日常开发中我们经常使用 Fragment 管理布局，使用起来非常方便，但是在简单的 API 背后隐藏了什么操作，很多人恐怕不了解。

如果你回答不出这些问题，那这篇文章可能就对你有些帮助：

• Fragment FragmentManager FragmentTransaction 的关系和作用
• Fragment 如何实现布局的添加替换
• 嵌套 Fragment 的原理

读完本文你将了解：

注意，这里分析的是 support-fragment-25.3.1-source.jar" 的support.v4.app.Fragment 及相关类。

Fragment 的使用

Fragment 的使用大家应该都熟悉，这里举个例子，要实现这样的类似饿了么点餐效果：

界面丑了点，但意思是差不多的哈，，左边一个列表，点击后切换右边的布局。我们就可以使用 Fragment 来实现。

实现也很简单，创建一个的布局，然后在 Activity 里点击时替换 Fragment。

mFragmentManager = getSupportFragmentManager();
mFragmentManager.beginTransaction().replace(R.id.fl_content, fragment).commitAllowingStateLoss();

代码很简单，核心就三步：

1. 创建 Fragment
2. 获取 FragmentManager
3. 调用事务，添加、替换

我们一步步来了解这背后的故事。

Fragment 大家应该比较熟悉，放到最后。

先来看看 FragmentManager。

FragmentManager

public abstract class FragmentManager {...}

FragmentManager 是一个抽象类，定义了一些和 Fragment 相关的操作和内部类/接口。

定义的操作

FragmentManager 中定义的方法如下：

//开启一系列对 Fragments 的操作
public abstract FragmentTransaction beginTransaction();//FragmentTransaction.commit() 是异步执行的，如果你想立即执行，可以调用这个方法
public abstract boolean executePendingTransactions();//根据 ID 找到从 XML 解析出来的或者事务中添加的 Fragment
//首先会找添加到 FragmentManager 中的，找不到就去回退栈里找
public abstract Fragment findFragmentById(@IdRes int id);//跟上面的类似，不同的是使用 tag 进行查找
public abstract Fragment findFragmentByTag(String tag);//弹出回退栈中栈顶的 Fragment，异步执行的
public abstract void popBackStack();//立即弹出回退栈中栈顶的，直接执行哦
public abstract boolean popBackStackImmediate();//返回栈顶符合名称的，如果传入的 name 不为空，在栈中间找到了 Fragment，那将弹出这个 Fragment 上面的所有 Fragment
//有点类似启动模式的 singleTask 的感觉
//如果传入的 name 为 null，那就和 popBackStack() 一样了
//异步执行
public abstract void popBackStack(String name, int flags);//同步版的上面
public abstract boolean popBackStackImmediate(String name, int flags);//和使用 name 查找、弹出一样 
//不同的是这里的 id 是 FragmentTransaction.commit() 返回的 id
public abstract void popBackStack(int id, int flags);
//你懂得
public abstract boolean popBackStackImmediate(int id, int flags);//获取回退栈中的元素个数
public abstract int getBackStackEntryCount();
//根据索引获取回退栈中的某个元素
public abstract BackStackEntry getBackStackEntryAt(int index);//添加或者移除一个监听器
public abstract void addOnBackStackChangedListener(OnBackStackChangedListener listener);
public abstract void removeOnBackStackChangedListener(OnBackStackChangedListener listener);//还定义了将一个 Fragment 实例作为参数传递
public abstract void putFragment(Bundle bundle, String key, Fragment fragment);
public abstract Fragment getFragment(Bundle bundle, String key);//获取 manager 中所有添加进来的 Fragment
public abstract List<Fragment> getFragments();

可以看到，定义的方法有很多是异步执行的，后面看看它究竟是如何实现的异步。

内部类/接口：

• BackStackEntry：Fragment 后退栈中的一个元素
• onBackStackChangedListener：后退栈变动监听器
• FragmentLifecycleCallbacks: FragmentManager 中的 Fragment 生命周期监听

//后退栈中的一个元素
public interface BackStackEntry {//栈中该元素的唯一标识public int getId();//获取 FragmentTransaction#addToBackStack(String) 设置的名称public String getName();@StringRespublic int getBreadCrumbTitleRes();@StringRespublic int getBreadCrumbShortTitleRes();public CharSequence getBreadCrumbTitle();public CharSequence getBreadCrumbShortTitle();
}

可以看到 BackStackEntry 的接口比较简单，关键信息就是 ID 和 Name。

//在 Fragment 回退栈中有变化时回调
public interface OnBackStackChangedListener {public void onBackStackChanged();
}

//FragmentManager 中的 Fragment 生命周期监听public abstract static class FragmentLifecycleCallbacks {public void onFragmentPreAttached(FragmentManager fm, Fragment f, Context context) {}public void onFragmentAttached(FragmentManager fm, Fragment f, Context context) {}public void onFragmentCreated(FragmentManager fm, Fragment f, Bundle savedInstanceState) {}public void onFragmentActivityCreated(FragmentManager fm, Fragment f,Bundle savedInstanceState) {}public void onFragmentViewCreated(FragmentManager fm, Fragment f, View v,Bundle savedInstanceState) {}public void onFragmentStarted(FragmentManager fm, Fragment f) {}public void onFragmentResumed(FragmentManager fm, Fragment f) {}public void onFragmentPaused(FragmentManager fm, Fragment f) {}public void onFragmentStopped(FragmentManager fm, Fragment f) {}public void onFragmentSaveInstanceState(FragmentManager fm, Fragment f, Bundle outState) {}public void onFragmentViewDestroyed(FragmentManager fm, Fragment f) {}public void onFragmentDestroyed(FragmentManager fm, Fragment f) {}public void onFragmentDetached(FragmentManager fm, Fragment f) {}}
}

熟悉 Fragment 生命周期的同学一定觉得很面熟，这个接口就是为我们提供一个 FragmentManager 所有 Fragment 生命周期变化的回调。

小结：

可以看到，FragmentManager 是一个抽象类，它定义了对一个 Activity/Fragment 中 添加进来的 Fragment 列表、Fragment 回退栈的操作、管理。

实现类 FragmentManagerImpl

FragmentManager 定义的任务是由 FragmentManagerImpl 实现的。

主要成员：

final class FragmentManagerImpl extends FragmentManager implements LayoutInflaterFactory {ArrayList<OpGenerator> mPendingActions;Runnable[] mTmpActions;boolean mExecutingActions;ArrayList<Fragment> mActive;ArrayList<Fragment> mAdded;ArrayList<Integer> mAvailIndices;ArrayList<BackStackRecord> mBackStack;ArrayList<Fragment> mCreatedMenus;// Must be accessed while locked.ArrayList<BackStackRecord> mBackStackIndices;ArrayList<Integer> mAvailBackStackIndices;ArrayList<OnBackStackChangedListener> mBackStackChangeListeners;private CopyOnWriteArrayList<Pair<FragmentLifecycleCallbacks, Boolean>> mLifecycleCallbacks;
//...
}

