Fragment 与 Activity 的联系和 Fragment 的管理
参考：从源码角度剖析Fragment核心知识点
版本：androidx 1.1.0
大部分都使用 AppCompatActivity，所以就以它的父类 FragmentActivity ① ^{①} ① 来进行分析。
FragmentActivity 通过使用 FragmentController 来对 Framgent 进行管理，不过 FragmentController 并没有真正实现对 Fragment 进行管理，而是通过内部持有的一个成员 FragmentHostCallback ② ^{②} ② 来间接管理。
final FragmentController mFragments = FragmentController.createController(new HostCallbacks());
FragmentActiviy 在创建的时候 new 了一个继承了 FragmentHostCallback 的类 HostCallback，它是FragmentActivity 的内部类，而 FragmentHostCallback 当中持有 Activity、Context、Handler 以及 FragmentManagerImpl ③ ^{③} ③ 的引用。
@Nullable private final Activity mActivity;
@NonNull private final Context mContext;
@NonNull private final Handler mHandler;
private final int mWindowAnimations; //这个不清楚是啥
final FragmentManagerImpl mFragmentManager = new FragmentManagerImpl();
以下是构造函数：
public FragmentHostCallback(@NonNull Context context, @NonNull Handler handler,
int windowAnimations) {
this(context instanceof Activity ? (Activity) context : null, context, handler,
windowAnimations);
}
FragmentHostCallback(@NonNull FragmentActivity activity) {
this(activity, activity /*context*/, new Handler(), 0 /*windowAnimations*/);
}
FragmentHostCallback(@Nullable Activity activity, @NonNull Context context,
@NonNull Handler handler, int windowAnimations) {
mActivity = activity;
mContext = Preconditions.checkNotNull(context, "context == null");
mHandler = Preconditions.checkNotNull(handler, "handler == null");
mWindowAnimations = windowAnimations;
}
FragmentActivity 用的是第二个构造函数：
public HostCallbacks() {
super(FragmentActivity.this /*fragmentActivity*/);
}
从上面得知，每一个 FragmentActivity 都有自己的 FragmentController，而 FragmentController 中的 FragmentHostCallback 中 mActivity 就是 FragmentActivity 自己，mContext 也是用的 mActivity 的 引用，
mHandler 是新建的。
另外 FragmentHostCallback 类头顶有这么一段注释：
/**
* Integration points with the Fragment host.
* Fragments may be hosted by any object; such as an {@link Activity}. In order to
* host fragments, implement {@link FragmentHostCallback}, overriding the methods
* applicable to the host.
*/
大意就是 Fragment 可以由任意对象托管，只要继承 FragmentHostCallback 即可，例如 Activity 。
FragmentManagerImpl 是 FragmentManager 的实现类，它实现了对 Fragment 的管理。FragmentManager 类当中有一堆的抽象方法，仅有一个静态的 FragmentFactroy 常量和公有的 getFragmentFactory() 方法，主要看它的实现类 FragmentManagerImpl 。
Google 对它的定义是：
/**
* Container for fragments associated with an activity.
*/
与 activity 关联的 Fragment 的容器。
由于 FragmentHostCallback 中持有的 FragmentManagerImpl 能对 Fragment 进行管理并且常量声明是包私有，FragmentController 提供了获取 FragmentManagerImpl 的方法，那么 FragmentActivity 就能对 Fragment 进行管理。
/*FragmentActivity*/
@NonNull
public FragmentManager getSupportFragmentManager() {
return mFragments.getSupportFragmentManager();
}
/*FragmentController*/
@NonNull
public FragmentManager getSupportFragmentManager() {
return mHost.mFragmentManager;
}
Fragment 的生命周期是受 FragmentActivity 的生命周期影响的，在 FragmentActivity 调用 onCreate、onStart 等函数时，FragmentActivity 会调用由 FragmentController.dispatchXXX 方法，对 Fragment 的生命周期进行控制。(实际上 FragmentController 只是作为代理，本质还是调用了 FragmentHostCallback.FragmentManagerImpl.dispatchXXX 方法)
以 onDestroy 为例：
/*FragmentActivity*/
@Override
protected void onDestroy() {
super.onDestroy();
mFragments.dispatchDestroy();
mFragmentLifecycleRegistry.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
}
/*FragmentController*/
public void dispatchDestroy() {
