Embedding Existing JavaScript Components Into Uno-WASM - Part 1

Leveraging the Uno.WASM Bootstrapper

At the heart of Uno-WASM, there's a package called Uno.Wasm.Bootstrap (Uno Bootstrapper) project. It contains the tooling required to build, package, deploy, run, and debug a .NET project in a web browser using WebAssembly. It's automatically included in the WASM head of an Uno app.

It is possible to add JavaScript files or CSS files into any Uno-WASM applications: they will be loaded automatically by the runtime. It is also possible to add any kind of downloadable assets as content.

HTML5 is a Rich and Powerful Platform

Uno fully embraces HTML5 as its display backend when targeting WebAssembly (WASM). As a result, it is possible to integrate with almost any existing JavaScript library to extend the behavior of an app.

Embedding assets

In the HTML world, everything running in the browser is assets that must be downloaded from a server. To integrate existing JavaScript frameworks, they can be either download those from another location on the Internet (usually from a CDN service) or embed and deployed with the app.

The Uno Bootstrapper can automatically embed any asset and deploy them with the app. Some of them (CSS & JavaScript) can also be loaded with the app. Here's how to declare them in a Uno Wasm project:

  1. JavaScript files should be in the WasmScripts folder: they will be copied to the output folder and loaded automatically by the bootstrapper when the page loads. They must be marked with the EmbeddedResources build action:

    <!-- .csproj file -->
      <EmbeddedResource Include="WasmScripts\javascriptfile.js" />
      <EmbeddedResource Include="WasmScripts\**\*.js" /> <!-- globing works too -->
  2. CSS Style files should be in the WasmCSS folder: they will be copied to the output folder and referenced in the HTML head of the application. They must be marked with the EmbeddedResources build action.

    <!-- .csproj file -->
      <EmbeddedResource Include="WasmCSS\stylefile.css" />
      <EmbeddedResource Include="WasmCSS\**\*.css" /> <!-- globing works too -->
  3. Asset files should be marked with the Content build action in the app. The file will be copied to the output folder and will preserve the same relative path.

    <!-- .csproj file -->
      <Content Include="Assets\image.png" />
  4. Alternatively, any kind of asset file can be placed directly in the wwwroot folder as with any standard ASP.NET Core project. They will be deployed with the app, but the application code is responsible for fetching and using them.

    Is it an ASP.NET Core "web" project? No, but it shares a common structure. Some of the deployment features, like the wwwroot folder, and the Visual Studio integration for running/debugging are reused in a similar way to an ASP.NET Core project. The C# code put in the project will run in the browser, using the .NET runtime. There is no need for a server side component in Uno-Wasm projects.

Uno-Wasm Controls Are Actual HTML5 Elements

The philosophy of Uno is to rely on native platforms where it makes sense. In the context of a browser, that's the HTML5 DOM. This means that each time is created, a class deriving from UIElement is creating a corresponding HTML element.

That also means that it is possible to control how this element is created. By default it is a <div>, but it can be changed in the constructor by providing the htmlTag parameter to the one required. For example:

public sealed partial class MyDivControl : FrameworkElement
    public MyDivControl() // will create a <div> HTML element (by default)

public sealed partial class MyInputControl : FrameworkElement
    public MyInputControl() // Will create an <input> HTML element

public sealed partial class MyInputControl : FrameworkElement
    public MyInputControl() // Will create a <span> HTML element

When using HtmlElementAttribute in a WebAssembly-only library, importing the Uno.WinUI.Runtime.WebAssembly is required. If multiple targets frameworks are needed, you'll need to create a cross-targeted library, as well as adding the Uno.WinUI.Runtime.WebAssembly library.

Once created, it is possible to interact directly with this element by calling helper methods available in Uno. Note that those methods are only available when targeting the Wasm platform. It is possible to use conditional code to use these methods in a multi-platform project.

