How do I setup a callback mechanism for RichEdit in win32

Question

In win32, how do I setup a callback mechanism for RichEdit I have not created myself?

PART 1

I'm reading from a textedit field in another application's GUI. This works just fine now, except after the first read I'd like to fetch only new or modified lines. In GTK+ or Qt I'd just install a callback on some signal the field edits when its changed, but how does it work on Win32?

My MSDN searches result with nothing useful, probably because I don't know the exact term to search for. The class of the textedit is RichText20W, and it has some messages that are probably used somehow, though that article just discusses using them for the parent of the class.

PART 2

Also, if there is no such "text changed, here is the newly inserted text" callback which returns the new content immediately, I need some way to easily detect what is new. From top-of-my-head:

1. Have a marker at the end of the text block we've read, and only read between that and the end.
2. Store what we've read previously, and after a second read, remove the duplicate part from the latter to have the newly inserted stuff.

Option 2 might not be viable, since the textedit can contain any amount of text. The marker part sounds doable, but yet again, my feeble Win32 skills and horrible Win32 function names prevent me from finding the right way to do it.

Note that all these must be doable for a textedit I do not own and have not created, they belong to a third party process.

Code samples in C++ highly appreciated.

Disclaimer

Obviously, if there is some better way of doing it, let me know. I only assumed callback would be the way to go based on my previous experience with GTK+/Qt. Feel free to show me the path :)

Answer 1

Win32 controls don't work on message-specific callbacks that you can subscribe to. They just send messages to their parent window when something happens, in this case EN_UPDATE, EN_CHANGE and all that. Even these events don't tell you what text changed. They only tell you that it did change.

You could subclass the parent, but the documentation for SetWindowLongPtr explicitly says you "should not subclass a window class created by another process." Something like this is probably possible with hooks, but I haven't used them enough to say for certain how you'd actually do it.

Answer 2

Based on Joel's answer, I quit looking for callbacks and just made a small class that hooks itself (not by a real API hook though) to the richedit and polls it once a second for content length, and if it has changed since the last poll, it asks for the content, compares that to previous known content and emits a signal with the changed content.

This seems to work OK for this purpose, but if someone knows a better way still (some real and tested way of doing it via API hooks or something), please post.