iOS/Objective-C equivalent of Android's AsyncTask

I'm familiar with using AsyncTask in Android: create a subclass, call execute on an instance of the subclass and onPostExecute is called on the UI thread or main thread. What's the equivalent in iOS?

Answer to Original Question:

Grand Central Dispatch (GCD) offers a mechanism to perform tasks in the background, though it works in a structurally different way than AsyncTask. To perform something asynchronously, you just need to create a queue (like a thread) and then pass a block to dispatch_async() to be performed in the background. I find it neater than AsyncTask, as there is no subclassing involved; it is more or less plug-and-play wherever you have code you'd like to execute in the background. An example:

dispatch_queue_t queue = dispatch_queue_create("com.yourdomain.yourappname", NULL);
dispatch_async(queue, ^{
    //code to be executed in the background
});

Other Points:

1) Callbacks

If you want to perform a task in the background and update the UI (or do something on another thread) when the background task is done, you can simply nest the dispatch calls:

dispatch_queue_t queue = dispatch_queue_create("com.yourdomain.yourappname", NULL);
dispatch_async(queue, ^{
    //code to be executed in the background
    dispatch_async(dispatch_get_main_queue(), ^{
        //code to be executed on the main thread when background task is finished
    });
});

2) Global Queues

When creating a queue, you can also use the dispatch_get_global_queue() function to get a global dispatch queue with a certain priority (such as DISPATCH_QUEUE_PRIORITY_HIGH). These queues are universally accessible and are useful when you want to assign multiple tasks to the same thread/queue. Note that memory is managed for you completely by iOS.

3) Memory

There is sometimes some confusion regarding memory management and dispatch queues because they have their own dispatch_retain/dispatch_release functions. However, rest assured that they are treated as Objective-C objects by ARC, so you don't need to worry about calling these functions. Referencing rob mayoff's great answer regarding GCD and ARC, you can see the documentation describe GCD queues' equivalence with Objective-C objects:

* By default, libSystem objects such as GCD and XPC objects are declared as
* Objective-C types when building with an Objective-C compiler. This allows
* them to participate in ARC, in RR management by the Blocks runtime and in
* leaks checking by the static analyzer, and enables them to be added to Cocoa
* collections.
*
* NOTE: this requires explicit cancellation of dispatch sources and xpc
*       connections whose handler blocks capture the source/connection object,
*       resp. ensuring that such captures do not form retain cycles (e.g. by
*       declaring the source as __weak).
*
* To opt-out of this default behavior, add -DOS_OBJECT_USE_OBJC=0 to your
* compiler flags.
*
* This mode requires a platform with the modern Objective-C runtime, the
* Objective-C GC compiler option to be disabled, and at least a Mac OS X 10.8
* or iOS 6.0 deployment target.

4) Multiple Tasks/Blocks

I'll add that GCD has a grouping interface supports synchronizing multiple asynchronous blocks if a task cannot continue until multiple asynchronous activities have completed. Jörn Eyrich and ɲeuroburɳ provide a generous explanation of this topic here. If you need this functionality, I would highly recommend taking a few minutes to read both of their answers closely and understand the differences between them.

The documentation has a wealth of information on the topic if you are so inclined.

There are no classes for that in iOS but you can simulate it using queues. You can call:

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    //Your code to execute in background...
});

for async tasks and inside your async code call next queue to do something in the view...:

dispatch_async(dispatch_get_main_queue(), ^{
    //Your code to execute on UIthread (main thread)
});

Then, using this two queues you can create a asyncTask class, add this class to your project to implement them:

---

//
//  AsyncTask.h
//
//  Created by Mansour Boutarbouch Mhaimeur on 25/10/13.
//

#import <Foundation/Foundation.h>

@interface AsyncTask : NSObject

- (void) executeParameters: (NSArray *) params;
- (void) preExecute;
- (NSInteger) doInBackground: (NSArray *) parameters;
- (void) postExecute: (NSInteger) result;
@end

---

//
//  AsyncTask.m
//
//  Created by Mansour Boutarbouch Mhaimeur on 25/10/13.
//

#import "AsyncTask.h"

@implementation AsyncTask

- (void) executeParameters: (NSArray *) params{
    [self preExecute];
    __block NSInteger result;
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        result = [self doInBackground:params];
        dispatch_async(dispatch_get_main_queue(), ^{
            [self postExecute:result];
        });
    });
}

- (void) preExecute{
    //Method to override
    //Run on main thread (UIThread)
}

- (NSInteger) doInBackground: (NSArray *) parameters{
    //Method to override
    //Run on async thread (Background)
    return 0;
}

- (void) postExecute: (NSInteger) result{
    //Method to override
    //Run on main thread (UIThread)
}

