Promises library for C# for management of asynchronous operations.
C#

README.md

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Promises library for C# for management of asynchronous operations.

Inspired by Javascript promises, but slightly different.

Used by Real Serious Games for building serious games and simulations on Unity3d.

If you are interested in using promises for game development and Unity please see this article.

Recent Updates

  • 8 March 2015
    • Transform function has been renamed to Then (another overload of Then).

Contents

Table of Contents generated with DocToc

Understanding Promises

To learn about promises:

Promises/A+ Spec

This promise library conforms to the Promises/A+ Spec (at least, as far as is possible with C#):

Getting the DLL

The DLL can be installed via nuget. Use the Package Manager UI or console in Visual Studio or use nuget from the command line.

See here for instructions on installing a package via nuget: http://docs.nuget.org/docs/start-here/using-the-package-manager-console

The package to search for is RSG.Promise.

Getting the Code

You can get the code by cloning the github repository. You can do this in a UI like SourceTree or you can do it from the command line as follows:

git clone https://github.com/Real-Serious-Games/C-Sharp-Promise.git

Alternately, to contribute please fork the project in github.

Creating a Promise for an Async Operation

Reference the DLL and import the namespace:

using RSG; 

Create a promise before you start the async operation:

var promise = new Promise<string>();

The type of the promise should reflect the result of the async op.

Then initiate your async operation and return the promise to the caller.

Upon completion of the async op the promise is resolved:

promise.Resolve(myValue);

The promise is rejected on error/exception:

promise.Reject(myException);

To see it in context, here is an example function that downloads text from a URL. The promise is resolved when the download completes. If there is an error during download, say unresolved domain name, then the promise is rejected:

public IPromise<string> Download(string url)
{
    var promise = new Promise<string>();    // Create promise.
    using (var client = new WebClient())
    {
        client.DownloadStringCompleted +=   // Monitor event for download completed.
            (s, ev) =>
            {
                if (ev.Error != null)
                {
                    promise.Reject(ev.Error);   // Error during download, reject the promise.
                }
                else
                {
                    promise.Resolve(ev.Result); // Downloaded completed successfully, resolve the promise.
                }
            };

        client.DownloadStringAsync(new Uri(url), null); // Initiate async op.
    }

    return promise; // Return the promise so the caller can await resolution (or error).
}

Creating a Promise, Alternate Method

There is another way to create a promise that replicates the JavaScript convention of passing a resolver function into the constructor. The resolver function is passed functions that resolve or reject the promise. This allows you to express the previous example like this:

var promise = new Promise<string>((resolve, reject) => 
{        
    using (var client = new WebClient())
    {
        client.DownloadStringCompleted +=   // Monitor event for download completed.
            (s, ev) =>
            {
                if (ev.Error != null)
                {
                    reject(ev.Error);       // Error during download, reject the promise.
                }
                else
                {
                    resolve(ev.Result);     // Downloaded completed successfully, resolve the promise.
                }
            };

        client.DownloadStringAsync(new Uri(url), null); // Initiate async op.
    }
});

Waiting for an Async Operation to Complete

The simplest usage is to register a completion handler to be invoked on completion of the async op:

Download("http://www.google.com")
    .Done(html =>
        Console.WriteLine(html)
    );

This snippet downloads the front page from Google and prints it to the console.

For all but the most trivial applications you will also want to register an error hander:

Download("http://www.google.com")
    .Catch(exception =>
        Console.WriteLine("An exception occured while downloading!")
    )
    .Done(html =>
        Console.WriteLine(html)
    );

The chain of processing for a promise ends as soon as an error/exception occurs. In this case when an error occurs the Catch handler would be called, but not the Done handler. If there is no error, then only Done is called.

Chaining Async Operations

Multiple async operations can be chained one after the other using Then:

Download("http://www.google.com")
    .Then(html =>
        return Download(ExtractFirstLink(html)) // Extract the first link and download it. 
    )
    .Catch(exception =>
        Console.WriteLine("An exception occured while downloading!")
    )
    .Done(firstLinkHtml =>
        Console.WriteLine(firstLinkHtml)
    );

Here we are chaining another download onto the end of the first download. The first link in the html is extracted and we then download that. Then expects the return value to be another promise. The chained promise can have a different result type.

Transforming the Results

Sometimes you will want to simply transform or modify the resulting value without chaining another async operation.

Download("http://www.google.com")
    .Then(html =>
        return ExtractAllLinks(html)) // Extract all links and return an array of strings.  
    )
    .Done(links =>                    // The input here is an array of strings.
        foreach (var link in links)
        {
            Console.WriteLine(link);
        }
    );

As is demonstrated the type of the value can also be changed during transformation. In the previous snippet a Promise<string> is transformed to a Promise<string[]>.

