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@kean
Created February 18, 2023 15:28
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ScrollViewPrefetcher
// The MIT License (MIT)
//
// Copyright (c) 2021 Alexander Grebenyuk (github.com/kean).
import SwiftUI
public protocol ScrollViewPrefetcherDelegate: AnyObject {
/// Returns all valid indices for the collection.
func getAllIndicesForPrefetcher(_ prefetcher: ScrollViewPrefetcher) -> Range<Int>
func prefetcher(_ prefetcher: ScrollViewPrefetcher, prefetchItemsAt indices: [Int])
func prefetcher(_ prefetcher: ScrollViewPrefetcher, cancelPrefechingForItemAt indices: [Int])
}
public final class ScrollViewPrefetcher {
private let prefetchWindowSize: Int
public weak var delegate: ScrollViewPrefetcherDelegate?
private var visibleIndices: Set<Int> = []
private var flushedVisibleIndices: Set<Int> = []
private var isRefreshScheduled = false
private var prefetchWindow = 0..<0
public init(prefetchWindowSize: Int = 12) {
self.prefetchWindowSize = prefetchWindowSize
}
public func onAppear(_ index: Int) {
visibleIndices.insert(index)
scheduleRefreshIfNeeded()
}
public func onDisappear(_ index: Int) {
visibleIndices.remove(index)
scheduleRefreshIfNeeded()
}
/// SwiftUI sometimes calls onAppear in unexpected order, buffer takes care of it.
private func scheduleRefreshIfNeeded() {
guard !isRefreshScheduled else { return }
isRefreshScheduled = true
DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(100)) { [weak self] in
self?.updatePrefetchWindow()
}
}
private func updatePrefetchWindow() {
isRefreshScheduled = false
guard !visibleIndices.isEmpty else { return } // Should never happen
if flushedVisibleIndices.isEmpty {
// Showing screen for the first time
let lowerBound = visibleIndices.max()! + 1
updatePrefetchWindow(lowerBound..<(lowerBound + prefetchWindowSize))
} else {
// Need to figure out in which direction we the user is scrolling
let isScrollingDown = visibleIndices.max()! > flushedVisibleIndices.max()!
if isScrollingDown || visibleIndices.contains(0) {
let lowerBound = visibleIndices.max()! + 1
updatePrefetchWindow(lowerBound..<(lowerBound + prefetchWindowSize))
} else {
let upperBound = visibleIndices.min()! - 1
updatePrefetchWindow((upperBound - prefetchWindowSize)..<upperBound)
}
}
flushedVisibleIndices = visibleIndices
}
private func updatePrefetchWindow(_ newPrefetchWindow: Range<Int>) {
let oldPrefetchIndices = Set(prefetchWindow)
let newPrefetchIndices = Set(newPrefetchWindow)
prefetchWindow = newPrefetchWindow
let allIndices = Set(delegate?.getAllIndicesForPrefetcher(self) ?? 0..<0)
let removedIndicides = oldPrefetchIndices
.subtracting(newPrefetchIndices)
.intersection(allIndices) // Only call for visible items
.sorted()
let addedIndices = newPrefetchIndices
.subtracting(oldPrefetchIndices)
.intersection(allIndices) // Only call for visible items
.sorted()
delegate?.prefetcher(self, prefetchItemsAt: addedIndices)
delegate?.prefetcher(self, cancelPrefechingForItemAt: removedIndicides)
}
}
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