StateMachine有自己单独的工作线程，
protected StateMachine(String name) {
mSmThread = new HandlerThread(name);
mSmThread.start();
Looper looper = mSmThread.getLooper();
initStateMachine(name, looper);
}StateMachine中一个很重要的角色就是SmHandler，SmHandler在构造伊始就添加了两个状态：
private SmHandler(Looper looper, StateMachine sm) {
super(looper);
mSm = sm;
addState(mHaltingState, null);
addState(mQuittingState, null);
}这两个状态意思是整个状态机的停止状态和退出状态，如下：
/** * State entered when transitionToHaltingState is called. */
private class HaltingState extends State {
@Override
public boolean processMessage(Message msg) {
mSm.haltedProcessMessage(msg);
return true;
}
}
/** * State entered when a valid quit message is handled. */
private class QuittingState extends State {
@Override
public boolean processMessage(Message msg) {
return NOT_HANDLED;
}
}可见这两个状态都是继承自State，这个状态机中的状态都是用State表示的，如下：
public class State implements IState {
protected State() {
}
@Override
public void enter() {
}
@Override
public void exit() {
}
@Override
public boolean processMessage(Message msg) {
return false;
}
@Override
public String getName() {
String name = getClass().getName();
int lastDollar = name.lastIndexOf('$');
return name.substring(lastDollar + 1);
}
}看起来很简单，主要就三个函数，enter表示进入状态的回调，exit表示离开状态的回调，processMessage表示收到消息的回调。
再来看State是如何添加到状态机中的，如下：
/** The map of all of the states in the state machine */
private HashMap<State, StateInfo> mStateInfo = new HashMap<State, StateInfo>();
private final StateInfo addState(State state, State parent) {
StateInfo parentStateInfo = null;
if (parent != null) {
parentStateInfo = mStateInfo.get(parent);
if (parentStateInfo == null) {
// Recursively add our parent as it's not been added yet.
parentStateInfo = addState(parent, null);
}
}
StateInfo stateInfo = mStateInfo.get(state);
if (stateInfo == null) {
stateInfo = new StateInfo();
mStateInfo.put(state, stateInfo);
}
// Validate that we aren't adding the same state in two different hierarchies.
if ((stateInfo.parentStateInfo != null)
&& (stateInfo.parentStateInfo != parentStateInfo)) {
throw new RuntimeException("state already added");
}
stateInfo.state = state;
stateInfo.parentStateInfo = parentStateInfo;
stateInfo.active = false;
return stateInfo;
}可见每个State都只能有最多一个parent，或者没有parent，那这种带层级的State状态机有什么意义呢？因为以往我们认为的状态机都是若干完全独立的状态之间互相切换，不会有状态层级关系的，接下来我们就来看看这种层级关系的奥秘，从状态切换入手：
private final void transitionTo(IState destState) {
mDestState = (State) destState;
}只是设置了一个变量，不免让人有些失望，我们看这个变量在哪引用的，结果是在performTransitions中，而这个函数是在SmHandler的handleMessage中：
/** true if construction of the state machine has not been completed */
private boolean mIsConstructionCompleted;
@Override
public final void handleMessage(Message msg) {
if (!mHasQuit) {
/** Save the current message */
mMsg = msg;
/** State that processed the message */
State msgProcessedState = null;
if (mIsConstructionCompleted) {
/** Normal path */
msgProcessedState = processMsg(msg);
} else if (!mIsConstructionCompleted && (mMsg.what == SM_INIT_CMD)
&& (mMsg.obj == mSmHandlerObj)) {
/** Initial one time path. */
mIsConstructionCompleted = true;
invokeEnterMethods(0);
} else {
throw new RuntimeException("StateMachine.handleMessage: "
+ "The start method not called, received msg: " + msg);
}
performTransitions(msgProcessedState, msg);
}
}这个mHasQuit从字面意思上就是状态机quit了，状态切换到QuitState时整个状态机就要destroy了，这时候再发消息也不会处理了。再看mIsConstructionCompleted，这个字面意思是状态机的初始化构造是否结束了，用到的地方也就是handleMessage中，而构造指令就是SM_INIT_CMD，发出指令的地方在状态机启动的时候，如下：
public void start() {
// mSmHandler can be null if the state machine has quit.
