Android StateMachine分析

1.文章介绍

在Android源码中有很多优秀的代码框架,其中StateMachine可以完成某一个消息在不同的状态下能做出不同的业务处理,这种模式在复杂的状态和业务切换时应用非常广泛,比如Android Wifi模块随处可见StateMachine的使用。

2.干货

frameworks\base\core\java\com\android\internal\util\StateMachine.java中有一部分注释说明:

/* <p>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. After construction the programmer calls <code>start</code>
 * which initializes and starts the state machine. The first action the StateMachine
 * is to the invoke <code>enter</code> for all of the initial state's hierarchy,
 * starting at its eldest parent. The calls to enter will be done in the context
 * of the StateMachines Handler not in the context of the call to start and they
 * will be invoked before any messages are processed. For example, given the simple
 * state machine below mP1.enter will be invoked and then mS1.enter. Finally,
 * messages sent to the state machine will be processed by the current state,
 * in our simple state machine below that would initially be mS1.processMessage.</p>
<code>
        mP1
       /   \
      mS2   mS1 ----> initial state
</code>
 * <p>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</p>
 *
 * <p>Each state in the state machine may have a zero or one parent states and if
 * a child state is unable to handle a message it may have the message processed
 * by its parent by returning false or NOT_HANDLED. If a message is never processed
 * <code>unhandledMessage</code> will be invoked to give one last chance for the state machine
 * to process the message.</p>
 *
 * <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>
 *
 * <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>
 *
 * <p>In addition to <code>processMessage</code> each <code>State</code> has
 * an <code>enter</code> method and <code>exit</exit> method which may be overridden.</p>
 *
 * <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>
 *
 * <p>Two other methods that states can use are <code>deferMessage</code> and
 * <code>sendMessageAtFrontOfQueue</code>. The <code>sendMessageAtFrontOfQueue</code> sends
 * a message but places it on the front of the queue rather than the back. The
 * <code>deferMessage</code> causes the message to be saved on a list until a
 * transition is made to a new state. At which time all of the deferred messages
 * will be put on the front of the state machine queue with the oldest message
 * at the front. These will then be processed by the new current state before
 * any other messages that are on the queue or might be added later. Both of
 * these are protected and may only be invoked from within a state machine.</p>
 *
 * <p>To illustrate some of these properties we'll use state machine with an 8
 * state hierarchy:</p>
<code>
          mP0
         /   \
        mP1   mS0
       /   \
      mS2   mS1
     /  \    \
    mS3  mS4  mS5  ---> initial state
</code>
 * <p>After starting mS5 the list of active states is mP0, mP1, mS1 and mS5.
 * So the order of calling processMessage when a message is received is mS5,
 * mS1, mP1, mP0 assuming each processMessage indicates it can't handle this
 * message by returning false or NOT_HANDLED.</p>
 *
 * <p>Now assume mS5.processMessage receives a message it can handle, and during
 * the handling determines the machine should change states. It could call
 * transitionTo(mS4) and return true or HANDLED. Immediately after returning from
 * processMessage the state machine runtime will find the common parent,
 * which is mP1. It will then call mS5.exit, mS1.exit, mS2.enter and then
 * mS4.enter. The new list of active states is mP0, mP1, mS2 and mS4. So
 * when the next message is received mS4.processMessage will be invoked.</p>
 *
 * <p>Now for some concrete examples, here is the canonical HelloWorld as a state machine.
 * It responds with "Hello World" being printed to the log for every message.</p>
<code>
class HelloWorld extends StateMachine {
    HelloWorld(String name) {
        super(name);
        addState(mState1);
        setInitialState(mState1);
    }

    public static HelloWorld makeHelloWorld() {
        HelloWorld hw = new HelloWorld("hw");
        hw.start();
        return hw;
    }

    class State1 extends State {
        @Override public boolean processMessage(Message message) {
            log("Hello World");
            return HANDLED;
        }
    }
    State1 mState1 = new State1();
}

void testHelloWorld() {
    HelloWorld hw = makeHelloWorld();
    hw.sendMessage(hw.obtainMessage());
}
</code>
 * <p>A more interesting state machine is one with four states
 * with two independent parent states.</p>
<code>
        mP1      mP2
       /   \
      mS2   mS1
</code>
 * <p>Here is a description of this state machine using pseudo code.</p>
 <code>
state mP1 {
     enter { log("mP1.enter"); }
     exit { log("mP1.exit");  }
     on msg {
         CMD_2 {
             send(CMD_3);
             defer(msg);
             transitonTo(mS2);
             return HANDLED;
         }
         return NOT_HANDLED;
     }
}

