I know the difference between the two kinds.
For async tasks, next task will run after the current one began, which means tasks will be dispatched to multiple threads if there are threads available.
For sync tasks, next task will run after the current one finished.
How could main thread run async tasks since it only has one thread?
It's confusing to me.
Thanks in advance.
Your definitions of asynchronous and synchronous aren't quite right.
In Grand Central Dispatch you consider queues, not threads. A queue is either a serial dispatch queue or a concurrent dispatch queue.
A serial dispatch queue can run one task at a time and tasks are dispatched one after the other (serially).
A concurrent dispatch queue can run several tasks simultaneously on multiple threads.
Tasks execute on threads and Grand Central Dispatch takes care of assigning tasks to threads for you.
The main queue is a special serial dispatch queue that only assigns tasks to the main thread. Other queues can assign tasks to any available thread, including the main thread.
Now, for synchronous versus asynchronous dispatch, the difference is whether the dispatch blocks the current thread until the dispatched task is complete (synchronous) or queues the task without blocking the current thread (asynchronous).
When you dispatch asynchronously onto the main queue, you are providing a unit of work that Grand Central Dispatch will assign to the main thread at some future point, but your code continues executing without waiting for that dispatched item to complete.
You can dispatch asynchronous tasks to the main queue while running on the main queue quite happily, since the dispatched task will execute later, when the main queue is ready for a task.
What you can't do is dispatch synchronously onto the main queue from the main queue (or more generally, dispatch synchronously onto any serial dispatch queue from that same queue) since you will create a deadlock.
Dispatching synchronously blocks the current thread/queue until the dispatched item is complete. On a serial dispatch queue the dispatched item can't execute since the dispatching queue is blocked.