28 KiB

Raw Blame History

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
Create and use Razor Components	guardrex	Learn how to create and use Razor Components, including how to bind to data, handle events, and manage component life cycles.	>= aspnetcore-3.0	riande	mvc	02/13/2019	razor-components/components

Create and use Razor Components

By Luke Latham, Daniel Roth, and Morné Zaayman

View or download sample code (how to download). See the Get started topic for prerequisites.

Razor Components apps are built using components. A component is a self-contained chunk of user interface (UI), such as a page, dialog, or form. A component includes HTML markup and the processing logic required to inject data or respond to UI events. Components are flexible and lightweight. They can be nested, reused, and shared among projects.

Component classes

Components are typically implemented in .cshtml files using a combination of C# and HTML markup. The UI for a component is defined using HTML. Dynamic rendering logic (for example, loops, conditionals, expressions) is added using an embedded C# syntax called Razor. When a Razor Components app is compiled, the HTML markup and C# rendering logic are converted into a component class. The name of the generated class matches the name of the file.

Members of the component class are defined in a @functions block (more than one @functions block is permissible). In the @functions block, component state (properties, fields) is specified along with methods for event handling or for defining other component logic.

Component members can then be used as part of the component's rendering logic using C# expressions that start with @. For example, a C# field is rendered by prefixing @ to the field name. The following example evaluates and renders:

_headingFontStyle to the CSS property value for font-style.
_headingText to the content of the <h1> element.

<h1 style="font-style:@_headingFontStyle">@_headingText</h1>

@functions {
    private string _headingFontStyle = "italic";
    private string _headingText = "Put on your new Blazor!";
}

After the component is initially rendered, the component regenerates its render tree in response to events. Razor Components then compares the new render tree against the previous one and applies any modifications to the browser's Document Object Model (DOM).

Using components

Components can include other components by declaring them using HTML element syntax. The markup for using a component looks like an HTML tag where the name of the tag is the component type.

The following markup renders a HeadingComponent (HeadingComponent.cshtml) instance:

[!code-cshtml]

Component parameters

Components can have component parameters, which are defined using non-public properties on the component class decorated with [Parameter]. Use attributes to specify arguments for a component in markup.

In the following example, the ParentComponent sets the value of the Title property of the ChildComponent:

ParentComponent.cshtml:

[!code-cshtml]

ChildComponent.cshtml:

[!code-cshtml]

Child content

Components can set the content of another component. The assigning component provides the content between the tags that specify the receiving component. For example, a ParentComponent can provide content for rendering by a Child component by placing the content inside <ChildComponent> tags.

ParentComponent.cshtml:

[!code-cshtml]

The Child component has a ChildContent property that represents a RenderFragment. The value of ChildContent is positioned in the child component's markup where the content should be rendered. In the following example, the value of ChildContent is received from the parent component and rendered inside the Bootstrap panel's panel-body.

ChildComponent.cshtml:

[!code-cshtml]

[!NOTE] The property receiving the RenderFragment content must be named ChildContent by convention.

Data binding

Data binding to both components and DOM elements is accomplished with the bind attribute. The following example binds the ItalicsCheck property to the check box's checked state:

<input type="checkbox" class="form-check-input" id="italicsCheck" 
    bind="@_italicsCheck" />

When the check box is selected and cleared, the property's value is updated to true and false, respectively.

The check box is updated in the UI only when the component is rendered, not in response to changing the property's value. Since components render themselves after event handler code executes, property updates are usually reflected in the UI immediately.

Using bind with a CurrentValue property (<input bind="@CurrentValue" />) is essentially equivalent to the following:

<input value="@CurrentValue" 
    onchange="@((UIChangeEventArgs __e) => CurrentValue = __e.Value)" />

When the component is rendered, the value of the input element comes from the CurrentValue property. When the user types in the text box, the onchange event is fired and the CurrentValue property is set to the changed value. In reality, the code generation is a little more complex because bind handles a few cases where type conversions are performed. In principle, bind associates the current value of an expression with a value attribute and handles changes using the registered handler.

Format strings

Data binding works with xref:System.DateTime format strings. Other format expressions, such as currency or number formats, aren't available at this time.

