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HeavyLifters Deferred Library

HLDeferred makes programming asynchronous processes with callbacks easy in Objective-C. Asynchronous data sources are scheduled on an NSOperationQueue and provide an HLDeferred object that accept callbacks. Callbacks are simple Objective-C blocks.

An HLDeferred object represents a result (or failure) that will become available once its data source retrieves it. This library includes support for creating data sources using concurrent or non-concurrent NSOperation objects, but you are not limited to this approach in any way.

HLDeferred is based on Deferred classes from Twisted Python. The Twisted pattern has been adapted to many languages and environments. HeavyLifters provides and uses implementations in Objective-C, Objective-J, JavaScript, Node.js, and Java.

Why should I use this?

Your classes are getting bloated and convoluted because of "delegate hell" (NSURLConnectionDelegate and friends). You just want to fetch some data, perhaps JSON using HTTP, act on it once it's available, and know if fetching fails. Tracing your code has become a chore because processes are split across many delegate methods, or you have to maintain complicated state machines to get nested asynchronous processes to work correctly.

You wish there was an easy way do to simple things like this. There is.


  • Xcode 3.2.5 with iOS 4.2 SDK (or later)
  • iOS 4.x devices are supported

Note: This code would probably work on Mac OS X 10.6+, but it hasn't been tested there.

Building HLDeferred

  • Clone HLDeferred from GitHub: git clone git://
  • Open HLDeferred-objc/HLDeferred/HLDeferred.xcodeproj in Xcode
  • Build

Using HLDeferred in your iOS app project

  • Drag-and-drop the HLDeferred project icon at the top of HLDeferred's Groups & Files pane into your app project
  • In your app target inspector (General tab), make your project dependent on the HLDeferred target in HLDeferred.xcodeproj
  • Drag the libHLDeferred.a static library (under HLDeferred.xcodeproj in your app project) inside the "Link Binary with Libraries" section of your app target.
  • Drag the "HLDeferred Headers" group from the HLDeferred project into your app project.
  • import and use the HLDeferred classes in your app

Running Unit Tests

How it works

You can create an HLDeferred object directly, but you typically request one from a data source. You get the data source's result by adding callbacks and/or errbacks to the callback chain of the HLDeferred object the data source provides. When the data source has the result (or determines failure), the HLDeferred object is sent a -takeResult: message (or -takeError:). At this point, the HLDeferred object's callback chain is fired, meaning each link in the chain (a callback or errback) is called in turn. The result is the input to the first callback, and its output is the input to the next callback (and so on).

If a callback (or errback) returns an exception, the next errback is called, otherwise the next callback is called.


The Most Basic Example

- (void) demonstration
    HLDeferred *d = [[HLDeferred alloc] init];
    NSLog(@"created HLDeferred object");

    // add a callback
    [d then: ^(id result) {
        NSLog(@"Hello, %@", result);
        return result;
    NSLog(@"added a callback to the HLDeferred's callback chain");

    // resolve the HLDeferred, which fires the callback chain
    [d takeResult: @"World"];
    NSLog(@"In the console, you should see 'Hello, World' in the line above");

    // Note: use [d takeError: @"DISASTER!"] to indicate failure

    [d release];

Adding callbacks and errbacks

Each link in a callback chain is a pair of Objective-C blocks, representing a callback and an errback. Firing the chain executes the callback OR errback of each link, in sequence. For each link, its callback is executed if its input is a result; the errback is executed if its input is a failure (failures are represented by HLFailure objects).

Adding (just) a callback

To append a link with a callback to an HLDeferred object, send it the -then: message, passing in a ThenBlock. Example:

[aDeferred then: ^(id result) {
    // do something useful with the result
    return result;

HLDeferred adds a link to its chain with your callback and a "passthrough" errback. The passthrough errback simply returns its exception parameter.