可以看到，FragmentManagerImpl 中定义了 添加的、活跃的。以及回退栈的列表，这和 FragmentManager 的要求一致。

int mCurState = Fragment.INITIALIZING;
FragmentHostCallback mHost;
FragmentContainer mContainer;
Fragment mParent;static Field sAnimationListenerField = null;boolean mNeedMenuInvalidate;
boolean mStateSaved;
boolean mDestroyed;
String mNoTransactionsBecause;
boolean mHavePendingDeferredStart;

接着还有当前的状态，当前 Fragment 的起始 mParent，以及 FragmentManager 的 mHost 和 mContainer。

FragmentContainer 就是一个接口，定义了关于布局的两个方法：

public abstract class FragmentContainer {@Nullablepublic abstract View onFindViewById(@IdRes int id);public abstract boolean onHasView();
}

而 FragmentHostCallback 就复杂一点了，它提供了 Fragment 需要的信息，也定义了 Fragment 宿主应该做的操作：

public abstract class FragmentHostCallback<E> extends FragmentContainer {private final Activity mActivity;final Context mContext;private final Handler mHandler;final int mWindowAnimations;final FragmentManagerImpl mFragmentManager = new FragmentManagerImpl();//...
}

我们知道，一般来说 Fragment 的宿主就两种：

1. Activity
2. Fragment

比如 FragmentActivity 的内部类 HostCallbacks 就实现了这个抽象类：

class HostCallbacks extends FragmentHostCallback<FragmentActivity> {public HostCallbacks() {super(FragmentActivity.this /*fragmentActivity*/);}//...@Overridepublic LayoutInflater onGetLayoutInflater() {return FragmentActivity.this.getLayoutInflater().cloneInContext(FragmentActivity.this);}@Overridepublic FragmentActivity onGetHost() {return FragmentActivity.this;}@Overridepublic void onStartActivityFromFragment(Fragment fragment, Intent intent, int requestCode) {FragmentActivity.this.startActivityFromFragment(fragment, intent, requestCode);}@Overridepublic void onStartActivityFromFragment(Fragment fragment, Intent intent, int requestCode, @Nullable Bundle options) {FragmentActivity.this.startActivityFromFragment(fragment, intent, requestCode, options);}@Overridepublic void onRequestPermissionsFromFragment(@NonNull Fragment fragment,@NonNull String[] permissions, int requestCode) {FragmentActivity.this.requestPermissionsFromFragment(fragment, permissions,requestCode);}@Overridepublic boolean onShouldShowRequestPermissionRationale(@NonNull String permission) {return ActivityCompat.shouldShowRequestPermissionRationale(FragmentActivity.this, permission);}@Overridepublic boolean onHasWindowAnimations() {return getWindow() != null;}@Overridepublic void onAttachFragment(Fragment fragment) {FragmentActivity.this.onAttachFragment(fragment);}@Nullable@Overridepublic View onFindViewById(int id) {return FragmentActivity.this.findViewById(id);}@Overridepublic boolean onHasView() {final Window w = getWindow();return (w != null && w.peekDecorView() != null);}
}

我们再看看他对 FragmentManager 定义的关键方法是如何实现的。

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

beginTransaction() 返回一个新的 BackStackRecord ，我们后面介绍。

前面提到了，popBackStack() 是一个异步操作，它是如何实现异步的呢？

@Override
public void popBackStack() {enqueueAction(new PopBackStackState(null, -1, 0), false);
}
public void enqueueAction(OpGenerator 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<>();}mPendingActions.add(action);scheduleCommit();}
}
private void scheduleCommit() {synchronized (this) {boolean postponeReady =mPostponedTransactions != null && !mPostponedTransactions.isEmpty();boolean pendingReady = mPendingActions != null && mPendingActions.size() == 1;if (postponeReady || pendingReady) {mHost.getHandler().removeCallbacks(mExecCommit);mHost.getHandler().post(mExecCommit);}}
}

可以看到，调用到最后，是调用宿主中的 Handler 来发送任务的，so easy 嘛。其他的异步执行也是类似，就不赘述了。

后退栈相关方法：

ArrayList<BackStackRecord> mBackStack;
@Override
public int getBackStackEntryCount() {return mBackStack != null ? mBackStack.size() : 0;
}@Override
public BackStackEntry getBackStackEntryAt(int index) {return mBackStack.get(index);
}

可以看到，开始事务和后退栈，返回/操作的都是 BackStackRecord，我们来了解了解它是何方神圣。

事务

BackStackRecord 继承了 FragmentTransaction：

final class BackStackRecord extends FragmentTransaction implementsFragmentManager.BackStackEntry, FragmentManagerImpl.OpGenerator {...}

先来看看 FragmentTransaction。

FragmentTransaction

FragmentTransaction 定义了一系列对 Fragment 的操作方法：

//它会调用 add(int, Fragment, String)，其中第一个参数传的是 0
public abstract FragmentTransaction add(Fragment fragment, String tag);//它会调用 add(int, Fragment, String)，其中第三个参数是 null
public abstract FragmentTransaction add(@IdRes int containerViewId, Fragment fragment);//添加一个 Fragment 给 Activity 的最终实现
//第一个参数表示 Fragment 要放置的布局 id
//第二个参数表示要添加的 Fragment，【注意】一个 Fragment 只能添加一次
//第三个参数选填，可以给 Fragment 设置一个 tag，后续可以使用这个 tag 查询它
public abstract FragmentTransaction add(@IdRes int containerViewId, Fragment fragment,@Nullable String tag);//调用 replace(int, Fragment, String)，第三个参数传的是 null
public abstract FragmentTransaction replace(@IdRes int containerViewId, Fragment fragment);//替换宿主中一个已经存在的 fragment
//这一个方法等价于先调用 remove(), 再调用 add()
public abstract FragmentTransaction replace(@IdRes int containerViewId, Fragment fragment,@Nullable String tag);//移除一个已经存在的 fragment
//如果之前添加到宿主上，那它的布局也会被移除
public abstract FragmentTransaction remove(Fragment fragment);//隐藏一个已存的 fragment
//其实就是将添加到宿主上的布局隐藏
public abstract FragmentTransaction hide(Fragment fragment);//显示前面隐藏的 fragment，这只适用于之前添加到宿主上的 fragment
public abstract FragmentTransaction show(Fragment fragment);//将指定的 fragment 将布局上解除
//当调用这个方法时，fragment 的布局已经销毁了
public abstract FragmentTransaction detach(Fragment fragment);//当前面解除一个 fragment 的布局绑定后，调用这个方法可以重新绑定
//这将导致该 fragment 的布局重建，然后添加、展示到界面上
public abstract FragmentTransaction attach(Fragment fragment);

