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Dispatch Group

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DispatchGroup allows for aggregate synchronization of work. You can use them to submit multiple different work items and track when they all complete, even though they might run on different queues. This behavior can be helpful when progress can’t be made until all of the specified tasks are complete.

A Scenario when this could be useful is if you have multiple webservice calls that all need to finish before continuing. For example, you need to download multiple sets of data that needs to be processed by some function. You have to wait for all webservices to complete before calling the function to process all the received data.

Swift 3 

func doLongTasksAndWait () {
    print("starting long running tasks")
    let group = DispatchGroup()          //create a group for a bunch of tasks we are about to do
    for i in 0...3 {                     //launch a bunch of tasks (eg a bunch of webservice calls that all need to be finished before proceeding to the next ViewController)
        group.enter()                    //let the group know that something is being added
        DispatchQueue.global().async {   //run tasks on a background thread
            sleep(arc4random() % 4)      //do some long task eg webservice or database lookup (here we are just sleeping for a random amount of time for demonstration purposes)
            print("long task \(i) done!")
            group.leave()                //let group know that the task is finished
        }
    }
    group.wait()                         //will block whatever thread we are on here until all the above tasks have finished (so maybe dont use this function on your main thread)
    print("all tasks done!")
}

Alternatively, if you do not want to wait for the groups to finish, but instead want to run a function once all the tasks have completed, use the notify function in place of the group.wait() 

group.notify(queue: DispatchQueue.main) { //the queue: parameter is which queue this block will run on, if you need to do UI updates, use the main queue
    print("all tasks done!")              //this will execute when all tasks have left the group
}

Example output: 

starting long running tasks
long task 0 done!
long task 3 done!
long task 1 done!
long task 2 done!
all tasks done!

For more info, refer to the Apple Docs or the related topic

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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
5 UIButton
6 UILocalNotification
7 UIImageView
8 Checking for Network Connectivity
9 Accessibility
10 UITableView
11 Auto Layout
12 UIView
13 UIAlertController
14 MKMapView
15 UIColor
16 NSAttributedString
17 CAAnimation
18 UITextView
19 UINavigationController
20 Concurrency
21 CAGradientLayer
22 UIGestureRecognizer
23 Custom UIViews from XIB files
24 Safari Services
25 UIStackView
26 UIImage
27 UIWebView
28 CALayer
29 Swift and Objective-C interoperability
30 NSDate
31 Custom Fonts
32 AVSpeechSynthesizer
33 UIBarButtonItem
34 UIScrollView
35 Localization
36 NSNotificationCenter
37 UITextField
38 Networking with Alamofire
39 UIViewController
40 iBeacon
41 CLLocation
42 NSURLSession
43 UISwitch
44 Checking iOS version
45 Universal Links
46 UICollectionView
47 PDF Creation
48 In-App Purchase
49 NSTimer
50 CGContext
51 UITabBarController
52 UISearchController
53 UIActivityViewController
54 Core Location
55 FacebookSDK
56 AFNetworking
57 CTCallCenter
58 UIImagePickerController
59 NSUserDefaults
60 UIControl - Event Handling with Blocks
61 UIBezierPath
62 UIPageViewController
63 UIAppearance
64 Push Notifications
65 Key Value Coding
66 Initialization idioms
67 Storyboard
68 Background Modes and Events
69 Fastlane
70 CAShapeLayer
71 WKWebView
72 UUID Universally Unique Identifier
73 Categories
74 Handling URL Schemes
75 Realm database
76 ARC Automatic Reference Counting
77 UIPickerView
78 Dynamic Type
79 NSURL
80 SWRevealViewController
81 Snapshot of UIView
82 DispatchGroup
83 GCD Grand Central Dispatch
84 Size Classes and Adaptivity
85 UIScrollView, AutoLayout
86 IBOutlets
87 AWS SDK
88 Debugging Crashes
89 UISplitViewController
90 UISplitViewController
91 UIDevice
92 CloudKit
93 GameplayKit
94 XCode Build Archive From Command Line
95 Unit Testing with XCTest
96 NSData
97 AVPlayer and AVPlayerViewController
98 Deep Linking
99 App Transport Security (ATS)
100 Core Graphics
101 Segues
102 UIDatePicker
103 NSPredicate
104 EventKit
105 NSBundle
106 SiriKit
107 Contacts Framework
108 Dynamically updating a UIStackView
109 Speech Recognition
110 NSURLConnection
111 StoreKit
112 Code signing
113 .ipa file to upload on appstore with ApplicationLoader
114 Resizing UIImage
115 Size Classes and Adaptivity
116 MKDistanceFormatter
117 3D Touch
118 GameCenter Leaderboards
119 Keychain
120 Handle Multiple Environment using Macro
121 Set View Background
122 Block
123 Content Hugging
124 Google Places API
125 Navigation Bar
126 UITextField Delegate
127 App-wide operations
128 UILabel text underlining
129 Cut a UIImage into a circle
130 Make selective UIView corners rounded
131 Convert HTML to NSAttributed string and vice versa
132 Convert NSAttributedString to UIImage
133 CoreImage Filters
134 Face Detection Using CoreImageOpenCV
135 MPMediaPickerDelegate
136 Graph Coreplot
137 NSHTTPCookieStorage
138 FCM Messaging
139 Create Custom Framework
140 Custom Keyboard
141 AirDrop
142 SLComposeViewController
143 AirPrint tutorial
144 UISlider
145 Carthage
146 HealthKit
147 Core SpotLight
148 UI Testing
149 Core Motion
150 QR Code Scanner
151 plist
152 NSInvocation
153 UIRefreshControl TableView
154 WCSessionDelegate
155 AppDelegate
156 App Submission Process
157 MVVM
158 UIStoryboard
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
167 BackgroundModes
168 NSArray
169 OpenGL
170 UIScrollView with StackView child
171 Cache online images
172 MVP Architecture
173 UIKit Dynamics
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
180 UIFont
181 Simulator Builds
182 Simulating Location Using GPX files
183 Custom methods of selection of UITableViewCells
184 Custom methods of selection of UITableViewCells
185 UISegmentedControl
186 SqlCipher integration
187 Custom UITextField
188 Custom UITextField
189 Security
190 Guideline to architecture patterns
191 UIFeedbackGenerator
192 UIKit Dynamics with UICollectionView
193 Multicast Delegates
194 Using image assets
195 UITableViewCell
196 Runtime in Objective-C
197 ModelPresentationStyles
198 CydiaSubstrate tweak
199 Create a video from images
200 Codable
201 FileHandle
202 NSUserActivity
203 Rich Notifications
204 Load images async
205 Adding Swift bridging header
206 Creating an App ID
207 Changing the rootview controller
208 Attributed text in UILabel
209 UITableViewController
210 Contributors