Sequencer is an iOS library for asynchronous flow control.

Sequencer turns complicated nested blocks logic into a clean, straightforward, and readable code.

Jumping straight to code

Let's output some strings to the Log Console:

Sequencer *sequencer = [[Sequencer alloc] init];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
    NSLog(@"This is the first step");
    completion(nil);
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
    NSLog(@"This is another step");
    completion(nil);
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
    NSLog(@"This step is going to do some async work...");
    int64_t delayInSeconds = 2.0;
    dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);
    dispatch_after(popTime, dispatch_get_main_queue(), ^(void){
        NSLog(@"finished the async work.");
        completion(nil);
    });
}];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
    NSLog(@"This is the last step");
    completion(nil);
}];
[sequencer run];

What does the above code do?

1. A Sequencer was created. There is no need to retain/hold-on-to-it. Trust me.
2. Four steps were enqueued to the Sequencer. The third step is async, but all the rest are plain sync code.
3. Each step finishes by calling completion() with a result object. This result is sent to the next step (in our case the result is nil).
4. We run the sequencer.

Note: Break the steps by just removing the call to completion(nil). Everything will be cleaned-up auto-magically.

A Bettter Example

Let's do something more useful.

We will call the app.net API to get the latest feed, read the last feed-item and get its URL and then read the HTML content of the URL.

(Using AFNetworking for all the networking stuff)

Sequencer *sequencer = [[Sequencer alloc] init];
[sequencer enqueueStep:^(id result, SequencerCompletion completion) {
    NSURL *url = [NSURL URLWithString:@"https://alpha-api.app.net/stream/0/posts/stream/global"];
    NSURLRequest *request = [NSURLRequest requestWithURL:url];
    AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
        completion(JSON);
    } failure:nil];
    [operation start];
}];
[sequencer enqueueStep:^(NSDictionary *feed, SequencerCompletion completion) {
    NSArray *data = [feed objectForKey:@"data"];
    NSDictionary *lastFeedItem = [data lastObject];
    NSString *cononicalURL = [lastFeedItem objectForKey:@"canonical_url"];

    NSURLRequest *request = [NSURLRequest requestWithURL:[NSURL URLWithString:cononicalURL]];
    AFHTTPRequestOperation *operation = [[AFHTTPRequestOperation alloc] initWithRequest:request];
    [operation setCompletionBlockWithSuccess:^(AFHTTPRequestOperation *operation, id responseObject) {
        completion(responseObject);
    } failure:nil];
    [operation start];
}];
[sequencer enqueueStep:^(NSData *htmlData, SequencerCompletion completion) {
    NSString *html = [[NSString alloc] initWithData:htmlData encoding:NSUTF8StringEncoding];
    NSLog(@"HTML Page: %@", html);
    completion(nil);
}];
[sequencer run];

1. The first step downloads a JSON object, turns it into an NSDictionary and sends it to the next step.
2. The second step reads the canonicalURL, downloads it and send it to the next step.
3. The third and last step turns the data into a string and outputs it.

License

Sequencer is available under the MIT license:

Copyright (c) 2013 Tal Bereznitskey

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Contact

Find me on github: Tal Bereznitskey

Follow me on Twitter: @ketacode