{"id":11542,"date":"2022-09-27T19:56:39","date_gmt":"2022-09-27T14:26:39","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-subscribing-to-a-future-observable\/"},"modified":"2022-09-27T19:56:39","modified_gmt":"2022-09-27T14:26:39","slug":"solved-subscribing-to-a-future-observable","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-subscribing-to-a-future-observable\/","title":{"rendered":"[Solved] Subscribing to a future observable"},"content":{"rendered":"

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The key problem is to find a way to keep an Observer subscribed to a stream whilst tearing down and replacing an underlying source. Let’s just focus on a single event source – you should be able to extrapolate from that.<\/p>\n

First of all, here is an example class we can use that has a single event SomeEvent<\/code> that follows the standard .NET pattern using an EventHandler<StringEventArgs><\/code> delegate. We will use this to create sources of events.<\/p>\n

Note I have intercepted the event add\/remove handlers in order to show you when Rx subscribes and unsubscribes from the events, and given the class a name property to let us track different instances:<\/p>\n

public class EventSource\n{\n    private string _sourceName;\n\n    public EventSource(string sourceName)\n    {\n        _sourceName = sourceName;\n    }\n\n    private event EventHandler<MessageEventArgs> _someEvent;\n\n    public event EventHandler<MessageEventArgs> SomeEvent\n    {\n        add\n        {\n            _someEvent = (EventHandler<MessageEventArgs>)\n                Delegate.Combine(_someEvent, value);\n            Console.WriteLine(\"Subscribed to SomeEvent: \" + _sourceName);\n        }\n        remove\n        {\n            _someEvent = (EventHandler<MessageEventArgs>)\n                Delegate.Remove(_someEvent, value);\n            Console.WriteLine(\"Unsubscribed to SomeEvent: \" + _sourceName);\n        }\n\n    }\n\n    public void RaiseSomeEvent(string message)\n    {\n        var temp = _someEvent;\n        if(temp != null)\n            temp(this, new MessageEventArgs(message));\n    }\n}\n\npublic class MessageEventArgs : EventArgs\n{\n    public MessageEventArgs(string message)\n    {\n        Message = message;\n    }\n\n    public string Message { get; set; }   \n\n    public override string ToString()\n    {\n        return Message;\n    }\n}\n<\/code><\/pre>\n
Solution Key Idea – StreamSwitcher<\/h3>\n
Now, here is the heart of the solution. We will use a Subject<IObservable<T>><\/code> to create a stream of streams. We can use the Observable.Switch()<\/code> operator to return only the most recent stream to Observers. Here’s the implementation, and an example of usage will follow:<\/p>\n
public class StreamSwitcher<T> : IObservable<T>\n{\n    private Subject<IObservable<T>> _publisher;\n    private IObservable<T> _stream;\n\n    public StreamSwitcher()\n    {\n        _publisher = new Subject<IObservable<T>>();\n        _stream = _publisher.Switch();\n    }\n\n    public IDisposable Subscribe(IObserver<T> observer)\n    {\n        return _stream.Subscribe(observer);\n    }\n\n    public void Switch(IObservable<T> newStream)\n    {\n        _publisher.OnNext(newStream);\n    }\n\n    public void Suspend()\n    {\n        _publisher.OnNext(Observable.Never<T>());\n    }\n\n    public void Stop()\n    {\n        _publisher.OnNext(Observable.Empty<T>());\n        _publisher.OnCompleted();\n    }\n}\n<\/code><\/pre>\n
Usage<\/h3>\n
With this class you can hook up a new stream on each occasion you want to start events flowing by using the Switch<\/code> method – which just sends the new event stream to the Subject<\/code>.<\/p>\n
You can unhook events using the Suspend<\/code> method, which sends an Observable.Never<T>()<\/code> to the Subject<\/code> effectively pausing the flow of events.<\/p>\n