@end

---

This is an example that I am using in a project:

---

#import "AsyncTask.h"
#import "Chat.h"

@interface SendChatTask : AsyncTask{
    NSArray * chatsNotSent;
}

@end

---

#import "SendChatTask.h"

@implementation SendChatTask

- (void) preExecute{
    //Method to override
}

- (NSInteger) doInBackground: (NSArray *) parameters{
    //Method to override
    NSString *sendChatsURL = [NSString stringWithFormat:@"%@%@%@",HOST, NAMESPACE,URL_SEND_CHAT];
    chatsNotSent = [parameters objectAtIndex:0];
    
    NSString *response;
    NSMutableDictionary *params = [[NSMutableDictionary alloc] init];
    //...
    NSError *error;
    NSData *jsonData = [NSJSONSerialization dataWithJSONObject:[ChatJSONParser wrapChatArray: chatsNotSent] options:0 error:&error];
    NSString *JSONString = [[NSString alloc] initWithBytes:[jsonData bytes] length:[jsonData length] encoding:NSUTF8StringEncoding];
    
    [params setObject:JSONString forKey:@"chats"];
    
    response = [HTTPClient executePOST:sendChatsURL parameters:params];
    
    if([respuesta isEqualToString:@"true"]){
        return 1;
    }else{
        return -1;
    }
}

- (void) postExecute: (NSInteger) result{
    //Method to override
    if (result == 1) {
        for (Chat *chat in chatsNotSent) {
            chat.state = STATE_NOT_SENT;
            [chat save];
            AppDelegate *appDelegate = (AppDelegate *)[[UIApplication sharedApplication] delegate];
            [appDelegate refreshChat];
        }
    } else {
        
    }
}

@end

---

And the following call:

[[[SendChatTask alloc] init] executeParameters:[NSArray arrayWithObjects: chatsNotSent, nil]];

You can add a publishProgress() update method and respective... I don't use it for the moment because I call my async task in background services.

I hope it's helpful.

if your targeting earlier iOS Version (than iOS 4 for Grand Central Dispatch) you could use the NSObject performSelector methods

• Execute on Background Thread performSelectorInBackground:withObject:

• And execute on MainThread performSelectorOnMainThread:withObject:waitUntilDone:

This is an example:

[self performSelectorInBackground:@selector(executeInBackground) withObject:nil];


-(void) executeInBackground
{
	NSLog(@"executeInBackground");
	
	[self performSelectorOnMainThread:@selector(executeOnMainThread) withObject:nil waitUntilDone:NO];
}

-(void) executeOnMainThread
{
	NSLog(@"executeOnMainThread");
}

#Swift 3

In Android when I wanted to run a task on a background thread and then update the UI when it finished, I used AsyncTask (example). Now when I am making iOS versions of my apps, I use Grand Central Dispatch (GCD) to do the same thing. Here is how it is done with Swift:

DispatchQueue.global(qos: .background).async {
    
    // code to be run on a background task

    DispatchQueue.main.async {

        // code to be run on the main thread after the background task is finished
    }
}

#Notes

• Grand Central Dispatch Tutorial for Swift

Here is a c# Xamarin.iOS version with PusblishProgress:

internal abstract class AsyncTask : NSObject
{
	protected abstract nint DoInBackground(NSArray parameters);

	protected abstract void PostExecute(nint result);

	public void ExecuteParameters(NSArray @params)
	{
		this.PreExecute();

		DispatchQueue.GetGlobalQueue(DispatchQueuePriority.Default).DispatchAsync(() =>
		{
            //We're on a Background thread
			var result = this.DoInBackground(@params);
			DispatchQueue.MainQueue.DispatchAsync(() => {
                // We're on the main thread
				this.PostExecute(result);
            });
		});

	}

	protected abstract void PreExecute();

    protected void PublishProgress(NSArray parameters)
    {
		InvokeOnMainThread(() => {
			// We're on the main thread
			this.OnProgressUpdate(parameters);
		});
	}

	protected abstract void OnProgressUpdate(NSArray parameters);
}

And implementation:

internal class MyAsyncTask : AsyncTask
{
    protected override void OnProgressUpdate(NSArray parameters)
    {
        // This runs on the UI Thread
    }
    
    protected override nint DoInBackground(NSArray parameters)
    {
        // Do some background work
        // ....
        var progress = NSArray.FromObjects(1, "Done step 1");
        PublishProgress(progress);
    
        return 0;
     }
    
     protected override void PostExecute(nint result)
     {
         // This runs on the UI Thread
    
     }
    
     protected override void PreExecute()
     {
        // This runs on the UI Thread
    
     }
}