SmHandler smh = mSmHandler;
if (smh == null) return;
/** Send the complete construction message */
smh.completeConstruction();
}我们参考AdapterState状态机，如下：
public static AdapterState make(AdapterService service, AdapterProperties adapterProperties) {
Log.d(TAG, "make() - Creating AdapterState");
AdapterState as = new AdapterState(service, adapterProperties);
as.start();
return as;
}就是创建好状态机之后调用其start函数启动状态机。我们接下来看看start函数中completeConstruction的实现，如下：
private final void completeConstruction() {
/** * Determine the maximum depth of the state hierarchy * so we can allocate the state stacks. */
int maxDepth = 0;
for (StateInfo si : mStateInfo.values()) {
int depth = 0;
for (StateInfo i = si; i != null; depth++) {
i = i.parentStateInfo;
}
if (maxDepth < depth) {
maxDepth = depth;
}
}
mStateStack = new StateInfo[maxDepth];
mTempStateStack = new StateInfo[maxDepth];
setupInitialStateStack();
/** Sending SM_INIT_CMD message to invoke enter methods asynchronously */
sendMessageAtFrontOfQueue(obtainMessage(SM_INIT_CMD, mSmHandlerObj));
}这里首先遍历所有的状态，算出最大的深度，然后初始化状态栈mStateStack和mTempStateStack，再调用setupInitialStateStack，最后才发送了SM_INIT_CMD到消息队列的头。
状态机的切换中非常重要的角色就是这个状态栈了，所以我们要重点关注状态栈的初始化。不过这里还是先分析SM_INIT_CMD，发消息的时候还带上了mSmHandlerObj，这个东西就是一个普通的Object，每次状态机自身发的消息都会带上这个Object以区分是外面的消息还是状态机自身的消息，比如关于INIT和QUIT都会带上这个Object。在handleMessage中收到SM_INIT_CMD后会给mIsConstructionCompleted置为true，表示初始化过了，然后调用invokeEnterMethods(0)，如下：
private final void invokeEnterMethods(int stateStackEnteringIndex) {
for (int i = stateStackEnteringIndex; i <= mStateStackTopIndex; i++) {
mStateStack[i].state.enter();
mStateStack[i].active = true;
}
}这个函数会从stateStackEnteringIndex到mStateStackTopIndex遍历调用enter函数，这个mStateStackTopIndex是什么呢？要搞清楚这个问题我们就得回到setupInitialStateStack函数了。
private final void setupInitialStateStack() {
StateInfo curStateInfo = mStateInfo.get(mInitialState);
for (mTempStateStackCount = 0; curStateInfo != null; mTempStateStackCount++) {
mTempStateStack[mTempStateStackCount] = curStateInfo;
curStateInfo = curStateInfo.parentStateInfo;
}
// Empty the StateStack
mStateStackTopIndex = -1;
moveTempStateStackToStateStack();
}这里从initial state开始遍历其父state一直到头，遍历路径记录在mTempStateStack中，而将StateStack栈顶index置为－1，然后moveTempStateStackToStateStack。
private final int moveTempStateStackToStateStack() {
int startingIndex = mStateStackTopIndex + 1;
int i = mTempStateStackCount - 1;
int j = startingIndex;
while (i >= 0) {
mStateStack[j] = mTempStateStack[i];
j += 1;
i -= 1;
}
mStateStackTopIndex = j - 1;
return startingIndex;
}这个函数相当于将TempStateStack倒序copy到StateStack中。在TempStateStack中先入栈的是底层的状态，后入栈的是顶层的状态，而StateStack刚好相反，先入栈的是顶层parent状态，后入栈的是底层状态。现在我们回到invokeEnterMethods(0)，这里会从StateStack的0开始到mStateStackTopIndex调用enter，也就是从状态机的初始状态的最顶层parent层层往下调到最底层状态的enter。
总结一下，状态机初始化时会先addState，然后设置好initial state，然后start，在start中初始化StateStack，将initial state从祖先开始依次入栈，然后再从祖先开始依次调用他们的enter回调。
