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Block registration

Blocks reach the palette in two ways: the built-in categories are registered as internal block packages when the runtime starts, and extensions register their blocks through the same pipeline that turns a getInfo() descriptor into real scratch-blocks. This page describes how that happens in scratch-vm/src/engine/runtime.js.

Built-in block packages​

The core categories (motion, looks, sound, events, control, sensing, operators, variables, and custom blocks) are plain classes in scratch-vm/src/blocks/scratch3_*.js. On construction, the runtime calls _registerBlockPackages(), which instantiates each package and collects three things from it:

  • getPrimitives(): a map of opcode to implementation function. These are stored in runtime._primitives, keyed by opcode (for example motion_movesteps), and are what the interpreter calls.
  • getHats(): metadata for hat blocks (for example whether the hat restarts existing threads), stored in runtime._hats.
  • getMonitored(): which reporters can be shown as stage monitors, merged into runtime.monitorBlockInfo.

Each package is also registered for the compiler via compilerRegisterExtension(name, object), which attaches it to the runtime as ext_<name> so compiled code can reach it.

An opcode is the category name and block name joined by an underscore, like looks_sayforsecs. The palette's block shape, label, and inputs come from the scratch-blocks definitions in the scratch-blocks package; the VM side only provides the opcode and its behavior.

Extension blocks​

Extensions do not edit those files. They describe their blocks in getInfo() and register with Scratch.extensions.register (see the Extension API). The extension manager runs getInfo() and hands the result to the runtime's _registerExtensionPrimitives(extensionInfo), which:

  1. Builds a category descriptor from the extension's id, name, colors (color1/color2/ color3, falling back to defaults), and icons, and pushes it onto runtime._blockInfo.
  2. Fills the category by converting each block descriptor with _convertForScratchBlocks, which turns the text, blockType, and arguments into the scratch-blocks XML the editor needs.
  3. Registers any custom field types, emitting EXTENSION_FIELD_ADDED for each.
  4. Emits EXTENSION_ADDED with the finished category so the GUI can add it to the palette.

Reloading an extension's blocks calls _refreshExtensionPrimitives, which rebuilds the category and emits BLOCKSINFO_UPDATE. Removing one calls _unregisterExtensionPrimitives and emits EXTENSION_REMOVED. See Events.

A block descriptor​

Each entry in the blocks array of getInfo() looks like this:

{
    opcode: 'doThing',
    blockType: Scratch.BlockType.COMMAND,
    text: 'do thing with [INPUT]',
    arguments: {
        INPUT: {
            type: Scratch.ArgumentType.STRING,
            defaultValue: 'hello'
        }
    }
}

The runtime namespaces the opcode with the extension ID, so doThing on extension myext becomes the opcode myext_doThing, and calls the extension instance's doThing method when the block runs. Placeholders in text (like [INPUT]) are matched to keys in arguments.

Compiled blocks​

The interpreter path uses the block methods directly. For extensions that want their blocks to run through RemixWarp's JavaScript compiler, vm.exports.compiler.register(extensionId, blocks) registers a descriptor per opcode with a type (one of any, number, numberOrNaN, string, boolean, command) and a compile function. See Compiled extensions.

See also​