作者：字节小站
1. 前言
Fragment真是一个非常老的家伙，它的第一条提交记录是在2010年，而最近的一条记录则是在2021年6月11号，足足11岁了，但是它却老当益壮，在Jetpack中大放异彩，Navigation组件就是基于Fragment的一个跳转组件，Google的单Activity项目结构就是一个Activity和多个Fragment项目结构。多年以来，一提到Fragment，大家脑海中的第一印象可能还停留在平板应用开发中了，它曾经在手机项目中高频使用Fragment的机会还真没那么多。一方面是因为手机项目一般都是多Activity结构实现的，不会涉及到那么多的Fragment使用，另一方面，Fragment也比Activity复杂，相比之下使用有点麻烦，甚至是痛苦，不好定位问题，所以在技术选型时，能避则避。它太难用了，以至于Square团队的Piwai大神(LeakCanary的作者)在2014年发表了一篇倡导反对使用Fragment的文章。不管怎么样，这么多年过去了，Android团队也一直没有放弃对Fragment的支持，甚至在Jetpack中，Fragment开始扮演越来越重要的角色了，最近关于Navigation的倡导使用越来越多了，加上我参与了一个平板项目，所以对Fragment主流程源码进行了一次全面的研究。收获颇多，相信将来对Navigation研究定是大有裨益。
2. 创建Fragment
根据官方文档，写一个简单的Demo。简单了解Fragment的使用。
2.1 新建一个Fragment类
class ExampleFragment extends Fragment {
public ExampleFragment() {
super(R.layout.example_fragment);
}
}
2.2 把Fragment添加到Activity上
把Fragment添加到Activity上可以通过xml文件和代码编程两种方式实现。本文只介绍第二种实现方式。
<!-- res/layout/example_activity.xml -->
<androidx.fragment.app.FragmentContainerView
xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/fragment_container_view"
android:layout_width="match_parent"
android:layout_height="match_parent" />
public class ExampleActivity extends AppCompatActivity {
public ExampleActivity() {
super(R.layout.example_activity);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.setReorderingAllowed(true)
.add(R.id.fragment_container_view, ExampleFragment.class, null)
.commit();
}
}
}
我们可以看到，主要是通过FragmentManager来将Fragment添加到fragment_container_view对应的布局中，Demo是蛮简单的，模板代码，网上拷贝一下就能用。但是FragmentManager、Transaction、commit()分别是什么意思，还有待深入探究一番。
3. 深入理解Fragment原理
3.1 Fragment本质是什么
3.1.1 Fragment本质是View
从前面的DEMO我们看到，Fragment会add到FragmentContainerView上面，能被添加到ViewGroup的组件，一定也是View或者ViewGroup。有源码为证：
//androidx.fragment.app.Fragment.java
ViewGroup mContainer;
// The View generated for this fragment.
View mView;
那么既然如此，何必大费周章呢，直接用View来替代Fragment不就好了，addView()、removeView()使用起来更简单直观。
当然了Fragment本质是View但是又不仅限于此， 在真实的项目中，界面往往很复杂，业务逻辑也很复杂，往往需要处理各种UI状态变化，比如：增加一组View，替换一组View，删除一组View，还需要和Activity协作。总之，如果让开发者自己去实现这一套逻辑，恐怕比使用Fragment要麻烦的多吧。尽管Fragment看起来蛮复杂的，但是还是远比我们自己去实现相同功能简单的多。
3.1.2 Fragment本质不仅限于View
Fragment作为一个View可以被添加到Activity的布局中去。但是他更强大，还有以下几个特性：
- 处理生命周期，比Activity生命周期还要复杂
- 通过FragmentTransaction操作多个Fragment
- 通过FragmentManager维护一个回退栈，回退到上一个FragmentTransaction操作的界面
//androidx.fragment.app.Fragment.java
static final int INITIALIZING = -1; // Not yet attached.
static final int ATTACHED = 0; // Attached to the host.
static final int CREATED = 1; // Created.
static final int VIEW_CREATED = 2; // View Created.
static final int AWAITING_EXIT_EFFECTS = 3; // Downward state, awaiting exit effects
static final int ACTIVITY_CREATED = 4; // Fully created, not started.
static final int STARTED = 5; // Created and started, not resumed.
static final int AWAITING_ENTER_EFFECTS = 6; // Upward state, awaiting enter effects
static final int RESUMED = 7; // Created started and resumed.