INITIAL
state mS1 parent mP1 {
     enter { log("mS1.enter"); }
     exit  { log("mS1.exit");  }
     on msg {
         CMD_1 {
             transitionTo(mS1);
             return HANDLED;
         }
         return NOT_HANDLED;
     }
}

state mS2 parent mP1 {
     enter { log("mS2.enter"); }
     exit  { log("mS2.exit");  }
     on msg {
         CMD_2 {
             send(CMD_4);
             return HANDLED;
         }
         CMD_3 {
             defer(msg);
             transitionTo(mP2);
             return HANDLED;
         }
         return NOT_HANDLED;
     }
}

state mP2 {
     enter {
         log("mP2.enter");
         send(CMD_5);
     }
     exit { log("mP2.exit"); }
     on msg {
         CMD_3, CMD_4 { return HANDLED; }
         CMD_5 {
             transitionTo(HaltingState);
             return HANDLED;
         }
         return NOT_HANDLED;
     }
}
</code>
 * <p>The implementation is below and also in StateMachineTest:</p>
<code>
class Hsm1 extends StateMachine {
    public static final int CMD_1 = 1;
    public static final int CMD_2 = 2;
    public static final int CMD_3 = 3;
    public static final int CMD_4 = 4;
    public static final int CMD_5 = 5;

    public static Hsm1 makeHsm1() {
        log("makeHsm1 E");
        Hsm1 sm = new Hsm1("hsm1");
        sm.start();
        log("makeHsm1 X");
        return sm;
    }

    Hsm1(String name) {
        super(name);
        log("ctor E");

        // Add states, use indentation to show hierarchy
        addState(mP1);
            addState(mS1, mP1);
            addState(mS2, mP1);
        addState(mP2);

        // Set the initial state
        setInitialState(mS1);
        log("ctor X");
    }

    class P1 extends State {
        @Override public void enter() {
            log("mP1.enter");
        }
        @Override public boolean processMessage(Message message) {
            boolean retVal;
            log("mP1.processMessage what=" + message.what);
            switch(message.what) {
            case CMD_2:
                // CMD_2 will arrive in mS2 before CMD_3
                sendMessage(obtainMessage(CMD_3));
                deferMessage(message);
                transitionTo(mS2);
                retVal = HANDLED;
                break;
            default:
                // Any message we don't understand in this state invokes unhandledMessage
                retVal = NOT_HANDLED;
                break;
            }
            return retVal;
        }
        @Override public void exit() {
            log("mP1.exit");
        }
    }

    class S1 extends State {
        @Override public void enter() {
            log("mS1.enter");
        }
        @Override public boolean processMessage(Message message) {
            log("S1.processMessage what=" + message.what);
            if (message.what == CMD_1) {
                // Transition to ourself to show that enter/exit is called
                transitionTo(mS1);
                return HANDLED;
            } else {
                // Let parent process all other messages
                return NOT_HANDLED;
            }
        }
        @Override public void exit() {
            log("mS1.exit");
        }
    }

    class S2 extends State {
        @Override public void enter() {
            log("mS2.enter");
        }
        @Override public boolean processMessage(Message message) {
            boolean retVal;
            log("mS2.processMessage what=" + message.what);
            switch(message.what) {
            case(CMD_2):
                sendMessage(obtainMessage(CMD_4));
                retVal = HANDLED;
                break;
            case(CMD_3):
                deferMessage(message);
                transitionTo(mP2);
                retVal = HANDLED;
                break;
            default:
                retVal = NOT_HANDLED;
                break;
            }
            return retVal;
        }
        @Override public void exit() {
            log("mS2.exit");
        }
    }

    class P2 extends State {
        @Override public void enter() {
            log("mP2.enter");
            sendMessage(obtainMessage(CMD_5));
        }
        @Override public boolean processMessage(Message message) {
            log("P2.processMessage what=" + message.what);
            switch(message.what) {
            case(CMD_3):
                break;
            case(CMD_4):
                break;
            case(CMD_5):
                transitionToHaltingState();
                break;
            }
            return HANDLED;
        }
        @Override public void exit() {
            log("mP2.exit");
        }
    }

