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Device interoperability layer for Windows and MacOS
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Latest commit 810f20b Mar 7, 2019

Scratch Link

Scratch Link is a helper application which allows Scratch 3.0 to communicate with hardware peripherals. Scratch Link replaces the Scratch Device Manager and Scratch Device Plug-in.

System Requirements:

macOS 10.10 "Yosemite"
Windows 10 Version 1709 (build 16299) "Fall Creators Update" or newer

Using Scratch Link with Scratch 3.0

To use Scratch Link with Scratch 3.0:

  1. Install and run Scratch Link
  2. Open Scratch 3.0
  3. Select the "Add Extension" button (looks like Scratch blocks with a + at the bottom of the block categories list)
  4. Select a compatible extension such as the micro:bit or LEGO EV3 extension.
  5. Follow the prompts to connect your peripheral.
  6. Build a project with the new extension blocks. Scratch Link will help Scratch communicate with your peripheral.

Development: Getting started


The general network protocol and all supported hardware protocols are documented in Markdown files in the Documentation subdirectory. Please note that network protocol stability and compatibility are high priorities for this project. Changes to the protocol are unlikely to be accepted without very strong justification combined with thorough documentation.

Please use markdownlint to check documentation changes before submitting a pull request.


These steps are necessary regardless of platform.

Scratch Link provides Secure WebSocket (WSS) communication and uses digital certificates to do so. These certificates are not provided in this repository.

To prepare certificates for Scratch Link development:

  • Obtain the raw certificates: see Certificates/ for details. Those outside the Scratch Team will need to provide their own certificates for unofficial builds.
  • cd Certificates and run ./ to prepare the necessary certificate files.
    • On Windows, this script can be run using Cygwin or WSL.


The macOS version of this project is in the macOS subdirectory. It uses Swift 4.1 and the Swift Package Manager.

Developer prerequisites on macOS, most of which are available through Homebrew:

The build is primarily controlled through make:

  • Build the app bundle with make, which will automatically:
    1. Compile Scratch Link code using swift build
    2. Create an app bundle at dist/Scratch
    3. Copy all necessary frameworks and dylibs into the app bundle
    4. Generate and/or copy other resources into the app bundle (certificates, icons, etc.)
  • Build PKG installers with make dist, which runs both of these:
    • Build a PKG for the Mac App Store with make dist-mas
    • Build a PKG for non-Store distribution ("Developer ID") with make dist-devid
  • Run the app in any of these ways:
    • Use Finder to activate the Scratch Link bundle in the dist directory
    • Run "dist/Scratch"
      • Debug output will appear in the terminal
    • Type open "dist/Scratch" in a terminal
      • Debug output will not appear in the terminal
  • Create an Xcode project file with make xcodeproj
    • If your workflow uses the Xcode project file (Xcode, AppCode, etc.) you should re-run this command each time you add or remove source files.
    • Any changes you make to the Xcode project file will be discarded when you run this command.
    • You may not be able to run the project using the Xcode project file, but completion and building should work.


The Windows version of this project is in the Windows subdirectory.


Build, run, and debug by opening the Solution (*.sln) file in Visual Studio 2017.

Signing the MSI installer

This section applies to Scratch Team members only.

To build and sign the ScratchLinkSetup installer (MSI), you must install the appropriate signing certificate. Contact another Scratch Team member to obtain the certificate, then install it with these steps:

  1. Open "Manage User Certificates"
  2. Expand "Personal"
  3. Right-click "Certificates" under "Personal"
  4. Select "Import..."
  5. Follow the steps to import the signing certificate.
    • You may need to change the file browser to Personal Information Exchange (*.pfx;*.p12).
    • When prompted, enter the password for the certificate file you're importing.
    • On the last step, make sure the certificate store is listed as "Personal"

You can verify that you've installed the correct certificate by comparing the thumbprint in the Certificate Manager to the one listed in the post-build event in the ScratchLinkSetup project.

Known Issues for Developers

  1. Building the ScratchLinkSetup project may fail with a System.IO.DirectoryNotFoundException if the Windows case sensitivity flag is enabled on any directory in the path to the Scratch Link project files. This flag can become enabled when WSL is used to create or manipulate directories.
    • Solution: Use fsutil file queryCaseSensitiveInfo myDirName to check if myDirName has its case sensitivity flag set. If so, use fsutil file setCaseSensitiveInfo myDirName disable to clear the flag.
    • More detail:
  2. The make step may fail if the path to the Scratch Link directory contains whitespace.
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