mHost.mFragmentManager.dispatchDestroy();
}
/*FragmentManagerImpl*/
public void dispatchDestroy() {
mDestroyed = true;
execPendingActions();
dispatchStateChange(Fragment.INITIALIZING);
mHost = null;
mContainer = null;
mParent = null;
if (mOnBackPressedDispatcher != null) {
// mOnBackPressedDispatcher can hold a reference to the host
// so we need to null it out to prevent memory leaks
mOnBackPressedCallback.remove();
mOnBackPressedDispatcher = null;
}
}
它调用了 dispatchStateXXX 的方法来改变所有被管理 Fragment 的状态。实际上变更的是 FragmentManagerImpl 的状态，然后引导 Fragment 进行状态变更。
private void dispatchStateChange(int nextState) {
try {
mExecutingActions = true;
moveToState(nextState, false);
} finally {
mExecutingActions = false;
}
execPendingActions();
}
接着它调用 moveToState（这部分的代码看不懂，菜是原罪）
void moveToState(int newState, boolean always)
第一个参数是 FragmentManagerImpl 要变更的新状态，第二个参数如果是 true ，新状态无论是什么样，即使和当前一样，也要把自己和管理的 Fragment 都刷新一遍。如果是 false，只有不相等的时候才刷新。
最终会进入 moveToState 五个参数的方法，在这个里面对 Fragment 进行状态迁移，调用 Fragment 的 生命周期方法。
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
boolean keepActive)
{
...
else if (f.mState > newState) {
switch (f.mState) {
...
case Fragment.CREATED:
if (newState < Fragment.CREATED) {
if (mDestroyed) {
// 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.
...
}
if (f.getAnimatingAway() != null || f.getAnimator() != 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);
boolean beingRemoved = f.mRemoving && !f.isInBackStack();
if (beingRemoved || mNonConfig.shouldDestroy(f)) {
...
f.performDestroy();
dispatchOnFragmentDestroyed(f, false);
} else {
f.mState = Fragment.INITIALIZING;
}
f.performDetach();
dispatchOnFragmentDetached(f, false);
...
}
}
}
}
}
这部分的描述引用了开头所给链接的文章内容：
fragment 的 state取值，为前面提到的七中状态，其中最低值是 INITIALIZING 状态，代表 fragment 刚创建，还未被 add， 最高状态值是 RESUMED,代表 fragment 处于前台。 所以 moveToState 内部分两条线，状态跃升，和状态降低，里面各有一个 switch 判断，switch 里每个 case 都没有 break，这意味着，状态可以持续变迁，比如从 INITIALIZING，一直跃升到 RESUMED，将每个 case 都走一遍，每次 case 语句内，都会改变 state 的值。
由于水平有限，moveToState 里面的具体细节不懂，所以就不往下分析了。
这仅仅只是 FragmentActivity 来控制 Fragment，还缺少 Fragment 单方面对 FragmentActivity 的访问。
在 FragmentActivity 的 onCreate 方法中调用了 FragmentController.attachHost，将 FragmentController 当中持有的 FragmentHostCallback 对象传递给了 FragmentManagerImpl，使 FragmentManagerImpl 也持有了 FragmentHostCallback 对象，并分发给由这个 FragmentManagerImpl 管理的 Fragment 持有从 FragmentActivity 传递过来的 FragmentHostCallback 对象。
/*FragmentActivity*/
mFragments.attachHost(null /*parent*/);
/*FragmentController*/
public void attachHost(@Nullable Fragment parent) {
mHost.mFragmentManager.attachController(
mHost, mHost /*container*/, parent);
}
/*FragmentManagerImpl*/
public void attachController(@NonNull FragmentHostCallback host,
@NonNull FragmentContainer container, @Nullable final Fragment parent) {
...
mHost = host;
...
}
然后 Fragment 当中有 FragmentHostCallback 的引用，它通过 FragmentManagerImpl 的 onCreateView 和 moveToState 获得
void moveToState(Fragment f, int newState, int transit, int transitionStyle,
boolean keepActive) {
...
if (f.mState <= newState) {
...
switch (f.mState) {
case Fragment.INITIALIZING:
if (newState > Fragment.INITIALIZING) {
...
f.mHost = mHost;
...
}
}
...
}
...
}
public View onCreateView(@Nullable View parent, @NonNull String name,
@NonNull Context context, @NonNull AttributeSet attrs) {
if (fragment == null) {
...
fragment.mHost = mHost;
...
} else if (fragment.mInLayout) {
...
