转载请标明出处：http://blog.csdn.net/shensky711/article/details/53132952
本文出自: 【HansChen的博客】

概述

在Fragment使用中，有时候需要对Fragment进行add、remove、show、hide、replace等操作来进行Fragment的显示隐藏等管理，这些管理是通过FragmentTransaction进行事务管理的。事务管理是对于一系列操作进行管理，一个事务包含一个或多个操作命令，是逻辑管理的工作单元。一个事务开始于第一次执行操作语句，结束于Commit。通俗地将，就是把多个操作缓存起来，等调用commit的时候，统一批处理。下面会对Fragmeng的事务管理做一个代码分析

分析入口

    /*** 显示Fragment，如果Fragment已添加过，则直接show，否则构造一个Fragment** @param containerViewId 容器控件id* @param clz             Fragment类*/protected void showFragment(@IdRes int containerViewId, Class<? extends Fragment> clz) {FragmentManager fm = getFragmentManager();FragmentTransaction ft = fm.beginTransaction();//开始事务管理Fragment f;if ((f = fm.findFragmentByTag(clz.getName())) == null) {try {f = clz.newInstance();ft.add(containerViewId, f, clz.getName());//添加操作} catch (Exception e) {e.printStackTrace();}} else {ft.show(f);//添加操作}ft.commit();//提交事务}

上面是一个简单的显示Fragment的栗子，简单判断一下Fragment是否已添加过，添加过就直接show，否则构造一个Fragment，最后提交事务。

代码分析

FragmentManager

上图是获取FragmentManager的大体过程

要管理Fragment事务，首先是需要拿到FragmentManager，在Activity中可以通过getFragmentManager()方法获取(使用兼容包的话，通过FragmentActivity#getSupportFragmentManager())，在这里我们就不对兼容包进行分析了

    final FragmentController mFragments = FragmentController.createController(new HostCallbacks());/*** Return the FragmentManager for interacting with fragments associated* with this activity.*/public FragmentManager getFragmentManager() {return mFragments.getFragmentManager();}

FragmentManager是一个抽象类，它是通过mFragments.getFragmentManager()来获取的，mFragments是FragmentController对象，它通过FragmentController.createController(new HostCallbacks())生成，这是一个静态工厂方法：

    public static final FragmentController createController(FragmentHostCallback<?> callbacks) {return new FragmentController(callbacks);}

在这里面直接new了一个FragmentController对象，注意FragmentController的构造方法需要传入一个FragmentHostCallback

FragmentController构造方法

    private final FragmentHostCallback<?> mHost;private FragmentController(FragmentHostCallback<?> callbacks) {mHost = callbacks;}

构造方法很简单，传入了一个FragmentHostCallback实例

FragmentController#getFragmentManager

    public FragmentManager getFragmentManager() {return mHost.getFragmentManagerImpl();}

这里又调用了mHost的getFragmentManagerImpl方法，希望童鞋们没有被绕晕，mHost是一个FragmentHostCallback实例，那我们回过头来看看它传进来的地方

FragmentHostCallback

这个FragmentHostCallback是一个抽象类，我们可以看到，在Activity中是传入了 Activity#HostCallbacks内部类，这个就是FragmentHostCallback的实现类

FragmentHostCallback#getFragmentManagerImpl

    final FragmentManagerImpl mFragmentManager = new FragmentManagerImpl();FragmentManagerImpl getFragmentManagerImpl() {return mFragmentManager;}

终于找到FragmentManager的真身FragmentManagerImpl了

FragmentManagerImpl#beginTransaction

    @Overridepublic FragmentTransaction beginTransaction() {return new BackStackRecord(this);}

可以看到，所谓的FragmentTransaction其实就是一个BackStackRecord。到现在，FragmentManager和FragmentTransaction我们都找到了。下图就是各个类之间的关系：

