Hello Readers, CoolMonkTechie heartily welcomes you in this article.
In this article, We will learn about three best available options to handle multithreading in iOS. Production applications will often need to perform heavier operations such as downloading high-resolution images or a executing non-cached database queries. To prevent stalling the main thread (and a hit in frame rate), Apple has provided a few tools to help us. We will discuss below available options to handle multithreading:
- Grand Central Dispatch
NSOperationandNSOperationQueue- performSelectorInBackground
A famous quote about Learning is :
” I am always ready to learn although I do not always like being taught. “
So Let’s begin.
1. Grand Central Dispatch
Grand Central Dispatch is a technology that abstracts away the low-level details of multithreading. When using GCD, we only have to think about the tasks we want to perform. These tasks can then be added to serial or concurrent queues. Moreover, we can add tasks to groups and run code after all tasks within the group complete.
Let’s walk through an example where we download an image from a remote URL and then use it to populate a UIImageView.
// Assume we have an `imageView` property on self
private func loadWallpaper() {
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) { [weak self] in
guard
let wallpaperURL = NSURL(string: "http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-157301.jpg"),
let imageData = NSData(contentsOfURL: wallpaperURL)
else {
return
}
dispatch_async(dispatch_get_main_queue()) {
self?.imageView.image = UIImage(data: imageData)
}
}
}Most uses of GCD start with a call to dispatch_async, which takes in a queue to use and the block to execute. In our example, we’d like to execute the wallpaper download on a background queue, so we make use of the system-defined global queue with a background quality of service (QoS), DISPATCH_QUEUE_PRIORITY_BACKGROUND. The flag passed into dispatch_get_global_queue should always be 0.
Now we have the block of work to execute. We construct a NSURL via its fail-able String initializer and then fetch the data associated with that resource via NSData(contentsOfURL:). If the above step completes successfully (else we just return from the block), we now have our data at hand.
To update imageView‘s image property, we need to make sure we return to the main thread via dispatch_async(dispatch_get_main_queue()) { /* ... */ }. Remember in iOS, all UI updates should be performed on the main thread. Inside the main thread block, we set the image using the NSData initializer on UIImage.
Now that we’ve seen a one-off block example, let’s dive into how we can accomplish groups of dependent tasks. Imagine we wanted to download multiple wallpapers and present an alert to the user when all of the images finish loading. Dispatch groups will be our best friends in these scenarios.
First, let’s refactor the loadWallpaper function from the previous example to accept a dispatch_group_t and a target URL.
private func loadWallpaper(group: dispatch_group_t, url: String) {
dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0)) { [weak self] in
defer {
dispatch_group_leave(group)
}
guard
let wallpaperURL = NSURL(string: url),
let imageData = NSData(contentsOfURL: wallpaperURL)
else {
// In production scenarios, we would want error handing here
return
}
// Use imageData in some manner, e.g. persisting to a cache, present in view hierarchy, etc.
print("Image downloaded \(url)")
}
}The function has been modified slightly to accept a parameter group of type dispatch_group_t (we’ll go into how to create these groups in the next snippet) and a target URL. Additionally, our previous call to dispatch_async has been replaced with dispatch_group_async, signalling that the block should be associated with group. Lastly, after completing our work with the resulting imageData we must notify group that the block is complete via dispatch_group_leave.
To use loadWallpaper(_:url:) a call site could look like so:
private func fetchAllWallpapers() {
let urls = [
"http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-329991.jpg",
"http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-329805.jpg",
"http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-330201.jpg"
]
let wallpaperGroup = dispatch_group_create()
urls.forEach {
dispatch_group_enter(wallpaperGroup)
loadWallpaper(wallpaperGroup, url: $0)
}
dispatch_group_notify(wallpaperGroup, dispatch_get_main_queue()) { [weak self] in
let alertController = UIAlertController(title: "Done!", message: "All images have downloaded", preferredStyle: .Alert)
alertController.addAction(UIAlertAction(title: "OK", style: .Default, handler: nil))
self?.presentViewController(alertController, animated: true, completion: nil)
}
}We start by creating a dispatch group, wallpaperGroup, using dispatch_group_create(). With the group in hand, we loop over all of the wallpaper URLs, first signalling to the group that we are about to start an operation by making a call to dispatch_group_enter(wallpaperGroup) (each group entry call must pair with a group leave call). We then proceed to call loadWallpaper(_:url:).