Here is a list of helper methods used to facilitate the integration with the HTML DOM:

  • The extension method element.SetCssStyle() can be used to set a CSS Style on the HTML element. Example:

    // Setting only one CSS style
    this.SetCssStyle("text-shadow", "2px 2px red");
    // Setting many CSS styles at once using C# tuples
    this.SetCssStyle(("text-shadow", "2px 2px blue"), ("color", "var(--app-fg-color1)"));
  • The element.ClearCssStyle() extension method can be used to set CSS styles to their default values. Example:

    // Reset text-shadow style to its default value
    // Reset both text-shadow and color to their default values
    this.ClearCssStyle("text-shadow", "color");
  • The element.SetHtmlAttribute() and element.ClearHtmlAttribute() extension methods can be used to set HTML attributes on the element:

    // Set the "href" attribute of an <a> element
    this.SetHtmlAttribute("href", "#section2");
    // Set many attributes at once (less interop)
    this.SetHtmlAttribute(("target", "_blank"), ("referrerpolicy", "no-referrer"));
    // Remove attribute from DOM element
    // Get the value of an attribute of a DOM element
    var href = this.GetHtmlAttribute("href");
  • The element.SetCssClass() and element.UnsetCssClass() extension methods can be used to add or remove CSS classes to the HTML Element:

    // Add the class to element
    // Add many classes at once (less interop)
    this.SetCssClass("warning", "level2");
    // Remove class from element
    // You can also set one class from a list of possible values.
    // Like a radio-button, like non-selected values will be unset
    var allClasses = new [] { "Small", "Medium", "Large"};
    this.SetCssClass(allClasses, 2); // set to "Large"
  • The element.SetHtmlContent() extension method can be used to set arbitrary HTML content as child of the control.

    this.SetHtmlContent("<h2>Welcome to Uno Platform!</h2>");

    IMPORTANT: This method should not be used when children "managed" controls are present: doing so can result in inconsistent runtime errors because of de-synchronized visual tree.

  • Finally, it is possible to invoke an arbitrary JavaScript code by using the static method WebAssembleRuntime.InvokeJS(). The script is directly executed in the context of the browser, giving the ability to perform anything that JavaScript can do. See next section for more details.

Invoke JavaScript code From C#

Whenever there's a need to invoke a JavaScript code in the browser, the Uno.Foundation.WebAssemblyRuntime static class should be used. There is also helpers you can call as extension methods on the elements.

// Invoke javascript synchronously
WebAssemblyRuntime.InvokeJS("alert(\"It works!\");");

// Use "string" return value of .InvokeJS()
var html = WebAssemblyRuntime.InvokeJS("document.getElementById('banner').innerHTML");

// Invoke javascript asynchronously and await completition
await WebAssemblyRuntime.InvokeAsync(
    "fetch('https://api.domain.tld/documents/1301', {method: 'DELETE'});");

// Invoke javascript asynchronously and await returned string
var str = await WebAssemblyRuntime.InvokeAsync(
 "(async () => \"It works asynchronously!\")();");

// Escape javascript data to prevent javascript script injection
var escapedUserId = WebAssemblyRuntime.EscapeJS(userId);

// Call javascript in the context os a specific UIElement.
// In this case, it will be available as "element" in the execution scope.
MyControl.ExecuteJavascript("element.toggleAttribute(\"readonly\");"); // sync
await MyControl.ExecuteJavascriptAsync("element.requestFullScreen();"); // async
  • InvokeAsync should return a Promise or, be an async method.
  • Any Promise rejected result will get translated into an ApplicationException exception.
  • Remember to always use InvariantCulture when generating JavaScript for numbers. There's also a helper in Uno.Core called .ToStringInvariant(): this dependency is already present in any Uno projects in the namespace Uno.Extensions.
  • Calling the javascript document.getElementById() with the element's HtmlId will only work when the element is actually loaded in the DOM. So it's better to call the extensions <element>.ExecuteJavascript() or <element>.ExecuteJavascriptAsync(): they will work all the time.

Invoke C# code From JavaScript

There's 2 ways to callback to managed C# code from JavaScript:

  1. Use Mono to wrap a dotnet static method into a JavaScript function like this:JavaScript_:

    // Register the method wrapper (should be cached)
    const sumMethod = Module.mono_bind_static_method(
        "[Assembly.Name] Fully.Qualified.ClassType:SumMethod");
    // Call the method
    const result = sumMethod(1, 2); // should return 3


    // In assembly "Assembly.Name"
    namespace Fully.Qualified
        public static class ClassType
            public static int SumMethod(int i1, int i2) => i1 + i2;
  2. Use HTML's Event or CustomEvent and dispatch them to managed code like this:


    // Generate a custom generic event from JavaScript/Typescript
    htmlElement.dispatchEvent(new Event("simpleEvent"));


    this.RegisterHtmlEventHandler("simpleEvent", (sender, evt)=> { [...] });

    For more information, see Handling custom HTML events.


    Currently, there's no easy way to asynchronously call managed (dotnet) code from JavaScript in the current version of Uno.

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