# Blazor
Blazor apps are based on *components*. A **component** in Blazor is an element of UI, such as a page, dialog, or data entry form.
Components are .NET C# classes built into .NET assemblies that:
- Define flexible UI rendering logic.
- Handle user events.
- Can be nested and reused.
- Can be shared and distributed as Razor class libraries or NuGet packages.
The component class is usually written in the form of a Razor markup page with a `.razor` file extension. Components in Blazor are formally referred to as *Razor components*.
## Project Structure & Important Files
### Blazor Server Project Structure
```txt
Project
|-Properties
| |- launchSettings.json
|
|-wwwroot --> static files
| |-css
| | |- site.css
| | |- bootstrap
| |
| |- favicon.ico
|
|-Pages
| |- _Host.cshtml --> fallback page
| |- Component.razor
| |- Index.razor
| |- ...
|
|-Shared
| |- MainLayout.razor
| |- MainLayout.razor.css
| |- ...
|
|- _Imports.razor --> @using imports
|- App.razor --> component root of the app
|
|- appsettings.json --> application settings
|- Program.cs --> App entry-point
|- Startup.cs --> services and middleware configs
```
### Blazor WASM Project Structure
```txt
Project
|-Properties
| |- launchSettings.json
|
|-wwwroot --> static files
| |-css
| | |- site.css
| | |- bootstrap
| |
| |- index.html
| |- favicon.ico
|
|-Pages
| |- Component.razor
| |- Index.razor
| |- ...
|
|-Shared
| |- MainLayout.razor
| |- MainLayout.razor.css
| |- ...
|
|- _Imports.razor --> @using imports
|- App.razor --> component root of the app
|
|- appsettings.json --> application settings
|- Program.cs --> App entry-point
```
### Blazor PWA Project Structure
```txt
Project
|-Properties
| |- launchSettings.json
|
|-wwwroot --> static files
| |-css
| | |- site.css
| | |- bootstrap
| |
| |- index.html
| |- favicon.ico
| |- manifest.json
| |- service-worker.js
| |- icon-512.png
|
|-Pages
| |- Component.razor
| |- Index.razor
| |- ...
|
|-Shared
| |- MainLayout.razor
| |- MainLayout.razor.css
| |- ...
|
|- _Imports.razor --> @using imports
|- App.razor --> component root of the app
|
|- appsettings.json --> application settings
|- Program.cs --> App entrypoint
```
### `manifest.json`, `service-worker.js` (Blazor PWA)
[PWA](https://web.dev/progressive-web-apps/)
[PWA MDN Docs](https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps)
[PWA Manifest](https://developer.mozilla.org/en-US/docs/Web/Manifest)
[Service Worker API](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API)
```json
// manifest.json
{
"name": "<App Name>",
"short_name": "<Short App Name>",
"start_url": "./",
"display": "standalone",
"background_color": "#ffffff",
"theme_color": "#03173d",
"icons": [
{
"src": "icon-512.png",
"type": "image/png",
"sizes": "512x512"
}
]
}
```
## Common Blazor Files
### `App.razor`
```cs
<Router AppAssembly="@typeof(Program).Assembly" PreferExactMatches="@true">
<Found Context="routeData">
<RouteView RouteData="@routeData" DefaultLayout="@typeof(MainLayout)" />
</Found>
<NotFound>
<LayoutView Layout="@typeof(MainLayout)">
<p>Sorry, there's nothing at this address.</p>
</LayoutView>
</NotFound>
</Router>
```
### `MainLayout.razor` (Blazor Server/WASM)
```cs
@inherits LayoutComponentBase
<div class="page">
<div class="sidebar">
<NavMenu /> // NavMenu Component
</div>
<div class="main">
<div class="top-row px-4">
</div>
<div class="content px-4">
@Body
</div>
</div>
</div>
```
### `_Host.cshtml` (Blazor Server)
```html
@page "/"
@namespace BlazorServerDemo.Pages
@addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers
@{
Layout = null;
}
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>BlazorServerDemo</title>
<base href="~/" />
<link rel="stylesheet" href="css/bootstrap/bootstrap.min.css" />
<link href="css/site.css" rel="stylesheet" />
<link href="BlazorServerDemo.styles.css" rel="stylesheet" />
</head>
<body>
<component type="typeof(App)" render-mode="ServerPrerendered" />
<div id="blazor-error-ui">
<environment include="Staging,Production">
An error has occurred. This application may no longer respond until reloaded.
</environment>
<environment include="Development">
An unhandled exception has occurred. See browser dev tools for details.
</environment>
<a href="" class="reload">Reload</a>
<a class="dismiss">🗙</a>
</div>
<script src="_framework/blazor.server.js"></script>
</body>
</html>
```
## Components (`.razor`)
[Blazor Components](https://docs.microsoft.com/en-us/aspnet/core/blazor/components/)
```cs
@page "/route/{RouteParameter}" // make component accessible from a URL
@page "/route/{RouteParameter?}" // specify route parameter as optional
@page "/route/{RouteParameter:<type>}" // specify route parameter type
@namespace <Namespace> // set the component namespace
@using <Namespace> // using statement
@inherits BaseType // inheritance
@attribute [Attribute] // apply an attribute
@inject Type objectName // dependency injection
// html of the page here
<Namespace.ComponentFolder.Component /> // access component w/o @using
<Component Property="value"/> // insert component into page, passing attributes
<Component @onclick="@CallbackMethod">
@ChildContent // segment of UI content
</Component>
@code {
// component model (Properties, Methods, ...)
