Add warning about .net standard project

[glennawatson]

Add warning message if using the .NET Standard platform.

It was previous happening before this version came in this file

ReactiveUI/src/ReactiveUI/Platforms/netstandard2.0/PlatformRegistrations.cs

Line 20 in 08503a7

if (registerFunction == null)

We have to be careful so we don't confuse WASM users as well.

[rgomez90]

Can give this a try if needed :)

[glennawatson]

@rgomez90 -- sure if you like. You can probably add a check in RxApp.cs -- if it's the DefaultScheduler it's likely to be wrong.

[rgomez90]

@glennawatson with warning message do you mean #warning or LogHost.Default.Warn("...");?

I suppose the second, so if I understand correctly, I should just check the Scheduler in RxApp and if it's the DefaultScheduler I should call LogHost.Default.Warn("ReactiveUI needs a platform to run, it cannot run on .NET Standard");

But where should I do the check? In the static ctor or in the MainThreadScheduler setter?

[glennawatson]

I would maybe even put it in the MainThreadScheduler getter with a bool value you set to avoid it happening excessively.

[glennawatson]

and LogHost is the correct approach., #warning would be compile time only.

[glennawatson]

so

if (!_hasSchedulerBeenChecked && schedulerType == ...)
{
   _hasSchedulerBeenChecked = true;

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[rgomez90]

@glennawatson thank you for the quick replies!

Yes, I already signed (like previous years), but this is not the reason I am contributing.

I wrote some sort of auto-measurement and device management app for my company (we produce transducers) and ported it to use ReactiveUI.

Since them I am in love with it, and helped me a lot to introduce myself in reactive programming....without it I wouldn't have put my nose in there...

That's the reason, I will use A LOT ReactiveUI and also want to give something back, to this really beautiful library 😃 I learned (and still learn) a lot from it / from you core contributors!

[rgomez90]

Something like this would be ok?

public static IScheduler MainThreadScheduler
{
    get
    {
        if (_unitTestMainThreadScheduler != null)
        {
            return _unitTestMainThreadScheduler;
        }

        // If Scheduler is DefaultScheduler, user is likely using .NET Standard
        if (!_hasSchedulerBeenChecked && _mainThreadScheduler == Scheduler.Default)
        {
            _hasSchedulerBeenChecked = true;
            LogHost.Default.Warn("ReactiveUi cannot run on .NET Standard! It needs a platform to run! (.Net Core, WPF, UWP...)");
        }

        return _mainThreadScheduler;
    }

    set
    {
      ...
    }
}

[glennawatson]

It can run in .NET Standard 2.0 but in very limited scenarios.

So web assembly projects such as Uno/Blazor will run the .net standard project in the web browser but those project types should be setting their own main thread scheduler as well so it should never be default.

So I would just say something along the lines of you need to add a nuget host package, such as ReactiveUI.WPF, ReactiveUI.Blazor etc.

We tend to do the I as upper case in the UI, so ReactiveUI.

[glennawatson]

So what I mean is in the blazor/uno case, the user should install ReactiveUI.Blazor or ReactiveUI.Uno, which will change the main thread scheduler.

[rgomez90]

@glennawatson thank you again for your explanation. I have close to no idea about the new Blazor and Uno. (Blazor is in my To-Do list)

So something like this should be better then

...
 if (!_hasSchedulerBeenChecked && _mainThreadScheduler == Scheduler.Default)
    {
        _hasSchedulerBeenChecked = true;
        LogHost.Default.Warn("It seems you are running .NET Standard, but there is no host package installed!\n");
        LogHost.Default.Warn("You will need to install the specific host package for your platform (ReactiveUI.WPF, ReactiveUI.Blazor, ...)\n");
        LogHost.Default.Warn("You can install the needed package via NuGet");
    }
...

[glennawatson]

Seems good with the message.

Btw the .net standard mode only happens when you use Blazor client mode, they have a server mode that runs asp.net and signalr which is .NET Core 3 based.