Defines a function to be called when this Node has input focus and the input method text has changed. If this function is not defined in this Node, then it receives the result string of the input method composition as a series of onKeyTyped function calls.

When the Node loses the input focus, the JavaFX runtime automatically commits the existing composed text if any.

Returns:  the event handler that is called when this Node has input focus and the input method text has changed