ASP.NET Core Blazor event handling

• 10/20/2020
• 7 minutes to read
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By Luke Latham and Daniel Roth

Razor components provide event handling features. For an HTML element attribute named @on{EVENT} (for example, @onclick) with a delegate-typed value, a Razor component treats the attribute's value as an event handler.

The following code calls the UpdateHeading method when the button is selected in the UI:

<button class="btn btn-primary" @onclick="UpdateHeading">
    Update heading
</button>

@code {
    private void UpdateHeading(MouseEventArgs e)
    {
        ...
    }
}

The following code calls the CheckChanged method when the check box is changed in the UI:

<input type="checkbox" class="form-check-input" @onchange="CheckChanged" />

@code {
    private void CheckChanged()
    {
        ...
    }
}

Event handlers can also be asynchronous and return a Task. There's no need to manually call StateHasChanged. Exceptions are logged when they occur.

In the following example, UpdateHeading is called asynchronously when the button is selected:

<button class="btn btn-primary" @onclick="UpdateHeading">
    Update heading
</button>

@code {
    private async Task UpdateHeading(MouseEventArgs e)
    {
        await ...
    }
}

Event argument types

For some events, event argument types are permitted. Specifying an event parameter in an event method definition is optional and only necessary if the event type is used in the method. In the following example, the MouseEventArgs event argument is used in the ShowMessage method to set message text:

private void ShowMessage(MouseEventArgs e)
{
    messageText = $"The mouse is at coordinates: {e.ScreenX}:{e.ScreenY}";
}

Supported EventArgs are shown in the following table.

For more information, see the following resources:

• EventArgs classes in the ASP.NET Core reference source (dotnet/aspnetcore master branch). The master branch represents API under development for the next ASP.NET Core release. For the current release, select the appropriate GitHub repository branch (for example, release/3.1).
• MDN web docs: GlobalEventHandlers: Includes information on which HTML elements support each DOM event.

Lambda expressions

Lambda expressions can also be used:

<button @onclick="@(e => Console.WriteLine("Hello, world!"))">Say hello</button>

It's often convenient to close over additional values, such as when iterating over a set of elements. The following example creates three buttons, each of which calls UpdateHeading passing an event argument (MouseEventArgs) and its button number (buttonNumber) when selected in the UI:

<h2>@message</h2>

@for (var i = 1; i < 4; i++)
{
    var buttonNumber = i;

    <button class="btn btn-primary"
            @onclick="@(e => UpdateHeading(e, buttonNumber))">
        Button #@i
    </button>
}

@code {
    private string message = "Select a button to learn its position.";

    private void UpdateHeading(MouseEventArgs e, int buttonNumber)
    {
        message = $"You selected Button #{buttonNumber} at " +
            $"mouse position: {e.ClientX} X {e.ClientY}.";
    }
}

EventCallback

A common scenario with nested components is the desire to run a parent component's method when a child component event occurs. An onclick event occurring in the child component is a common use case. To expose events across components, use an EventCallback. A parent component can assign a callback method to a child component's EventCallback.

The ChildComponent in the sample app (Components/ChildComponent.razor) demonstrates how a button's onclick handler is set up to receive an EventCallback delegate from the sample's ParentComponent. The EventCallback is typed with MouseEventArgs, which is appropriate for an onclick event from a peripheral device:

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" @onclick="OnClickCallback">
        Trigger a Parent component method
    </button>
</div>

@code {
    [Parameter]
    public string Title { get; set; }

    [Parameter]
    public RenderFragment ChildContent { get; set; }

    [Parameter]
    public EventCallback<MouseEventArgs> OnClickCallback { get; set; }
}

The ParentComponent sets the child's EventCallback<TValue> (OnClickCallback) to its ShowMessage method.

Pages/ParentComponent.razor:

@page "/ParentComponent"

<h1>Parent-child example</h1>

<ChildComponent Title="Panel Title from Parent"
                OnClickCallback="@ShowMessage">
    Content of the child component is supplied
    by the parent component.
</ChildComponent>

<p><b>@messageText</b></p>

@code {
    private string messageText;

    private void ShowMessage(MouseEventArgs e)
    {
        messageText = $"Blaze a new trail with Blazor! ({e.ScreenX}, {e.ScreenY})";
    }
}

When the button is selected in the ChildComponent:

• The ParentComponent's ShowMessage method is called. messageText is updated and displayed in the ParentComponent.
• A call to StateHasChanged isn't required in the callback's method (ShowMessage). StateHasChanged is called automatically to rerender the ParentComponent, just as child events trigger component rerendering in event handlers that execute within the child. For more information, see ASP.NET Core Blazor component rendering.

EventCallback and EventCallback<TValue> permit asynchronous delegates. EventCallback is weakly typed and allows passing any type argument in InvokeAsync(Object). EventCallback<TValue> is strongly typed and requires passing a T argument in InvokeAsync(T) that's assignable to TValue.

<ChildComponent 
    OnClickCallback="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

Invoke an EventCallback or EventCallback<TValue> with InvokeAsync and await the Task:

await OnClickCallback.InvokeAsync(arg);

Use EventCallback and EventCallback<TValue> for event handling and binding component parameters.

Prefer the strongly typed EventCallback<TValue> over EventCallback. EventCallback<TValue> provides better error feedback to users of the component. Similar to other UI event handlers, specifying the event parameter is optional. Use EventCallback when there's no value passed to the callback.

Prevent default actions

Use the @on{EVENT}:preventDefault directive attribute to prevent the default action for an event.

When a key is selected on an input device and the element focus is on a text box, a browser normally displays the key's character in the text box. In the following example, the default behavior is prevented by specifying the @onkeypress:preventDefault directive attribute. The counter increments, and the + key isn't captured into the <input> element's value:

<input value="@count" @onkeypress="KeyHandler" @onkeypress:preventDefault />

@code {
    private int count = 0;

    private void KeyHandler(KeyboardEventArgs e)
    {
        if (e.Key == "+")
        {
            count++;
        }
    }
}

Specifying the @on{EVENT}:preventDefault attribute without a value is equivalent to @on{EVENT}:preventDefault="true".

The value of the attribute can also be an expression. In the following example, shouldPreventDefault is a bool field set to either true or false:

<input @onkeypress:preventDefault="shouldPreventDefault" />

Stop event propagation

Use the @on{EVENT}:stopPropagation directive attribute to stop event propagation.

In the following example, selecting the check box prevents click events from the second child <div> from propagating to the parent <div>:

<label>
    <input @bind="stopPropagation" type="checkbox" />
    Stop Propagation
</label>

<div @onclick="OnSelectParentDiv">
    <h3>Parent div</h3>

    <div @onclick="OnSelectChildDiv">
        Child div that doesn't stop propagation when selected.
    </div>

    <div @onclick="OnSelectChildDiv" @onclick:stopPropagation="stopPropagation">
        Child div that stops propagation when selected.
    </div>
</div>

@code {
    private bool stopPropagation = false;

    private void OnSelectParentDiv() => 
        Console.WriteLine($"The parent div was selected. {DateTime.Now}");
    private void OnSelectChildDiv() => 
        Console.WriteLine($"A child div was selected. {DateTime.Now}");
}

<input @ref="exampleInput" />

<button @onclick="ChangeFocus">Focus the Input Element</button>

@code {
    private ElementReference exampleInput;

    private async Task ChangeFocus()
    {
        await exampleInput.FocusAsync();
    }
}