This method spawns the new thread and invokes the receiver’s main method on the new thread. If you initialized the receiver with a target and selector, the default main method invokes that selector automatically.
If this thread is the first thread detached in the application, this method posts the NSWillBecomeMultiThreadedNotification with object nil to the default notification center.
spawn:卵、子；产生；
/**
* A {@code Thread} is a concurrent unit of execution. It has its own call stack
* for methods being invoked, their arguments and local variables. Each application
* has at least one thread running when it is started, the main thread, in the main
* {@link ThreadGroup}. The runtime keeps its own threads in the system thread
* group.
*
* <p>There are two ways to execute code in a new thread.
* You can either subclass {@code Thread} and overriding its {@link #run()} method,
* or construct a new {@code Thread} and pass a {@link Runnable} to the constructor.
* In either case, the {@link #start()} method must be called to actually execute
* the new {@code Thread}.
*
* <p>Each {@code Thread} has an integer priority that affect how the thread is
* scheduled by the OS. A new thread inherits the priority of its parent.
* A thread's priority can be set using the {@link #setPriority(int)} method.
*/