To run code after completion of the group, we specify a block in a dispatch_group_notify call. In our case, we’ll simply present a UIAlertController letting the user know that all of the downloads have finished.
While GCD can be extremely powerful, it can be a bit cumbersome to work with in practice. To help with this, we can use Swifty GCD wrapper .
protocol ExcutableQueue {
var queue: dispatch_queue_t { get }
}
extension ExcutableQueue {
func execute(closure: () -> Void) {
dispatch_async(queue, closure)
}
}
enum Queue: ExcutableQueue {
case Main
case UserInteractive
case UserInitiated
case Utility
case Background
var queue: dispatch_queue_t {
switch self {
case .Main:
return dispatch_get_main_queue()
case .UserInteractive:
return dispatch_get_global_queue(QOS_CLASS_USER_INTERACTIVE, 0)
case .UserInitiated:
return dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)
case .Utility:
return dispatch_get_global_queue(QOS_CLASS_UTILITY, 0)
case .Background:
return dispatch_get_global_queue(QOS_CLASS_BACKGROUND, 0)
}
}
}
enum SerialQueue: String, ExcutableQueue {
case DownLoadImage = "myApp.SerialQueue.DownLoadImage"
case UpLoadFile = "myApp.SerialQueue.UpLoadFile"
var queue: dispatch_queue_t {
return dispatch_queue_create(rawValue, DISPATCH_QUEUE_SERIAL)
}
}Using this wrapper, our example above could be rewritten as:
Queue.Background.execute {
guard
let url = NSURL(string: "http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-157301.jpg"),
let data = NSData(contentsOfURL: url)
else {
return
}
Queue.Main.execute { [weak self] in
self?.imageView.image = UIImage(data: data)
}
}
2. NSOperation and NSOperationQueue
NSOperations and NSOperationQueues provide you with a higher-level API, when compared to GCD. They were first introduced in iOS 4 and are actually implemented with GCD under the hood. Typically, we’ll want to use this API over GCD, unless you’re performing a simple unit of work on a specific queue. NSOperations provide us with powerful functionality such as cancellation and dependencies.
To start, we’ll port the wallpaper downloading example to use an NSBlockOperation. NSBlockOperation is a simple wrapper on a block of work that can be added to a queue.
private func loadWallpaper(queue: NSOperationQueue, url: String) {
guard let wallpaperURL = NSURL(string: url) else { return }
let downloadOperation = NSBlockOperation {
guard let imageData = NSData(contentsOfURL: wallpaperURL) else { return }
NSOperationQueue.mainQueue().addOperationWithBlock { [weak self] in
self?.imageView.image = UIImage(data: imageData)
}
}
queue.addOperation(downloadOperation)
}The initializer for NSBlockOperation simply takes a block to run. In our case, we’ll download the data from wallpaperURL and return to the main queue to set the image property on imageView. After initializing downloadOperation, we add it to queue.
When creating an NSOperationQueue, we have a few points of customization.
let queue = NSOperationQueue()
queue.maxConcurrentOperationCount = 1
// If you want to hold the queue, use the `suspended` property
queue.suspended = trueThe maxConcurrentOperationCount property allows us to set a limit on how many operations may run concurrently in a given queue. Setting this to 1, implies our queue will be serial (queing order may not be preserved, as operations only run when their ready flag is set to true). If this property isn’t set, it defaults to NSOperationQueueDefaultMaxConcurrentOperationCount, which is dictated by system conditions.
By default, all operations that are ready (ready property is true) are run when added to a queue. We can halt all execution on a queue by setting the suspended property to true.
NSOperations become really powerful when we separate them out into operation subclasses. To demonstrate this, let’s make a wallpaper resizing operation. We’ll need to subclass a custom wrapper of NSOperation that has the proper KVO notifications in place.
class ResizeImageOperation: Operation {
enum Error {
case FileReadError
case ResizeError
case WriteError
}
let targetSize: CGSize
let path: NSURL
var error: Error?
