Logging

ReactiveUI comes with its own logging framework which can be used to debug your applications as well as ReactiveUI itself. You may ask yourself, "Seriously, another logging framework?". The reason RxUI does this itself is for portability - none of the common popular logging frameworks support all of the platforms that ReactiveUI supports, and many are server-oriented frameworks and ill-suited for simple mobile app logging.

this.Log() and IEnableLogger

ReactiveUI's logger works a bit differently than other frameworks - its design is inspired by Rails 'logger'. To use it, make your class implement the IEnableLogger interface:

public class MyClass : IEnableLogger
{
    // IEnableLogger doesn't actually require anything of us
}

Now, you can call the Log method on your class. Because of how extension methods work, you must prepend this to it:

this.Log().Info("Downloaded {0} tweets", tweets.Count);

There are five levels of logging, Debug, Info, Warn, Error, and Fatal. Additionally, there are special methods to log exceptions - for example, this.Log().InfoException(ex, "Failed to post the message").

This trick doesn't work for static methods though, you have to settle for an alternate method, LogHost.Default.Info(...).

Debugging Observables

ReactiveUI has several helpers for debugging IObservables. The most straightforward one is Log, which logs events that happen to an Observable:

// Note: Since Log acts like another Rx operator like Select or Where,
// it won't do anything by itself unless someone Subscribes to it.
this.WhenAny(x => x.Name, x => x.Value)
    .SelectMany(async x => GoogleForTheName(x))
    .Log(this, "Result of Search")
    .Subscribe();

Another useful method to debug Observables is LoggedCatch. This method works identically to Rx's Catch operator, except that it also logs the exception to the Logger. For example:

var userAvatar = await FetchUserAvatar()
    .LoggedCatch(this, Observable.Return(default(Avatar)));

Configuring the logger

To configure the logger, register an implementation of ILogger (there are several built-in ones, such as DebugLogger). Here's an example where we use a built-in logger, but a custom log level:

// I only want to hear about errors
var logger = new DebugLogger() { LogLevel = LogLevel.Error };
RxApp.MutableResolver.RegisterConstant(logger, typeof(ILogger));

If you really need to control how things are logged, you can implement IFullLogger, which will allow you to control every logging overload.

Getting log messages to Visual Studio's IntelliTrace

  1. Modify Visual Studio's IntelliTrace settings via Debug>IntelliTrace>Open IntelliTrace Settings>IntelliTrace Events>Tracing> Check everything except Assertion under tracing. If you leave the default settings, you won't pick up Debug level tracing - by checking all of the Trace events, you will pick up Debug level messages.

  2. Make sure you have the reactiveui-nlog package installed to your unit test assembly (unfortunately, you are out of luck if you are using a Windows Store Test Library, but a "normal" unit test library works fine)

  3. Add a file called nlog.config to your unit test project. Make sure you set the "copy to output directory" property to "Copy if newer" or "Copy always" If you leave the default action of "Do not copy" then the nlog.config file won't be copied to your bin directory and nlog won't be able to find its config file, which means it won't know to write to the trace listener.

  4. Here is the nlog.config file

<?xml version="1.0" encoding="utf-8" ?>
<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" >
  <targets>
    <target name="trace" xsi:type="trace"  layout="RxUI:${message}"/>
  </targets>
  <rules>
    <logger name="ReactiveUI.*"  writeTo="trace" />
  </rules>
</nlog>
  1. Register NLogger at the start of your unit test with:
var logManager = RxApp.MutableResolver.GetService<ILogManager>();
RxApp.MutableResolver.RegisterConstant(logManager.GetLogger<NLogLogger>(),typeof(IFullLogger));

Hint: An easy way to filter the IntelliTrace view to only show ReactiveUI events is to type RxUI into the IntelliTrace window search box