我们再回到handleMessage，在invokeEnterMethods之后还调用了performTransitions(msgProcessedState, msg); 这个是检查状态切换的，如果设置过mDestState则这里要切换状态了：
private void performTransitions(State msgProcessedState, Message msg) {
State destState = mDestState;
if (destState != null) {
/** * Process the transitions including transitions in the enter/exit methods */
while (true) {
/** * Determine the states to exit and enter and return the * common ancestor state of the enter/exit states. Then * invoke the exit methods then the enter methods. */
StateInfo commonStateInfo = setupTempStateStackWithStatesToEnter(destState);
invokeExitMethods(commonStateInfo);
int stateStackEnteringIndex = moveTempStateStackToStateStack();
invokeEnterMethods(stateStackEnteringIndex);
/** * Since we have transitioned to a new state we need to have * any deferred messages moved to the front of the message queue * so they will be processed before any other messages in the * message queue. */
moveDeferredMessageAtFrontOfQueue();
if (destState != mDestState) {
// A new mDestState so continue looping
destState = mDestState;
} else {
// No change in mDestState so we're done
break;
}
}
mDestState = null;
}
/** * After processing all transitions check and * see if the last transition was to quit or halt. */
if (destState != null) {
if (destState == mQuittingState) {
/** * Call onQuitting to let subclasses cleanup. */
mSm.onQuitting();
cleanupAfterQuitting();
} else if (destState == mHaltingState) {
/** * Call onHalting() if we've transitioned to the halting * state. All subsequent messages will be processed in * in the halting state which invokes haltedProcessMessage(msg); */
mSm.onHalting();
}
}
}由于是初始化，所以msgProcessedState为null，而且mDestState也为null，所以这里其实什么也没有做。再回到handleMessage，如果是初始化之后则会调processMsg，如下
private final State processMsg(Message msg) {
StateInfo curStateInfo = mStateStack[mStateStackTopIndex];
if (isQuit(msg)) {
transitionTo(mQuittingState);
} else {
while (!curStateInfo.state.processMessage(msg)) {
/** * Not processed */
curStateInfo = curStateInfo.parentStateInfo;
if (curStateInfo == null) {
/** * No parents left so it's not handled */
mSm.unhandledMessage(msg);
break;
}
}
}
return (curStateInfo != null) ? curStateInfo.state : null;
}从状态栈中取出栈顶状态，将msg交给该state处理，如果处理了返回true则直接返回当前处理的state，否则交给父state处理，如果一直没有哪个state可以处理的话就调用SmHandler的unhandledMessage。
从processMsg返回到handleMessage后，还会调用performTransitions，不过可以看到如果mDestState为空则什么也不做。而mDestState是调transitionTo设置的，所以如果是单纯的发消息不会涉及状态的切换。
通常transitionState都是在某个state的processMsg中，这样在processMsg返回后继续调performTransitions时就会检查切换状态了。
不过注意的是切换状态会依次将当前状态栈出栈并将新状态链入栈，不过如果两者有共同的祖先结点，那祖先结点就没必要折腾了，只是下面不同的子状态才exit。我们分析performTransitions函数，首先通过setupTempStateStackWithStatesToEnter找到最低公共祖先，这里面就是从目标state开始往上遍历直到发现state是active为止，因为当前state的链上肯定都是active的。
接下来从当前状态开始调用exit直到最低公共祖先，注意不包括这个祖先，路上的state的active都标为false。
我们总结一下，当调enter的时候是从上往下，调exit的时候是从下往上，处理msg的时候也是从下往上。不过也可以理解，初始化的时候是先从上开始，退出的时候是反着来。处理消息也是先让下处理，处理不好才往上走。