对 fragment 的操作基本就这几步，我们知道，要完成对 fragment 的操作，最后还需要提交一下：

mFragmentManager.beginTransaction().replace(R.id.fl_child, getChildFragment())
//        .commit().commitAllowingStateLoss();

事务的四种提交方式

事务最终的提交方法有四种：

1. commit()
2. commitAllowingStateLoss()
3. commitNow()
4. commitNowAllowingStateLoss()

它们之间的特点及区别如下：

public abstract int commit();

commit() 在主线程中异步执行，其实也是 Handler 抛出任务，等待主线程调度执行。

注意：
commit() 需要在宿主 Activity 保存状态之前调用，否则会报错。
这是因为如果 Activity 出现异常需要恢复状态，在保存状态之后的 commit() 将会丢失，这和调用的初衷不符，所以会报错。

public abstract int commitAllowingStateLoss();

commitAllowingStateLoss() 也是异步执行，但它的不同之处在于，允许在 Activity 保存状态之后调用，也就是说它遇到状态丢失不会报错。

因此我们一般在界面状态出错是可以接受的情况下使用它。

public abstract void commitNow();

commitNow() 是同步执行的，立即提交任务。

前面提到 FragmentManager.executePendingTransactions() 也可以实现立即提交事务。但我们一般建议使用 commitNow(), 因为另外那位是一下子执行所有待执行的任务，可能会把当前所有的事务都一下子执行了，这有可能有副作用。

此外，这个方法提交的事务可能不会被添加到 FragmentManger 的后退栈，因为你这样直接提交，有可能影响其他异步执行任务在栈中的顺序。

和 commit() 一样，commitNow() 也必须在 Activity 保存状态前调用，否则会抛异常。

public abstract void commitNowAllowingStateLoss();

同步执行的 commitAllowingStateLoss()。

OK，了解了 FragmentTransaction 定义的操作，去看看我们真正关心的、 beginTransaction() 中返回的 BackStackRecord:

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

事务真正实现/回退栈 BackStackRecord

BackStackRecord 既是对 Fragment 进行操作的事务的真正实现，也是 FragmentManager 中的回退栈的实现：

final class BackStackRecord extends FragmentTransaction implementsFragmentManager.BackStackEntry, FragmentManagerImpl.OpGenerator {...}

它的关键成员：

final FragmentManagerImpl mManager;//Op 可选的状态值
static final int OP_NULL = 0;
static final int OP_ADD = 1;
static final int OP_REPLACE = 2;
static final int OP_REMOVE = 3;
static final int OP_HIDE = 4;
static final int OP_SHOW = 5;
static final int OP_DETACH = 6;
static final int OP_ATTACH = 7;ArrayList<Op> mOps = new ArrayList<>();
static final class Op {int cmd;    //状态Fragment fragment;int enterAnim;int exitAnim;int popEnterAnim;int popExitAnim;
}int mIndex = -1;    //栈中最后一个元素的索引

可以看到 Op 就是添加了状态和动画信息的 Fragment， mOps 就是栈中所有的 Fragment。

事务定义的方法它是如何实现的呢。

先看添加一个 Fragment 到布局 add() 的实现：

@Override
public FragmentTransaction add(int containerViewId, Fragment fragment) {doAddOp(containerViewId, fragment, null, OP_ADD);return this;
}@Override
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) {final Class fragmentClass = fragment.getClass();final int modifiers = fragmentClass.getModifiers();if (fragmentClass.isAnonymousClass() || !Modifier.isPublic(modifiers)|| (fragmentClass.isMemberClass() && !Modifier.isStatic(modifiers))) {throw new IllegalStateException("Fragment " + fragmentClass.getCanonicalName()+ " must be a public static class to be  properly recreated from"+ " instance state.");}//1.修改添加的 fragmentManager 为当前栈的 managerfragment.mFragmentManager = mManager;if (tag != null) {if (fragment.mTag != null && !tag.equals(fragment.mTag)) {throw new IllegalStateException("Can't change tag of fragment "+ fragment + ": was " + fragment.mTag+ " now " + tag);}fragment.mTag = tag;}if (containerViewId != 0) {if (containerViewId == View.NO_ID) {throw new IllegalArgumentException("Can't add fragment "+ fragment + " with tag " + tag + " to container view with no id");}if (fragment.mFragmentId != 0 && fragment.mFragmentId != containerViewId) {throw new IllegalStateException("Can't change container ID of fragment "+ fragment + ": was " + fragment.mFragmentId+ " now " + containerViewId);}//2.设置宿主 ID 为布局 IDfragment.mContainerId = fragment.mFragmentId = containerViewId;}//3.构造 OpOp op = new Op();op.cmd = opcmd;    //状态op.fragment = fragment;//4.添加到数组列表中addOp(op);
}
void addOp(Op op) {mOps.add(op);op.enterAnim = mEnterAnim;op.exitAnim = mExitAnim;op.popEnterAnim = mPopEnterAnim;op.popExitAnim = mPopExitAnim;
}

可以看到添加一个 Fragment 到布局很简单，概况一下就是：
修改 fragmentManager 和 ID，构造成 Op，设置状态信息，然后添加到列表里。

添加完了看看替换 replace 的实现：

@Override
public FragmentTransaction replace(int containerViewId, Fragment fragment) {return replace(containerViewId, fragment, null);
}@Override
public FragmentTransaction replace(int containerViewId, Fragment fragment, String tag) {if (containerViewId == 0) {throw new IllegalArgumentException("Must use non-zero containerViewId");}doAddOp(containerViewId, fragment, tag, OP_REPLACE);return this;
}

太可怕了，也是调用上面刚提到的 doAddOp()，不同之处在于第四个参数为 OP_REPLACE，看来之前小看了这个状态值！

再看其他方法的实现就很简单了，无非就是构造一个 Op，设置对应的状态值。

@Override
public FragmentTransaction remove(Fragment fragment) {Op op = new Op();op.cmd = OP_REMOVE;op.fragment = fragment;addOp(op);return this;
}@Override
public FragmentTransaction hide(Fragment fragment) {Op op = new Op();op.cmd = OP_HIDE;op.fragment = fragment;addOp(op);return this;
}@Override
public FragmentTransaction show(Fragment fragment) {Op op = new Op();op.cmd = OP_SHOW;op.fragment = fragment;addOp(op);return this;
}