    @Override
    void onHalting() {
        log("halting");
        synchronized (this) {
            this.notifyAll();
        }
    }

    P1 mP1 = new P1();
    S1 mS1 = new S1();
    S2 mS2 = new S2();
    P2 mP2 = new P2();
}
</code>
 * <p>If this is executed by sending two messages CMD_1 and CMD_2
 * (Note the synchronize is only needed because we use hsm.wait())</p>
<code>
Hsm1 hsm = makeHsm1();
synchronize(hsm) {
     hsm.sendMessage(obtainMessage(hsm.CMD_1));
     hsm.sendMessage(obtainMessage(hsm.CMD_2));
     try {
          // wait for the messages to be handled
          hsm.wait();
     } catch (InterruptedException e) {
          loge("exception while waiting " + e.getMessage());
     }
}
</code>
 * <p>The output is:</p>
<code>
D/hsm1    ( 1999): makeHsm1 E
D/hsm1    ( 1999): ctor E
D/hsm1    ( 1999): ctor X
D/hsm1    ( 1999): mP1.enter
D/hsm1    ( 1999): mS1.enter
D/hsm1    ( 1999): makeHsm1 X
D/hsm1    ( 1999): mS1.processMessage what=1
D/hsm1    ( 1999): mS1.exit
D/hsm1    ( 1999): mS1.enter
D/hsm1    ( 1999): mS1.processMessage what=2
D/hsm1    ( 1999): mP1.processMessage what=2
D/hsm1    ( 1999): mS1.exit
D/hsm1    ( 1999): mS2.enter
D/hsm1    ( 1999): mS2.processMessage what=2
D/hsm1    ( 1999): mS2.processMessage what=3
D/hsm1    ( 1999): mS2.exit
D/hsm1    ( 1999): mP1.exit
D/hsm1    ( 1999): mP2.enter
D/hsm1    ( 1999): mP2.processMessage what=3
D/hsm1    ( 1999): mP2.processMessage what=4
D/hsm1    ( 1999): mP2.processMessage what=5
D/hsm1    ( 1999): mP2.exit
D/hsm1    ( 1999): halting
</code>
*/

不得不说,Google写的注释还真是详细,通过注释对StateMachine的用法有个基本了解。

StateMachine状态

接下来看看StateMachine到底是如何实现状态管理的:

    /**
     * Initialize.
     *
     * @param looper for this state machine
     * @param name of the state machine
     */
    private void initStateMachine(String name, Looper looper) {
        mName = name;
        mSmHandler = new SmHandler(looper, this);
    }

    /**
     * Constructor creates a StateMachine with its own thread.
     *
     * @param name of the state machine
     */
    protected StateMachine(String name) {
        mSmThread = new HandlerThread(name);
        mSmThread.start();
        Looper looper = mSmThread.getLooper();

        initStateMachine(name, looper);
    }

    /**
     * Constructor creates a StateMachine using the looper.
     *
     * @param name of the state machine
     */
    protected StateMachine(String name, Looper looper) {
        initStateMachine(name, looper);
    }

    /**
     * Constructor creates a StateMachine using the handler.
     *
     * @param name of the state machine
     */
    protected StateMachine(String name, Handler handler) {
        initStateMachine(name, handler.getLooper());
    }

    /**
     * Add a new state to the state machine
     * @param state the state to add
     * @param parent the parent of state
     */
    protected final void addState(State state, State parent) {
        mSmHandler.addState(state, parent);
    }

