Java 官网对Looper对象的说明：
public class Looperextends Object
Class used to run a message loop for a thread. Threads by default do not have a message loop associated with them; to create one, call prepare() in the thread that is to run the loop, and then loop() to have it process messages until the loop is stopped.
Most interaction with a message loop is through the Handler class.
This is a typical example of the implementation of a Looper thread, using the separation of prepare() and loop() to create an initial Handler to communicate with the Looper.
static void loop() ： Run the message queue in this thread.
static void prepare() ： Initialize the current thread as a looper.
