How to do client-side UI events in Blazor

I want to add a more up-to-date (as of 0.9.0) example of calling a JavaScript function to set the focus to another control after some event, like clicking on a button. This might be helpful for someone just starting out with Blazor (like me).

This example builds on the example code in the Blazor documentation "Build Your First Blazor Components App" at https://docs.microsoft.com/en-us/aspnet/core/tutorials/build-your-first-razor-components-app?view=aspnetcore-3.0

First, follow all the instructions in the documentation. When you have a working To-Do List page, then add the following:

1. At the bottom of Index.html, under wwwroot, and below the script tag that loads the webassembly.js, add the following script:

<script>
        window.MySetFocus = (ctrl) => {
            document.getElementById(ctrl).focus();
            return true;
        }
</script>

2. At the top of your todo.cshtml page, add the following using statement:

@inject IJSRuntime JsRuntime;

3. In the @functions section of your todo.cshtml page, add the following function:

    async void Focus(string controlId)
    {
        var obj = JsRuntime.InvokeAsync<string>(
            "MySetFocus", controlId);
    }

4. In the AddToDo() function, just below the line where you set the "newToDo" variable to an empty string, add a call to the Focus function, passing in the string id of the input control. (The example in the docs does not assign an ID to the input control, so just add one yourself. I named mine "todoItem").

void AddTodo()
    {
        if (!string.IsNullOrWhiteSpace(newTodo))
        {
            todos.Add(new TodoItem { Title = newTodo });
            newTodo = string.Empty;
            Focus("todoItem"); // this is the new code
        }
    }

5. Build and run your app. When you click the add new item button, the new item should be added to the list, the input control blanked out, and the focus should be back in the input control, ready for another item to be added.

Blazor is just the replacement (to be more precise "value addition") to JavaScript. It is a client-side only solution (but it might add some easy binding to ASP.NET in the future).

Still, it's completely based on HTML and CSS. C# is replacing the JS part using web assembly. So nothing has changed on how you access / modify HTML controls.

As of now (version 0.1.0) you have to rely on HTML DOM focus() Method to do what you intend to do (yes you have to use JavaScript as of now :( ).

// Not tested code
// This is JavaScript. 
// Put this inside the index.html. Just below <script type="blazor-boot"></script>
<script>
    Blazor.registerFunction('Focus', (controlId) => {
      return document.getElementById(controlId).focus();
    });
</script>

//and then wrap it for calls from .NET:    
// This is C#

public static object Focus(string controlId)
{
    return RegisteredFunction.Invoke<object>("Focus", controlId);
    //object type is used since Invoke does not have a overload for void methods. Don't know why. 
    //this will return undefined according to js specs
}

For more information, you can refer to below.

If you want to improve the packaging of JS neatly, you can do something like this: https://stackoverflow.com/a/49521216/476609

public class BlazorExtensionScripts : Microsoft.AspNetCore.Blazor.Components.BlazorComponent
{
    protected override void BuildRenderTree(Microsoft.AspNetCore.Blazor.RenderTree.RenderTreeBuilder builder)
    {
        builder.OpenElement(0, "script");
        builder.AddContent(1, "Blazor.registerFunction('Focus', (controlId) => { document.getElementById(controlId).focus(); });");
        builder.CloseElement();
    }

    public static void Focus(string controlId)
    {
        RegisteredFunction.Invoke<object>("Focus", controlId);
    }
}

then add this component to the root: (App.cshtml):

<BlazorExtensionScripts></BlazorExtensionScripts>
<Router AppAssembly=typeof(Program).Assembly />

From .NET 5 Preview 8

Set UI focus in Blazor apps

Blazor now has a FocusAsync convenience method on ElementReference for setting the UI focus on that element.

<button @onclick="() => textInput.FocusAsync()">Set focus</button>
<input @ref="textInput"/>