Blazor components

Use components for a modular Blazor application.

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Blazor’s Razor Components are at the heart of Blazor development. A component is a piece of a user interface with processing logic to enable dynamic behavior.

Component Lifecycle

There are several events that you can tap into to access data or perform UI operations on your components. Each event except for ShouldRender has a corresponding async event. In general, it’s better to use async operations for scalability and a smoother responding UI.

  • OnInitialized/OnInitializedAsync – Use these events to fetch data and present the user interface.
  • OnParametersSet/OnParametersSetAsync – This event runs when the component has received all parameters. It executes every time the parameters are updated.
  • OnAfterRender/OnAfterRenderAsync – Once the component has finished rendering and all HTML has been displayed, this event is raised. This is the time to manipulate DOM elements.
  • ShouldRender/ShouldRender – Called each time a component is rendered. Override and use this to manipulate UI refreshes by setting the return value of ShouldRender to true.

With these events you can manage the lifecycle of a component as needed. Note that some events are only raised upon the first time a client accesses a page. Additionally, there is a workflow to how components are rendered. Keep in mind, a page is a component and each page in an app will likely contain multiple components. For more in-depth knowledge on the component lifecycle, visit Microsoft's ASP.NET Core Razor component lifecycle documentation.

Data Binding

Data binding enables us to synchronize data between a component and a data store. Data binding can be one-way or two-way.

One-way data binding is simply setting the value of a DOM element to some value in a property or variable, often from a data store, or simply displaying the value of a property or variable.

Two-way data binding means that the DOM element that is data bound participates in the HTTP POST request when data is sent to an API. Two-way data binding is done by using the @bind or @bind-Value attributes in an element. You may bind to a property or to an event.

We'll revisit data binding in the Forms tutorial step in Blazor.

Passing Parameters to Components

Any time you have a hierarchy or nesting of components (virtually every UI framework), you'll need to share data from the between them. In Blazor, this is the job of parameters. When using a component, parameters behave exactly the same as HTML attributes. Therefore, a call to a component that displays a message might look like the following line of code:

<Message MessageText=”An error has occurred.”/>

MessageText is the parameter, so at runtime "An error has occurred" is passed to the Message component for display.

The following code from the Message component (in Message.razor) renders the message. Inside the div elements are references to the @MessageText parameter. Notice in the @code section of the component that the MessageText property is both public and decorated with the Parameter attribute, so it matches the MessageText attribute in the call.

<div class="row">
 <div class="col">
     <div class="alert">@MessageText</div>
 </div>
</div>

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

Referencing Component Data

Sometimes, rather than using declarative HTML you need to programmatically access the component in the @code block to call one of the component's method. You can do this by applying the @ref attribute to the component's declaration, then using it in the code.

Take a look at the Logger component declaration and the call to its log method below. This code is found in a calling page, such as Index.razor:


<button @onclick="@(() => logger?.Log(5))">Log</button>

<Logger @ref="logger" />

@code {
private Logger? logger;
}

Inside the component, Logger.razor:

@using Microsoft.Extensions.Logging
@inject ILogger<Logger> logger

@code {
    private string logValue;
    public void Log(int value)
    {
        logger.LogInformation("{Value}", value);
    }
}

Conclusion

It's no secret in software development that components enable modular development and promote code reuse since each component is a discrete unit of code and markup. In Blazor, components are controlled through lifecycle events, where you can retrieve data and render displays.

See Also

Razor Pages
Routing