This parameter may be set to be a one argument function that is called whenever a unicode character is generated. The function will be called with an integer representing the character. Use this rather than relying on key codes when text input is desired. Note that it is often desirable to have some form of key bindings present even when the char-callback function is set (e.g. pressing escape may exit a text field), but if no other key bindings are desired, an empty list may be passed to push-key-bindings.