Cpp code generated by qmlcachegen relies on stable metaobject layouts. They will, however, inevitably change in a incompatible way every now and then. This renders the cache files compiled with a previous version and stored on a user's machine invalid and unsafe to use. There is currently no mechanism to invalidate the cache files if a meta object breaks compatibility, by adding a signal for instance. To address this, we can record hashes of the meta objects a compilation unit relies on and invalidate it when a mismatch is found at runtime before executing it. We then fall back to other mechanisms to run the code. This first step adds the logic to compute the hash of a metaobject to moc and adds it to its json output. The hashes are of the form <hash_revision>$<base64_hash> Task-number: QTBUG-142186 Change-Id: Ifdd56b6259874024341a2b2623d088a45816b0a1 Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>