How does touch events get the control of main-thread ?

[duduWang20]

We all known that all touch events are sent directly to main thread without runloop's dispatch mechanism . How does this happens ?
It there a mechanism , something like runloop , trigger it directly on the main thread ?
Or it is the hardware or the main thread's or the main run loop's special mechanism ?

1   UIKit                               0x0000000108e2261d -[UIWindow _sendTouchesForEvent:] + 2317
2   UIKit                               0x0000000108e23f2a -[UIWindow sendEvent:] + 4124
3   UIKit                               0x0000000108dc7365 -[UIApplication sendEvent:] + 352
4   UIKit                               0x0000000109713a1d __dispatchPreprocessedEventFromEventQueue + 2809
5   UIKit                               0x0000000109716672 __handleEventQueueInternal + 5957
6   UIKit                               0x0000000109716a76 __handleEventQueueInternal + 6985
7   CoreFoundation                      0x0000000108631101 __CFRUNLOOP_IS_CALLING_OUT_TO_A_SOURCE0_PERFORM_FUNCTION__ + 17
8   CoreFoundation                      0x00000001086d0f71 __CFRunLoopDoSource0 + 81
9   CoreFoundation                      0x0000000108615a6b __CFRunLoopDoSources0 + 267
10  CoreFoundation                      0x0000000108614fff __CFRunLoopRun + 1279
11  CoreFoundation                      0x0000000108614889 CFRunLoopRunSpecific + 409
12  GraphicsServices                    0x000000010ed809c6 GSEventRunModal + 62
13  UIKit                               0x0000000108dab5d6 UIApplicationMain + 159
14  tt                                  0x000000010741433f main + 111
15  libdyld.dylib                       0x000000010c355d81 start + 1
16  ???                                 0x0000000000000001 0x0 + 1