Select Many in Rx

Question

Please any one let me know how the SelectMany operator in Rx works. I don't know more about this operator in Linq either.

Please explain this with the help of a simple example, and also in what occasion we will use this operator in Rx.

http://stackoverflow.com/questions/3869329/rx-enumerableex-for-vs-enumerable-selectmany — Mitch Wheat, Apr 09 '12 at 09:19
It seems bit complicated.. i am pretty new to the technology. can u provide a simple sample — StezPet, Apr 09 '12 at 09:33

score 25 · Answer 1 · answered Apr 11 '12 at 01:18

25

SelectMany is just:

source.Select(selector).Merge();

In other words, it selects the source input into a stream of Observables, then flattens each Observable into a stream of results.

answered Apr 11 '12 at 01:18

Ana Betts

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score 23 · Accepted Answer · answered Apr 09 '12 at 12:22

SelectMany combines projection and flattening into a single step. Suppose you have a number of lists like { {1, 2}, {3, 4, 5}, { 6, 7 } } you can use SelectMany to flatten it into a single list like: { 1, 2, 3, 4, 5, 6, 7}

SelectMany in Rx can flatten multiple sequences into one observable (there are actually several overloads).

For a practical example, suppose you have a function DownloadFile(filename) which gives you an Observable which produces a value when the file completes downloading. You can now write:

string[] files = { "http://.../1", "http://.../2", "http://.../3" };

files.ToObservable()
                 .SelectMany(file => DownloadFile(file))
                 .Take(3)
                 .Subscribe(c => Console.WriteLine("Got " + c) , ()=>  Console.WriteLine("Completed!"));

All 3 observables of DownloadFile are flattened into one, so you can wait for 3 values to arrive to see that all downloads are completed.

I couldn't understand why you used SelectMany here in your code, you don't have list of lists -> you didn't get any flatten, I mean, how will the result be different in your code if you just used select? — Mohammed Noureldin, Dec 27 '16 at 00:02
With `Select`, you'd end up with an observable of observables. — Asti, Dec 27 '16 at 00:17
Could you explain more? that was not clear for me, I am still new in LINQ and RX — Mohammed Noureldin, Dec 27 '16 at 00:57

score 10 · Answer 3 · answered Apr 11 '12 at 22:22

10

I found this short video helpful in understanding SelectMany for Rx (and as a more advanced use of marble diagrams): http://channel9.msdn.com/Blogs/J.Van.Gogh/Reactive-Extensions-API-in-depth-SelectMany

answered Apr 11 '12 at 22:22

yamen

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Select Many in Rx

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