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KeyboardLayoutBase class

  • ASCII_TO_KEYCODE is a bytes string with the keycode for each low-ascii character at the ascii value place. The keycode for "u" is at: ASCII_TO_KEYCODE[ord("u")]. Since the keycode value is below 0x80, the high bit is used to encode if the shift key is needed.
  • NEED_ALTGR lists characters that require the altgr key in addition.
  • HIGHER_ASCII is a dictionary that associates the ord() int value of high ascii and utf8 characters of one or more bytes to the keycode.
  • COMBINED_KEYS is a table (dictionary) of characters like Ñ that can be accessed by typing first a dead key (like ~) followed by a regular key (like N) - on a Windows French keyboard for example.
    • Indexes are ord() int values of each character.
    • The value is an int encoding 2 bytes.
    • The upper byte is the keycode of the first key, including the shift bit.
    • The lower byte is the ascii code of the second key (a-z, A-Z or space).
    • The lower byte's high bit also encodes whether or not the first key needs altgr, so as not to pack NEEDS_ALTGR with a bunch of characters.

With the Ñ example:

  • ~ is altgr + é, é is keycode 0x1F
  • N is ascii code 0x4E, with the altgr bit becomes 0xCE
  • Ñ is therefore 0xD1: 0x1FCE,
  • When typing, "N" is looked up: ASCII_TO_KEYCODE[0x4E] == 0x91 (shift+0x11)
  • So write("Ñ") sends the keycodes: (0xE6, 0x1F) (altgr+é) then (0xE1, 0x11) (shift+n)

Using int values to index the dictionaries, and as values for the COMBINED_KEYS table reduces the memory size compared to strings and bytestrings.

Layouts and KeyboardLayout classes

Every layout class is called KeyboardLayout, to make changing layout easier without having a different class name in every file, and have to guess or lookup the class name (is it CamelCase of the module name ? Is it US or Us ?). For compatibility, I added KeyboardLayout = KeyboardLayoutUS in the US keyboard layout. Importing multiple layouts is possible by using the full module.KeyboardLayout name, or import KeyboardLayout as ...