<input bind="@StartDate" format-value="yyyy-MM-dd" />

@functions {
    [Parameter]
    private DateTime StartDate { get; set; } = new DateTime(2020, 1, 1);
}

The format-value attribute specifies the date format to apply to the value of the input element. The format is also used to parse the value when an onchange event occurs.

Component parameters

Binding also recognizes component parameters, where bind-{property} can bind a property value across components.

The following component uses ChildComponent and binds the ParentYear parameter from the parent to the Year parameter on the child component:

Parent component:

@page "/ParentComponent"

<h1>Parent Component</h1>

<p>ParentYear: @ParentYear</p>

<ChildComponent bind-Year="@ParentYear" />

<button class="btn btn-primary" onclick="@ChangeTheYear">
    Change Year to 1986
</button>

@functions {
    [Parameter]
    private int ParentYear { get; set; } = 1978;

    void ChangeTheYear()
    {
        ParentYear = 1986;
    }
}

Child component:

<h2>Child Component</h2>

<p>Year: @Year</p>

@functions {
    [Parameter]
    private int Year { get; set; }

    [Parameter]
    private Action<int> YearChanged { get; set; }
}

The Year parameter is bindable because it has a companion YearChanged event that matches the type of the Year parameter.

Loading the ParentComponent produces the following markup:

<h1>Parent Component</h1>

<p>ParentYear: 1978</p>

<h2>Child Component</h2>

<p>Year: 1978</p>

If the value of the ParentYear property is changed by selecting the button in the ParentComponent, the Year property of the ChildComponent is updated. The new value of Year is rendered in the UI when the ParentComponent is rerendered:

<h1>Parent Component</h1>

<p>ParentYear: 1986</p>

<h2>Child Component</h2>

<p>Year: 1986</p>

Event handling

Razor Components provide event handling features. For an HTML element attribute named on<event> (for example, onclick, onsubmit) with a delegate-typed value, Razor Components treats the attribute's value as an event handler. The attribute's name always starts with on.

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>

@functions {
    void UpdateHeading(UIMouseEventArgs e)
    {
        ...
    }
}

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

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

@functions {
    void CheckboxChanged()
    {
        ...
    }
}

Event handlers can also be asynchronous and return a xref:System.Threading.Tasks.Task. There's no need to manually call StateHasChanged(). Exceptions are logged when they occur.

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

@functions {
    async Task UpdateHeading(UIMouseEventArgs e)
    {
        ...
    }
}

For some events, event-specific event argument types are permitted. If access to one of these event types isn't necessary, it isn't required in the method call.

The list of supported event arguments is:

UIEventArgs
UIChangeEventArgs
UIKeyboardEventArgs
UIMouseEventArgs

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 (UIMouseEventArgs) 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>
}

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

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

Capture references to components

Component references provide a way get a reference to a component instance so that you can issue commands to that instance, such as Show or Reset. To capture a component reference, add a ref attribute to the child component and then define a field with the same name and the same type as the child component.

<MyLoginDialog ref="loginDialog" ... />

@functions {
    MyLoginDialog loginDialog;

    void OnSomething()
    {
        loginDialog.Show();
    }
}

When the component is rendered, the loginDialog field is populated with the MyLoginDialog child component instance. You can then invoke .NET methods on the component instance.

[!IMPORTANT] The loginDialog variable is only populated after the component is rendered and its output includes the MyLoginDialog element. Until that point, there's nothing to reference. To manipulate components references after the component has finished rendering, use the OnAfterRenderAsync or OnAfterRender methods.

While capturing component references uses a similar syntax to capturing element references, it isn't a JavaScript interop feature. Component references aren't passed to JavaScript code; they're only used in .NET code.

[!NOTE] Do not use component references to mutate the state of child components. Instead, use normal declarative parameters to pass data to child components. This causes child components to rerender at the correct times automatically.

