ValueTree doesn’t rely on message-thread. It has synchronous callback behaviour (on the same thread where the changes happen), while others like ChangeBroadcaster are asynchronous, and callback on the message-thread.
This is simply a matter of definition and should be taken into account when programming.
I agree, there should be better tools (like wrappers for thread-safe access to / and callbacks from value-trees) and warnings do deal with it.
Sure, it doesn’t have to be the message thread, but it has to be single threaded (read the second part of my comment…).
The reason is - logically, once the callback is called you’re gonna be reading the value, and doing read-access into a ValueTree is not thread safe.
What you want to prevent is not callbacks run at a certain thread, but read/write access happen at the same time.
This would surely happen if you start writing/dispatching callbacks in another thread and expecting any read to be valid after that.
I’ve pushed some changes to ListenerList, I don’t expect this to fix anything and everything but it would be good to see how much of an issue it still is for others.
Unfortunately I had hoped to add some additional debug assertions to help catch common issues but it seems due to the way some hosts work it results in a too many false positives so I’ve had to leave it out for now.