init(size: CGSize, path: NSURL) {
self.targetSize = size
self.path = path
}
override func execute() {
// Need to signal KVO notifications for operation completion
defer {
finish()
}
guard let sourceImage = UIImage(contentsOfFile: path.absoluteString) else {
error = Error.FileReadError
return
}
let finalWidth: CGFloat, finalHeight: CGFloat
let ratio = sourceImage.size.width / sourceImage.size.height
// Scale aspect fit the image
if sourceImage.size.width >= sourceImage.size.height {
finalWidth = targetSize.width
finalHeight = finalWidth / ratio
} else {
finalHeight = targetSize.height
finalWidth = finalHeight * ratio
}
let imageSize = CGSize(width: finalWidth, height: finalHeight)
UIGraphicsBeginImageContextWithOptions(imageSize, true, 0.0)
defer { UIGraphicsEndImageContext() }
let rect = CGRect(origin: .zero, size: imageSize)
sourceImage.drawInRect(rect)
guard
let resizedImage = UIGraphicsGetImageFromCurrentImageContext(),
let imageData = UIImageJPEGRepresentation(resizedImage, 1.0)
else {
error = Error.ResizeError
return
}
guard imageData.writeToFile(path.absoluteString, atomically: true) else {
error = Error.WriteError
return
}
}
}- To help with error handling, we add a nested
Errorenum with a few cases. ResizeImageOperationcan be initialized with a target size and path to write,- The meat of the operation is placed in the
executemethod (overridden fromOperation). We need to make sure todefera call tofinish(), so that theOperationsuperclass can signal the proper KVO notifications. - We then proceed with the resizing the image (scale aspect fit) and saving it to disk.
Now that we have a resizing operation in hand, let’s refactor our download operation a bit to work with it:
private func downloadWallpaper(url: NSURL, path: NSURL) -> NSOperation {
return NSBlockOperation {
guard
let imageData = NSData(contentsOfURL: url),
let image = UIImage(data: imageData)
else { return }
UIImageJPEGRepresentation(image, 1.0)?.writeToFile(path.absoluteString, atomically: true)
}
}We now return an NSOperation and have the operation write the image data to disk. Lastly, to make the download and resize operations dependent, we can use them like so:
// Assume self has `imageView` and `wallpaperQueue` properties
if
let cacheDirectory = NSSearchPathForDirectoriesInDomains(.CachesDirectory, .UserDomainMask, true).first,
let cacheDirectoryURL = NSURL(string: cacheDirectory)
{
let targetURL = cacheDirectoryURL.URLByAppendingPathComponent("wallpaper.jpg")
let downloadOperation = downloadWallpaper(NSURL(string: "http://wallpapers.wallhaven.cc/wallpapers/full/wallhaven-329991.jpg")!, path: targetURL)
let resizeOperation = ResizeImageOperation(size: CGSize(width: imageView.bounds.size.width * 2, height: imageView.bounds.size.height * 2), path: targetURL)
resizeOperation.addDependency(downloadOperation)
resizeOperation.completionBlock = { [weak self, weak resizeOperation] in
if let error = resizeOperation?.error {
print(error)
return
}
guard
let path = resizeOperation?.path,
let imageData = NSData(contentsOfFile: path.absoluteString)
else {
return
}
NSOperationQueue.mainQueue().addOperationWithBlock {
self?.imageView.image = UIImage(data: imageData)
}
}
wallpaperQueue.suspended = true
wallpaperQueue.addOperation(downloadOperation)
wallpaperQueue.addOperation(resizeOperation)
wallpaperQueue.suspended = false
}- The key line to notice is
resizeOperation.addDependency(downloadOperation). That’s how we express the resizing operation’s dependency ondownloadOperation. - Moreover, in the completion block of
resizeOperation, we check for errors and proceed with displaying the resized image. - Note: we make sure to suspend the queue first, then add the operations. This prevents the operations from beginning immediately upon addition.
3. PerformSelectorInBackground
To wrap up, let’s show a simple example of performSelectorInBackground. Assuming self has a method sleepAndPrint(_:), we can make the following call:
performSelectorInBackground("sleepAndPrint:", withObject: "supsup")If our target selector had no argument, the selector would simply be "sleepAndPrint").
func sleepAndPrint(message: String) {
NSThread.sleepForTimeInterval(1)
print(message)
}That’s all about in this article.
Conclusion
In this article, We understood about three best available options to handle multithreading in iOS. We’ve discussed about GCD, NSoperations, and NSObject‘s performSelectorInBackground method as means of performing work in a multithreaded fashion. If we have small units of work to perform, we’ll want to reach for GCD or performSelectorInBackground. On the other hand, if we have larger operations that may have dependencies, NSOperation should be our tool of choice.
Thanks for reading ! I hope you enjoyed and learned about three best available options to handle multithreading in iOS. Reading is one thing, but the only way to master it is to do it yourself.
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