PubFactory

Creating a custom publisher with Apple's Combine framework is a task that involves many lines of code. But the code between custom publishers is often very similar. In Combine only the class Future allows to build a publisher just from a simple closure. Unfortunately, Future does not allow to cancel the task within that closure. Furthermode, Future allows exactly one output item only.

PubFactory provides the class Create to build custom publishers which are fully cancellable and which can emit a stream of items.

Each asynchronous call can be easily transformed into a custom publisher with PubFactory.Create. In its simplest form PubFactory.Create is initialized with a closure. The closure has one parameter (subscriber) that gives the closure the ability to send out items and a completion (finished or failure) to the connected subscriber down-streams. The only task of the closure is to start an asynchronous operation and to return a cancellable that is able to stop this operation. The class Create requires two generics: The output type of the publisher and its error type.

let publisher = Create<Int, Never> { subscriber in
    // Start asynchronous operation
    return AnyCancellable {
        // Action to stop asynchronous operation
    }
}

The asynchronous operation can use the subscriber object to notify the subscriber down-streams. The following code sends the integer sequence 4, 2, 7 to the connected subscriber. At the end it completes the stream without failure.

subscriber.receive(4)
subscriber.receive(2)
subscriber.receive(7)
subscriber.receive(completion: .finished)

Let's look at an example. We implement our own version of URLSession.dataTaskPublisher by using PubFactory.Create:

extension URLSession {
    func myDataTaskPublisher(for url: URL) -> Create<Data?, Error> {
        Create<Data?, Error> { subscriber in
            let task = URLSession.shared.dataTask(with: url) { dataOrNil, _, errorOrNil in
                if let error = errorOrNil {
                    subscriber.receive(completion: .failure(error))
                }
                else {
                    subscriber.receive(dataOrNil)
                    subscriber.receive(completion: .finished)
                }
            }
            task.resume()
            return AnyCancellable { task.cancel() }
        }
    }
}

In this example the closure creates and starts a URLSession.dataTask to load a web site. The data task itself has a completion closure that is called if the web site content has been loaded or if an error occured. In case of error, the closure sends a failure to the subscriber by calling:

subscriber.receive(completion: .failure(error))

If no error occured, it sends the data to the subscriber and completes the stream immediately afterwards:

subscriber.receive(dataOrNil)
subscriber.receive(completion: .finished)

The closure returns a cancellable that stops the task as soon the subscription is cancelled:

return AnyCancellable { task.cancel() }

In this particular case you could use Future from the Combine framework instead. The disadvantage of using Future is, that the data task cannot be cancelled.

Future<Data?, Error> { subscriber in
    let task = URLSession.shared.dataTask(with: url) { dataOrNil, _, errorOrNil in
        if let error = errorOrNil {
            subscriber(.failure(error))
        }
        else {
            subscriber(.success(dataOrNil))
        }
    }
    task.resume()
}

In the following example we define a publisher that sends incrementally ascending integer numbers starting with 0.

let numberGenerator = Create<Int, Never> { subscriber in
    thread = Thread {
        var i = 0
        while !Thread.current.isCancelled {
            subscriber.receive(i)
            i += 1
        }
    }
    thread?.start()
    return AnyCancellable { thread?.cancel() }
}

To do so the closure starts its own thread which increments the variable i. With each change the content of i is send to subscriber. This process continues until the thread is cancelled. Therefore the closure returns a AnyCancellable that will stop the thread if the subscription has been cancelled.

It is very important that the closure does not block. If it would be blocking then the Cancellable which should stop the task inside the closure would never be constructed. Thus the following code is invalid:

let numberGenerator = Create<Int, Never> { subscriber in
    var isCancelled = false
    var i = 0
    while !isCancelled {
        subscriber.receive(i)
        i += 1
    }
    return AnyCancellable { isCancelled = true }
}

Combine has backpressure handling integrated. Create provides basic backpressure handling as well. If the demand of the subscriber is .none then all items sent by subscriber.receive(:) are simply ignored until the demand is increased again.

There is a second possibility to create a publisher with Create. It is made for use cases like the number generator. A closure with two parameters is used. The first parameter is a subscriber and the second parameter is a context. This closure does not return a cancellable as it is expected that the closure stops itself based on the status of the context:

let publisher = Create<Int, Never> { subscriber, context in
    var i = 0
    while !context.cancelled {
        subscriber.receive(i)
        i += 1
    }
}

The context provides the information about the state of the subscription:

protocol Context {
    var paused: Bool { get }
    var cancelled: Bool { get }
    func waitIfPaused()
}

It also provides backpressure support. In the case the subscriber's demand if .none, the number generator can easily be stopped temporarily by just adding one more command:

let publisher = Create<Int, Never> { subscriber, context in
    var i = 0
    while !context.cancelled {
        context.waitIfPaused()
        subscriber.receive(i)
        i += 1
    }
}

The package can be installed by using the Swift Package Manager.

MIT License

Copyright 2020 Patrick Sturm

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.

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