session-ios/SignalServiceKit/src/Util/ReverseDispatchQueue.swift
2018-12-13 11:44:29 -05:00

75 lines
2 KiB
Swift

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