do I have to read the source for each object to learns its methods?
Yes, you will have to read the source code to see what methods are available.
How can I send a fixed text as if it were what the user would enter in the textfield of the composer?
action:composer.submit and the necessary server side hooks. For example
@baris Could you explain how I can do to send text as if it were the editor so that the composer does not throw error that it enters more than 0 characters?