claude-code/packages/modifiers-napi/src/index.ts

const FLAG_SHIFT = 0x20000;
const FLAG_CONTROL = 0x40000;
const FLAG_OPTION = 0x80000;
const FLAG_COMMAND = 0x100000;

const modifierFlags: Record<string, number> = {
  shift: FLAG_SHIFT,
  control: FLAG_CONTROL,
  option: FLAG_OPTION,
  command: FLAG_COMMAND,
};

// kCGEventSourceStateCombinedSessionState = 0
const kCGEventSourceStateCombinedSessionState = 0;

let cgEventSourceFlagsState: ((stateID: number) => number) | null = null;

function loadFFI(): void {
  if (cgEventSourceFlagsState !== null || process.platform !== "darwin") {
    return;
  }

  try {
    const ffi = require("bun:ffi") as typeof import("bun:ffi");
    const lib = ffi.dlopen(
      `/System/Library/Frameworks/Carbon.framework/Carbon`,
      {
        CGEventSourceFlagsState: {
          args: [ffi.FFIType.i32],
          returns: ffi.FFIType.u64,
        },
      }
    );
    cgEventSourceFlagsState = (stateID: number): number => {
      return Number(lib.symbols.CGEventSourceFlagsState(stateID));
    };
  } catch {
    // If loading fails, keep the function null so isModifierPressed returns false
    cgEventSourceFlagsState = null;
  }
}

export function prewarm(): void {
  loadFFI();
}

export function isModifierPressed(modifier: string): boolean {
  if (process.platform !== "darwin") {
    return false;
  }

  loadFFI();

  if (cgEventSourceFlagsState === null) {
    return false;
  }

  const flag = modifierFlags[modifier];
  if (flag === undefined) {
    return false;
  }

  const currentFlags = cgEventSourceFlagsState(
    kCGEventSourceStateCombinedSessionState
  );
  return (currentFlags & flag) !== 0;
}