那这些状态值的不同是什么时候起作用的呢？

别忘了我们操作 Fragment 还有最后一步，提交。

看看这两个是怎么实现的：

@Override
public int commit() {return commitInternal(false);
}@Override
public int commitAllowingStateLoss() {return commitInternal(true);
}
int commitInternal(boolean allowStateLoss) {if (mCommitted) throw new IllegalStateException("commit already called");//...mCommitted = true;if (mAddToBackStack) {mIndex = mManager.allocBackStackIndex(this);    //更新 index 信息} else {mIndex = -1;}mManager.enqueueAction(this, allowStateLoss);    //异步任务入队return mIndex;
}
public void enqueueAction(OpGenerator 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<>();}mPendingActions.add(action);scheduleCommit();    //发送任务}
}
private void scheduleCommit() {synchronized (this) {boolean postponeReady =mPostponedTransactions != null && !mPostponedTransactions.isEmpty();boolean pendingReady = mPendingActions != null && mPendingActions.size() == 1;if (postponeReady || pendingReady) {mHost.getHandler().removeCallbacks(mExecCommit);mHost.getHandler().post(mExecCommit);}}
}

前面已经介绍过了，FragmentManager.enqueueAction() 最终是使用 Handler 实现的异步执行。

现在的问题是执行的任务是啥?

答案就是 Handler 发送的任务 mExecCommit:

Runnable mExecCommit = new Runnable() {@Overridepublic void run() {execPendingActions();}
};
/*** Only call from main thread!* 更新 UI 嘛，肯定得在主线程*/
public boolean execPendingActions() {ensureExecReady(true);boolean didSomething = false;while (generateOpsForPendingActions(mTmpRecords, mTmpIsPop)) {mExecutingActions = true;try {optimizeAndExecuteOps(mTmpRecords, mTmpIsPop);    //这里是入口} finally {cleanupExec();}didSomething = true;}doPendingDeferredStart();return didSomething;
}
private void optimizeAndExecuteOps(ArrayList<BackStackRecord> records,ArrayList<Boolean> isRecordPop) {if (records == null || records.isEmpty()) {return;}if (isRecordPop == null || records.size() != isRecordPop.size()) {throw new IllegalStateException("Internal error with the back stack records");}// Force start of any postponed transactions that interact with scheduled transactions:executePostponedTransaction(records, isRecordPop);final int numRecords = records.size();int startIndex = 0;for (int recordNum = 0; recordNum < numRecords; recordNum++) {final boolean canOptimize = records.get(recordNum).mAllowOptimization;if (!canOptimize) {// execute all previous transactionsif (startIndex != recordNum) {//这里将 Ops 过滤一遍executeOpsTogether(records, isRecordPop, startIndex, recordNum);}// execute all unoptimized pop operations together or one add operation//...}if (startIndex != numRecords) {executeOpsTogether(records, isRecordPop, startIndex, numRecords);}
}
private void executeOpsTogether(ArrayList<BackStackRecord> records,ArrayList<Boolean> isRecordPop, int startIndex, int endIndex) {final boolean allowOptimization = records.get(startIndex).mAllowOptimization;boolean addToBackStack = false;if (mTmpAddedFragments == null) {mTmpAddedFragments = new ArrayList<>();} else {mTmpAddedFragments.clear();}if (mAdded != null) {mTmpAddedFragments.addAll(mAdded);}for (int recordNum = startIndex; recordNum < endIndex; recordNum++) {final BackStackRecord record = records.get(recordNum);final boolean isPop = isRecordPop.get(recordNum);if (!isPop) {record.expandReplaceOps(mTmpAddedFragments);    //修改状态} else {record.trackAddedFragmentsInPop(mTmpAddedFragments);    }addToBackStack = addToBackStack || record.mAddToBackStack;}mTmpAddedFragments.clear();if (!allowOptimization) {FragmentTransition.startTransitions(this, records, isRecordPop, startIndex, endIndex,false);}//真正处理的入口executeOps(records, isRecordPop, startIndex, endIndex);int postponeIndex = endIndex;if (allowOptimization) {ArraySet<Fragment> addedFragments = new ArraySet<>();addAddedFragments(addedFragments);postponeIndex = postponePostponableTransactions(records, isRecordPop,startIndex, endIndex, addedFragments);makeRemovedFragmentsInvisible(addedFragments);    //名字就能看出来作用}if (postponeIndex != startIndex && allowOptimization) {// need to run something nowFragmentTransition.startTransitions(this, records, isRecordPop, startIndex,postponeIndex, true);moveToState(mCurState, true);}//...
}
//修改 Ops 状态，这一步还没有真正处理状态
expandReplaceOps(ArrayList<Fragment> added) {for (int opNum = 0; opNum < mOps.size(); opNum++) {final Op op = mOps.get(opNum);switch (op.cmd) {case OP_ADD:case OP_ATTACH:added.add(op.fragment);break;case OP_REMOVE:case OP_DETACH:added.remove(op.fragment);break;case OP_REPLACE: {     Fragment f = op.fragment;int containerId = f.mContainerId;boolean alreadyAdded = false;for (int i = added.size() - 1; i >= 0; i--) {Fragment old = added.get(i);if (old.mContainerId == containerId) {if (old == f) {alreadyAdded = true;} else {Op removeOp = new Op();removeOp.cmd = OP_REMOVE;    //可以看到，替换也是通过删除实现的removeOp.fragment = old;removeOp.enterAnim = op.enterAnim;removeOp.popEnterAnim = op.popEnterAnim;removeOp.exitAnim = op.exitAnim;removeOp.popExitAnim = op.popExitAnim;mOps.add(opNum, removeOp);added.remove(old);opNum++;}}}if (alreadyAdded) {mOps.remove(opNum);opNum--;} else {op.cmd = OP_ADD;added.add(f);}}break;}}
}
//设置将要被移除的 Fragment 为不可见的最终实现
private void makeRemovedFragmentsInvisible(ArraySet<Fragment> fragments) {final int numAdded = fragments.size();for (int i = 0; i < numAdded; i++) {final Fragment fragment = fragments.valueAt(i);if (!fragment.mAdded) {final View view = fragment.getView();    //获取 Fragment 的布局，设置状态if (Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {fragment.getView().setVisibility(View.INVISIBLE);} else {        //高版本设置透明度fragment.mPostponedAlpha = view.getAlpha();view.setAlpha(0f);}}}
}