} else {
...
fragment.mHost = mHost;
...
}
}
onCreateView 方法主要用于定义在 xml 中的 fragment。这样 Fragment 就可以通过使用 FragmentHostCallback 提供获取 FragmentActivity 实例的方法来获取 FragmentActivity 来使用，并且它本身也封装了一些方法供 Fragment 使用。
以上是 FragmentActivity 与 Fragment 之间的联系。
因为需要根据情况展示界面，所以不得不对 Fragment 进行一些操作，因此一个东西油然而生：FragmentTransaction。
API for performing a set of Fragment operations.
用于执行一组 Fragment 操作的API。
换句话说，就是平时写代码的时候使用的 add、show、replace 等操作。
它使用了一个可变长度的数组来存储一次事务提交的操作：
ArrayList mOps = new ArrayList<>()
Op 就是一次操作的参数储存的容器。
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;
static final int OP_SET_PRIMARY_NAV = 8;
static final int OP_UNSET_PRIMARY_NAV = 9;
static final int OP_SET_MAX_LIFECYCLE = 10;
static final class Op {
int mCmd;
Fragment mFragment;
int mEnterAnim;
int mExitAnim;
int mPopEnterAnim;
int mPopExitAnim;
Lifecycle.State mOldMaxState;
Lifecycle.State mCurrentMaxState;
Op() {
}
Op(int cmd, Fragment fragment) {
this.mCmd = cmd;
this.mFragment = fragment;
this.mOldMaxState = Lifecycle.State.RESUMED;
this.mCurrentMaxState = Lifecycle.State.RESUMED;
}
Op(int cmd, @NonNull Fragment fragment, Lifecycle.State state) {
this.mCmd = cmd;
this.mFragment = fragment;
this.mOldMaxState = fragment.mMaxState;
this.mCurrentMaxState = state;
}
}
存放了操作的命令 id、操作的 Fragment 引用、动画以及生命周期状态。
add、show、replace 等操作使用的是第二个构造函数，add，replace 调用 doAddOp 方法添加到数组中，其他操作是直接添加的。
void doAddOp(int containerViewId, Fragment fragment, @Nullable String tag, int opcmd) {
...
addOp(new Op(opcmd, fragment));
}
添加到返回栈的操作它仅仅只是把 mAddToBackStack 置为 true，传入的 name 算是一个标识，用于返回栈弹出的判断。mAllowAddToBackStack 可以通过 disallowAddToBackStack 方法置为 false 使得不能进行添加到返回栈的操作。
@NonNull
public FragmentTransaction addToBackStack(@Nullable String name) {
if (!mAllowAddToBackStack) {
throw new IllegalStateException(
"This FragmentTransaction is not allowed to be added to the back stack.");
}
mAddToBackStack = true;
mName = name;
return this;
}
不过它只还是一个抽象类，BackStackRecord 类继承了它，并且它还持有了一个 FragmentManagerImpl 对象，因为它只是操作的记录者，由 FragmentManagerImpl 来进行操作。
之后使用 BackStackRecord.commit() 提交事务，就开始运作了。
@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");
if (FragmentManagerImpl.DEBUG) {
Log.v(TAG, "Commit: " + this);
LogWriter logw = new LogWriter(TAG);
PrintWriter pw = new PrintWriter(logw);
dump(" ", pw);
pw.close();
}
mCommitted = true;
if (mAddToBackStack) {
mIndex = mManager.allocBackStackIndex(this);
} else {
mIndex = -1;
}
mManager.enqueueAction(this, allowStateLoss);
return mIndex;
}
如果有添加到返回栈的操作，FragmentManagerImpl 会返回一个下标（下标这一部分不懂就不再讲解）过来，然后调用 enqueueAction。
public void enqueueAction(OpGenerator action, boolean allowStateLoss) {
if (!allowStateLoss) {
checkStateLoss();
}
synchronized (this) {
if (mDestroyed || mHost == null) {
if (allowStateLoss) {
// This FragmentManager isn't attached, so drop the entire transaction.