从代码我们可以看出，有8种状态。分为两类，View相关的以及和Activity状态相关的。后文会详细介绍，此处点到为止。
3.2 FragmentTransaction
3.2.1 FragmentTransaction概述
软件工程中有一门叫数据库的课程，数据库中有一个叫"事务"的概念，它表示多个操作是原子性的，要么一起成功，要么一起失败，典型的例子就是转账，张三给李四转账100元这么一个事务，存在两个操作，从张三的账户中扣除100元，给李四的账户增加100元。因为有了事务，不会存在只有某一个操作成功，而另一个操作失败的情况，否则后果不堪设想。
在Android中SharedPreference也有commit方法。
SharedPreferences sharedPreferences= getSharedPreferences("data",Context.MODE_PRIVATE);
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString("name", “Tom”);
editor.putInt("age", 28);
editor.putBoolean("marrid",false);
editor.commit();
上述功能，一次提交做了三件事情，putString，putInt，putBoolean。
而FragmentTransaction和SharedPreference类似。它可以同时操作多个Fragment。
假设有布局文件如下：
<LinearLayout android:orientation="vertical">
<ViewGroup1>
<Fragment1 />
</ViewGroup1>
<ViewGroup2>
<Fragment2 />
</ViewGroup2>
<ViewGroup3>
<Fragment3 />
</ViewGroup3>
</LinearLayout>
假设我们想在ViewGroup1上增加Fragment1，ViewGroup2上用Fragment2替换掉，把ViewGroup3上的Fragment移除掉。
伪代码如下:
getSupportFragmentManager().beginTransaction()
.setReorderingAllowed(true)
.add(ViewGroup1, Fragment1.class, null)
.replace(ViewGroup2,Fragment2.class,null)
.remove(Fragment3)
.commit();
问：这三个操作我为什么要放到一个事务中呢？我连续执行这三个操作不行吗？
答：有些场景下可以，有些场景下不可以。涉及到回退栈的时候，是否放到事务中，按返回键会有很大的区别。我们把上述三个操作分别命名为OP1、OP2、OP3。如果调用了FragmentTransaction.addToBackStack()，三个操作放到事务中时， 按返回键时会执行与OP1、OP2、OP3相反的操作，即ViewGroup1、ViewGroup2、ViewGroup3同时恢复到处理事务之前的状态。如果三个操作不放到事务中时， 则会依次恢复 ViewGroup3、ViewGroup2、ViewGroup1的状态
3.2.2 FragmentTransaction源码分析
3.2.2.1 支持的操作OP_CMD
//FragmentTransaction.java
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;
挑几个一眼就能看懂的。
- OP_ADD表示增加Fragment
- OP_REPLACE表示替换某个ViewGroup上的Fragment
- OP_REMOVE删除Fragment
- OP_HIDE隐藏Fragment，等同于View.setVisibility(View.GONE)
- OP_SHOW显示Fragment，等同于View.setVisibility(View.VISIBLE)
- OP_DETACH detach Fragment
- OP_ATTACH attach Fragment
对应的方法分别是：
//FragmentTransaction.java
public FragmentTransaction add(@NonNull Fragment fragment, @Nullable String tag) {
doAddOp(0, fragment, tag, OP_ADD);
return this;
}
public FragmentTransaction replace(@IdRes int containerViewId, @NonNull Fragment fragment) {
return replace(containerViewId, fragment, null);
}
public FragmentTransaction remove(@NonNull Fragment fragment) {
addOp(new Op(OP_REMOVE, fragment));
return this;
}
3.2.2.2 OP类
public abstract class FragmentTransaction {
ArrayList<Op> mOps = new ArrayList<>();
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;
}
}
}
从源码中我们可以看到，FragmentTransaction中有ArrayList mOps。它表示一个事务中需要处理几个操作。而OP类最重要的两个字段就是mCmd和mFragment。表示对某个Fragment执行指定的操作。
3.2.2.3 FragmentTransaction.commit()方法
//FragmentTransaction.java
public abstract class FragmentTransaction {
public abstract int commit();
}
我们看到commit()方法是个抽象方法。它由BackStackRecord类实现。
final class BackStackRecord extends FragmentTransaction implements
FragmentManager.BackStackEntry, FragmentManager.OpGenerator {
@Override
public int commit() {
return commitInternal(false);
}
}
3.3 BackStackRecord
3.3.1 BackStackRecord概述
BackStackRecord是FragmentTransaction的子类，表示它支持同时操作多个Fragment。同时，顾名思义，它会被放到BackStack(回退栈)中。回退栈定义在FragmentManager中，是一个ArrayList集合。
//FragmentManager.java
ArrayList<BackStackRecord> mBackStack;
一言以蔽之，BackStackRecord支持事务操作，同时又会被放入到回退栈中
3.3.2 BackStackRecord commit流程
int commitInternal(boolean allowStateLoss) {
if (mCommitted) throw new IllegalStateException("commit already called");
if (FragmentManager.isLoggingEnabled(Log.VERBOSE)) {
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();
} else {
mIndex = -1;
}
mManager.enqueueAction(this, allowStateLoss);
return mIndex;
}
该方法最主要的就是调用了FragmentManager.enqueueAction方法
3.4 FragmentManager
3.4.1 FragmentManager概述
从前文，我们知道，对多个Fragment的操作会被记录到FragmentTransaction中，最终调用FragmentManager.enqueueAction方法，真正执行Fragment操作。
3.4.2 操作调用流程
1. FragmentManager.enqueueAction()
void enqueueAction(@NonNull OpGenerator action, boolean allowStateLoss) {
if (!allowStateLoss) {
if (mHost == null) {
if (mDestroyed) {
throw new IllegalStateException("FragmentManager has been destroyed");
} else {
throw new IllegalStateException("FragmentManager has not been attached to a "
+ "host.");
}
}
checkStateLoss();
}
synchronized (mPendingActions) {
if (mHost == null) {
if (allowStateLoss) {
// This FragmentManager isn't attached, so drop the entire transaction.