Adding (just) an errback

To append a link with an errback to an HLDeferred object, send it the -fail: message, passing in a FailBlock. Example:

[aDeferred fail: ^(HLFailure *failure) {
    // optionally do something useful with [failure value]
    return failure;

HLDeferred adds a link to its chain with your errback and a "passthrough" callback. The passthrough callback simply returns its result parameter.

Adding a callback and an errback

To add a link with a callback and errback to an HLDeferred object, send it either the -then:fail: message or the both: message.

Use -then:fail: when you want different behaviour in the case of success or failure:

[aDeferred then: ^(id result) {
    // do something useful with the result
    return result;
} fail: ^(HLFailure *failure) {
    // optionally do something useful with [failure value]
    return failure;

Use -both: when you intend to do the same thing in either case:

[aDeferred both: ^(id result) {
    // in the case of failure, result is an HLFailure
    // do something in either case
    return result;

HLDeferred in practice

By convention, names of methods returning an HLDeferred object are prefixed with "request", such as:

// result is a MyThing object
- (HLDeferred *) requestDistantInformation;

It might be nice if Objective-C had support for generic types so you could specify the expected type of the result of the HLDeferred. Since that isn't the case, you should document that information in your header file or elsewhere.

Data Sources

This library includes several concurrent NSOperation classes that act as data sources and provide a HLDeferred object.

Using the included data sources

Create a data source operation, then schedule it on an NSOperationQueue by sending it the -requestStartOnQueue: message, such as:

NSOperation *op = [[HLURLDataSource alloc] initWithContext: ctx];
HLDeferred *d = [op requestStartOnQueue: [NSOperationQueue mainQueue]];

Note: You don't have to use [NSOperationQueue mainQueue], you can create your own queue and use that. Be aware that HLDeferredDataSource executes its HLDeferred object's callback chain on the main thread using GCD's dispatch_async.

Making your own data sources

See the comments in HLDeferredDataSource.h and HLDeferredConcurrentDataSource.h for more information.

Data Source Example

- (HLDeferred *) requestURLFromString: (NSString *)urlString
    // Fetch an URL and do something when it's complete
    NSOperation *op = [[HLURLDataSource alloc] initWithURLString: urlString];
    NSOperationQueue *queue = [NSOperationQueue mainQueue];
    HLDeferred *d = [op requestStartOnQueue: queue];
    [op release];

    // the "then" block runs if the operation succeeds
    [d then: ^(id result) {
        // result is NSData
        // download succeeded, do something
        return @"SUCCESS!";
    } fail: ^(HLFailure *failure) { // runs if the operations fails
        // request failed
        return failure;
    return d;

- (void) main
    HLDeferred *d = [self requestURLFromString: @""];
    // both means "in the case of success or failure"
    [d both: ^(id result) {
        // run after the callback specified in requestURLFromString
        NSLog(@"result is: %@", [result description]);
        return result;
    // perhaps return d.
    // The caller could add more callbacks

Composing HLDeferred objects arbitrarily

You can return an HLDeferred object from the callback or errback of another HLDeferred object. If you do, the next link of the callback chain will not be executed until the callback chain of the returned HLDeferred is fired. The input to the next link of the original HLDeferred object's callback chain will be the output of the returned HLDeferred object.

This effectively builds a dependency tree of HLDeferred objects. Using this approach makes it easy to compose arbitrary trees of HLDeferred objects without having to manage complicated state machines.

Quick note about memory management

When using a subclass of HLDeferredDataSource, memory management is simple; you do not need to explicitly retain/release the HLDeferred provided by -requestStartOnQueue:.

The HLDeferredDataSource owns the HLDeferred object returned by -requestStartOnQueue:. Its callback chain will be fired (on the main thread) prior to the operation being marked as complete. When the operation is marked as complete, the queue releases it, which in turn releases the HLDeferred object.

As the NSOperationQueue retains the HLDeferredDataSource until it is complete, you can safely release the data source object once after calling -requestStartOneQueue:.

Assemble a firing squad with HLDeferredList

HLDeferredList waits for a list of HLDeferred objects to finish firing before firing its callback chain.