return;
}
throw new IllegalStateException("Activity has been destroyed");
}
if (mPendingActions == null) {
mPendingActions = new ArrayList<>();
}
mPendingActions.add(action);
scheduleCommit();
}
}
这里使用了一个变量来存储等待处理的操作：
ArrayList mPendingActions;
接下来又调用了 scheduleCommit，用一个 Runnable 在 FragmentActivity 获取的 handler 中运行了 execPendingActions。
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);
updateOnBackPressedCallbackEnabled();
}
}
}
Runnable mExecCommit = new Runnable() {
@Override
public void run() {
execPendingActions();
}
};
public boolean execPendingActions() {
ensureExecReady(true);
boolean didSomething = false;
while (generateOpsForPendingActions(mTmpRecords, mTmpIsPop)) {
mExecutingActions = true;
try {
removeRedundantOperationsAndExecute(mTmpRecords, mTmpIsPop);
} finally {
cleanupExec();
}
didSomething = true;
}
updateOnBackPressedCallbackEnabled();
doPendingDeferredStart();
burpActive();
return didSomething;
}
mTmpRecords 存储了 BackStackRecord 提交的变量，mTmpIsPop 是记录与之对应的BackStackRecord 是否是出栈操作的变量，根据注释所述，是用于除去冗余操作的。
// Temporary vars for removing redundant operations in BackStackRecords:
ArrayList mTmpRecords;
ArrayList mTmpIsPop;
然后调用 generateOpsForPendingActions 对 mPendingActions 当中的成员调用自身的 generateOps 生成操作。
private boolean generateOpsForPendingActions(ArrayList records,
ArrayList isPop) {
boolean didSomething = false;
synchronized (this) {
if (mPendingActions == null || mPendingActions.size() == 0) {
return false;
}
final int numActions = mPendingActions.size();
for (int i = 0; i < numActions; i++) {
didSomething |= mPendingActions.get(i).generateOps(records, isPop);
}
mPendingActions.clear();
mHost.getHandler().removeCallbacks(mExecCommit);
}
return didSomething;
}
generateOps 是一个接口的方法，在 BackStackRecord 和 PopBackStackState 类都实现了它。
/*FragmentManagerImpl.OpGenerator*/
/**
* An add or pop transaction to be scheduled for the UI thread.
*/
interface OpGenerator {
/**
* Generate transactions to add to {@code records} and whether or not the transaction
* is an add or pop to {@code isRecordPop}.
*
* records and isRecordPop must be added equally so that each transaction in records
* matches the boolean for whether or not it is a pop in isRecordPop.
*
* @param records A list to add transactions to.
* @param isRecordPop A list to add whether or not the transactions added to records is a pop transaction.
* @return true if something was added or false otherwise.
*/
boolean generateOps(ArrayList records, ArrayList isRecordPop);
}
/*FragmentManagerImpl.PopBackStackState*/
/**
* A pop operation OpGenerator. This will be run on the UI thread and will generate the
* transactions that will be popped if anything can be popped.
*/
private class PopBackStackState implements OpGenerator {
...
@Override
public boolean generateOps(ArrayList records,
ArrayList isRecordPop) {
if (mPrimaryNav != null // We have a primary nav fragment
&& mId < 0 // No valid id (since they're local)
&& mName == null) { // no name to pop to (since they're local)
final FragmentManager childManager = mPrimaryNav.getChildFragmentManager();
if (childManager.popBackStackImmediate()) {
// We didn't add any operations for this FragmentManager even though
// a child did do work.
return false;
}
}
return popBackStackState(records, isRecordPop, mName, mId, mFlags);
}
}
/*BackStackRecord*/
public boolean generateOps(ArrayList records, ArrayList isRecordPop) {
if (FragmentManagerImpl.DEBUG) {
Log.v(TAG, "Run: " + this);
}
records.add(this);
isRecordPop.add(false);
if (mAddToBackStack) {
mManager.addBackStackState(this);
}
return true;
}
PopBackStackState.generateOps 是对所提交的 BackStackRecord 进行弹出的操作。调用 popBackStack 的时候就相当于提交一次弹出的操作。
@Override
public void popBackStack(final int id, final int flags) {
if (id < 0) {
throw new IllegalArgumentException("Bad id: " + id);
}
enqueueAction(new PopBackStackState(null, id, flags), false);
}
popBackStackState 方法把要弹出的 BackStackRecord 进行了如下操作：
- 如果 name 、id 没有设置，且没有设置 POP_BACK_STACK_INCLUSIVE 位，那么就把返回栈最后一个 BackStackRecord 移除，并添加到要执行操作序列 records 中去，isRecordPop 置为 true。
- 如果有一个参数及以上被设置过，那么将从栈顶开始进行匹配，直到有一个参数相等为止。之后对匹配下标之后的 BackStackRecord 进行和情况 1 一样的操作；如果设置了 POP_BACK_STACK_INCLUSIVE 位，那么从匹配的下标开始再接着找，直到任何一个参数都不相等的时候停止，从栈顶到当前匹配的下标（包括自身）都进行和情况 1 一样的操作。
- 没有参数被设置过但设置了 POP_BACK_STACK_INCLUSIVE 位，那么返回栈内的所有 BackStackRecord 都会进行和情况 1 一样的操作。
`
ArrayList mBackStack; \\返回栈
@SuppressWarnings("unused")
boolean popBackStackState(ArrayList records, ArrayList isRecordPop,
String name, int id, int flags) {
if (mBackStack == null) {
return false;
}
if (name == null && id < 0 && (flags & POP_BACK_STACK_INCLUSIVE) == 0) {
int last = mBackStack.size() - 1;
if (last < 0) {
return false;
}
records.add(mBackStack.remove(last));
isRecordPop.add(true);
} else {
int index = -1;
if (name != null || id >= 0) {
// If a name or ID is specified, look for that place in
// the stack.