代码多了一点，但我们终于找到了最终的实现：Handler 异步发到主线，调度执行后，聚合、修改 Ops 的状态，然后遍历、修改 Fragment 栈中的 View 的状态。

真正处理的部分

前面主要是对 Fragment 的包装类 Ops 进行一些状态修改，真正根据 Ops 状态进行操作在这个部分：

/*** Executes the operations contained within this transaction. The Fragment states will only* be modified if optimizations are not allowed.*/
void executeOps() {final int numOps = mOps.size();for (int opNum = 0; opNum < numOps; opNum++) {final Op op = mOps.get(opNum);final Fragment f = op.fragment;f.setNextTransition(mTransition, mTransitionStyle);switch (op.cmd) {case OP_ADD:f.setNextAnim(op.enterAnim);mManager.addFragment(f, false);break;case OP_REMOVE:f.setNextAnim(op.exitAnim);mManager.removeFragment(f);break;case OP_HIDE:f.setNextAnim(op.exitAnim);mManager.hideFragment(f);break;case OP_SHOW:f.setNextAnim(op.enterAnim);mManager.showFragment(f);break;case OP_DETACH:f.setNextAnim(op.exitAnim);mManager.detachFragment(f);break;case OP_ATTACH:f.setNextAnim(op.enterAnim);mManager.attachFragment(f);break;default:throw new IllegalArgumentException("Unknown cmd: " + op.cmd);}if (!mAllowOptimization && op.cmd != OP_ADD) {mManager.moveFragmentToExpectedState(f);}}if (!mAllowOptimization) {// Added fragments are added at the end to comply with prior behavior.mManager.moveToState(mManager.mCurState, true);}
}

FragmentManager 对这些方法的实现也很简单，修改 Fragment 的状态值，比如 remove(Fragment):

public void removeFragment(Fragment fragment) {if (DEBUG) Log.v(TAG, "remove: " + fragment + " nesting=" + fragment.mBackStackNesting);final boolean inactive = !fragment.isInBackStack();if (!fragment.mDetached || inactive) {if (mAdded != null) {mAdded.remove(fragment);}if (fragment.mHasMenu && fragment.mMenuVisible) {mNeedMenuInvalidate = true;}fragment.mAdded = false;    //设置属性值fragment.mRemoving = true;}
}

