Add UseWhenExtensions and UseWhenExtensionsTests

[tuespetre]

Addresses #351

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[kevinchalet]

Nice, I had planned to add UseWhen to the aspnet-contrib packages, but if it's part of ASP.NET Core, that's cool too 😄

IMHO, using Predicate instead of a delegate accepting a HttpContext prevents tons of useful scenarios.

Here's my own version of UseWhen, reviewed by @Tratcher last year: https://gist.github.com/PinpointTownes/6d0492cd3d1b3a64a670

[tuespetre]

Haha, reinvented wheels :)

The 'Predicate' is just an alias for Func<HttpContext,bool>. I just copied the MapWhenExtensions file and changed it (same for the tests.)

[kevinchalet]

The 'Predicate' is just an alias for Func.

Sure, but this doesn't allow you to support path-specific middleware registrations.

E.g

// Create a new branch where the registered middleware will be executed only for non API calls.
app.UseWhen(context => !context.Request.Path.StartsWithSegments("/api"), branch => {
    branch.UseCookieAuthentication(new CookieAuthenticationOptions { ... });
});

[kevinchalet]

Okay, my bad. I had not realized you had added an alias at the top of the file 😄

[tuespetre]

Oops, GitHub took my brackets for HTML. I'm on my phone and don't seem to have the backtick key needed for an inline code span. That should have read 'Func(HttpContext,bool)'

[kevinchalet]

Defining custom aliases with common names (like Predicate) is a nice trap 😄

On a related, an async overload might be nice too.

[Tratcher]

This covers the if (condition) { } scenario, but is there also a need for if (condition) { } else { } or even if (condition) { } elseif (condition) { } elseif (condition) { } else { } ?

[Tratcher]

if (condition) { } else { } can be done as if (condition) { } if (!condition) { }, but it may be inefficient to evaluate the conditional twice.

if (condition) { } elseif (condition) else { } can be done as if (condition) { } else { if (condition) { } else { } }

[tuespetre]

@Tratcher as far as I see it this fills the need of the original issue, if the additional use case(s) ever become in demand they can be designed then...

[Tratcher]

Rebased and merged, thanks.

[tuespetre]

Thanks!

[kevinchalet]

nit: the async/await could have been removed here to save a state machine allocation.

[tuespetre]

😱 true, true