It can optionally fire when the first result is obtained from the list, or when the first error is encountered, or can consume errors.

Composition example

- (HLDeferred *) requestLogin
    if ([MyApp isLoggedIn]) {
        return [HLDeferred deferredWithResult: @"ok"];
    } else {
        NSDictionary *ctx = ...;
        NSOperation *op = [[HLDeferredURLRequestConcurrentOperation alloc] initWithContext: ctx];
        NSOperationQueue *queue = [NSOperationQueue mainQueue];
        HLDeferred *d = [op requestStartOnQueue: queue];
        [op release];

        [d then: ^(id result) {
            NSLog(@"login succeeded");
            return @"ok";

        return d;

// assume these do network requests of some sort
- (HLDeferred *) requestStuff1 { ... }
- (HLDeferred *) requestStuff2 { ... }

// if successful, the result is a dictionary with keys stuff1 and stuff2
- (HLDeferred *) requestMyStuff
    // call requestLogin and add a callback to its callback chain
    // return that deferred, which the caller can add its own
    // callbacks and/or errbacks to.
    return [[self requestLogin] then: ^(id result) {
        if ([@"ok" isEqualToString: [result description]]) {
            NSArray *stuffs = [NSArray arrayWithObjects:
                [self requestStuff1],
                [self requestStuff2],
            HLDeferredList *ds = [[HLDeferredList alloc] initWithDeferreds: stuffs
                                                          fireOnFirstError: YES];
            [ds then: ^(id result) {
                // result is an NSArray containing 2 objects
                //  - the result of requestStuff1's HLDeferred
                //  - the result of requestStuff2's HLDeferred
                NSDictionary *dict = [[NSDictionary alloc] init];
                [dict setObject: [result objectAtIndex: 0] forKey: @"stuff1"];
                [dict setObject: [result objectAtIndex: 1] forKey: @"stuff2"];
                return [dict autorelease];
            return [ds autorelease];
        } else {
            // returning an exception switches the execution of the
            // callback chain from the "then branch" to the "fail branch"
            return [HLFailure wrap: result];

- (void) main
    [[self requestMyStuff] then: ^(id result) {
        // at this point, we know requestStuff1 and requestStuff2
        // completed successfully, and result is an NSDictionary
        return result;
    } fail: (HLFailure *failure) {
        // the process failed
        return failure;

Handling Failure with composition

Unless you require fine-grained handling of failures, specifiy errbacks only in the top-level code that calls the first method returning an HLDeferred object. An IBAction method is a good example of "top-level code"...

- (IBAction) signIn: (id)sender
    [[self requestSignInForUser: userid
                   withPassword: password] then: ^(id result) {
        // success!
        return @"ok";
    } fail: (HLFailure *failure) {
        // oh noes!
        [UIAlertView ...];
        return failure;

If -requestSignInForUser:withPassword: displayed a UIAlertView as well, the user would see multiple alerts!

There's more (leftovers we haven't written about yet)

HLDeferred also has support for finalizers (which run after the callback chain is exhausted). Check out -thenFinally:, -failFinally:, -thenFinally:failFinally: and -bothFinally: in HLDeferred.m for more information. (Eventually we'll write better docs about this - or you can and send us a pull request :-))

If you use an HLDeferredOperation, you can cancel the operation by sending its HLDeferred object the -cancel message. HLDeferredOperation does this by conforming to the HLDeferredCancellable protocol and setting itself as the cancelTarget of the HLDeferred object - thus it is sent the -cancel message when you send -cancel to the HLDeferred object.

Also, timeouts are currently supported, but this will probably be removed soon, as we haven't used this functionality. We implemented it because it was part of the Twisted implementation, but since then the timeout functionality has been removed from Twisted as its use was long discouraged.


How to contribute





Copyright 2011 HeavyLifters Network Ltd. Licensed under the terms of the MIT license. See included LICENSE file.

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