下面开始真正的事务管理分析，我们先选择一个事务add来进行分析

FragmentTransaction#add

    public FragmentTransaction add(int containerViewId, Fragment fragment, String tag) {doAddOp(containerViewId, fragment, tag, OP_ADD);return this;}private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {//设置fragment的FragmentManagerImpl，mManager其实就是Activity#HostCallbacks中的成员变量fragment.mFragmentManager = mManager;//设置fragment的tagif (tag != null) {if (fragment.mTag != null && !tag.equals(fragment.mTag)) {throw new IllegalStateException("...");}fragment.mTag = tag;}if (containerViewId != 0) {if (containerViewId == View.NO_ID) {throw new IllegalArgumentException("...");}if (fragment.mFragmentId != 0 && fragment.mFragmentId != containerViewId) {throw new IllegalStateException("");}//设置fragment的mContainerId以及mFragmentIdfragment.mContainerId = fragment.mFragmentId = containerViewId;}//新增一个操作Op op = new Op();op.cmd = opcmd;op.fragment = fragment;//添加操作addOp(op);}//插入到链表的最后void addOp(Op op) {if (mHead == null) {mHead = mTail = op;} else {op.prev = mTail;mTail.next = op;mTail = op;}op.enterAnim = mEnterAnim;op.exitAnim = mExitAnim;op.popEnterAnim = mPopEnterAnim;op.popExitAnim = mPopExitAnim;mNumOp++;}

add的操作步骤为：

1. 设置fragment的FragmentManagerImpl
2. 设置fragment的tag
3. 设置fragment的mContainerId以及mFragmentId
4. 插入一个类型为OP_ADD的操作到链表最后

这里用到了一个类：

    static final class Op {Op next;//下一操作节点Op prev;//上一操作节点int cmd;//操作类型，可选有：OP_NULL|OP_ADD|OP_REPLACE|OP_REMOVE|OP_HIDE|OP_SHOW|OP_DETACH|OP_ATTACHFragment fragment;//操作的Fragment对象int enterAnim;//入场动画int exitAnim;//出场动画int popEnterAnim;//弹入动画int popExitAnim;//弹出动画ArrayList<Fragment> removed;}

这是一个操作链表节点。所有add、remove、hide等事物最终会形成一个操作链

FragmentTransaction#commit

等所有操作都插入后，最后我们需要调用FragmentTransaction的commit方法，操作才会真正地执行。

    public int commit() {return commitInternal(false);}int commitInternal(boolean allowStateLoss) {//防止重复commitif (mCommitted) {throw new IllegalStateException("commit already called");}//DEBUG代码统统不管if (FragmentManagerImpl.DEBUG) {Log.v(TAG, "Commit: " + this);LogWriter logw = new LogWriter(Log.VERBOSE, TAG);PrintWriter pw = new FastPrintWriter(logw, false, 1024);dump("  ", null, pw, null);pw.flush();}mCommitted = true;//只有调用了addToBackStack方法之后，这个标记才会为trueif (mAddToBackStack) {mIndex = mManager.allocBackStackIndex(this);} else {mIndex = -1;}//插入事物队列mManager.enqueueAction(this, allowStateLoss);return mIndex;}

FragmentManagerImpl#enqueueAction

    /*** Adds an action to the queue of pending actions.** @param action the action to add* @param allowStateLoss whether to allow loss of state information* @throws IllegalStateException if the activity has been destroyed*/public void enqueueAction(Runnable action, boolean allowStateLoss) {if (!allowStateLoss) {checkStateLoss();}synchronized (this) {if (mDestroyed || mHost == null) {throw new IllegalStateException("Activity has been destroyed");}if (mPendingActions == null) {mPendingActions = new ArrayList<Runnable>();}mPendingActions.add(action);if (mPendingActions.size() == 1) {mHost.getHandler().removeCallbacks(mExecCommit);mHost.getHandler().post(mExecCommit);}}}

这里把操作添加到mPendingActions列表里去。并通过mHost.getHandler()获取Handler发送执行请求。从上面的分析知道，mHost就是Activity的HostCallbacks，构造方法中把Activity的mHandler传进去了，这里执行的mHost.getHandler()获取到的也就是Activity中的mHandler，这样做是因为需要在主线程中执行

final Handler mHandler = new Handler();