index = mBackStack.size()-1;
while (index >= 0) {
BackStackRecord bss = mBackStack.get(index);
if (name != null && name.equals(bss.getName())) {
break;
}
if (id >= 0 && id == bss.mIndex) {
break;
}
index--;
}
if (index < 0) {
return false;
}
if ((flags&POP_BACK_STACK_INCLUSIVE) != 0) {
index--;
// Consume all following entries that match.
while (index >= 0) {
BackStackRecord bss = mBackStack.get(index);
if ((name != null && name.equals(bss.getName()))
|| (id >= 0 && id == bss.mIndex)) {
index--;
continue;
}
break;
}
}
}
if (index == mBackStack.size()-1) {
return false;
}
for (int i = mBackStack.size() - 1; i > index; i--) {
records.add(mBackStack.remove(i));
isRecordPop.add(true);
}
}
return true;
}
BackStackRecord.generateOps 进行添加操作了，并且如果添加到返回栈的话它会去调用 FragmentManagerImpl 的 addBackStackState 方法。
void addBackStackState(BackStackRecord state) {
if (mBackStack == null) {
mBackStack = new ArrayList();
}
mBackStack.add(state);
}
回到 execPendingActions 方法，它调用 removeRedundantOperationsAndExecute 方法进行操作。个人能力有限，其他的不会，我就只讲有个比较重要的方法 executeOps 中的 BackStackRecord.executeOps，BackStackRecord.executePopOps：
/*FragmentManagerImpl*/
private static void executeOps(ArrayList records,
ArrayList isRecordPop, int startIndex, int endIndex) {
for (int i = startIndex; i < endIndex; i++) {
final BackStackRecord record = records.get(i);
final boolean isPop = isRecordPop.get(i);
if (isPop) {
record.bumpBackStackNesting(-1);
// Only execute the add operations at the end of
// all transactions.
boolean moveToState = i == (endIndex - 1);
record.executePopOps(moveToState);
} else {
record.bumpBackStackNesting(1);
record.executeOps();
}
}
}
/*BackStackRecord*/
void executeOps() {
final int numOps = mOps.size();
for (int opNum = 0; opNum < numOps; opNum++) {
final Op op = mOps.get(opNum);
final Fragment f = op.mFragment;
if (f != null) {
f.setNextTransition(mTransition, mTransitionStyle);
}
switch (op.mCmd) {
case OP_ADD:
f.setNextAnim(op.mEnterAnim);
mManager.addFragment(f, false);
break;
case OP_REMOVE:
f.setNextAnim(op.mExitAnim);
mManager.removeFragment(f);
break;
case OP_HIDE:
f.setNextAnim(op.mExitAnim);
mManager.hideFragment(f);
break;
case OP_SHOW:
f.setNextAnim(op.mEnterAnim);
mManager.showFragment(f);
break;
case OP_DETACH:
f.setNextAnim(op.mExitAnim);
mManager.detachFragment(f);
break;
case OP_ATTACH:
f.setNextAnim(op.mEnterAnim);
mManager.attachFragment(f);
break;
case OP_SET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(f);
break;
case OP_UNSET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(null);
break;
case OP_SET_MAX_LIFECYCLE:
mManager.setMaxLifecycle(f, op.mCurrentMaxState);
break;
default:
throw new IllegalArgumentException("Unknown cmd: " + op.mCmd);
}
if (!mReorderingAllowed && op.mCmd != OP_ADD && f != null) {
mManager.moveFragmentToExpectedState(f);
}
}
if (!mReorderingAllowed) {
// Added fragments are added at the end to comply with prior behavior.
