This document is intended to name and describe the concepts, functions and data structures inside Kaleidoscope.

It is, as yet, incredibly incomplete.

Entries should be included in dictionary order. When describing an identifier of any kind from the codebase, it should be written using identical capitalization to its use in the code and surrounded by backticks: identifierName

Firmware Terminology

These terms commonly arise when discussing the firmware.


The loop method in one’s sketch file is the heart of the firmware. It runs - as the name suggests - in a loop. We call these runs cycles. A lot of things happen within a cycle: from key scanning, through key event handling, LED animations, and so on and so forth.

Event handler

A function, usually provided by a Plugin that is run by a Hook.

At the time of this writing, the following event handlers are run by hooks:

  • onSetup: Run once, when the plugin is initialised during Kaleidoscope.setup().

  • beforeEachCycle: Run as the first thing at the start of each cycle.

  • onKeyswitchEvent: Run for every non-idle key, in each cycle the key isn’t idle in. If a key gets pressed, released, or is held, it is not considered idle, and this event handler will run for it too.

  • beforeReportingState: Runs each cycle right before sending the various reports (keys pressed, mouse events, etc) to the host.

  • afterEachCycle: Runs at the very end of each cycle.


A point where the core firmware calls event handlers, allowing plugins to augment the firmware behaviour, by running custom code.


An Arduino library prepared to work with Kaleidoscope. They implement methods from the kaleidoscope::Plugin (usually a subset of them). See event handlers above for a list of methods.


These terms arise when discussing the testing framworks and related tests.

Sim Harness

An abstraction used in testing to inject events into or invoke actions on the simulated firmware. This abstraction comprises half of the interface to the test simulator.

Sim State

An abstraction used in testing to encapsulate, snapshot, and examine firmware state. This abstraction comprises half of the interface to the test simulator.


An indivial assertion or expectation that must hold for a test case to pass.

Test Case

An indivual TEST* macro invocation. Its body consists of one or tests and optionally other code, e.g. to invoke the test harness. Note that gtest uses the non-standard term Test for what we call a Test Case.

Test File

An individual file containing one or more test suites.

Test Fixture

A class comprising setup, teardown, and other code and common state to make writing test cases easieer. A fresh object of the fixture class associated with a test suite is constructed for each run of each teset case in the test suite.

Test Simulator

An abstraction wrapping a virtual firmware build that allows performing actions against the virtual firmware and reading state out of the virtual firmware. The interface to the test simular is comprised of the sim harness and the sim state.

Test Suite

A collection of related test cases, optionally with an associated test fixture.