问题
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.
回答1:
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.
回答2:
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.
回答3:
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
来源：https://stackoverflow.com/questions/10071187/select-many-in-rx