Some thoughts on asynchrony in a modern world, inspired by Reactive Extensions and await in C#, for the Obj-C audience at CocoaHeads Stockholm June 2012. Please read the notes on each slide to make sense of them, the slides are not understandable by themselves.

1. Moar tools for asynchrony!
@nevyn

2. • Background computation
Building image,
database search,
(anything threaded)
• I/O Network
Disk
touch app activation
• Events
value changes (KVO)
keyboard
wifi connected

3. while(event = getNextEvent()) { // async event
// possibly async IO, polling
NSURL *pictureURL = [[Backend currentUser] profilePictureURL];
// possibly async IO
NSData *pictureData = [NSData dataWithContentsOfURL:pictureURL];
// computation
UIImage *profilePicture = [UIImage imageWithData:pictureData];
if ([event touches:someRect]) {
// IO
UIImage *highlight = [UIImage imageNamed:@"profile-picture-highlight"];
// computation
profilePicture = [highlight imageCompositedOver:profilePicture];
}
[profilePicture drawInRect:someRect];
}

4. - (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions;
AudioSessionAddPropertyListener(
kAudioSessionProperty_AudioRouteChange,
audioRouteChangeListenerCallback,
NULL
);
-[NSOperation setCompletionBlock:(void (^)(void))block];

5. // async event
- (void)touchesBegan:(NSArray*)touches withEvent:(UIEvent*)event {
// async IO
[[Backend currentUser] getProfilePictureURL:^ (NSURL*url) {
// async IO
[HTTPLibrary fetchURL:url onCompletion:^ (NSData *data) {
// async processing
dispatch_async(dispatch_get_global_queue(0,0), ^{
UIImage *profilePicture = [UIImage imageWithData:data];
… highlight …
dispatch_async(dispatch_get_main_queue(), ^{
imageView.image = profilePicture;
};
}
} error:^(NSError* err) {
...
}];
} onError:(NSError *err) {
…
}];
}

6. NSString *filename = ...;
NSData *maybeData = MALazyFuture(^{
return [NSData dataWithContentsOfFile: filename];
});
[object doSomethingOrNotWithData: maybeData];
NSString *filename = ...;
NSData *futureData = MABackgroundFuture(^{
return [NSData dataWithContentsOfFile: filename];
});
[object doSomethingLaterWithData: futureData];

7. NSString *filename = ...;
Task *futureData = TCBackgroundTask(^{
return [NSData dataWithContentsOfFile: filename];
});
futureData.onDoneCallback = ^ (NSData *data) {
[object doSomethingLaterWithData: futureData];
};
[futureData startOnQueue:dispatch_get_global_queue(0,0)];

8. Task *task = [[Backend currentUser] profilePictureURL];
NSURL *profilePictureURL = [task runSynchronously];

9. // I/O
Task *pictureTask = [UIImage imageFromURL:profilePictureURL];
// computation
Task *badgeTask = [UIImage imageNamedTask:@"profile-picture-highlight"];
// I/O
Task *colorTask = [[Backend currentUser] favoriteColor];
// Wait for all of the above to complete
[Task waitAll:pictureTask, badgeTask, colorTask, nil
cancelOn:[CancellationToken cancelOnDismissed:self]];
// Use the fetched or computed values
UIImage *badgedImage = [[badgeTask value] composeOnTopOf: [pictureTask value]];
UIImage *coloredImage = [badgedImage tintedImage: [colorTask value]];

10. private async Task<byte[]> GetURLContentsAsync(string url)
{
var content = new MemoryStream();
HttpWebRequest webReq = WebRequest.Create(url);
// GetResponseAsync returns a Task<WebResponse>.
WebResponse response = await webReq.GetResponseAsync(); // “Pause” execution
Stream responseStream = response.GetResponseStream();
// CopyToAsync returns a Task
await responseStream.CopyToAsync(content); // “Pause” execution
return content.ToArray();
}

11. private async Task<byte[]> GetURLContentsAsync(string url)
{
var content = new MemoryStream();
HttpWebRequest webReq = WebRequest.Create(url);
// GetResponseAsync returns a Task<WebResponse>.
WebResponse response = await webReq.GetResponseAsync();
Stream responseStream = response.GetResponseStream();
// CopyToAsync returns a Task
await responseStream.CopyToAsync(content);
private Task<byte[]> GetURLContentsAsync(string url)
{
return content.ToArray(); var content = new MemoryStream();
} HttpWebRequest webReq = WebRequest.Create(url);
Task<WebResponse> responseTask = webReq.GetResponseAsync();
Task task2 = responseTask.startAndCallback(^(WebResponse
response) {
Stream responseStream = response.GetResponseStream();
Task copyTask = responseStream.CopyToAsync(content);
Task<byte[]> bytesTask = copyTask.startAndCallback(^ {
return content.ToArray();
});
return bytesTask;
};
return task2;
}

12. TCTask* GetURLContentsAsImageAsync(NSURL *url) {
return [[[NSURLConnection requestURL:url] then:^ (id response) {
return [response getDataAsync];
} then:^ (NSData *data) {
return TCBackgroundTask(^{
[UIImage imageWithData:data];
});
}];
}

13. [[[TCRACSocket connectTo:@"localhost" port:1236]
selectMany:^id<RACSubscribable>(id x) {
return [x lines];
}] subscribeNext:^(id x) {
self.label.text = x;
} error:^(NSError *error) {
NSLog(@"Failure: %@", error);
}];

14. • Objects <3
• Get un-stuck!