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.NET MAUI HybridWebView experiment

This repo has the experimental .NET MAUI HybridWebView control, which enables hosting arbitrary HTML/JS/CSS content in a WebView and enables communication between the code in the WebView (JavaScript) and the code that hosts the WebView (C#/.NET). For example, if you have an existing React JS application, you could host it in a cross-platform .NET MAUI native application, and build the back-end of the application using C# and .NET.

Example usage of the control:

<hwv:HybridWebView
    HybridAssetRoot="hybrid_root"
    MainFile="hybrid_app.html"
    RawMessageReceived="OnHybridWebViewRawMessageReceived" />

And here's how .NET code can call a JavaScript method:

var sum = await myHybridWebView.InvokeJsMethodAsync<int>("JsAddNumbers", 123, 456);

And the reverse, JavaScript code calling a .NET method:

HybridWebView.SendInvokeMessageToDotNet("CallMeFromScript", ["msg from js", 987]);

With JavaScript you can also asynchronously call a .NET method and get a result:

HybridWebView.SendInvokeMessageToDotNetAsync("CallMeFromScriptAsync", ["msg from js", 987])
	.then(result => console.log("Got result from .NET: " + result));

or

const result = await HybridWebView.SendInvokeMessageToDotNetAsync("CallMeFromScriptAsync", ["msg from js", 987]);

In addition to method invocation, sending "raw" messages is also supported.

What's in this repo?

Projects in this repo:

  • HybridWebView --> a cross-platform .NET MAUI control that can load static web assets (HTML, JS, CSS, etc.) and send messages between JavaScript and .NET
  • MauiCSharpInteropWebView --> a sample app that shows the basic functionality of sending messages and calling methods between JavaScript and .NET
  • MauiReactJSHybridApp --> a sample app that incorporates a pre-existing React-based todo application into a .NET MAUI cross-platform application

Discussions/questions/comments

See the main discussion topic here: dotnet/maui#12009

Or please log an issue in this repo for any other topics.

Getting Started

To get started, you'll need a .NET MAUI 8 project, then add the HybridWebView control, and add some web content to it.

Note: If you'd like to check out an already completed sample, go to https://github.com/Eilon/SampleMauiHybridWebViewProject

  1. Ensure you have Visual Studio 2022 with the .NET MAUI workload installed
  2. Create a .NET MAUI App project that targets .NET 8 (or use an existing one)
  3. Add a reference to the EJL.MauiHybridWebView package (NuGet package):
    1. Right-click on the Dependencies node in Solution Explorer and select Manage NuGet Packages
    2. Select the Browse tab
    3. Ensure the Include prerelease checkbox is checked
    4. In the search box enter ejl.mauihybridwebview
    5. Select the matching package, and click the Install button
  4. Register and add the HybridWebView control to a page in your app:
    1. In MauiProgram.cs add the line builder.Services.AddHybridWebView(); to register the HybridWebView components
    2. Open the MainPage.xaml file
    3. Delete the <ScrollView> control and all of its contents
    4. Add a xmlns:ejl="clr-namespace:HybridWebView;assembly=HybridWebView" declaration to the top-level <ContentPage ....> tag
    5. Add the markup <ejl:HybridWebView HybridAssetRoot="hybrid_root" RawMessageReceived="OnHybridWebViewRawMessageReceived" /> inside the <ContentPage> tag
    6. Open the MainPage.xaml.cs file
    7. Delete the count field, and the OnCounterClicked method, and replace it with the following code: 
      private async void OnHybridWebViewRawMessageReceived(object sender, HybridWebView.HybridWebViewRawMessageReceivedEventArgs e)
{
    await Dispatcher.DispatchAsync(async () =>
    {
        await DisplayAlert("JavaScript message", e.Message, "OK");
    });
}
  5. Now add some web content to the app:
    1. In Solution Explorer expand the Resources / Raw folder
    2. Create a new sub-folder called hybrid_root
    3. In the hybrid_root folder add a new file called index.html and replace its contents with: 
      <!DOCTYPE html>