最终会调用 moveToState()，我们直接来看它的实现：

void moveToState(Fragment f, int newState, int transit, int transitionStyle,boolean keepActive) {//还没有添加的 Fragment 处于 onCreate() 状态if ((!f.mAdded || f.mDetached) && newState > Fragment.CREATED) {newState = Fragment.CREATED;}if (f.mRemoving && newState > f.mState) {// While removing a fragment, we can't change it to a higher state.newState = f.mState;}//推迟启动的设置为 stopif (f.mDeferStart && f.mState < Fragment.STARTED && newState > Fragment.STOPPED) {newState = Fragment.STOPPED;}if (f.mState < newState) {// For fragments that are created from a layout, when restoring from// state we don't want to allow them to be created until they are// being reloaded from the layout.if (f.mFromLayout && !f.mInLayout) {return;}if (f.getAnimatingAway() != null) {// The fragment is currently being animated...  but!  Now we// want to move our state back up.  Give up on waiting for the// animation, move to whatever the final state should be once// the animation is done, and then we can proceed from there.f.setAnimatingAway(null);//如果当前 Fragment 正有动画，直接修改为最终状态moveToState(f, f.getStateAfterAnimating(), 0, 0, true);}switch (f.mState) {case Fragment.INITIALIZING:if (DEBUG) Log.v(TAG, "moveto CREATED: " + f);if (f.mSavedFragmentState != null) {f.mSavedFragmentState.setClassLoader(mHost.getContext().getClassLoader());f.mSavedViewState = f.mSavedFragmentState.getSparseParcelableArray(FragmentManagerImpl.VIEW_STATE_TAG);f.mTarget = getFragment(f.mSavedFragmentState,FragmentManagerImpl.TARGET_STATE_TAG);if (f.mTarget != null) {f.mTargetRequestCode = f.mSavedFragmentState.getInt(FragmentManagerImpl.TARGET_REQUEST_CODE_STATE_TAG, 0);}f.mUserVisibleHint = f.mSavedFragmentState.getBoolean(FragmentManagerImpl.USER_VISIBLE_HINT_TAG, true);if (!f.mUserVisibleHint) {f.mDeferStart = true;if (newState > Fragment.STOPPED) {newState = Fragment.STOPPED;}}}f.mHost = mHost;f.mParentFragment = mParent;f.mFragmentManager = mParent != null? mParent.mChildFragmentManager : mHost.getFragmentManagerImpl();dispatchOnFragmentPreAttached(f, mHost.getContext(), false);f.mCalled = false;f.onAttach(mHost.getContext());    //调用 Fragment 生命周期方法if (!f.mCalled) {throw new SuperNotCalledException("Fragment " + f+ " did not call through to super.onAttach()");}if (f.mParentFragment == null) {mHost.onAttachFragment(f);} else {f.mParentFragment.onAttachFragment(f);}dispatchOnFragmentAttached(f, mHost.getContext(), false);if (!f.mRetaining) {f.performCreate(f.mSavedFragmentState); //调用 Fragment 生命周期方法dispatchOnFragmentCreated(f, f.mSavedFragmentState, false);} else {f.restoreChildFragmentState(f.mSavedFragmentState);f.mState = Fragment.CREATED;}f.mRetaining = false;if (f.mFromLayout) {    //从布局解析来的// For fragments that are part of the content view// layout, we need to instantiate the view immediately// and the inflater will take care of adding it.f.mView = f.performCreateView(f.getLayoutInflater(    //调用 Fragment 生命周期方法f.mSavedFragmentState), null, f.mSavedFragmentState);if (f.mView != null) {f.mInnerView = f.mView;if (Build.VERSION.SDK_INT >= 11) {ViewCompat.setSaveFromParentEnabled(f.mView, false);} else {f.mView = NoSaveStateFrameLayout.wrap(f.mView);}if (f.mHidden) f.mView.setVisibility(View.GONE);f.onViewCreated(f.mView, f.mSavedFragmentState);    //调用 Fragment 生命周期方法dispatchOnFragmentViewCreated(f, f.mView, f.mSavedFragmentState, false);} else {f.mInnerView = null;}}case Fragment.CREATED:if (newState > Fragment.CREATED) {if (DEBUG) Log.v(TAG, "moveto ACTIVITY_CREATED: " + f);if (!f.mFromLayout) {ViewGroup container = null;if (f.mContainerId != 0) {if (f.mContainerId == View.NO_ID) {throwException(new IllegalArgumentException("Cannot create fragment "+ f+ " for a container view with no id"));}container = (ViewGroup) mContainer.onFindViewById(f.mContainerId);if (container == null && !f.mRestored) {String resName;try {resName = f.getResources().getResourceName(f.mContainerId);} catch (NotFoundException e) {resName = "unknown";}throwException(new IllegalArgumentException("No view found for id 0x"+ Integer.toHexString(f.mContainerId) + " ("+ resName+ ") for fragment " + f));}}f.mContainer = container;f.mView = f.performCreateView(f.getLayoutInflater( //调用 Fragment 生命周期方法f.mSavedFragmentState), container, f.mSavedFragmentState);if (f.mView != null) {f.mInnerView = f.mView;if (Build.VERSION.SDK_INT >= 11) {ViewCompat.setSaveFromParentEnabled(f.mView, false);} else {f.mView = NoSaveStateFrameLayout.wrap(f.mView);}if (container != null) {container.addView(f.mView);       //将 Fragment 的布局添加到父布局中}if (f.mHidden) {f.mView.setVisibility(View.GONE);}f.onViewCreated(f.mView, f.mSavedFragmentState);//调用 Fragment 生命周期方法dispatchOnFragmentViewCreated(f, f.mView, f.mSavedFragmentState,false);// Only animate the view if it is visible. This is done after// dispatchOnFragmentViewCreated in case visibility is changedf.mIsNewlyAdded = (f.mView.getVisibility() == View.VISIBLE)&& f.mContainer != null;} else {f.mInnerView = null;}}f.performActivityCreated(f.mSavedFragmentState); //调用 Fragment 生命周期方法dispatchOnFragmentActivityCreated(f, f.mSavedFragmentState, false);if (f.mView != null) {f.restoreViewState(f.mSavedFragmentState);}f.mSavedFragmentState = null;}case Fragment.ACTIVITY_CREATED:if (newState > Fragment.ACTIVITY_CREATED) {f.mState = Fragment.STOPPED;}case Fragment.STOPPED:if (newState > Fragment.STOPPED) {if (DEBUG) Log.v(TAG, "moveto STARTED: " + f);f.performStart();dispatchOnFragmentStarted(f, false);}case Fragment.STARTED:if (newState > Fragment.STARTED) {if (DEBUG) Log.v(TAG, "moveto RESUMED: " + f);f.performResume();dispatchOnFragmentResumed(f, false);f.mSavedFragmentState = null;f.mSavedViewState = null;}}} else if (f.mState > newState) {switch (f.mState) {case Fragment.RESUMED:if (newState < Fragment.RESUMED) {if (DEBUG) Log.v(TAG, "movefrom RESUMED: " + f);f.performPause();dispatchOnFragmentPaused(f, false);}case Fragment.STARTED:if (newState < Fragment.STARTED) {if (DEBUG) Log.v(TAG, "movefrom STARTED: " + f);f.performStop();dispatchOnFragmentStopped(f, false);}case Fragment.STOPPED:if (newState < Fragment.STOPPED) {if (DEBUG) Log.v(TAG, "movefrom STOPPED: " + f);f.performReallyStop();}case Fragment.ACTIVITY_CREATED:if (newState < Fragment.ACTIVITY_CREATED) {if (DEBUG) Log.v(TAG, "movefrom ACTIVITY_CREATED: " + f);if (f.mView != null) {// Need to save the current view state if not// done already.if (mHost.onShouldSaveFragmentState(f) && f.mSavedViewState == null) {saveFragmentViewState(f);}}f.performDestroyView();dispatchOnFragmentViewDestroyed(f, false);if (f.mView != null && f.mContainer != null) {Animation anim = null;if (mCurState > Fragment.INITIALIZING && !mDestroyed&& f.mView.getVisibility() == View.VISIBLE&& f.mPostponedAlpha >= 0) {anim = loadAnimation(f, transit, false,transitionStyle);}f.mPostponedAlpha = 0;if (anim != null) {final Fragment fragment = f;f.setAnimatingAway(f.mView);f.setStateAfterAnimating(newState);final View viewToAnimate = f.mView;anim.setAnimationListener(new AnimateOnHWLayerIfNeededListener(viewToAnimate, anim) {@Overridepublic void onAnimationEnd(Animation animation) {super.onAnimationEnd(animation);if (fragment.getAnimatingAway() != null) {fragment.setAnimatingAway(null);moveToState(fragment, fragment.getStateAfterAnimating(),0, 0, false);}}});f.mView.startAnimation(anim);}f.mContainer.removeView(f.mView);}f.mContainer = null;f.mView = null;f.mInnerView = null;}case Fragment.CREATED:if (newState < Fragment.CREATED) {if (mDestroyed) {if (f.getAnimatingAway() != null) {// The fragment's containing activity is// being destroyed, but this fragment is// currently animating away.  Stop the// animation right now -- it is not needed,// and we can't wait any more on destroying// the fragment.View v = f.getAnimatingAway();f.setAnimatingAway(null);v.clearAnimation();}}if (f.getAnimatingAway() != null) {// We are waiting for the fragment's view to finish// animating away.  Just make a note of the state// the fragment now should move to once the animation// is done.f.setStateAfterAnimating(newState);newState = Fragment.CREATED;} else {if (DEBUG) Log.v(TAG, "movefrom CREATED: " + f);if (!f.mRetaining) {f.performDestroy();dispatchOnFragmentDestroyed(f, false);} else {f.mState = Fragment.INITIALIZING;}f.performDetach();dispatchOnFragmentDetached(f, false);if (!keepActive) {if (!f.mRetaining) {makeInactive(f);} else {f.mHost = null;f.mParentFragment = null;f.mFragmentManager = null;}}}}}}if (f.mState != newState) {Log.w(TAG, "moveToState: Fragment state for " + f + " not updated inline; "+ "expected state " + newState + " found " + f.mState);f.mState = newState;}
}

代码很长，但做的事情很简单：

1. 根据状态调用对应的生命周期方法
2. 如果是新创建的，就把布局添加到 ViewGroup 中

Fragment 是什么

Fragment 是什么，从官网、别人博客上看到的都是他人之言，我们还是得去看源码才能得到答案。

public class Fragment implements ComponentCallbacks, OnCreateContextMenuListener {...}