mManager.moveToState(mManager.mCurState, true);
}
}
/*BackStackRecord*/
void executePopOps(boolean moveToState) {
for (int opNum = mOps.size() - 1; opNum >= 0; opNum--) {
final Op op = mOps.get(opNum);
Fragment f = op.mFragment;
if (f != null) {
f.setNextTransition(FragmentManagerImpl.reverseTransit(mTransition),
mTransitionStyle);
}
switch (op.mCmd) {
case OP_ADD:
f.setNextAnim(op.mPopExitAnim);
mManager.removeFragment(f);
break;
case OP_REMOVE:
f.setNextAnim(op.mPopEnterAnim);
mManager.addFragment(f, false);
break;
case OP_HIDE:
f.setNextAnim(op.mPopEnterAnim);
mManager.showFragment(f);
break;
case OP_SHOW:
f.setNextAnim(op.mPopExitAnim);
mManager.hideFragment(f);
break;
case OP_DETACH:
f.setNextAnim(op.mPopEnterAnim);
mManager.attachFragment(f);
break;
case OP_ATTACH:
f.setNextAnim(op.mPopExitAnim);
mManager.detachFragment(f);
break;
case OP_SET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(null);
break;
case OP_UNSET_PRIMARY_NAV:
mManager.setPrimaryNavigationFragment(f);
break;
case OP_SET_MAX_LIFECYCLE:
mManager.setMaxLifecycle(f, op.mOldMaxState);
break;
default:
throw new IllegalArgumentException("Unknown cmd: " + op.mCmd);
}
if (!mReorderingAllowed && op.mCmd != OP_REMOVE && f != null) {
mManager.moveFragmentToExpectedState(f);
}
}
if (!mReorderingAllowed && moveToState) {
mManager.moveToState(mManager.mCurState, true);
}
}
不过也不用多说什么了，之前所提交的操作都在这里消费了，executeOps 方法中我们提交了什么操作就通过FragmentManagerImpl 对 Fragment 执行什么操作，executePopOps 则是反过来，因为作为弹出操作肯定是按原路返回恢复到之前的状态。
这样事务的添加和返回栈的操作就到这里了。下面就是 Fragment 和 FragmentManagerImpl 的 数据保存和恢复。
在 FragmentManagerImpl.enqueueAction 中 allowStateLoss 参数为 true 那就认为允许数据丢失，为 false 认为不允许丢失，即如果调用过 onSaveInstanceState 方法或 onStop 方法在 FragmentManagerImpl 就会抛出异常。
private void checkStateLoss() {
if (isStateSaved()) {
throw new IllegalStateException(
"Can not perform this action after onSaveInstanceState");
}
}
@Override
public boolean isStateSaved() {
// See saveAllState() for the explanation of this. We do this for
// all platform versions, to keep our behavior more consistent between
// them.
return mStateSaved || mStopped;
}
这里的 state 指的是 FragmentManagerImpl 的一系列信息，它通过 saveAllState 来进行存储操作。
Parcelable saveAllState() {
...
execPendingActions();
mStateSaved = true;
if (mActive.isEmpty()) {
return null;
}
int size = mActive.size();
ArrayList active = new ArrayList<>(size);
boolean haveFragments = false;
for (Fragment f : mActive.values()) {
if (f != null) {
...
haveFragments = true;
FragmentState fs = new FragmentState(f);
active.add(fs);
if (f.mState > Fragment.INITIALIZING && fs.mSavedFragmentState == null) {
fs.mSavedFragmentState = saveFragmentBasicState(f);
if (f.mTargetWho != null) {
Fragment target = mActive.get(f.mTargetWho);
if (target == null) {
throwException(new IllegalStateException(
"Failure saving state: " + f
+ " has target not in fragment manager: "
+ f.mTargetWho));
}
if (fs.mSavedFragmentState == null) {
fs.mSavedFragmentState = new Bundle();
}
putFragment(fs.mSavedFragmentState,
FragmentManagerImpl.TARGET_STATE_TAG, target);
if (f.mTargetRequestCode != 0) {
fs.mSavedFragmentState.putInt(
FragmentManagerImpl.TARGET_REQUEST_CODE_STATE_TAG,
f.mTargetRequestCode);
}
}
} else {
fs.mSavedFragmentState = f.mSavedFragmentState;
}
}
}
if (!haveFragments) {
if (DEBUG) Log.v(TAG, "saveAllState: no fragments!");
return null;
}
ArrayList added = null;
BackStackState[] backStack = null;
// Build list of currently added fragments.
