Custom Tracing of DotVVM Events

This guide describes how to achieve something similar like the MiniProfiler widget, but for any tracing technology you like.

IRequestTracer interface

You can implement a IRequestTracer interface and register the instance in the ServiceCollection and DotVVM will automatically call the methods on it.

TraceEvent

The main method of the tracer is TraceEvent method. It receives name of the event as a string. We use string just to allow anyone to extend the set of events, but DotVVM itself will call you with following events.

• BeginRequest - when DotVVM gets the request from Asp.Net Core or OWIN. It's called before a matching route is selected, so the context.Route is going to be null.
• ViewInitialized - when the initial control tree is created, but before DotVVM initializes the view model
• ViewModelCreated - when the view model is creates, before it is deserialized and before Init is called
• InitCompleted - after Init is called on view model and all controls
• ViewModelDeserialized - after the view model is deserialized (only called for post backs)
• LoadCompleted - after Load is called on view model and all controls
• CommandExecuted - after the command is executed (only called for post backs)
• PreRenderCompleted - after PreRender is called on view model and all controls
• ViewModelSerialized - after the view model is serialized to JSON
• OutputRendered - when output HTML or JSON is written to the response body
• StaticCommandExecuted - after static command method is executed

EndRequest

The EndRequest(IDotvvmRequestContext context) is called in case the request is handled successfully. The other overload EndRequest(IDotvvmRequestContext context, Exception exception) is used, in case an exception occurs.

All these method return a Task, so you can do any async operation. However, we encourage you to avoid long running operations since the tracing is called quite number of times during the request. If you want to monitor timing between the events of a DotVVM application, slow tracing may ruin your measurements.

Sample tracer

With this knowledge, we can create a simple tracer, that will just write the timings to standard output. We'll start a Stopwatch when we get the BeginRequest event and then just print the elapsed time.

In our sample, we don't use async IO as writing using Console.WriteLine is more convenient.

public class SampleRequestTracer : IRequestTracer
{
    Stopwatch sw = new Stopwatch();
    public Task TraceEvent(string eventName, IDotvvmRequestContext context)
    {
        if (eventName == "BeginRequest")
            sw.Start();
        Console.WriteLine($"Trace {sw.Elapsed}: {eventName}");
        return Task.CompletedTask;
    }

    public Task EndRequest(IDotvvmRequestContext context)
    {
        Console.WriteLine($"Trace {sw.Elapsed}: End of request");
        return Task.CompletedTask;
    }

    public Task EndRequest(IDotvvmRequestContext context, Exception exception)
    {
        Console.WriteLine($"Trace {sw.Elapsed}: Error occured ({exception})");
        return Task.CompletedTask;
    }
}

It should be registered IServiceCollection (in the ConfigureServices method of Asp.Net Core startup class):

services.AddScoped<IRequestTracer, SampleRequestTracer>();

Also note that only requests to the page itself are traced. Requests for resources are not included in DotVVM tracing.