Message

* Defines a message containing a description and arbitrary data object that can be
* sent to a {@link Handler}.  This object contains two extra int fields and an
* extra object field that allow you to not do allocations in many cases.  

/**
 * User-defined message code so that the recipient can identify 
 * what this message is about. Each {@link Handler} has its own name-space
 * for message codes, so you do not need to worry about yours conflicting
 * with other handlers.
 */
public int what;

/*package*/ Bundle data;

/*package*/ Handler target;

/*package*/ Runnable callback;

转载于:https://www.cnblogs.com/feng9exe/p/5950320.html