Error Handling

An error raised in a callback aborts the function and all subsequent callbacks in the chain:

promise.Then(v => Something())   // <--- An error here aborts all subsequent callbacks... 
    .Then(v => SomethingElse())
    .Then(v => AnotherThing())
    .Catch(e => HandleError(e))  // <--- Until the error handler is invoked here. 

Unhandled Errors

When Catch is omitted exceptions go silently unhandled. This is an acknowledged issue with the Promises pattern.

We handle this in a similar way to the JavaScript Q library. The Done method is used to terminate a chain, it registers a default catch handler that propagates unhandled exceptions to a default error handling mechanism that can be hooked into by the user.

Terminating a Promise chain using Done:

promise.Then(v => Something()) 
    .Then(v => SomethingElse())
    .Then(v => AnotherThing())
    .Done();    // <--- Terminate the pipeline and propagate unhandled exceptions. 

To use the Done you must apply the following rule: When you get to the end of a chain of promises, you should either return the last promise or end the chain by calling Done.

To hook into the unhandled exception stream:

Promise.UnhandledException += Promise_UnhandledException;

Then forward the exceptions to your own logging system:

private void Promise_UnhandledException(object sender, ExceptionEventArgs e)
{
    Log.Error(e.Exception, "An unhandled proimses exception occured!"); 
}

Promises that are already Resolved/Rejected

For convenience or testing you will at some point need to create a promise that starts out in the resolved or rejected state. This is easy to achieve using Resolved and Rejected functions:

var resolvedPromise = Promise<string>.Resolved("some result");

var rejectedPromise = Promise<string>.Rejected(someException);

Interfaces

The class Promise implements the following interfaces:

  • IPromise<T> Interface to await promise resolution.
  • IPendingPromise<T> Interface that can resolve or reject the promise.

Combining Multiple Async Operations

The All function combines multiple async operations to run in parallel. It converts a collection of promises or a variable length parameter list of promises into a single promise that yields a collection.

Say that each promise yields a value of type T, the resulting promise then yields a collection with values of type T.

Here is an example that extracts links from multiple pages and merges the results:

var urls = new List<string>();
urls.Add("www.google.com");
urls.Add("www.yahoo.com");

Promise<string[]>
    .All(url => Download(url))  // Download each URL.
    .Then(pages =>              // Receives collection of downloaded pages.
        pages.SelectMany(
            page => ExtractAllLinks(page) // Extract links from all pages then flatten to single collection of links.
        )
    )
    .Done(links =>              // Receives the flattened collection of links from all pages at once.
    {
        foreach (var link in links)
        {
            Console.WriteLine(link);
        }
    });

Chaining Multiple Async Operations

The ThenAll function is a convenient way of chaining multiple promise onto an existing promise:

promise
    .Then(result => SomeAsyncOperation(result)) // Chain a single async operation
    .ThenAll(result =>                          // Chain multiple async operations.
        new IPromise<string>[]                  // Return an enumerable of promises. 
        {                   
            SomeAsyncOperation1(result),        
            SomeAsyncOperation2(result),
            SomeAsyncOperation3(result)
        }
    )
    .Done(collection => ...);                   // Final promise resolves 
                                                // with a collection of values 
                                                // when all operations have completed.  

Racing Asynchronous Operations

The Race and ThenRace functions are similar to the All and ThenAll functions, but it is the first async operation that completes that wins the race and it's value resolves the promise.

promise
    .Then(result => SomeAsyncOperation(result)) // Chain an async operation.
    .ThenRace(result =>                         // Race multiple async operations.
        new IPromise<string>[]                  // Return an enumerable of promises. 
        {                   
            SomeAsyncOperation1(result),        
            SomeAsyncOperation2(result),
            SomeAsyncOperation3(result)
        }
    )
    .Done(result => ...);                       // The result has come from whichever of
                                                // the async operations completed first. 

Chaining Synchronous Actions that have no Result

The Then function can be used to chain synchronous operations that yield no result.

var promise = ...
promise
    .Then(result => SomeAsyncOperation(result))     // Chain an async operation.
    .Then(result => Console.WriteLine(result))      // Chain a sync operation that yields no result.
    .Done(result => ...);  // Result from previous ascync operation skips over the *Do* and is passed through.

Promises that have no Results (a non-value promise)

What about a promise that has no result? This represents an asynchronous operation that promises only to complete, it doesn't promise to yield any value as a result. I call this a non-value promise, as opposed to a value promise, which is a promise that does yield a value. This might seem like a curiousity but it is actually very useful for sequencing visual effects.