Lifecycle methods

OnInitAsync and OnInit execute code to initialize the component. To perform an asynchronous operation, use OnInitAsync and the await keyword on the operation:

protected override async Task OnInitAsync()
{
    await ...
}

For a synchronous operation, use OnInit:

protected override void OnInit()
{
    ...
}

OnParametersSetAsync and OnParametersSet are called when a component has received parameters from its parent and the values are assigned to properties. These methods are executed after component initialization and then each time the component is rendered:

protected override async Task OnParametersSetAsync()
{
    await ...
}

protected override void OnParametersSet()
{
    ...
}

OnAfterRenderAsync and OnAfterRender are called after a component has finished rendering. Element and component references are populated at this point. Use this stage to perform additional initialization steps using the rendered content, such as activating third-party JavaScript libraries that operate on the rendered DOM elements.

protected override async Task OnAfterRenderAsync()
{
    await ...
}

protected override void OnAfterRender()
{
    ...
}

SetParameters can be overridden to execute code before parameters are set:

public override void SetParameters(ParameterCollection parameters)
{
    ...

    base.SetParameters(parameters);
}

If base.SetParameters isn't invoked, the custom code can interpret the incoming parameters value in any way required. For example, the incoming parameters aren't required to be assigned to the properties on the class.

ShouldRender can be overridden to suppress refreshing of the UI. If the implementation returns true, the UI is refreshed. Even if ShouldRender is overridden, the component is always initially rendered.

protected override bool ShouldRender()
{
    var renderUI = true;

    return renderUI;
}

Component disposal with IDisposable

If a component implements xref:System.IDisposable, the Dispose method is called when the component is removed from the UI. The following component uses @implements IDisposable and the Dispose method:

@using System
@implements IDisposable

...

@functions {
    public void Dispose()
    {
        ...
    }
}

Routing

Routing in Razor Components is achieved by providing a route template to each accessible component in the app.

When a .cshtml file with an @page directive is compiled, the generated class is given a xref:Microsoft.AspNetCore.Mvc.RouteAttribute specifying the route template. At runtime, the router looks for component classes with a RouteAttribute and renders whichever component has a route template that matches the requested URL.

Multiple route templates can be applied to a component. The following component responds to requests for /BlazorRoute and /DifferentBlazorRoute:

[!code-cshtml]

Route parameters

Components can receive route parameters from the route template provided in the @page directive. The router uses route parameters to populate the corresponding component parameters.

RouteParameter.cshtml:

[!code-cshtml]

Optional parameters aren't supported, so two @page directives are applied in the example above. The first permits navigation to the component without a parameter. The second @page directive takes the {text} route parameter and assigns the value to the Text property.

Base class inheritance for a "code-behind" experience

Component files (.cshtml) mix HTML markup and C# processing code in the same file. The @inherits directive can be used to provide Razor Components apps with a "code-behind" experience that separates component markup from processing code.

The sample app shows how a component can inherit a base class, BlazorRocksBase, to provide the component's properties and methods.

BlazorRocks.cshtml:

[!code-cshtml]

BlazorRocksBase.cs:

[!code-csharp]

The base class should derive from BlazorComponent.

Razor support

Razor directives

Razor directives are shown in the following table.

Directive	Description
@functions	Adds a C# code block to a component.
`@implements`	Implements an interface for the generated component class.
@inherits	Provides full control of the class that the component inherits.
@inject	Enables service injection from the service container. For more information, see Dependency injection into views.
`@layout`	Specifies a layout component. Layout components are used to avoid code duplication and inconsistency.
@page	Specifies that the component should handle requests directly. The `@page` directive can be specified with a route and optional parameters. Unlike Razor Pages, the `@page` directive doesn't need to be the first directive at the top of the file. For more information, see Routing.
@using	Adds the C# `using` directive to the generated component class.
@addTagHelper	Use `@addTagHelper` to use a component in a different assembly than the app's assembly.

Conditional attributes

Attributes are conditionally rendered based on the .NET value. If the value is false or null, the attribute isn't rendered. If the value is true, the attribute is rendered minimized.

In the following example, IsCompleted determines if checked is rendered in the control's markup:

<input type="checkbox" checked="@IsCompleted" />

@functions {
    [Parameter]
    private bool IsCompleted { get; set; }
}

If IsCompleted is true, the check box is rendered as:

<input type="checkbox" checked />

If IsCompleted is false, the check box is rendered as:

<input type="checkbox" />

Additional information on Razor

For more information on Razor, see the Razor syntax reference.

