Custom URL Protocols: The Secret to Opening Desktop Apps from the Web

Ever wondered how websites can open desktop applications like Notion, Steam, or others directly from the browser? The magic lies in Custom URL Protocols. This method is widely used by popular apps to provide seamless transitions between web and desktop environments, ensuring a smooth user experience. In this article, I’ll walk you through how Custom URL Protocols work and show you how to set one up for your own application using Windows Registry.

What Are Custom URL Protocols?

A Custom URL Protocol is a unique identifier registered in the system’s registry. When a web browser encounters a link starting with a registered protocol (e.g., notion://), it can invoke a dialog to open the specified desktop application, if installed. This approach is ideal for launching desktop applications directly from web pages, enhancing user interaction across platforms.

Here are a few examples of commonly registered URL protocols:

• Notion: notion://

• Calculator: ms-calculator://

• Notepad: ms-notepad://

These protocols are recognized by Windows (and other operating systems) and can be launched by simply clicking a corresponding link in the browser. Now, let's dive into the process of creating your own Custom URL Protocol.

Registering a Custom URL Protocol in Windows

To register a Custom URL Protocol in Windows, we’ll work with the Windows Registry. Here’s a step-by-step guide to creating your own protocol.

Steps to Register Your Custom URL Protocol

1. Open the Registry Editor

   • Press Win + R, type regedit, and press Enter to open the Windows Registry Editor.

2. Navigate to HKEY_CLASSES_ROOT

   • In the Registry Editor, go to HKEY_CLASSES_ROOT. This location stores all the registered protocols and file associations.

3. Create a New Key for Your Protocol

   • Under HKEY_CLASSES_ROOT, create a new key with the name of your protocol. For this example, let’s call it my-custprotocol.

4. Add a URL Protocol String Value

   • Inside the my-custprotocol key, create a new String Value named URL Protocol. This value doesn’t require any specific data; simply leave it blank. Adding this key informs Windows that this entry is a protocol handler.

5. Define the Launch Command

   • Inside the my-custprotocol key, create a subkey named shell.

   • Under shell, create another subkey named open.

   • Under open, create a final subkey called command.

   • Set the (Default) value of command to the path of the executable you want to open, with the following format:

       "path\to\your\application.exe" "%1"
     

   • For example, if your application is located at D:\TestApp\CustomProtocol.Tester.exe, you would enter:

       "D:\TestApp\CustomProtocol.Tester.exe" "%1"
     

The %1 argument allows the protocol to pass any query string data from the browser to your application, enabling custom handling based on the URL parameters.

Now, when you or any user navigates to my-custprotocol://example, the browser will prompt to open your application, passing along any query strings as arguments.

Handling URL Arguments in Your Application

In most frameworks, such as Electron, WPF, Tkinter, or JavaFX, you can access command-line arguments in the application startup process. You can then parse the %1 argument to extract parameters from the query string, allowing you to implement custom logic based on the URL.

For instance, if you navigate to my-custprotocol://open?doc=123, the query doc=123 will be passed to your application, and you could handle this in your app to open a specific document.

Automating the Registry Key Creation

Adding a Custom URL Protocol manually can be tedious, especially if you want this protocol to register on each installation. The registration process can be automated using scripts in various programming languages, such as PowerShell, C#, or Python.

Here's a simple PowerShell script to automate the registry addition for my-custprotocol:

$protocolName = "my-custprotocol"
$exePath = "D:\TestApp\CustomProtocol.Tester.exe"

New-Item -Path "HKCR:\$protocolName" | Out-Null
New-ItemProperty -Path "HKCR:\$protocolName" -Name "URL Protocol" -Value "" | Out-Null
New-Item -Path "HKCR:\$protocolName\shell" | Out-Null
New-Item -Path "HKCR:\$protocolName\shell\open" | Out-Null
New-Item -Path "HKCR:\$protocolName\shell\open\command" | Out-Null
Set-ItemProperty -Path "HKCR:\$protocolName\shell\open\command" -Name "(Default)" -Value "`"$exePath`" `"%1`""

Run this script as part of your application installation process to register the protocol automatically.

Conclusion

Custom URL Protocols offer a powerful way to bridge the web and desktop application worlds, creating a smooth and integrated experience for users. By following these steps, you can create and register your own Custom URL Protocol, enabling you to launch your desktop applications directly from the web. With the ability to pass arguments through URL query strings, this approach can add significant functionality to your application, making it highly responsive to user actions initiated from a browser.

So, the next time you need to invoke an app from the web, remember that setting up a Custom URL Protocol might be the key!