Promise is very similar to Promise<T> and implements the similar interfaces: IPromise and IPendingPromise.

Promise<T> functions that affect the resulting value have no relevance for the non-value promise and have been removed.

As an example consider the chaining of animation and sound effects as we often need to do in game development:

RunAnimation("Foo")                         // RunAnimation returns a promise that 
    .Then(() => RunAnimation("Bar"))        // is resolved when the animation is complete.
    .Then(() => PlaySound("AnimComplete"));

Convert a value promise to a non-value promise

From time to time you might want to convert a value promise to a non-value promise or vice versa. Both Promise and Promise<T> have overloads of Then and ThenAll that do this conversion. You just need to return the appropriate type of promise (for Then) or enumerable of promises (for ThenAll).

As an example consider a recursive link extractor and file downloader function:

public IPromise DownloadAll(string url) 
{
    return DownloadURL(url)                     // Yields a value, the HTML text downloaded.
        .Then(html => ExtractLinks(html))   // Convert HTML into an enumerable of links.
        .ThenAll(links =>                       // Process each link. 
        {
            // Determine links that should be followed, then follow them.
            var linksToFollow = links.Where(link => IsLinkToFollow(link)); 
            var linksFollowing = linksToFollow.Select(link => DownloadAll(link));

            // Determine links that are files to be downloaded, then download them.
            var linksToDownload = links.Where(link => IsLinkToDownload(link));
            var linksDownloading = linksToDownload.Select(link => DownloadFile(link));

            // Return an enumerable of promises.
            // This combines the recursive link following and any files we want to download.                
            // Because we are returning an enumerable of non-value promises, the resulting
            // chained promises is also non-value. 
            return linksToFollow.Concat(linksDownloading);
        });         
}

Usage:

DownloadAll("www.somewhere.com")
    .Done(() =>
        Console.WriteLine("Recursive download completed."); 
    );

Running a Sequence of Operations

The Sequence and ThenSequence functions build a single promise that wraps multiple sequential operations that will be invoked one after the other.

Multiple promise-yielding functions are provided as input, these are chained one after the other and wrapped in a single promise that is resolved once the sequence has completed.

var sequence = Promise.Sequence(
    () => RunAnimation("Foo"),
    () => RunAnimation("Bar"),
    () => PlaySound("AnimComplete")
);

The inputs can also be passed as a collection:

var operations = ...
var sequence = Promise.Sequence(operations);

This might be used, for example, to play a variable length collection of animations based on data:

var animationNames = ... variable length array of animation names loaded from data...
var animations = animationNames.Select(animName => (Func<IPromise>)(() => RunAnimation(animName)));
var sequence = Promise.Sequence(animations);
sequence
    .Done(() =>
    {
        // All animations have completed in sequence.
    });

Unfortunately we find that we have reached the limits of what is possible with C# type inference, hence the use of the ugly cast (Func<IPromise>).

The cast can easily be removed by converting the inner anonymous function to an actual function which I'll call PrepAnimation:

private Func<IPromise> PrepAnimation(string animName) 
{
    return () => RunAnimation(animName);
}

var animations = animationNames.Select(animName => PrepAnimation(animName));
var sequence = Promise.Sequence(animations);
sequence
    .Done(() =>
    {
        // All animations have completed in sequence.
    });

Holy cow... we've just careened into functional programming territory, herein lies very powerful and expressive programming techniques.

Combining Parallel and Sequential Operations

We can easily combine sequential and parallel operations to build very expressive logic.

Promise.Sequence(               // Play operations 1 and 2 sequently.
    () => Promise.All(              // Operation 1: Play animation and sound at same time.
        RunAnimation("Foo"),
        PlaySound("Bar")
    ),
    () => Promise.All(
        RunAnimation("One"),        // Operation 2: Play animation and sound at same time.
        PlaySound("Two")      
    )
);

I'm starting to feel like we are defining behavior trees.

Examples

  • Example1
    • Example of downloading text from a URL using a promise.
  • Example2
    • Example of a promise that is rejected because of an error during
    • the async operation.
  • Example3
    • This example downloads search results from google then transforms the result to extract links.
    • Includes both error handling and a completion handler.
  • Example4
    • This example downloads search results from google, extracts the links and follows only a single first link, downloads its then prints the result.
    • Includes both error handling and a completion handler.
  • Example5
    • This example downloads search results from google, extracts the links, follows all (absolute) links and combines all async operations in a single operation using the All function.