Raw HTML

Strings are normally rendered using DOM text nodes, which means that any markup they may contain is ignored and treated as literal text. To render raw HTML, wrap the HTML content in a MarkupString value. The value is parsed as HTML or SVG and inserted into the DOM.

[!WARNING] Rendering raw HTML constructed from any untrusted source is a security risk and should be avoided!

The following example shows using the MarkupString type to add a block of static HTML content to the rendered output of a component:

@((MarkupString)myMarkup)

@functions {
    string myMarkup = "<p class='markup'>This is a <em>markup string</em>.</p>";
}

Templated components

Templated components are components that accept one or more UI templates as parameters, which can then be used as part of the component's rendering logic. Templated components allow you to author higher-level components that are more reusable than regular components. A couple of examples include:

A table component that allows a user to specify templates for the table's header, rows, and footer.
A list component that allows a user to specify a template for rendering items in a list.

Template parameters

A templated component is defined by specifying one or more component parameters of type RenderFragment or RenderFragment<T>. A render fragment represents a segment of UI that is rendered by the component. A render fragment optionally takes a parameter that can be specified when the render fragment is invoked.

Components/TableTemplate.cshtml:

[!code-cshtml]

When using a templated component, the template parameters can be specified using child elements that match the names of the parameters (TableHeader and RowTemplate in the following example):

<TableTemplate Items="@pets">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate>
        <td>@context.PetId</td>
        <td>@context.Name</td>
    </RowTemplate>
</TableTemplate>

Template context parameters

Component arguments of type RenderFragment<T> passed as elements have an implicit parameter named context (for example from the preceding code sample, @context.PetId), but you can change the parameter name using the Context attribute on the child element. In the following example, the RowTemplate element's Context attribute specifies the pet parameter:

<TableTemplate Items="@pets">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate Context="pet">
        <td>@pet.PetId</td>
        <td>@pet.Name</td>
    </RowTemplate>
</TableTemplate>

Alternatively, you can specify the Context attribute on the component element. The specified Context attribute applies to all specified template parameters. This can be useful when you want to specify the content parameter name for implicit child content (without any wrapping child element). In the following example, the Context attribute appears on the TableTemplate element and applies to all template parameters:

<TableTemplate Items="@pets" Context="pet">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate>
        <td>@pet.PetId</td>
        <td>@pet.Name</td>
    </RowTemplate>
</TableTemplate>

Generic-typed components

Templated components are often generically typed. For example, a generic List View Template component can be used to render IEnumerable<T> values. To define a generic component, use the @typeparam directive to specify type parameters.

Components/ListViewTemplate.cshtml:

[!code-cshtml]

When using generic-typed components, the type parameter is inferred if possible:

<ListViewTemplate Items="@pets">
    <ItemTemplate Context="pet">
        <li>@pet.Name</li>
    </ItemTemplate>
</ListViewTemplate>

Otherwise, the type parameter must be explicitly specified using an attribute that matches the name of the type parameter. In the following example, TItem="Pet" specifies the type:

<ListViewTemplate Items="@pets" TItem="Pet">
    <ItemTemplate Context="pet">
        <li>@pet.Name</li>
    </ItemTemplate>
</ListViewTemplate>

Cascading values and parameters

In some scenarios, it's inconvenient to flow data from an ancestor component to a descendent component using component parameters, especially when there are several component layers. Cascading values and parameters solve this problem by providing a convenient way for an ancestor component to provide a value to all of its descendent components. Cascading values and parameters also provide an approach for components to coordinate.

Theme example

In the following Theme example from the sample app, the ThemeInfo class specifies the theme information to flow down the component hierarchy so that all of the buttons within a given part of the app share the same style.

UIThemeClasses/ThemeInfo.cs:

public class ThemeInfo
{
    public string ButtonClass { get; set; }
}

An ancestor component can provide a cascading value using the Cascading Value component. The Cascading Value component wraps a subtree of the component hierarchy and supplies a single value to all components within that subtree.

For example, the sample app specifies theme information (ThemeInfo) in one of the app's layouts as a cascading parameter for all components that make up the layout body of the @Body property. ButtonClass is assigned a value of btn-success in the layout component. Any descendent component can consume this property through the ThemeInfo cascading object.