return;
}
throw new IllegalStateException("Activity has been destroyed");
}
mPendingActions.add(action);
scheduleCommit();
}
}
该方法最终调用到scheduleCommit()方法
2. FragmentManager.scheduleCommit()
void scheduleCommit() {
synchronized (mPendingActions) {
boolean postponeReady =
mPostponedTransactions != null && !mPostponedTransactions.isEmpty();
boolean pendingReady = mPendingActions.size() == 1;
if (postponeReady || pendingReady) {
mHost.getHandler().removeCallbacks(mExecCommit);
mHost.getHandler().post(mExecCommit);
updateOnBackPressedCallbackEnabled();
}
}
}
该方法最终通过Handler执行mExecCommit
3. FragmentManager.mExecCommit
private Runnable mExecCommit = new Runnable() {
@Override
public void run() {
execPendingActions(true);
}
};
该方法最终调用execPendingActions
4. FragmentManager.execPendingActions()
boolean execPendingActions(boolean allowStateLoss) {
ensureExecReady(allowStateLoss);
boolean didSomething = false;
while (generateOpsForPendingActions(mTmpRecords, mTmpIsPop)) {
mExecutingActions = true;
try {
removeRedundantOperationsAndExecute(mTmpRecords, mTmpIsPop);
} finally {
cleanupExec();
}
didSomething = true;
}
updateOnBackPressedCallbackEnabled();
doPendingDeferredStart();
mFragmentStore.burpActive();
return didSomething;
}
重点关注removeRedundantOperationsAndExecute(mTmpRecords, mTmpIsPop)
5. FragmentManager.removeRedundantOperationsAndExecute()
private void removeRedundantOperationsAndExecute(@NonNull ArrayList<BackStackRecord> records,
@NonNull ArrayList<Boolean> isRecordPop) {
}
方法比较长省略代码，最终调用executeOpsTogether
6. FragmentManager.executeOpsTogether
private void executeOpsTogether(@NonNull ArrayList<BackStackRecord> records,
@NonNull ArrayList<Boolean> isRecordPop, int startIndex, int endIndex) {
}
方法比较长省略代码，最终调用executeOps方法
7. FragmentManager.executeOps()
private static void executeOps(@NonNull ArrayList<BackStackRecord> records,
@NonNull ArrayList<Boolean> 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();
}
}
}
该方法分为两种情况，入栈和出栈，对应commit()和FragmentManager.popBackStack()操作。
8. FragmentManager.executeOps()
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.setPopDirection(false);
f.setNextTransition(mTransition);
f.setSharedElementNames(mSharedElementSourceNames, mSharedElementTargetNames);
}
switch (op.mCmd) {
case OP_ADD:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.setExitAnimationOrder(f, false);
mManager.addFragment(f);
break;
case OP_REMOVE:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.removeFragment(f);
break;
case OP_HIDE:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.hideFragment(f);
break;
case OP_SHOW:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.setExitAnimationOrder(f, false);
mManager.showFragment(f);
break;
case OP_DETACH:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.detachFragment(f);
break;
case OP_ATTACH:
f.setAnimations(op.mEnterAnim, op.mExitAnim, op.mPopEnterAnim, op.mPopExitAnim);
mManager.setExitAnimationOrder(f, false);
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) {
if (!FragmentManager.USE_STATE_MANAGER) {
mManager.moveFragmentToExpectedState(f);
}
}
}
if (!mReorderingAllowed && !FragmentManager.USE_STATE_MANAGER) {
// Added fragments are added at the end to comply with prior behavior.
mManager.moveToState(mManager.mCurState, true);
}
}
该方法做了两件事，其一：根据OP.mCmd，操作FragmentManager对应的方法，该步骤并不会真正执行操作，也只是做记录操作，其二：调用mManager.moveToState(mManager.mCurState, true)
最终会调用到mManager.moveToState(Fragment f, int newState)方法，它是Fragment框架中真正核心方法
3.5 FragmentManager的核心方法void moveToState(Fragment f, int newState)
作为核心方法，并没有放在3.4章节中，只是为了突出它的核心地位。
void moveToState(@NonNull Fragment f, int newState) {
FragmentStateManager fragmentStateManager = mFragmentStore.getFragmentStateManager(f.mWho);
if (fragmentStateManager == null) {
// Ideally, we only call moveToState() on active Fragments. However,
// in restoreSaveState() we can call moveToState() on retained Fragments
// just to clean them up without them ever being added to mActive.