再看看mExecCommit中做了什么操作：

    Runnable mExecCommit = new Runnable() {@Overridepublic void run() {execPendingActions();}};/*** Only call from main thread!*/public boolean execPendingActions() {if (mExecutingActions) {throw new IllegalStateException("Recursive entry to executePendingTransactions");}//再次检测是否主线程if (Looper.myLooper() != mHost.getHandler().getLooper()) {throw new IllegalStateException("Must be called from main thread of process");}boolean didSomething = false;while (true) {int numActions;synchronized (this) {//参数检测if (mPendingActions == null || mPendingActions.size() == 0) {break;}numActions = mPendingActions.size();if (mTmpActions == null || mTmpActions.length < numActions) {mTmpActions = new Runnable[numActions];}mPendingActions.toArray(mTmpActions);mPendingActions.clear();mHost.getHandler().removeCallbacks(mExecCommit);}mExecutingActions = true;//遍历执行待处理的事务操作for (int i=0; i<numActions; i++) {mTmpActions[i].run();mTmpActions[i] = null;}mExecutingActions = false;didSomething = true;}doPendingDeferredStart();return didSomething;}

插入了事物之后，就是在主线程中把需要处理的事务统一处理，处理事务是通过执行mTmpActions[i].run()进行的，这个mTmpActions[i]就是前面我们通过enqueueAction方法插入的BackStackRecord，童鞋们可能没注意到，它可是一个Runnable，我们来看看它的定义

final class BackStackRecord extends FragmentTransaction implementsFragmentManager.BackStackEntry, Runnable {static final String TAG = FragmentManagerImpl.TAG;... ...
}

兜兜转转，我们又回到了BackStackRecord

BackStackRecord#run

    public void run() {......if (mManager.mCurState >= Fragment.CREATED) {SparseArray<Fragment> firstOutFragments = new SparseArray<Fragment>();SparseArray<Fragment> lastInFragments = new SparseArray<Fragment>();calculateFragments(firstOutFragments, lastInFragments);beginTransition(firstOutFragments, lastInFragments, false);}//遍历链表，根据cmd事务类型依次处理事务Op op = mHead;while (op != null) {switch (op.cmd) {case OP_ADD: {Fragment f = op.fragment;f.mNextAnim = op.enterAnim;mManager.addFragment(f, false);}break;case OP_REPLACE: {Fragment f = op.fragment;int containerId = f.mContainerId;if (mManager.mAdded != null) {for (int i = mManager.mAdded.size() - 1; i >= 0; i--) {Fragment old = mManager.mAdded.get(i);if (old.mContainerId == containerId) {if (old == f) {op.fragment = f = null;} else {if (op.removed == null) {op.removed = new ArrayList<Fragment>();}op.removed.add(old);old.mNextAnim = op.exitAnim;if (mAddToBackStack) {old.mBackStackNesting += 1;}mManager.removeFragment(old, mTransition, mTransitionStyle);}}}}if (f != null) {f.mNextAnim = op.enterAnim;mManager.addFragment(f, false);}}break;case OP_REMOVE: {Fragment f = op.fragment;f.mNextAnim = op.exitAnim;mManager.removeFragment(f, mTransition, mTransitionStyle);}break;case OP_HIDE: {Fragment f = op.fragment;f.mNextAnim = op.exitAnim;mManager.hideFragment(f, mTransition, mTransitionStyle);}break;case OP_SHOW: {Fragment f = op.fragment;f.mNextAnim = op.enterAnim;mManager.showFragment(f, mTransition, mTransitionStyle);}break;case OP_DETACH: {Fragment f = op.fragment;f.mNextAnim = op.exitAnim;mManager.detachFragment(f, mTransition, mTransitionStyle);}break;case OP_ATTACH: {Fragment f = op.fragment;f.mNextAnim = op.enterAnim;mManager.attachFragment(f, mTransition, mTransitionStyle);}break;default: {throw new IllegalArgumentException("Unknown cmd: " + op.cmd);}}op = op.next;}mManager.moveToState(mManager.mCurState, mTransition,mTransitionStyle, true);if (mAddToBackStack) {mManager.addBackStackState(this);}}

到这一步，提交的事务就被真正执行了，我们知道，即使commit了事务之后，也不是同步执行的，是通过Handler发送到主线程执行的。

所有事务的处理都是在run方法里面执行，但是我们留意到，想要搞清楚add、remove等事务背后真正做了什么，还需要深入了解FragmentManagerImpl。

本文主要讲解Fragment事务的流程，FragmentManagerImpl的分析准备放到下一篇分析文章 「Fragment源码分析」 中，相信通过分析之后，就可以对Fragment的生命周期也有一个很好的认识了