size = mAdded.size();
if (size > 0) {
added = new ArrayList<>(size);
for (Fragment f : mAdded) {
added.add(f.mWho);
if (f.mFragmentManager != this) {
throwException(new IllegalStateException(
"Failure saving state: active " + f
+ " was removed from the FragmentManager"));
}
if (DEBUG) {
Log.v(TAG, "saveAllState: adding fragment (" + f.mWho
+ "): " + f);
}
}
}
// Now save back stack.
if (mBackStack != null) {
size = mBackStack.size();
if (size > 0) {
backStack = new BackStackState[size];
for (int i = 0; i < size; i++) {
backStack[i] = new BackStackState(mBackStack.get(i));
if (DEBUG) Log.v(TAG, "saveAllState: adding back stack #" + i
+ ": " + mBackStack.get(i));
}
}
}
FragmentManagerState fms = new FragmentManagerState();
fms.mActive = active;
fms.mAdded = added;
fms.mBackStack = backStack;
if (mPrimaryNav != null) {
fms.mPrimaryNavActiveWho = mPrimaryNav.mWho;
}
fms.mNextFragmentIndex = mNextFragmentIndex;
return fms;
}
Fragment （FragmentState）信息，返回栈（BackStackState）信息，FragmentManager （FragmentManagerState）自己的一些信息都被保存下来。
/*FragmentController*/
@Nullable
public Parcelable saveAllState() {
return mHost.mFragmentManager.saveAllState();
}
/*FragmentActivity*/
@Override
protected void onSaveInstanceState(@NonNull Bundle outState) {
...
Parcelable p = mFragments.saveAllState();
if (p != null) {
outState.putParcelable(FRAGMENTS_TAG, p);
}
...
}
然后 FragmentActivity 通过调用 restoreSaveState 来进行数据的恢复：
/*FragmentActivity*/
@SuppressWarnings("deprecation")
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
mFragments.attachHost(null /*parent*/);
if (savedInstanceState != null) {
Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
mFragments.restoreSaveState(p);
...
}
...
}
/*FragmentController*/
public void restoreSaveState(@Nullable Parcelable state) {
if (!(mHost instanceof ViewModelStoreOwner)) {
throw new IllegalStateException("Your FragmentHostCallback must implement "
+ "ViewModelStoreOwner to call restoreSaveState(). Call restoreAllState()
+ " if you're still using retainNestedNonConfig().");
}
mHost.mFragmentManager.restoreSaveState(state);
}
//放一下 ViewModelStoreOwner 源码
/**
* A scope that owns {@link ViewModelStore}.
* A responsibility of an implementation of this interface is to retain owned
* ViewModelStore during the configuration changes and call {@link
* ViewModelStore#clear()}, when this scope is going to be destroyed.
*/
@SuppressWarnings("WeakerAccess")
public interface ViewModelStoreOwner {
/**
* Returns owned {@link ViewModelStore}
*
* @return a {@code ViewModelStore}
*/
@NonNull
ViewModelStore getViewModelStore();
}
/*FragmentManagerImpl*/
void restoreSaveState(Parcelable state) {
if (state == null) return;
FragmentManagerState fms = (FragmentManagerState)state;
if (fms.mActive == null) return;
...
mActive.clear();
for (FragmentState fs : fms.mActive) {
...
}
mAdded.clear();
if (fms.mAdded != null) {
for (String who : fms.mAdded) {
...
}
}
if (fms.mBackStack != null) {
...
}
...
}
注释：
① 我仔细看了一下 FragmentActivity 的父类 Activity，里面有类似 FragmentActivity 持有 FragmentController 的操作，不过好像被弃用了。
② FragmentHostCallback 还实现了 FragmentContainer 接口，提供了回调到 Fragment 的容器（Google 的注释是这么说的。里面有三个方法，其中一个弃用了，一个是 View onFindViewById(@IdRes int id) ，用来找 Fragment 里面的视图，boolean onHasView() 来检查是否持有视图）。
③ Fragment 中也有自己的 FragmentManagerImpl，这样就实现了 Fragment 的嵌套。