从整个StateMachine的构造和初始化来看,SmHandler职责是比较重要的:

private static class SmHandler extends Handler {
        ...
        /** State used when state machine is halted */
        private HaltingState mHaltingState = new HaltingState();

        /** State used when state machine is quitting */
        private QuittingState mQuittingState = new QuittingState();

        /** Reference to the StateMachine */
        private StateMachine mSm;

        /**
         * Information about a state.
         * Used to maintain the hierarchy.
         */
        private class StateInfo {
            /** The state */
            State state;

            /** The parent of this state, null if there is no parent */
            StateInfo parentStateInfo;

            /** True when the state has been entered and on the stack */
            boolean active;
        }

        /** The map of all of the states in the state machine */
        private HashMap<State, StateInfo> mStateInfo = new HashMap<State, StateInfo>();

        /** The initial state that will process the first message */
        private State mInitialState;

        /** The destination state when transitionTo has been invoked */
        private State mDestState;

        /** The list of deferred messages */
        private ArrayList<Message> mDeferredMessages = new ArrayList<Message>();
        /**
         * Constructor
         *
         * @param looper for dispatching messages
         * @param sm the hierarchical state machine
         */
        private SmHandler(Looper looper, StateMachine sm) {
            super(looper);
            mSm = sm;
            addState(mHaltingState, null);
            addState(mQuittingState, null);
        }
        ...

在SmHandler 中有很多数据成员,其中HaltingState和QuittingState都是从State派生,重写了processMessage方法,用于状态为停止后的逻辑处理:

        /**
         * 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;
            }
        }

下面列出SmHandler核心接口实现:

        /**
         * Handle messages sent to the state machine by calling
         * the current state's processMessage. It also handles
         * the enter/exit calls and placing any deferred messages
         * back onto the queue when transitioning to a new state.
         */
        @Override
        public final void handleMessage(Message msg) {
           ...
        }

        /**
         * Do any transitions
         * @param msgProcessedState is the state that processed the message
         */
        private void performTransitions(State msgProcessedState, Message msg) {
            ...
        }
         /**
         * Complete the construction of the state machine.
         */
        private final void completeConstruction() {
            ...
        }
        /**
         * Process the message. If the current state doesn't handle
         * it, call the states parent and so on. If it is never handled then
         * call the state machines unhandledMessage method.
         * @return the state that processed the message
         */
        private final State processMsg(Message msg) {
            ...
        }
        /**
         * Initialize StateStack to mInitialState.
         */
        private final void setupInitialStateStack() {
            ...
        }
        /**
         * Add a new state to the state machine. Bottom up addition
         * of states is allowed but the same state may only exist
         * in one hierarchy.
         *
         * @param state the state to add
         * @param parent the parent of state
         * @return stateInfo for this state
         */
        private final StateInfo addState(State state, State parent) {
             ...
        }
        /** @see StateMachine#setInitialState(State) */
        private final void setInitialState(State initialState) {
           ...
        }

        /** @see StateMachine#transitionTo(IState) */
        private final void transitionTo(IState destState) {
            mDestState = (State) destState;
            if (mDbg) mSm.log("transitionTo: destState=" + mDestState.getName());
        }

        /** @see StateMachine#deferMessage(Message) */
        private final void deferMessage(Message msg) {
            if (mDbg) mSm.log("deferMessage: msg=" + msg.what);

            /* Copy the "msg" to "newMsg" as "msg" will be recycled */
            Message newMsg = obtainMessage();
            newMsg.copyFrom(msg);

            mDeferredMessages.add(newMsg);
        }

        /** @see StateMachine#quit() */
        private final void quit() {
            if (mDbg) mSm.log("quit:");
            sendMessage(obtainMessage(SM_QUIT_CMD, mSmHandlerObj));
        }

        /** @see StateMachine#quitNow() */
        private final void quitNow() {
            if (mDbg) mSm.log("quitNow:");
            sendMessageAtFrontOfQueue(obtainMessage(SM_QUIT_CMD, mSmHandlerObj));
        }

