>>how does the source tell the application that something is ready?
it sends you a message.
>>..how a scanner knows that there is something to scan ..
you post EnableDs to the source. at that point you’ve specificed with or without Ui and if the Ui doesn’t come up scanning should being or if the Ui does come up there is usually a button that says Scan and after the button is clicked scanning should begin.
>>the SCAN button… ….is not very easy to implement.
it isn’t covered in the twain spec but last time i looked at it, it seemed to me that a solution should be possible with deviceEvents and STI (the Still Image Api) but yeah – it looked like more work then it was worth to me.