Your Razor Components can implement interfaces even if you’re not using code-behind files. Given that a basic components looks like the following:

@page "/"

<h1>Hello, world!</h1>

Welcome to your new app.

@functions {

}

It’s not directly obvious how the implementation happens. But for that, we have the “@implements” directive:

@page "/"
@implements IMainPage

We can do the actual implementation in the functions-block:

@functions {
    public void UpdateText()
    {
        // Implementation logic
    }
}

Here’s a full example:

@page "/"
@implements IMainPage

<h1>Hello, world!</h1>

Welcome to your new app.

@functions {
    public void UpdateText()
    {
        // Implementation logic
    }
}

In some child & parent component situations it would be good if you could render or at least get an access to an instance of a Razor Component.

Given the following scenario where we try to render an instance of the Counter-component inside the Index-component:

<h1>Hello, world!</h1>

Welcome to your new app.

@counterInstance

@functions{
    Counter counterInstance = new Counter();
}

The following doesn’t work as it only draws the type name of the component:

image

There are couple options around this.

Capturing reference

In many (if not most?) cases you should let Razor Components to take care of creating the instance of your component and then use the “ref” keyword to capture the rendered instance:

<h1>Hello, world!</h1>

Welcome to your new app.

@DrawCounter()

@functions{

    private Counter _myCounter = null;

    RenderFragment DrawCounter()
    {
        return new RenderFragment(builder =>
        {
            builder.OpenComponent<Counter>(0);
            builder.AddComponentReferenceCapture(1, inst => { _myCounter = (Counter)inst; });
            builder.CloseComponent();
        });
    }

Reflection

If you really want, you can access the RenderFragment of the instance using reflection and then draw that:

Welcome to your new app.

@RenderContent(counterInstance)

@functions{
    Counter counterInstance = new Counter();

    RenderFragment RenderContent(ComponentBase instance)
    {
        var fragmentField = GetPrivateField(instance.GetType(), "_renderFragment");

        var value = (RenderFragment)fragmentField.GetValue(instance);

        return value;
    }

    //https://stackoverflow.com/a/48551735/66988
    private static FieldInfo GetPrivateField(Type t, String name)
    {
        const BindingFlags bf = BindingFlags.Instance |
                                BindingFlags.NonPublic |
                                BindingFlags.DeclaredOnly;

        FieldInfo fi;
        while ((fi = t.GetField(name, bf)) == null && (t = t.BaseType) != null) ;

        return fi;
    }

0 Comments

Blazor.EventAggregator is a lightweight Event Aggregator for Razor Components. Razor Components (and formerly known as Blazor) is an upcoming technology included in ASP.NET Core 3.0 (currently in Preview 2).

Event aggregator is used for indirect component to component communication. In event aggregator pattern you have message/event publishers and subscribers. In the case of Razor Components, component can publish its events and other component(s) can react to those events.

Note:

Blazor.EventAggregator is completely based on the work done in Caliburn.Micro. The source code was copied from it and then altered to work with Blazor.

Getting Started

First register EventAggregator as singleton in app’s ConfigureServices:

        public void ConfigureServices(IServiceCollection services)
        {
            services.AddSingleton<EventAggregator.Blazor.IEventAggregator, EventAggregator.Blazor.EventAggregator>();
        }

The rest depends on if you’re using components with code-behinds (inheritance) or without inheritance. The following guidance is for code-behind scenarios. I’ll add another example of using event aggregator without the code-behind model.

Creating the publisher

To create an event publishing component, first inject IEventAggregator:

        [Inject]
        private IEventAggregator _eventAggregator { get; set; }

Then publish the message when something interesting happens:

await _eventAggregator.PublishAsync(new CounterIncreasedMessage());

Here’s a full example of a publisher:

    public class CounterComponent : ComponentBase
    {
        [Inject]
        private IEventAggregator _eventAggregator { get; set; }

        public int currentCount = 0;

        public async Task IncrementCountAsync()
        {
            currentCount++;
            await _eventAggregator.PublishAsync(new CounterIncreasedMessage());
        }
    }

    public class CounterIncreasedMessage
    {
    }

Creating the subscriber

To create an event subscriber, also start by injecting IEventAggregator:

        [Inject]
        private IEventAggregator _eventAggregator { get; set; }

Then make sure to add and implement the IHandle<TMessageType> interface for all the event’s your component is interested in:

public class CounterListenerComponent : ComponentBase, IHandle<CounterIncreasedMessage>
...
public Task HandleAsync(CounterIncreasedMessage message)
{
    currentCount += 1;
    return Task.CompletedTask;
}

Here’s full example of a subscriber:

    public class CounterListenerComponent : ComponentBase, IHandle<CounterIncreasedMessage>
    {
        [Inject]
        private IEventAggregator _eventAggregator { get; set; }

        public int currentCount = 0;

        protected override void OnInit()
        {
            _eventAggregator.Subscribe(this);
        }

        public Task HandleAsync(CounterIncreasedMessage message)
        {
            currentCount += 1;
            return Task.CompletedTask;
        }
    }

Samples

The project site contains a full working sample of the code-behind model in the samples-folder.

Project Location

Source code is available through GitHub: https://github.com/mikoskinen/Blazor.EventAggregator

Requirements

The library has been developed and tested using the following tools:

  • .NET Core 3.0 Preview 2
  • ASP.NET Core 3.0 Preview 2
  • Visual Studio 2019

Acknowledgements

Work is based on the code available in Caliburn.Micro.

0 Comments

We wanted to run integration tests in VSTS’ release phase. Our application is ASP.NET Core 2.0 based and the integration tests are on .NET Core 2.0 projects. Unfortunately we were always met with the following error:

Testhost process exited with error: A fatal error was encountered. The library 'hostpolicy.dll' required to execute the application was not found in 'D:\a\r1\a\IntegrationTests\'.

It turned out that the problem was that we were publishing integration tests in the build phase without providing the framework and runtime arguments. We had to provide both of these arguments to the publish command to get things working:

-c $(BuildConfiguration) -o $(Build.ArtifactStagingDirectory) -r "win10-x64" -f "netcoreapp2.0"

0 Comments

ASP.NET Core ViewComponents usually come in pairs: There’s the class which inherits ViewComponent and there’s the Razor view which is returned by the class:

image

In most of the situations the ViewComponent runs some logic and then returns the view:

    public class MyTest : ViewComponent
    {
        public Task<IViewComponentResult> InvokeAsync(string content)
        {
            return Task.FromResult<IViewComponentResult>(View("Default"));
        }
    }

But what if you need to return the view only in some cases based on the ViewComponent’s parameters or because of some other validation? In these situation you can skip returning the view by returning empty content instead:

    public class MyTest : ViewComponent
    {
        public Task<IViewComponentResult> InvokeAsync(string content)
        {

            if (string.IsNullOrWhiteSpace(content))
            {
                return Task.FromResult<IViewComponentResult>(Content(string.Empty));
            }

            return Task.FromResult<IViewComponentResult>(View("Default", content));
        }
    }