Reactiveui: ThrownExceptions still lets exceptions unhandled

[jorisvergeer]

Hi,

This week i was debugging some crashes in an app that are related to networking (and in particular when the WiFi is disabled _System.Net.WebException: Error: ConnectFailure (Network is unreachable) ---> System.Net.Sockets.SocketException: Network is unreachable_ ). The culprit was an unhanded exception from within the async function of a ReactiveCommand.

var LoadSomeData = ReactiveCommand.CreateAsyncTask(async _ =>
{
    var client = Locator.Current.GetService<ISomeRefitClient>();
    return await client.GetSomeData();
});
LoadSomeData.ThrownExceptions.Subscribe(this.Log().Error);
LoadSomeData
    .ObserveOn(RxApp.MainThreadScheduler)
    .Select(result =>
    {
        ...
    }
...
LoadSomeData.ExecuteAsync();

If I am correct, the app should not crash since I am subscribed to the ThrownExceptions observable. But when I write another exception handler inside the async function, everything keeps running fine.

var LoadSomeData = ReactiveCommand.CreateAsyncTask(async _ =>
{
    var client = Locator.Current.GetService<ISomeRefitClient>();

    try{
        return await client.GetSomeData();
    }
    catch (Exception e)
    {
        this.Log().Error(e);
        return null;
    }
});
LoadSomeData.ThrownExceptions.Subscribe(this.Log().Error);
LoadSomeData
    .Where(result => result != null)
    .ObserveOn(RxApp.MainThreadScheduler)
    .Select(result =>
    {
        ...
    }
...
LoadSomeData.ExecuteAsync();

Did I do something wrong?

[mteper]

What does GetSomeData return? Any chance it's an IEnumerable that gets lazily executed in the subscription?

[jorisvergeer]

client is a refit client. (https://github.com/paulcbetts/refit)

Its signature looks like:

public interface ISomeRefitClient
{
    [Get("/some/entrypoint")]
    Task<List<SomeDataObject>> GetSomeData();

    [Get("/some/other/entrypoint")]
    IObservable<IEnumerable<SomeDataObject>> GetSomeOtherData();
}

Both methods (one with task, and one with observable) have this odd behaviour.

[kentcb]

@jorisvergeer are you able to provide a stand-alone repro, preferably as a unit test? It certainly seems like invalid behavior to me, and I'd be interested in checking it against the new ReactiveCommand implementation in v7.

[jorisvergeer]

Hi,

I created an unit test that shows this issue
https://gist.github.com/jorisvergeer/0056f696f5025fedc7f6

But while writing it I came across this issue and pull request

685 #686

and quoting @paulcbetts:

The expected behavior is that both:

• The exception ends up as an OnNext to ThrownExceptions
• command.ExecuteAsync should throw the Exception

I was under the impression that ExecuteAsync should not throw the exception when ThrownSubscriptions is Subscribed.

[kentcb]

Ah, that makes sense. And the behavior is the same in RxUI 7. Closing this then, but please reopen if you don't think it's resolved.

[JFMG]

Found a way to handle these exceptions :-)

For every Execute you need to add a Catch to return a value.
I implemented it like this:

The command property:

public ReactiveCommand<Unit, MyDataObject> LoadSomeData { get; }

The command initialization with ThrownExceptions:

LoadSomeData = ReactiveCommand.CreateFromTask<Unit, MyDataObject>(_ => OnLoadDataAsync());
LoadSomeData.ThrownExceptions.Subscribe(ex => ex.HandleException(...));

The Execute with a Catch statement:

await LoadSomeData.Execute().Catch(Observable.Return(new MyDataObject());