Finally you can stop altogether by called to Stop<\/code> to push an Observable.Empty<T>() and<\/code>OnComplete()` the subject.<\/p>\n
The best part is that this technique will cause Rx to do the right thing and properly unsubscribe from the underlying event sources each time you Switch<\/code>, Suspend<\/code> or Stop<\/code>. Note also, that once Stopped<\/code> no more events will flow, even if you Switch<\/code> again.<\/p>\n
Here’s an example program:<\/p>\n
static void Main()\n{\n    \/\/ create the switch to operate on\n    \/\/ an event type of EventHandler<MessageEventArgs>()\n    var switcher = new StreamSwitcher<EventPattern<MessageEventArgs>>();\n\n\n    \/\/ You can expose switcher using Observable.AsObservable() [see MSDN]\n    \/\/ to hide the implementation but here I just subscribe directly to\n    \/\/ the OnNext and OnCompleted events.\n    \/\/ This is how the end user gets their uninterrupted stream:\n    switcher.Subscribe(\n        Console.WriteLine,\n        () => Console.WriteLine(\"Done!\"));\n\n    \/\/ Now I'll use the example event source to wire up the underlying\n    \/\/ event for the first time\n    var source = new EventSource(\"A\");\n    var sourceObservable = Observable.FromEventPattern<MessageEventArgs>(\n        h => source.SomeEvent += h,\n        h => source.SomeEvent -= h);\n\n\n    \/\/ And we expose it to our observer with a call to Switch\n    Console.WriteLine(\"Subscribing\");\n    switcher.Switch(sourceObservable);\n\n    \/\/ Raise some events\n    source.RaiseSomeEvent(\"1\");\n    source.RaiseSomeEvent(\"2\");\n\n    \/\/ When we call Suspend, the underlying event is unwired\n    switcher.Suspend();\n    Console.WriteLine(\"Unsubscribed\");\n\n    \/\/ Just to prove it, this is not received by the observer\n    source.RaiseSomeEvent(\"3\");\n\n    \/\/ Now pretend we want to start events again\n    \/\/ Just for kicks, we'll use an entirely new source of events\n    \/\/ ... but we don't have to, you could just call Switch(sourceObservable)\n    \/\/ with the previous instance.\n    source = new EventSource(\"B\");\n    sourceObservable = Observable.FromEventPattern<MessageEventArgs>(\n        h => source.SomeEvent += h,\n        h => source.SomeEvent -= h);\n\n    \/\/ Switch to the new event stream\n    Console.WriteLine(\"Subscribing\");\n    switcher.Switch(sourceObservable);\n\n    \/\/ Prove it works\n    source.RaiseSomeEvent(\"3\");\n    source.RaiseSomeEvent(\"4\");\n\n    \/\/ Finally unsubscribe\n    switcher.Stop();\n}\n<\/code><\/pre>\n
This gives output like this:<\/p>\n
Subscribing\nSubscribed to SomeEvent: A\n1\n2\nUnsubscribed to SomeEvent: A\nUnsubscribed\nSubscribing\nSubscribed to SomeEvent: B\n3\n4\nUnsubscribed to SomeEvent: B\nDone!\n<\/code><\/pre>\n
Note it doesn’t matter when the end user subscribes – I did it up front, but they can Subscribe any time and they’ll start getting events at that point.<\/p>\n
Hope that helps! Of course you’ll need to pull together the various event types of the Geolocator API into a single convenient wrapper – but this should enable you to get there. <\/p>\n
If you have several events you want to combine into a single stream using this technique, look at operators like Merge<\/code>, which requires you to project the source streams into a common type, with Select<\/code> maybe, or something like CombineLatest<\/code> – this part of the problem shouldn’t be too tricky.<\/p>\n<\/p><\/div>\n
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            4<\/span> <\/p><\/div>\n<\/div>\n
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solved Subscribing to a future observable <\/p>\n","protected":false},"excerpt":{"rendered":"
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