可以看到，Fragment 没有继承任何类，只是实现了这两个接口，第二个不太重要，第一个是在内存不足时可以收到回调。

没有什么特别信息，我们还是去看看它的主要成员。

Fragment 的主要成员

static final int INITIALIZING = 0;     // Not yet created.
static final int CREATED = 1;          // Created.
static final int ACTIVITY_CREATED = 2; // The activity has finished its creation.
static final int STOPPED = 3;          // Fully created, not started.
static final int STARTED = 4;          // Created and started, not resumed.
static final int RESUMED = 5;          // Created started and resumed.//当前 Fragment 的状态值
int mState = INITIALIZING;
//...
// True if the fragment is in the list of added fragments.
boolean mAdded;// If set this fragment is being removed from its activity.
boolean mRemoving;// Set to true if this fragment was instantiated from a layout file.
boolean mFromLayout;// Set to true when the view has actually been inflated in its layout.
boolean mInLayout;// True if this fragment has been restored from previously saved state.
boolean mRestored;// Number of active back stack entries this fragment is in.
int mBackStackNesting;// Set to true when the app has requested that this fragment be hidden
// from the user.
boolean mHidden;// Set to true when the app has requested that this fragment be deactivated.
boolean mDetached;// If set this fragment would like its instance retained across
// configuration changes.
boolean mRetainInstance;// If set this fragment is being retained across the current config change.
boolean mRetaining;// If set this fragment has menu items to contribute.
boolean mHasMenu;// Set to true to allow the fragment's menu to be shown.
boolean mMenuVisible = true;// Used to verify that subclasses call through to super class.
boolean mCalled;

一堆标志位和状态值。然后就是关键的成员了：

// The fragment manager we are associated with.  Set as soon as the
// fragment is used in a transaction; cleared after it has been removed
// from all transactions.
FragmentManagerImpl mFragmentManager;//Fragmemt 绑定的对象，一半就是 Activity 和 Fragment
FragmentHostCallback mHost;
//管理子 Fragment
FragmentManagerImpl mChildFragmentManager;// For use when restoring fragment state and descendant fragments are retained.
// This state is set by FragmentState.instantiate and cleared in onCreate.
FragmentManagerNonConfig mChildNonConfig;
//如果这个 Fragment 绑定的是另一个 Fragment，就需要设置这个值
Fragment mParentFragment;
//容器 Fragment 的ID
int mFragmentId;
//容器 View 的ID
int mContainerId;//父布局
ViewGroup mContainer;//当前 Fragment 的布局
View mView;//真正保存状态的内部布局
View mInnerView;

看到这里，结合前面的，我们就清晰了一个 Fragment 的创建、添加过程：

在 onCreateView() 中返回一个 布局，然后在 FragmentManager 中拿到这个布局，添加到要绑定容器（Activity/Fragment）的 ViewGroup 中，然后设置相应的状态值。

生命周期方法

Fragment 的生命周期大家都清楚，官方提供了一张很清晰的图：

总共 11 个方法，这里我们看一下各个方法的具体源码。

1. onAttach(Context)

@CallSuper
public void onAttach(Context context) {mCalled = true;final Activity hostActivity = mHost == null ? null : mHost.getActivity();if (hostActivity != null) {mCalled = false;onAttach(hostActivity);}
}@Deprecated
@CallSuper
public void onAttach(Activity activity) {mCalled = true;
}

onAttach() 是一个 Fragment 和它的 Context 关联时第一个调用的方法，这里我们可以获得对应的 Context 或者 Activity，可以看到这里拿到的 Activity 是 mHost.getActivity()，后面我们介绍 FragmentManager 时介绍这个方法。

2. onCreate(Bundle)

public void onCreate(@Nullable Bundle savedInstanceState) {mCalled = true;restoreChildFragmentState(savedInstanceState);if (mChildFragmentManager != null&& !mChildFragmentManager.isStateAtLeast(Fragment.CREATED)) {mChildFragmentManager.dispatchCreate();}
}
void restoreChildFragmentState(@Nullable Bundle savedInstanceState) {if (savedInstanceState != null) {Parcelable p = savedInstanceState.getParcelable(FragmentActivity.FRAGMENTS_TAG);if (p != null) {if (mChildFragmentManager == null) {instantiateChildFragmentManager();}mChildFragmentManager.restoreAllState(p, mChildNonConfig);mChildNonConfig = null;mChildFragmentManager.dispatchCreate();}}
}

onCreate() 在 onAttach() 后调用，用于做一些初始化操作。

需要注意的是，Fragment 的 onCreate() 调用时关联的 Activity 可能还没创建好，所以这里不要有依赖外部 Activity 布局的操作。如果有依赖 Activity 的操作，可以放在 onActivityCreate() 中。

从上面的代码还可以看到，如果是从旧状态中恢复，会执行子 Fragment 状态的恢复，此外还在 onCreate() 中调用了子 Fragment 管理者的创建。

3. onCreateView(LayoutInflater, ViewGroup, Bundle)

@Nullable
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,@Nullable Bundle savedInstanceState) {return null;
}

在 onCreate() 后就会执行 onCreatView()，这个方法返回一个 View，默认返回为 null。

当我们需要在 Fragment 中显示布局时，需要重写这个方法，返回要显示的布局。

后面布局销毁时就会调用 onDestroyView()。

3.1. onViewCreated

public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
}

onViewCreate() 不是生命周期中的方法，但是却很有用。

它会在 onCreateView() 返回后立即执行，参数中的 view 就是之前创建的 View，因此我们可以在 onViewCreate() 中进行布局的初始化，比如这样：

@Override
public void onViewCreated(final View view, @Nullable final Bundle savedInstanceState) {if (view == null) {return;}mTextView = (TextView) view.findViewById(R.id.tv_content);mBtnSwitchChild = (Button) view.findViewById(R.id.btn_switch_child);Bundle arguments = getArguments();if (arguments != null && mTextView != null && !TextUtils.isEmpty(arguments.getString(KEY_TITLE))) {mTextView.setText(arguments.getString(KEY_TITLE));}mBtnSwitchChild.setOnClickListener(new View.OnClickListener() {@Overridepublic void onClick(final View v) {//...});
}

4. onActivityCreated(Bundle)

@CallSuper
public void onActivityCreated(@Nullable Bundle savedInstanceState) {mCalled = true;
}

onActivityCreated() 会在 Fragment 关联的 Activity 创建好、Fragment 的布局结构初始化完成后调用。

可以在这个方法里做些和布局、状态恢复有关的操作。

4.1 onViewStateRestored(Bundle)

