一个state的基本构造，processMessage 以及可选的enter exit 和getName。
processMessager是用于处理数据。 enter 和exit 则是类似于 面向编程的构造和析构方法。
* <p>A state is a <code>State</code> object and must implement
* <code>processMessage</code> and optionally <code>enter/exit/getName</code>.
* The enter/exit methods are equivalent to the construction and destruction
* in Object Oriented programming and are used to perform initialization and
* cleanup of the state respectively
什么时候会调用exit这个方法。
当从一个state transitionTo 另一个State 的时候是否会调用exit方法？
如果按照面向对象的思想对应过去， 之后在这个state消失的时候才会调用exit
那么transitionTo 是否会导致state的销毁？理论上应该是会的， 因为StateMachine只能处于一个state，所以在转移到别的state的同时会销毁当前state
也就是会调用exit方法。
以上仅仅是自己的逻辑推理结构， 不保证正确， 有空的时候自己做个实验验证下。
构建hsm，利用setInitialState来设定初始状态。
When a state machine is created <code>addState</code> is used to build the
* hierarchy and <code>setInitialState</code> is used to identify which of these
* is the initial state.
使用obtainMessage 来创建消息， 使用sendMessage发送给StateMachine， 当StateMachine收到消息时会调用当前状态的processMessage。
After the state machine is created and started, messages are sent to a state
* machine using <code>sendMessage</code> and the messages are created using
* <code>obtainMessage</code>. When the state machine receives a message the
* current state's <code>processMessage</code> is invoked. In the above example
* mS1.processMessage will be invoked first. The state may use <code>transitionTo</code>
* to change the current state to a new state
如果子状态不能处理某个消息时，可以通过返回false或者NOT_HANDLED交给其父状态处理
* a child state is unable to handle a message it may have the message processed
* by its parent by returning false or NOT_HANDLED.
这个消息转为父类处理是自动的还是手动的？理论上来说应该有一个transitionTo的过程。
如果说不需要的话，那么说说明构造hsm的时候addState会建立其子状态和父状态之间的特殊关系。
那么有个deferMessage这个是处理什么消息的呢？
当消息处理完了之后， 会转移到Halting的状态
* <p>When all processing is completed a state machine may choose to call
* <code>transitionToHaltingState</code>. When the current <code>processingMessage</code>
* returns the state machine will transfer to an internal <code>HaltingState</code>
* and invoke <code>halting</code>. Any message subsequently received by the state
* machine will cause <code>haltedProcessMessage</code> to be invoked.</p>
停止stateMachine的方法，quit
* <p>If it is desirable to completely stop the state machine call <code>quit</code> or
* <code>quitNow</code>. These will call <code>exit</code> of the current state and its parents,
* call <code>onQuiting</code> and then exit Thread/Loopers.</p>
这个地方明确的说明了会调用当前State和其父类state的exit方法，有两个疑问：
1、其父类是指的其直系父类还是会迭代调用
2、这个地方直接说明了exit的调用条件，是否说明之前的推断是错误的：|
exit 方法是为了保证StateMachine的环境正确性，感觉其主要工作应该是清楚当前状态对于整个环境的改变，相当于复位功能。
这个机制就保证了各个状态之间的独立性， 也说明了为什么在退出的时候会迭代的调用父状态的exit方法。
介绍了状态之间的切换规则， 主要的标准是沿着HSM这条线进行切换。
* <p>Since the states are arranged in a hierarchy transitioning to a new state
* causes current states to be exited and new states to be entered. To determine
* the list of states to be entered/exited the common parent closest to
* the current state is found. We then exit from the current state and its
* parent's up to but not including the common parent state and then enter all
* of the new states below the common parent down to the destination state.
* If there is no common parent all states are exited and then the new states
* are entered.</p>