<html lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta charset="utf-8" />
    <title></title>
    <script src="_hwv/HybridWebView.js"></script>
    <script src="myapp.js"></script>
</head>
<body>
    <div>
        Your message: <input type="text" id="messageInput" />
    </div>
    <div>
        <button onclick="SendMessageToCSharp()">Send to C#!</button>
    </div>
</body>
</html>
    4. In the same folder add another file myapp.js with the following contents: 
      function SendMessageToCSharp() {
    var message = document.getElementById('messageInput').value;
    HybridWebView.SendRawMessageToDotNet(message);
}
  6. You can now run your .NET MAUI app with the HybridWebView control!
    1. You can run it on Windows, Android, iOS, or macOS
    2. When you launch the app, type text into the textbox and click the button to receive the message in C# code

Proxy URLs

The HybridWebView control can redirect URL requests to native code, and allow custom responses streams to be set. This allows scenarios such as dynamically generating content, loading content from compressed files like ZIP or SQLite, or loading content from the internet that doesn't support CORS.

To use this feature, handle the ProxyRequestReceived event in the HybridWebView control. When the event handler is called set the ResponseStream and optionally the ResponseContentType of the HybridWebViewProxyEventArgs object received in the OnProxyRequest method.

The HybridWebViewProxyEventArgs has the following properties:

Property Type Description
QueryParams IDictionary<string, string> The query string parameters of the request. Note that all values will be strings.
ResponseContentType string The content type of the response body. Default: "text/plain"
ResponseStream Stream The stream to use as the response body.
Url string The full URL that was requested. 
myWebView.ProxyRequestReceived += async (args) =>
{
    //Use the query string parameters to determine what to do.
    if (args.QueryParams.TryGetValue("myParameter", out var myParameter))
	{
        //Add your logic to determine what to do. 
		if (myParameter == "myValue")
		{
            //Add logic to get your content (e.g. from a database, or generate it).
            //Can be anything that can be turned into a stream.

            //Set the response stream and optionally the content type.
			args.ResponseStream = new MemoryStream(Encoding.UTF8.GetBytes("This is the file content"));
			args.ResponseContentType = "text/plain";
		}
	}
};

In your web app, you can make requests to the proxy URL by either using relative paths like /proxy?myParameter=myValue or an absolute path by appending the relative path tot he pages origin location window.location.origin + '/proxy?myParameter=myValue'. Be sure to encode the query string parameters so that they are properly handled. Here are some ways to implement proxy URLs in your web app:

  1. Use proxy URLs with HTML tags. 
    <img src="/proxy?myParameter=myValue" />
  2. Use proxy URLs in JavaScript. 
    var request = window.location.origin + '/proxy?myParameter=' + encodeURIComponent('myValue');

fetch(request)
    .then(response => response.text())
    .then(data => console.log(data));
  3. Use proxy URLs with other JavaScript libraries. Some libraries only allow you to pass in string URLs. If you want to create the response in C# (for example, to generate content, or load from a compressed file), you can use a proxy URL to allow you to fulfil the response in C#.

NOTE: Data from the webview can only be set in the proxy query string. POST body data is not supported as the native WebView in platforms do not support it.

How to run the source code in this repo

To run this app you need to have Visual Studio for Windows or Mac, including the .NET MAUI workload. Then clone this repo, open the solution, and run one of the sample projects.

MauiReactJSHybridApp React JS app

The MauiReactJSHybridApp sample contains portions of a pre-existing Todo App built using React JS.

The original React JS Todo app sample used here is based on this sample: https://github.com/mdn/todo-react. I created a fork at https://github.com/Eilon/todo-react that incorporates some small changes to call the .NET API from JavaScript to synchronize the Todo list between the two parts of the app.

To make changes to the fork and update the .NET MAUI app, here's what I do:

  1. Clone of the forked repo and open a terminal/console window in that folder
  2. Run yarn to ensure the JavaScript dependencies are installed
  3. Run set PUBLIC_URL=/ to establish the root of the app as / because that's the root of the .NET MAUI HybridWebView app
  4. Run npm run build to compile the app and produce a static version of it
    • If you get this error: Error: error:0308010C:digital envelope routines::unsupported
    • Then run set NODE_OPTIONS=--openssl-legacy-provider
    • And run this again: npm run build
  5. This will build the HTML/JS/CSS output into a new ./build folder
  6. Go to the MauiReactJSHybridApp's Resources/Raw/ReactTodoApp folder and delete all the existing files, and replace with the files from the previous step's ./build folder
  7. Then run the MauiReactJSHybridApp from Visual Studio

This project is licensed under the MIT License. However, portions of the incorporated source code may be subject to other licenses:

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