Blazor Components: A Deep Dive

Last updated: October 26, 2023 | By: Microsoft Docs Team

Welcome to the deep dive into Blazor components! In this tutorial, we'll explore the intricacies of building, managing, and optimizing reusable UI elements in your Blazor applications.

Understanding Component Architecture

Blazor components are the building blocks of your user interface. They are typically Razor files (.razor) that combine HTML markup with C# code. This hybrid approach allows for dynamic and interactive web UIs built entirely with C#.

Component Anatomy

A typical Blazor component consists of:

• Markup: HTML structure defines the visual layout.
• Code: C# code handles logic, state, and event handling.
• Parameters: Properties that allow parent components to pass data down to child components.
• Event Callbacks: Mechanisms for child components to communicate events back to their parent components.

Component Hierarchy

Components can be nested, forming a tree-like structure. This hierarchy is fundamental to how data flows and events propagate throughout the application.

Advanced Component Features

Generics and Component Type Safety

Blazor supports generic components, enabling you to create highly reusable components that can operate on different data types. This enhances type safety and reduces boilerplate code.

@typeparam TItem

<ul>
    @foreach (var item in Items)
    {
        <li>@ItemTemplate(item)</li>
    }
</ul>

@code {
    [Parameter]
    public List<TItem> Items { get; set; }

    [Parameter]
    public RenderFragment<TItem> ItemTemplate { get; set; }
}
            

RenderFragments

RenderFragment is a powerful type that represents a piece of UI that can be rendered. It's often used to pass HTML content or other components as parameters.

Cascading Values and Parameters

CascadingValue and CascadingParameter enable you to pass values down the component tree without explicitly passing them through every intermediate component. This is invaluable for themes, authentication status, or configuration.

Component Lifecycle Management

Understanding the component lifecycle is crucial for efficient and predictable Blazor applications. Key methods include:

• OnInitialized(): Called when the component is initialized.
• OnParametersSet(): Called after OnInitialized() and whenever parameters are set by a parent.
• ShouldRender(): Allows you to optimize rendering by controlling whether the component needs to re-render.
• OnAfterRender(): Called after the component has been rendered to the DOM.

Optimizing Render Performance

Use ShouldRender() judiciously. Avoid unnecessary re-renders by comparing current parameter values with previous ones.

Event Handling and Data Flow

Passing Data Down: Parameters

As mentioned, parameters are the primary way to pass data from parent to child components.

// Parent Component
<MyChildComponent Message="Hello from Parent!" />

// Child Component (MyChildComponent.razor)
@code {
    [Parameter]
    public string Message { get; set; }
}
<p>@Message</p>
            

Propagating Events Up: Event Callbacks

EventCallback allows child components to notify their parents about events.

// Parent Component
<MyChildComponent OnAction="@HandleChildAction" />

@code {
    private void HandleChildAction()
    {
        // Do something when the child triggers the action
    }
}

// Child Component (MyChildComponent.razor)
@code {
    [Parameter]
    public EventCallback OnAction { get; set; }

    private void TriggerAction()
    {
        OnAction.InvokeAsync();
    }
}
<button @onclick="TriggerAction">Perform Action</button>
            

Component Communication Patterns

Besides parameters and event callbacks, Blazor offers other patterns for component communication:

// Example: EventAggregatorService
public class EventAggregatorService
{
    public event Action<string> MessageReceived;

    public void PublishMessage(string message)
    {
        MessageReceived?.Invoke(message);
    }
}

// Component subscribing to messages
@inject EventAggregatorService EventAggregator
@implements IDisposable

protected override void OnInitialized()
{
    EventAggregator.MessageReceived += HandleMessage;
}

private void HandleMessage(string message)
{
    // Update UI with message
    StateHasChanged();
}

public void Dispose()
{
    EventAggregator.MessageReceived -= HandleMessage;
}
                

@inject NavigationManager NavigationManager

void NavigateToAbout()
{
    NavigationManager.NavigateTo("/about");
}
                

Best Practices for Component Development

• Keep Components Small and Focused: Each component should ideally do one thing well.
• Favor Composition over Inheritance: Use composition to build complex UIs from simpler components.
• Use Meaningful Names: For components, parameters, and methods.
• Document Your Components: Especially for reusable libraries.
• Test Your Components: Write unit and integration tests.

Conclusion

Mastering Blazor components is key to building scalable, maintainable, and performant web applications. By understanding their lifecycle, communication patterns, and advanced features, you can create sophisticated UIs with confidence.

Continue your learning by exploring Routing and Navigation.