Essential Books

Dispatch Semaphore

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DispatchSemaphore provides an efficient implementation of a traditional counting semaphore, which can be used to control access to a resource across multiple execution contexts.

A scenario for when to use a semaphore could be if you are doing some file reading/writing, if multiple tasks are trying to read and write from file at the same time, it could increase your performance to make each task wait its turn so as to not overburden the I/O controller.

Swift 3 

func do2TasksAtATime () {
    print("starting long running tasks (2 at a time)")
    let sem = DispatchSemaphore(value: 2)            //this semaphore only allows 2 tasks to run at the same time (the resource count)
    for i in 0...7 {                                 //launch a bunch of tasks
        DispatchQueue.global().async {               //run tasks on a background thread
            sem.wait()                               //wait here if no resources available
            sleep(2)                                 //do some long task eg file access (here we are just sleeping for a 2 seconds for demonstration purposes)
            print("long task \(i) done! \(Date())")
            sem.signal()                             //let the semaphore know this resource is now available
        }
    }
}

Example output: (notice the time stamps) 

starting long running tasks (2 at a time)
long task 0 done! 2017-02-16 07:11:53 +0000
long task 1 done! 2017-02-16 07:11:53 +0000
long task 2 done! 2017-02-16 07:11:55 +0000
long task 3 done! 2017-02-16 07:11:55 +0000
long task 5 done! 2017-02-16 07:11:57 +0000
long task 4 done! 2017-02-16 07:11:57 +0000
long task 6 done! 2017-02-16 07:11:59 +0000
long task 7 done! 2017-02-16 07:11:59 +0000

For more info, refer to the Apple Docs

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GCD Grand Central Dispatch:
* GCD Grand Central Dispatch
* Dispatch Semaphore
* Dispatch Group
* Create a dispatch queue
* Getting the Main Queue
* Serial vs Concurrent Dispatch Queues
Table Of Contents
0 Getting started with iOS
1 UILabel
2 Change Status Bar Color
3 Passing Data between View Controllers
4 Managing the Keyboard
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84 Size Classes and Adaptivity
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108 Dynamically updating a UIStackView
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135 MPMediaPickerDelegate
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143 AirPrint tutorial
144 UISlider
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154 WCSessionDelegate
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156 App Submission Process
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159 Basic text file I/O
160 Text To Speech (TTS)
161 MPVolumeView
162 Objective-C Associated Objects
163 Passing Data between View Controllers
164 UIPhoenix
165 Blocks in Queue with MKBlockQueue
166 Simulator
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171 Cache online images
172 MVP Architecture
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174 Configure Beacons with CoreBluetooth
175 Core Data
176 Extension for rich Push Notification
177 Profiling with Instruments
178 Application rating review request
179 MyLayout
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182 Simulating Location Using GPX files
183 Custom methods of selection of UITableViewCells
184 Custom methods of selection of UITableViewCells
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191 UIFeedbackGenerator
192 UIKit Dynamics with UICollectionView
193 Multicast Delegates
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198 CydiaSubstrate tweak
199 Create a video from images
200 Codable
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205 Adding Swift bridging header
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207 Changing the rootview controller
208 Attributed text in UILabel
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210 Contributors