How many blocks are running on a queue?

问题

Any way to check how many blocks are running on a concurrent queue?

I want to limit running blocks in a concurrent queue, and add a new block only if this number is lower i.e. 3. Any way to do it?

The reason I read here, that maximum number of threads is 64. Above it app will freeze.

来源：https://stackoverflow.com/questions/29731285/how-many-blocks-are-running-on-a-queue