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Web DOM with Web Assembly

Frank A. Krueger edited this page Mar 17, 2018 · 21 revisions

This guide will show you how to use Ooui.Wasm to build an app deployed in a web assembly.

First we will build the default template project that prints "Hello World!" to the console.

Once that's working, we will build a UI using the HTML DOM.

You may also wish to read Xamarin.Forms with Web Assembly to see how to create web assembly apps using Xamarin.Forms.

All of these steps are written using the command line on Mac, but you can instead use Visual Studio and Windows to accomplish the same thing!

1. Create a new app

mkdir MyApp
cd MyApp

dotnet new console

2. Reference Ooui.Wasm

Ooui.Wasm is a package that contains a build target to generate the web assembly build.

dotnet add package Ooui.Wasm

3. Print hello world

Make sure Program.cs looks something like:

using System;

namespace MyApp
    class Program
        static void Main(string[] args)
            Console.WriteLine("Hello World!");

4. Build the app

dotnet build

(The first build will be slow because it's downloading the mono wasm SDK.)

In addition to building your app, this will also generate all the files you need to run your app in a web assembly. Those files are put in the dist directory. Let's take a look:

ls -al bin/Debug/netcoreapp2.0/dist/

-rw-r--r--  1 fak  staff      960 Mar 14 11:44 index.html
drwxr-xr-x  4 fak  staff      128 Mar 14 11:44 managed
-rw-r--r--  1 fak  staff   167240 Feb 28 20:39 mono.js
-rw-r--r--  1 fak  staff  1769591 Feb 28 20:39 mono.wasm
-rw-r--r--  1 fak  staff    14304 Mar 14 11:44 ooui.js
-rw-r--r--  1 fak  staff      284 Feb 28 20:39
  • index.html is a static web page that is custom generated for your projects and lists all of its dependencies and its entry point.
  • mono.js is a bridge between the browser's javascript world and mono running in the web assembly. You can blissfully ignore its banality.
  • mono.wasm this is it - the big enchilada - mono running in a web assembly.
  • ooui.js is the standard Ooui JS library that makes working with the DOM easy.
  • is a little web server to host these files (though dotnet-serve is recommended).
  • managed is a directory that contains your app's assemblies. Let's look:
ls -al bin/Debug/netcoreapp2.0/dist/managed/

-rw-r--r--  1 fak  staff     4096 Mar 14 11:44 MyApp.dll
-rw-r--r--  1 fak  staff  3675136 Feb 28 20:39 mscorlib.dll

We can see that our app is only 4k, but it depends on mscorlib (as all apps do). The total app size is less than 4 MB and caches nicely. (Yes, yes, we can do better!)

5. Run the web assembly app

dotnet serve -p 8000 bin/Debug/netcoreapp2.0/dist

This command requires that you install dotnet-serve - a great little web server you can run anywhere.

Alternatively, you can run this python script:

cd bin/Debug/netcoreapp2.0/dist

Your app is now running on http://localhost:8000

If you open that page you will briefly see "Loading..." and then a blank screen. That's because our app doesn't have a UI and just prints to the console.

If you open the browser's console and refresh the page, then you should see Hello World!

Hit Ctrl+C to kill the web server.

6. Build a UI

Edit Program.cs to be:

using System;
using Ooui;

namespace MyApp
    class Program
        static void Main(string[] args)
            // Create the UI
            var button = new Button("Click me!");

            // Add some logic to it
            var count = 0;
            button.Click += (s, e) => {
                button.Text = $"Clicked {count} times";

            // Publish a root element to be displayed
            UI.Publish ("/", button);

This is a little app with a button that counts its click - written using the HTML DOM.

The last line of Main is important - all web assembly apps must Publish a root element to be displayed.

7. Build and run the UI

dotnet build

dotnet serve -p 8000 bin/Debug/netcoreapp2.0/dist

Now when you go to http://localhost:8000 you will see the counter UI!

8. Deploying your Web App

Since web assembly apps run locally in the browser, serving them is a breeze.

Copy the dist directory to your favorite static web server. That server can be anything from Azure, to Amazon S3, to some Apache Thing running Linux Something.

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