編輯:關於Android編程
StateMachine有自己單獨的工作線程,
protected StateMachine(String name) { mSmThread = new HandlerThread(name); mSmThread.start(); Looper looper = mSmThread.getLooper(); initStateMachine(name, looper); }
StateMachine中一個很重要的角色就是SmHandler,SmHandler在構造伊始就添加了兩個狀態:
private SmHandler(Looper looper, StateMachine sm) { super(looper); mSm = sm; addState(mHaltingState, null); addState(mQuittingState, null); }
這兩個狀態意思是整個狀態機的停止狀態和退出狀態,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; } }
可見這兩個狀態都是繼承自State,這個狀態機中的狀態都是用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); } }
看起來很簡單,主要就三個函數,enter表示進入狀態的回調,exit表示離開狀態的回調,processMessage表示收到消息的回調。
再來看State是如何添加到狀態機中的,如下:
/** The map of all of the states in the state machine */ private HashMapmStateInfo = new HashMap (); 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; }
可見每個State都只能有最多一個parent,或者沒有parent,那這種帶層級的State狀態機有什麼意義呢?因為以往我們認為的狀態機都是若干完全獨立的狀態之間互相切換,不會有狀態層級關系的,接下來我們就來看看這種層級關系的奧秘,從狀態切換入手:
private final void transitionTo(IState destState) { mDestState = (State) destState; }
只是設置了一個變量,不免讓人有些失望,我們看這個變量在哪引用的,結果是在performTransitions中,而這個函數是在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); } }
這個mHasQuit從字面意思上就是狀態機quit了,狀態切換到QuitState時整個狀態機就要destroy了,這時候再發消息也不會處理了。再看mIsConstructionCompleted,這個字面意思是狀態機的初始化構造是否結束了,用到的地方也就是handleMessage中,而構造指令就是SM_INIT_CMD,發出指令的地方在狀態機啟動的時候,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(); }
我們參考AdapterState狀態機,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; }
就是創建好狀態機之後調用其start函數啟動狀態機。我們接下來看看start函數中completeConstruction的實現,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)); }
這裡首先遍歷所有的狀態,算出最大的深度,然後初始化狀態棧mStateStack和mTempStateStack,再調用setupInitialStateStack,最後才發送了SM_INIT_CMD到消息隊列的頭。
狀態機的切換中非常重要的角色就是這個狀態棧了,所以我們要重點關注狀態棧的初始化。不過這裡還是先分析SM_INIT_CMD,發消息的時候還帶上了mSmHandlerObj,這個東西就是一個普通的Object,每次狀態機自身發的消息都會帶上這個Object以區分是外面的消息還是狀態機自身的消息,比如關於INIT和QUIT都會帶上這個Object。在handleMessage中收到SM_INIT_CMD後會給mIsConstructionCompleted置為true,表示初始化過了,然後調用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; } }
這個函數會從stateStackEnteringIndex到mStateStackTopIndex遍歷調用enter函數,這個mStateStackTopIndex是什麼呢?要搞清楚這個問題我們就得回到setupInitialStateStack函數了。
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(); }
這裡從initial state開始遍歷其父state一直到頭,遍歷路徑記錄在mTempStateStack中,而將StateStack棧頂index置為-1,然後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; }
這個函數相當於將TempStateStack倒序copy到StateStack中。在TempStateStack中先入棧的是底層的狀態,後入棧的是頂層的狀態,而StateStack剛好相反,先入棧的是頂層parent狀態,後入棧的是底層狀態。現在我們回到invokeEnterMethods(0),這裡會從StateStack的0開始到mStateStackTopIndex調用enter,也就是從狀態機的初始狀態的最頂層parent層層往下調到最底層狀態的enter。
總結一下,狀態機初始化時會先addState,然後設置好initial state,然後start,在start中初始化StateStack,將initial state從祖先開始依次入棧,然後再從祖先開始依次調用他們的enter回調。
我們再回到handleMessage,在invokeEnterMethods之後還調用了performTransitions(msgProcessedState, msg); 這個是檢查狀態切換的,如果設置過mDestState則這裡要切換狀態了:
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(); } } }
由於是初始化,所以msgProcessedState為null,而且mDestState也為null,所以這裡其實什麼也沒有做。再回到handleMessage,如果是初始化之後則會調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; }
從狀態棧中取出棧頂狀態,將msg交給該state處理,如果處理了返回true則直接返回當前處理的state,否則交給父state處理,如果一直沒有哪個state可以處理的話就調用SmHandler的unhandledMessage。
從processMsg返回到handleMessage後,還會調用performTransitions,不過可以看到如果mDestState為空則什麼也不做。而mDestState是調transitionTo設置的,所以如果是單純的發消息不會涉及狀態的切換。
通常transitionState都是在某個state的processMsg中,這樣在processMsg返回後繼續調performTransitions時就會檢查切換狀態了。
不過注意的是切換狀態會依次將當前狀態棧出棧並將新狀態鏈入棧,不過如果兩者有共同的祖先結點,那祖先結點就沒必要折騰了,只是下面不同的子狀態才exit。我們分析performTransitions函數,首先通過setupTempStateStackWithStatesToEnter找到最低公共祖先,這裡面就是從目標state開始往上遍歷直到發現state是active為止,因為當前state的鏈上肯定都是active的。
接下來從當前狀態開始調用exit直到最低公共祖先,注意不包括這個祖先,路上的state的active都標為false。
我們總結一下,當調enter的時候是從上往下,調exit的時候是從下往上,處理msg的時候也是從下往上。不過也可以理解,初始化的時候是先從上開始,退出的時候是反著來。處理消息也是先讓下處理,處理不好才往上走。
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