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Adds the ability to integrate with collection/table view prefetch
Prefetch operations are requested from CellActions and executed in a private OperationQueue. The actual fetching/cancelling is delayed by a short period of time because iOS may trigger rapid fetch/cancel operations and this is an attempt to reduce the amount of fetch/cancel/fetch of the same IndexPath
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| Original file line number | Diff line number | Diff line change |
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| // | ||
| // FunctionalTableData+UITableViewDataSourcePrefetching.swift | ||
| // FunctionalTableData | ||
| // | ||
| // Created by Geoffrey Foster on 2019-03-25. | ||
| // Copyright © 2019 Shopify. All rights reserved. | ||
| // | ||
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| import Foundation | ||
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| /// A custom implementation of the `UITableViewDataSourcePrefetching` and `UICollectionViewDataSourcePrefetching` protocols that hooks into the `CellAction` type and uses its `prefetchAction: PrefetchAction?` | ||
| class DataSourcePrefetching: NSObject, UITableViewDataSourcePrefetching, UICollectionViewDataSourcePrefetching { | ||
| private struct PrefetchOperationWrapper { | ||
| let observations: (finished: NSKeyValueObservation, cancelled: NSKeyValueObservation) | ||
| let operation: Operation | ||
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| init(operation: Operation, observations: (finished: NSKeyValueObservation, cancelled: NSKeyValueObservation)) { | ||
| self.operation = operation | ||
| self.observations = observations | ||
| } | ||
| } | ||
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| private let queue = OperationQueue() | ||
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| private var operations: [ItemPath: PrefetchOperationWrapper] = [:] | ||
| private var prefetched: Set<ItemPath> = [] | ||
| private let operationsLock = NSLock() | ||
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| private var operationItemPathsToPrefetch: Set<ItemPath> = [] | ||
| private var operationItemPathsToCancel: Set<ItemPath> = [] | ||
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| var sections: [TableSection] = [] | ||
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| private let delay: (prefetch: TimeInterval, cancel: TimeInterval) | ||
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| init(delay: (prefetch: TimeInterval, cancel: TimeInterval) = (0.1, 0.1)) { | ||
| self.delay = delay | ||
| } | ||
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| // MARK: - UITableViewDataSourcePrefetching | ||
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| func tableView(_ tableView: UITableView, prefetchRowsAt indexPaths: [IndexPath]) { | ||
| perform(.prefetch, indexPaths: indexPaths) | ||
| } | ||
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| func tableView(_ tableView: UITableView, cancelPrefetchingForRowsAt indexPaths: [IndexPath]) { | ||
| perform(.cancel, indexPaths: indexPaths) | ||
| } | ||
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| // MARK: - UICollectionViewDataSourcePrefetching | ||
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| func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) { | ||
| perform(.prefetch, indexPaths: indexPaths) | ||
| } | ||
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| func collectionView(_ collectionView: UICollectionView, cancelPrefetchingForItemsAt indexPaths: [IndexPath]) { | ||
| perform(.cancel, indexPaths: indexPaths) | ||
| } | ||
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| // MARK: - | ||
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| private enum PrefetchOperationKind { | ||
| case prefetch | ||
| case cancel | ||
| } | ||
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| private func perform(_ kind: PrefetchOperationKind, indexPaths: [IndexPath]) { | ||
| // Because we can get rapid fire cancel/prefetch calls that contain the same IndexPath values we queue up ones to be cancelled and delay the cancel and queue up the fetches and delay the fetch | ||
| let itemPaths = sections.itemPaths(from: indexPaths) | ||
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| let performDelay: TimeInterval | ||
| switch kind { | ||
| case .prefetch: | ||
| operationItemPathsToPrefetch.formUnion(itemPaths) | ||
| operationItemPathsToCancel.subtract(itemPaths) | ||
| performDelay = delay.prefetch | ||
| case .cancel: | ||
| operationItemPathsToCancel.formUnion(itemPaths) | ||
| operationItemPathsToPrefetch.subtract(itemPaths) | ||
| performDelay = delay.cancel | ||
| } | ||
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| NSObject.cancelPreviousPerformRequests(withTarget: self, selector: #selector(run), object: nil) | ||
| perform(#selector(run), with: nil, afterDelay: performDelay) | ||
| } | ||
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| private func registerOperation(_ operation: Operation, forItemPath itemPath: ItemPath) -> Operation { | ||
| func operationFinished(for itemPath: ItemPath, successfully: Bool) { | ||
| operationsLock.withLock { | ||
| _ = operations.removeValue(forKey: itemPath) | ||
| if successfully { | ||
| prefetched.insert(itemPath) | ||
| } | ||
| } | ||
| } | ||
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| let finishedObservation = operation.observe(\.isFinished, options: .new) { (operation, change) in | ||
| if change.newValue == true { | ||
| operationFinished(for: itemPath, successfully: true) | ||
| } | ||
| } | ||
| let cancelledObservation = operation.observe(\.isCancelled, options: .new) { (operation, change) in | ||
| if change.newValue == true { | ||
| operationFinished(for: itemPath, successfully: false) | ||
| } | ||
| } | ||
| let prefetchOperation = PrefetchOperationWrapper(operation: operation, observations: (finished: finishedObservation, cancelled: cancelledObservation)) | ||
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| operationsLock.withLock { | ||
| operations[itemPath] = prefetchOperation | ||
| } | ||
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| return operation | ||
| } | ||
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| @objc private func run() { | ||
| /// Returns `true` if and only if an operation for the given `ItemPath` doesn't already exist and also hasn't already been run. | ||
| /// | ||
| /// - Parameter itemPath: | ||
| /// - Returns: `true` if a prefetch operation should be created and run for the given `ItemPath`. | ||
| func shouldPrefetch(itemPath: ItemPath) -> Bool { | ||
| return operationsLock.withLock { | ||
| operations[itemPath] == nil && !prefetched.contains(itemPath) | ||
| } | ||
| } | ||
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| var newOperations: [Operation] = [] | ||
| for itemPath in operationItemPathsToPrefetch where shouldPrefetch(itemPath: itemPath) { | ||
| guard let prefetchAction = sections[itemPath]?.actions.prefetchAction else { continue } | ||
| let operation = prefetchAction() | ||
| newOperations.append(registerOperation(operation, forItemPath: itemPath)) | ||
| } | ||
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| let cancelledOperations = operationsLock.withLock { | ||
| operations.filter { operationItemPathsToCancel.contains($0.key) }.map { $0.value } | ||
| } | ||
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| operationItemPathsToPrefetch.removeAll() | ||
| operationItemPathsToCancel.removeAll() | ||
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| queue.addOperations(newOperations, waitUntilFinished: false) | ||
| cancelledOperations.forEach { $0.operation.cancel() } | ||
| } | ||
| } | ||
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| private extension NSLocking { | ||
| func withLock<T> (_ body: () throws -> T) rethrows -> T { | ||
| self.lock() | ||
| defer { self.unlock() } | ||
| return try body() | ||
| } | ||
| } |
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