ReactiveCocoa Tutorial – The Definitive Introduction: Part 2/2

Throttling

You might have noticed that every time you type a new character, a Twitter search executes immediately. If you’re a fast typer (or simply hold down the delete key), this can result in the application performing several searches a second. This is not ideal for a couple of reasons: firstly, you’re hammering the Twitter search API and simultaneously throwing away most of the results. Secondly, you’re constantly updating the results which is rather distracting for the user!

A better approach would be to perform a search only if the search text is unchanged for a short amount of time, say 500 milliseconds.

As you’ve probably guessed, ReactiveCocoa makes this task incredibly simple!

Open RWSearchFormViewController.m and update the pipeline at the end of viewDidLoad by adding a throttle step just after the filter:

[[[[[[[self requestAccessToTwitterSignal]
  then:^RACSignal *{
    @strongify(self)
    return self.searchText.rac_textSignal;
  }]
  filter:^BOOL(NSString *text) {
    @strongify(self)
    return [self isValidSearchText:text];
  }]
  throttle:0.5]
  flattenMap:^RACStream *(NSString *text) {
    @strongify(self)
    return [self signalForSearchWithText:text];
  }]
  deliverOn:[RACScheduler mainThreadScheduler]]
  subscribeNext:^(NSDictionary *jsonSearchResult) {
    NSArray *statuses = jsonSearchResult[@"statuses"];
    NSArray *tweets = [statuses linq_select:^id(id tweet) {
      return [RWTweet tweetWithStatus:tweet];
    }];
    [self.resultsViewController displayTweets:tweets];
  } error:^(NSError *error) {
    NSLog(@"An error occurred: %@", error);
  }]; 

The throttle operation will only send a next event if another next event isn’t received within the given time period. It’s really that simple!

Build and run to confirm that the search results only update if you stop typing for 500 milliseconds. Feels much better doesn’t it? Your users will think so too.

And…with that final step your Twitter Instant application is complete. Give yourself a pat on the back and do a happy dance.

If you got lost somewhere in the tutorial you can download the final project (Don’t forget to run pod install from the project’s directory before opening), or you can obtain the code from GitHub where there is a commit for each Build & Run step in this tutorial.

Wrap Up

Before heading off and treating yourself to a victory cup of coffee, it’s worth admiring the final application pipeline:

That’s quite a complicated data flow, all expressed concisely as a single reactive pipeline. It’s a beautiful sight to see! Can you imagine how much more complex this application would be using non-reactive techniques? And how much harder it would be to see the data flows in such an application? Sounds very cumbersome, and now you don’t have to go down that road ever again!

Now you know that ReactiveCocoa is really quite awesome!

One final point, ReactiveCocoa makes it possible to use the Model View ViewModel, or the MVVM design pattern, which provides better separation of application logic and view logic. If anyone is interested in a follow-up article on MVVM with ReactiveCocoa, please let me know in the comments. I’d love to hear your thoughts and experiences!