Shared/CascadingValuesParametersLayout.cshtml:

@inherits BlazorLayoutComponent
@using BlazorSample.UIThemeClasses

<div class="container-fluid">
    <div class="row">
        <div class="col-sm-3">
            <NavMenu />
        </div>
        <div class="col-sm-9">
            <CascadingValue Value="@theme">
                <div class="content px-4">
                    @Body
                </div>
            </CascadingValue>
        </div>
    </div>
</div>

@functions {
    ThemeInfo theme = new ThemeInfo { ButtonClass = "btn-success" };
}

To make use of cascading values, components declare cascading parameters using the [CascadingParameter] attribute or based on a string name value:

<CascadingValue Value=@PermInfo Name="UserPermissions">...</CascadingValue>

[CascadingParameter(Name = "UserPermissions")] PermInfo Permissions { get; set; }

Binding with a string name value is relevant if you have multiple cascading values of the same type and need to differentiate them within the same subtree.

Cascading values are bound to cascading parameters by type.

In the sample app, the Cascading Values Parameters Theme component binds to the ThemeInfo cascading value to a cascading parameter. The parameter is used to set the CSS class for one of the buttons displayed by the component.

Pages/CascadingValuesParametersTheme.cshtml:

@page "/cascadingvaluesparameterstheme"
@layout CascadingValuesParametersLayout
@using BlazorSample.UIThemeClasses

<h1>Cascading Values & Parameters</h1>

<p>Current count: @currentCount</p>

<p>
    <button class="btn" onclick="@IncrementCount">
        Increment Counter (Unthemed)
    </button>
</p>

<p>
    <button class="btn @ThemeInfo.ButtonClass" onclick="@IncrementCount">
        Increment Counter (Themed)
    </button>
</p>

@functions {
    int currentCount = 0;

    [CascadingParameter] protected ThemeInfo ThemeInfo { get; set; }

    void IncrementCount()
    {
        currentCount++;
    }
}

TabSet example

Cascading parameters also enable components to collaborate across the component hierarchy. For example, consider the following TabSet example in the sample app.

The sample app has an ITab interface that tabs implement:

[!code-cs]

The Cascading Values Parameters TabSet component uses the Tab Set component, which contains several Tab components:

[!code-cshtml]

The child Tab components aren't explicitly passed as parameters to the Tab Set. Instead, the child Tab components are part of the child content of the Tab Set. However, the Tab Set still needs to know about each Tab component so that it can render the headers and the active tab. To enable this coordination without requiring additional code, the Tab Set component can provide itself as a cascading value that is then picked up by the descendent Tab components.

Components/TabSet.cshtml:

[!code-cshtml]

The descendent Tab components capture the containing Tab Set as a cascading parameter, so the Tab components add themselves to the Tab Set and coordinate on which tab is active.

Components/Tab.cshtml:

[!code-cshtml]

Razor templates

Render fragments can be defined using Razor template syntax. Razor templates are a way to define a UI snippet and assume the following format:

@<tag>...</tag>

The following example illustrates how to specify RenderFragment and RenderFragment<T> values.

RazorTemplates.cshtml:

@{
    RenderFragment template = @<p>The time is @DateTime.Now.</p>;
    RenderFragment<Pet> petTemplate = (pet) => @<p>Your pet's name is @pet.Name.</p>;
}

Render fragments defined using Razor templates can be passed as arguments to templated components or rendered directly. For example, the previous templates are directly rendered with the following Razor markup:

@template

@petTemplate(new Pet { Name = "Rex" })

Rendered output:

The time is 10/04/2018 01:26:52.

Your pet's name is Rex.

28 KiB Raw Blame History

Create and use Razor Components

Component classes

Using components

Component parameters

Child content

Data binding

Event handling

Capture references to components

Lifecycle methods

Component disposal with IDisposable

Routing

Route parameters

Base class inheritance for a "code-behind" experience

Razor support

Raw HTML

Templated components

Template parameters

Template context parameters

Generic-typed components

Cascading values and parameters

Theme example

TabSet example

Razor templates

28 KiB

Raw Blame History