll.delegate?.scrollViewWillBeginDragging?(scroll)
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probe.prepared = true
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probe.completeReturn()
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XCTAssertEqual(scroll.contentOffset.y, 5000)
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}
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}
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#endif

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