[Parameter] // capture attribute
public Type Property { get; set; } = defaultValue;
[Parameter] // capture route parameters
public type RouteParameter { get; set;}
[Parameter] // segment of UI content
public RenderFragment ChildContent { get; set;}
private void CallbackMethod() { }
}
```
## State Management
### Blazor WASM
```cs
// setup state singleton
builder.Services.AddSingleton<StateContainer>();
```
```cs
// StateContainer singleton
using System;
public class StateContainer
{
private int _counter;
public string Property
{
get => _counter;
set
{
_counter = value;
NotifyStateChanged(); // will trigger StateHasChanged(), causing a render
}
}
public event Action OnChange;
private void NotifyStateChanged() => OnChange?.Invoke();
}
```
```cs
// component that changes the state
@inject StateContainer State
// Delegate event handlers automatically trigger a UI render
<button @onClick="@HandleClick">
Change State
</button>
@code {
private void HandleClick()
{
State.Property += 1; // update state
}
}
```
```cs
// component that should be update on state change
@implements IDisposable
@inject StateContainer State
<p>Property: <b>@State.Property</b></p>
@code {
// StateHasChanged notifies the component that its state has changed.
// When applicable, calling StateHasChanged causes the component to be rerendered.
protected override void OnInitialized()
{
State.OnChange += StateHasChanged;
}
public void Dispose()
{
State.OnChange -= StateHasChanged;
}
}
```
## Data Binding & Events
```cs
<p>
<button @on{DOM EVENT}="{DELEGATE}" />
<button @on{DOM EVENT}="{DELEGATE}" @on{DOM EVENT}:preventDefault /> // prevent default action
<button @on{DOM EVENT}="{DELEGATE}" @on{DOM EVENT}:preventDefault="{CONDITION}" /> // prevent default action if CONDITION is true
<button @on{DOM EVENT}="{DELEGATE}" @on{DOM EVENT}:stopPropagation />
<button @on{DOM EVENT}="{DELEGATE}" @on{DOM EVENT}:stopPropagation="{CONDITION}" /> // stop event propagation if CONDITION is true
<button @on{DOM EVENT}="@(e => Property = value)" /> // change internal state w/ lambda
<button @on{DOM EVENT}="@(e => DelegateAsync(e, value))" /> // invoke delegate w/ lambda
<input @ref="elementReference" />
<input @bind="{PROPERTY}" /> // updates variable on ONCHANGE event (focus loss)
<input @bind="{PROPERTY}" @bind:event="{DOM EVENT}" /> // updates value on DOM EVENT
<input @bind="{PROPERTY}" @bind:format="{FORMAT STRING}" /> // use FORMAT STRING to display value
<ChildComponent @bind-{PROPERTY}="{PROPERTY}" @bind-{PROPERTY}:event="{EVENT}" /> // bind to child component {PROPERTY}
<ChildComponent @bind-{PROPERTY}="{PROPERTY}" @bind-{PROPERTY}:event="{PROPERTY}Changed" /> // bind to child component {PROPERTY}, listen for custom event
</p>
@code {
private ElementReference elementReference;
public string Property { get; set; }
public EventCallback<Type> PropertyChanged { get; set; } // custom event {PROPERTY}Changed
// invoke custom event
public async Task DelegateAsync(EventArgs e, Type argument)
{
/* ... */
await PropertyChanged.InvokeAsync(e, argument); // notify parent bound prop has changed
await elementReference.FocusAsync(); // focus an element in code
}
}
```
> **Note**: When a user provides an unparsable value to a data-bound element, the unparsable value is automatically reverted to its previous value when the bind event is triggered.
## Javascript/.NET Interop
[Call Javascript from .NET](https://docs.microsoft.com/en-us/aspnet/core/blazor/call-javascript-from-dotnet)
[Call .NET from Javascript](https://docs.microsoft.com/en-us/aspnet/core/blazor/call-dotnet-from-javascript)
### Render Blazor components from JavaScript [C# 10]
To render a Blazor component from JavaScript, first register it as a root component for JavaScript rendering and assign it an identifier:
```cs
// Blazor Server
builder.Services.AddServerSideBlazor(options =>
{
options.RootComponents.RegisterForJavaScript<Counter>(identifier: "counter");
});
// Blazor WebAssembly
builder.RootComponents.RegisterForJavaScript<Counter>(identifier: "counter");
```
Load Blazor into the JavaScript app (`blazor.server.js` or `blazor.webassembly.js`) and then render the component from JavaScript into a container element using the registered identifier, passing component parameters as needed:
```js
let containerElement = document.getElementById('my-counter');
await Blazor.rootComponents.add(containerElement, 'counter', { incrementAmount: 10 });
```
### Blazor custom elements [C# 10]
Experimental support is also now available for building custom elements with Blazor using the Microsoft.AspNetCore.Components.CustomElements NuGet package.
Custom elements use standard HTML interfaces to implement custom HTML elements.
To create a custom element using Blazor, register a Blazor root component as custom elements like this:
```cs
options.RootComponents.RegisterAsCustomElement<Counter>("my-counter");
```