可以看到SmHandler是消息转发和处理的核心。
接下来根据StateMachine的标准使用流程一步一步分析:

class HelloWorld extends StateMachine {
    HelloWorld(String name) {
        super(name);
        addState(mState1);
        setInitialState(mState1);
    }

    public static HelloWorld makeHelloWorld() {
        HelloWorld hw = new HelloWorld("hw");
        hw.start();
        return hw;
    }

    class State1 extends State {
        @Override 
        public boolean processMessage(Message message) {
            log("Hello World");
            return HANDLED;
        }
    }
    State1 mState1 = new State1();
}

void testHelloWorld() {
    HelloWorld hw = makeHelloWorld();
    hw.sendMessage(hw.obtainMessage());
}

1.在StateMachine初始化时,创建SmHandler对象

  private void initStateMachine(String name, Looper looper) {
        mName = name;
        mSmHandler = new SmHandler(looper, this);
    }

2.在SmHandler构造时,默认添加了mHaltingState、mQuittingState

        private SmHandler(Looper looper, StateMachine sm) {
            super(looper);
            mSm = sm;

            addState(mHaltingState, null);
            addState(mQuittingState, null);
        }

3.调用SmHandler的addState,格式化树形层次结构的mStateInfo

        /**
         * Add a new state to the state machine. Bottom up addition
         * of states is allowed but the same state may only exist
         * in one hierarchy.
         *
         * @param state the state to add
         * @param parent the parent of state
         * @return stateInfo for this state
         */
        private final StateInfo addState(State state, State parent) {
            if (mDbg) {
                mSm.log("addStateInternal: E state=" + state.getName() + ",parent="
                        + ((parent == null) ? "" : parent.getName()));
            }
            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;
            if (mDbg) mSm.log("addStateInternal: X stateInfo: " + stateInfo);
            return stateInfo;
        }

那么目前的树形层次结构(不计mHaltingState、mQuittingState,这两个状态是负责内部逻辑处理的)中就只有mState1.
4.设置状态机状态setInitialState

        /** @see StateMachine#setInitialState(State) */
        private final void setInitialState(State initialState) {
            if (mDbg) mSm.log("setInitialState: initialState=" + initialState.getName());
            mInitialState = initialState;
        }

5.StateMachine准备完毕,开始启动,调整mStateStack中各种State堆栈的次序

    /**
     * Start the state machine.
     */
    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();
    }
        /**
         * Complete the construction of the state machine.
         */
        private final void completeConstruction() {
            if (mDbg) mSm.log("completeConstruction: E");

            /**
             * 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;
                }
            }
            if (mDbg) mSm.log("completeConstruction: maxDepth=" + maxDepth);

            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));

            if (mDbg) mSm.log("completeConstruction: X");
        }

         /**
         * Initialize StateStack to mInitialState.
         */
        private final void setupInitialStateStack() {
            if (mDbg) {
                mSm.log("setupInitialStateStack: E mInitialState=" + mInitialState.getName());
            }

            StateInfo curStateInfo = mStateInfo.get(mInitialState);
            for (mTempStateStackCount = 0; curStateInfo != null; mTempStateStackCount++) {
                mTempStateStack[mTempStateStackCount] = curStateInfo;
                curStateInfo = curStateInfo.parentStateInfo;
            }

            // Empty the StateStack
            mStateStackTopIndex = -1;

            moveTempStateStackToStateStack();
        }

6.StateMachine消息处理sendMessage

    /**
     * Enqueue a message to this state machine.
     *
     * Message is ignored if state machine has quit.
     */
    public final void sendMessage(int what) {
        // mSmHandler can be null if the state machine has quit.
        SmHandler smh = mSmHandler;
        if (smh == null) return;

        smh.sendMessage(obtainMessage(what));
    }

SmHandler是从Handler派生的,因此对应于sendMessage的是handleMessage:

        /**
         * Handle messages sent to the state machine by calling
         * the current state's processMessage. It also handles
         * the enter/exit calls and placing any deferred messages
         * back onto the queue when transitioning to a new state.
         */
        @Override
        public final void handleMessage(Message msg) {
            if (!mHasQuit) {
                if (mDbg) mSm.log("handleMessage: E msg.what=" + msg.what);

