异步如何触发的呢？
register_transport
-->transport_write_action(transport_registration_send, &m)

remove_transport
-->transport_write_action(transport_registration_send, &m)

此时会将事件写入socket pair的写入端，而接收端：

void fdevent_loop()
{
...

for(;;) {

while((fde = fdevent_plist_dequeue())) {
unsigned events = fde->events;
fde->events = 0;
fde->state &= (~FDE_PENDING);
dump_fde(fde, "callback");

//这个回调函数是在：fdevent_install 函数中注册的：fde->func = func;
fde->func(fde->fd, events, fde->arg);
}
}
}

然后利用transport_read_action读取异步事情，那么数据处理在哪里呢？

transport_registration_func
-->
/* don't create transport threads for inaccessible devices */
if (t->connection_state != CS_NOPERM) {

if(adb_thread_create(&input_thread_ptr, input_thread, t)){
fatal_errno("cannot create input thread");
}

if(adb_thread_create(&output_thread_ptr, output_thread, t)){
fatal_errno("cannot create output thread");
}
}

在这里会创建两个线程 output thread和 input thread用于做异步 IO，