@CallSuper
public void onViewStateRestored(@Nullable Bundle savedInstanceState) {mCalled = true;
}

onViewStateRestored() 方法会在 onActivityCreated() 结束后调用，用于一个 Fragment 在从旧的状态恢复时，获取状态 saveInstanceState 恢复状态，比如恢复一个 check box 的状态。

经过这四步，Fragment 创建完成，同步于 Activity 的创建过程。

5. onStart()

@CallSuper
public void onStart() {mCalled = true;if (!mLoadersStarted) {mLoadersStarted = true;if (!mCheckedForLoaderManager) {mCheckedForLoaderManager = true;mLoaderManager = mHost.getLoaderManager(mWho, mLoadersStarted, false);}if (mLoaderManager != null) {mLoaderManager.doStart();}}
}

onStart() 当 Fragment 可见时调用，同步于 Activity 的 onStart()。

6. onResume()

@CallSuper
public void onResume() {mCalled = true;
}

onResume() 当 Fragment 可见并且可以与用户交互时调用。

它和 Activity 的 onResume() 同步。

7. onPause()

@CallSuper
public void onPause() {mCalled = true;
}

onPause() 当 Fragment 不再可见时调用。

也和 Activity 的 onPause() 同步。

8. onStop()

@CallSuper
public void onStop() {mCalled = true;
}

onStop() 当 Fragment 不再启动时调用，和 Activity.onStop() 一致。

9. onDestroyView()

@CallSuper
public void onDestroyView() {mCalled = true;
}

当 onCreateView() 返回的布局（不论是不是 null）从 Fragment 中解除绑定时调用 onDestroyView() 。

下次 Fragment 展示时，会重新创建布局。

10. onDestroy()

@CallSuper
public void onDestroy() {mCalled = true;//Log.v("foo", "onDestroy: mCheckedForLoaderManager=" + mCheckedForLoaderManager//        + " mLoaderManager=" + mLoaderManager);if (!mCheckedForLoaderManager) {mCheckedForLoaderManager = true;mLoaderManager = mHost.getLoaderManager(mWho, mLoadersStarted, false);}if (mLoaderManager != null) {mLoaderManager.doDestroy();}
}

当 Fragment 不再使用时会调用 onDestroy()，它是一个 Fragment 生命周期的倒数第二步。

可以看到这里，调用了 mLoaderManager.doDestroy()，后面介绍它。

11. onDetach()

@CallSuper
public void onDetach() {mCalled = true;
}

Fragment 生命周期的最后一个方法，当 Fragment 不再和一个 Activity 绑定时调用。

Fragment 的 onDestroyView(), onDestroy(), onDetach() 三个对应 Activity 的 onDestroyed() 方法。

总结

OK，看完这篇文章，相信对开头提出的问题你已经有了答案，这里再总结一下。

Fragment、FragmentManager、FragmentTransaction 关系

• Fragment
• 其实是对 View 的封装，它持有 view, containerView, fragmentManager, childFragmentManager 等信息
• FragmentManager
• 是一个抽象类，它定义了对一个 Activity/Fragment 中 添加进来的 Fragment 列表、Fragment 回退栈的操作、管理方法
• 还定义了获取事务对象的方法
• 具体实现在 FragmentImpl 中
• FragmentTransaction
• 定义了对 Fragment 添加、替换、隐藏等操作，还有四种提交方法
• 具体实现是在 BackStackRecord 中

Fragment 如何实现布局的添加替换

通过获得当前 Activity/Fragment 的 FragmentManager/ChildFragmentManager，进而拿到事务的实现类 BackStackRecord，它将目标 Fragment 构造成 Ops（包装Fragment 和状态信息），然后提交给 FragmentManager 处理。

如果是异步提交，就通过 Handler 发送 Runnable 任务，FragmentManager 拿到任务后，先处理 Ops 状态，然后调用 moveToState() 方法根据状态调用 Fragment 对应的生命周期方法，从而达到 Fragment 的添加、布局的替换隐藏等。

下面这张图从下往上看就是一个 Fragment 创建经历的方法：

嵌套 Fragment 的原理

也比较简单，Fragment 内部有一个 childFragmentManager，通过它管理子 Fragment。

在添加子 Fragment 时，把子 Fragment 的布局 add 到父 Fragment 即可。

---

2018.10.08 添加

• 开发中，使用不同事务操作，需要及时提交
• 提交事务尽量使用 commitAllowingStateLoss，否则会在状态需要保存时报错
• 提交时有必要加 try-catch

1.getChildFragmentManager() 要 try-catch，被移除后，Fragment 的 mHost 为空，初始化 ChildFragmentManager 会抛异常：

void instantiateChildFragmentManager() {if (mHost == null) {throw new IllegalStateException("Fragment has not been attached yet.");}//...
}

2.FragmentStatePagerAdapter 的 finishUpdate 方法可能会抛异常，需要创建子类重写这个方法，catch 住。

//自己创建的 FragmentStatePagerAdapter 子类
@Override
public void finishUpdate(ViewGroup container) {try {super.finishUpdate(container);} catch (Exception e) {e.printStackTrace();if (ConstantsOpenSdk.isDebug) {throw e;} else {CrashReport.postCatchedException(e);}}
}

病根：

@Override
public void finishUpdate(ViewGroup container) {if (mCurTransaction != null) {mCurTransaction.commitNowAllowingStateLoss();mCurTransaction = null;}
}
@Override
public void commitNowAllowingStateLoss() {disallowAddToBackStack();mManager.execSingleAction(this, true);
}
@Override
public FragmentTransaction disallowAddToBackStack() {if (mAddToBackStack) {throw new IllegalStateException("This transaction is already being added to the back stack");}mAllowAddToBackStack = false;return this;
}

3.DialogFragment 和 Dialog 的区别

DialogFragment 内部有一个 Dialog，它可以当做 Dialog 使用，也可以当做一个普通的 fragment 使用。也就是说比 ```Dialog`` 使用场景更多。

当把一个 DialogFragment 作为参数传入给 FragmentTransaction.add(int, Fragment) 时，会自动调用 setShowsDialog(false) 方法。

/*
* <p>This is normally set for you based on whether the fragment is
* associated with a container view ID passed to
* {@link FragmentTransaction#add(int, Fragment) FragmentTransaction.add(int, Fragment)}.
* If the fragment was added with a container, setShowsDialog will be
* initialized to false; otherwise, it will be true.
*
* @param showsDialog If true, the fragment will be displayed in a Dialog.
* If false, no Dialog will be created and the fragment's view hierarchly
* left undisturbed.
*/
public void setShowsDialog(boolean showsDialog) {mShowsDialog = showsDialog;
}