                /** 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);

                // We need to check if mSm == null here as we could be quitting.
                if (mDbg && mSm != null) mSm.log("handleMessage: X");
            }
        }

实际的消息处理调用了processMsg:

        /**
         * Process the message. If the current state doesn't handle
         * it, call the states parent and so on. If it is never handled then
         * call the state machines unhandledMessage method.
         * @return the state that processed the message
         */
        private final State processMsg(Message msg) {
            StateInfo curStateInfo = mStateStack[mStateStackTopIndex];
            if (mDbg) {
                mSm.log("processMsg: " + curStateInfo.state.getName());
            }

            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;
                    }
                    if (mDbg) {
                        mSm.log("processMsg: " + curStateInfo.state.getName());
                    }
                }
            }
            return (curStateInfo != null) ? curStateInfo.state : null;
        }

        /** Validate that the message was sent by quit or quitNow. */
        private final boolean isQuit(Message msg) {
            return (msg.what == SM_QUIT_CMD) && (msg.obj == mSmHandlerObj);
        }

在这个函数中可以看到,如果是显示调用了quit和quitNow接口,就会把Status状态转换为mQuittingState;其他正常调用时,首先会判断当前Status是否HANDLED了消息,如果是NOT_HANDLED状态,就会递归到父类Status中处理消息。
7.消息分发后同步状态机performTransitions

         /**
         * Do any transitions
         * @param msgProcessedState is the state that processed the message
         */
        private void performTransitions(State msgProcessedState, Message msg) {
            /**
             * If transitionTo has been called, exit and then enter
             * the appropriate states. We loop on this to allow
             * enter and exit methods to use transitionTo.
             */
            State orgState = mStateStack[mStateStackTopIndex].state;

            /**
             * Record whether message needs to be logged before we transition and
             * and we won't log special messages SM_INIT_CMD or SM_QUIT_CMD which
             * always set msg.obj to the handler.
             */
            boolean recordLogMsg = mSm.recordLogRec(mMsg) && (msg.obj != mSmHandlerObj);

            if (mLogRecords.logOnlyTransitions()) {
                /** Record only if there is a transition */
                if (mDestState != null) {
                    mLogRecords.add(mSm, mMsg, mSm.getLogRecString(mMsg), msgProcessedState,
                            orgState, mDestState);
                }
            } else if (recordLogMsg) {
                /** Record message */
                mLogRecords.add(mSm, mMsg, mSm.getLogRecString(mMsg), msgProcessedState, orgState,
                        mDestState);
            }

            State destState = mDestState;
            if (destState != null) {
                /**
                 * Process the transitions including transitions in the enter/exit methods
                 */
                while (true) {
                    if (mDbg) mSm.log("handleMessage: new destination call exit/enter");

                    /**
                     * 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();
                }
            }
        }

        /** @see StateMachine#transitionTo(IState) */
        private final void transitionTo(IState destState) {
            mDestState = (State) destState;
            if (mDbg) mSm.log("transitionTo: destState=" + mDestState.getName());
        }

        /** @see StateMachine#deferMessage(Message) */
        private final void deferMessage(Message msg) {
            if (mDbg) mSm.log("deferMessage: msg=" + msg.what);

            /* Copy the "msg" to "newMsg" as "msg" will be recycled */
            Message newMsg = obtainMessage();
            newMsg.copyFrom(msg);

            mDeferredMessages.add(newMsg);
        }

        /**
         * Move the deferred message to the front of the message queue.
         */
        private final void moveDeferredMessageAtFrontOfQueue() {
            /**
             * The oldest messages on the deferred list must be at
             * the front of the queue so start at the back, which
             * as the most resent message and end with the oldest
             * messages at the front of the queue.
             */
            for (int i = mDeferredMessages.size() - 1; i >= 0; i--) {
                Message curMsg = mDeferredMessages.get(i);
                if (mDbg) mSm.log("moveDeferredMessageAtFrontOfQueue; what=" + curMsg.what);
                sendMessageAtFrontOfQueue(curMsg);
            }
            mDeferredMessages.clear();
        }

