I. Begin from Apple Document
Apple describes the priciples of runloop in Threading Programming Guide, you can view the details from this link.
II. Video share
Video share from this link, it gives the basic description about runloop.
All screenshot in the video from this link, you can review all content from the video quickly.
III. Listenning the block of main thread
In this link, there are two methods to listen the block of main thread, this first one is NSTimer, and the other one is sephoremore. It also supplies 'RunloopMonitorDemo'.
IV. A fantastic blog to deep learning runloop
From this link, you can learn a lot about runloop. Below is the screenshot of its index.
The author of this blog has another userfull blog, it mainly talks about how to keep user interface smoothly.
V. Other references