75 lines
2.0 KiB
Swift
75 lines
2.0 KiB
Swift
// Copyright (c) 2018 Open Whisper Systems. All rights reserved.
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import Foundation
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import SignalCoreKit
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// This is intended to be a drop-in replacement for DispatchQueue
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// that processes its queue in reverse order.
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@objc
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public class ReverseDispatchQueue: NSObject {
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private static let isVerbose: Bool = false
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private let label: String
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private let serialQueue: DispatchQueue
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// TODO: We could allow creation with various QOS.
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@objc
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public required init(label: String) {
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self.label = label
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serialQueue = DispatchQueue(label: label)
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super.init()
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}
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public typealias WorkBlock = () -> Void
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private class Item {
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let workBlock: WorkBlock
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let index: UInt64
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required init(workBlock : @escaping WorkBlock, index: UInt64) {
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self.workBlock = workBlock
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self.index = index
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}
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}
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// These properties should only be accessed on serialQueue.
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private var items = [Item]()
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private var indexCounter: UInt64 = 0
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@objc
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public func async(workBlock : @escaping WorkBlock) {
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serialQueue.async {
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self.indexCounter = self.indexCounter + 1
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let index = self.indexCounter
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let item = Item(workBlock: workBlock, index: index )
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self.items.append(item)
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if ReverseDispatchQueue.isVerbose {
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Logger.verbose("Enqueued[\(self.label)]: \(item.index)")
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}
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self.process()
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}
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}
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private func process() {
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serialQueue.async {
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// Note that we popLast() so that we process
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// the queue in the _reverse_ order from
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// which it was enqueued.
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guard let item = self.items.popLast() else {
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// No enqueued work to do.
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return
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}
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if ReverseDispatchQueue.isVerbose {
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Logger.verbose("Processing[\(self.label)]: \(item.index)")
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}
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item.workBlock()
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self.process()
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}
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}
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}
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