这个阶段是Status的enter/exit逻辑处理:

        private final StateInfo setupTempStateStackWithStatesToEnter(State destState) {
            /**
             * Search up the parent list of the destination state for an active
             * state. Use a do while() loop as the destState must always be entered
             * even if it is active. This can happen if we are exiting/entering
             * the current state.
             */
            mTempStateStackCount = 0;
            StateInfo curStateInfo = mStateInfo.get(destState);
            do {
                mTempStateStack[mTempStateStackCount++] = curStateInfo;
                curStateInfo = curStateInfo.parentStateInfo;
            } while ((curStateInfo != null) && !curStateInfo.active);

            if (mDbg) {
                mSm.log("setupTempStateStackWithStatesToEnter: X mTempStateStackCount="
                        + mTempStateStackCount + ",curStateInfo: " + curStateInfo);
            }
            return curStateInfo;
        }
        /**
         * Call the exit method for each state from the top of stack
         * up to the common ancestor state.
         */
        private final void invokeExitMethods(StateInfo commonStateInfo) {
            while ((mStateStackTopIndex >= 0)
                    && (mStateStack[mStateStackTopIndex] != commonStateInfo)) {
                State curState = mStateStack[mStateStackTopIndex].state;
                if (mDbg) mSm.log("invokeExitMethods: " + curState.getName());
                curState.exit();
                mStateStack[mStateStackTopIndex].active = false;
                mStateStackTopIndex -= 1;
            }
        }

从setupTempStateStackWithStatesToEnter可以看出查找的是当前子类State层级中的未激活的顶层父类State,而invokeExitMethods可以看出首先会调用子类State的exit(),递归调用父类State的exit(),直至匹配到指定State(注意!curStateInfo.active这个限制条件);

        /**
         * Move the contents of the temporary stack to the state stack
         * reversing the order of the items on the temporary stack as
         * they are moved.
         *
         * @return index into mStateStack where entering needs to start
         */
        private final int moveTempStateStackToStateStack() {
            int startingIndex = mStateStackTopIndex + 1;
            int i = mTempStateStackCount - 1;
            int j = startingIndex;
            while (i >= 0) {
                if (mDbg) mSm.log("moveTempStackToStateStack: i=" + i + ",j=" + j);
                mStateStack[j] = mTempStateStack[i];
                j += 1;
                i -= 1;
            }

            mStateStackTopIndex = j - 1;
            if (mDbg) {
                mSm.log("moveTempStackToStateStack: X mStateStackTop=" + mStateStackTopIndex
                        + ",startingIndex=" + startingIndex + ",Top="
                        + mStateStack[mStateStackTopIndex].state.getName());
            }
            return startingIndex;
        }

        /**
         * Invoke the enter method starting at the entering index to top of state stack
         */
        private final void invokeEnterMethods(int stateStackEnteringIndex) {
            for (int i = stateStackEnteringIndex; i <= mStateStackTopIndex; i++) {
                if (mDbg) mSm.log("invokeEnterMethods: " + mStateStack[i].state.getName());
                mStateStack[i].state.enter();
                mStateStack[i].active = true;
            }
        }

从setupInitialStateStack和moveTempStateStackToStateStack接口可知,mTempStateStack是子类在栈底模式; mStateStack是顶层父类在栈底模式;
举例一幅层级图描述如下:

State树结构层次图

那么在sendMessage前mTempStateStack的堆栈情况:

mTempStateStack堆栈状态

mStateStack的堆栈情况:

mStateStack堆栈状态

因此在此阶段是先调用顶层父类State的enter()接口,然后再递归到子类的enter()逻辑处理。
最后,补一张针对Android测试用例的时序分析图,根据这张图,相信对StateMachine框架会有一个更好的理解
(测试源码在之前介绍已经贴出,模型如下)
 mP1    mP2
 /  \
mS2 mS1

测试用例时序分析图

3.结束语

如果你能清晰的分析出测试代码的Log输出,那么你已经掌握了StateMachine状态分发和处理机制原理,在自己的项目中就可以使用这种高级用法来规范代码解决复杂的状态处理。

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