// For these cases, a brand new FragmentStateManager is enough.
fragmentStateManager = new FragmentStateManager(mLifecycleCallbacksDispatcher,
mFragmentStore, f);
// Only allow this FragmentStateManager to go up to CREATED at the most
fragmentStateManager.setFragmentManagerState(Fragment.CREATED);
}
// When inflating an Activity view with a resource instead of using setContentView(), and
// that resource adds a fragment using the <fragment> tag (i.e. from layout and in layout),
// the fragment will move to the VIEW_CREATED state before the fragment manager
// moves to CREATED. So when moving the fragment manager moves to CREATED and the
// inflated fragment is already in VIEW_CREATED we need to move new state up from CREATED
// to VIEW_CREATED. This avoids accidentally moving the fragment back down to CREATED
// which would immediately destroy the Fragment's view. We rely on computeExpectedState()
// to pull the state back down if needed.
if (f.mFromLayout && f.mInLayout && f.mState == Fragment.VIEW_CREATED) {
newState = Math.max(newState, Fragment.VIEW_CREATED);
}
newState = Math.min(newState, fragmentStateManager.computeExpectedState());
if (f.mState <= newState) {
// If we are moving to the same state, we do not need to give up on the animation.
if (f.mState < newState && !mExitAnimationCancellationSignals.isEmpty()) {
// The fragment is currently being animated... but! Now we
// want to move our state back up. Give up on waiting for the
// animation and proceed from where we are.
cancelExitAnimation(f);
}
switch (f.mState) {
case Fragment.INITIALIZING:
if (newState > Fragment.INITIALIZING) {
fragmentStateManager.attach();
}
// fall through
case Fragment.ATTACHED:
if (newState > Fragment.ATTACHED) {
fragmentStateManager.create();
}
// fall through
case Fragment.CREATED:
// We want to unconditionally run this anytime we do a moveToState that
// moves the Fragment above INITIALIZING, including cases such as when
// we move from CREATED => CREATED as part of the case fall through above.
if (newState > Fragment.INITIALIZING) {
fragmentStateManager.ensureInflatedView();
}
if (newState > Fragment.CREATED) {
fragmentStateManager.createView();
}
// fall through
case Fragment.VIEW_CREATED:
if (newState > Fragment.VIEW_CREATED) {
fragmentStateManager.activityCreated();
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState > Fragment.ACTIVITY_CREATED) {
fragmentStateManager.start();
}
// fall through
case Fragment.STARTED:
if (newState > Fragment.STARTED) {
fragmentStateManager.resume();
}
}
} else if (f.mState > newState) {
switch (f.mState) {
case Fragment.RESUMED:
if (newState < Fragment.RESUMED) {
fragmentStateManager.pause();
}
// fall through
case Fragment.STARTED:
if (newState < Fragment.STARTED) {
fragmentStateManager.stop();
}
// fall through
case Fragment.ACTIVITY_CREATED:
if (newState < Fragment.ACTIVITY_CREATED) {
if (isLoggingEnabled(Log.DEBUG)) {
Log.d(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) {
fragmentStateManager.saveViewState();
}
}
}
// fall through
case Fragment.VIEW_CREATED:
if (newState < Fragment.VIEW_CREATED) {
FragmentAnim.AnimationOrAnimator anim = null;
if (f.mView != null && f.mContainer != null) {
// Stop any current animations:
f.mContainer.endViewTransition(f.mView);
f.mView.clearAnimation();
// If parent is being removed, no need to handle child animations.
if (!f.isRemovingParent()) {
if (mCurState > Fragment.INITIALIZING && !mDestroyed
&& f.mView.getVisibility() == View.VISIBLE
&& f.mPostponedAlpha >= 0) {
anim = FragmentAnim.loadAnimation(mHost.getContext(),
f, false, f.getPopDirection());
}
f.mPostponedAlpha = 0;
// Robolectric tests do not post the animation like a real device
// so we should keep up with the container and view in case the
// fragment view is destroyed before we can remove it.
ViewGroup container = f.mContainer;
View view = f.mView;
if (anim != null) {
FragmentAnim.animateRemoveFragment(f, anim,
mFragmentTransitionCallback);
}
container.removeView(view);
if (FragmentManager.isLoggingEnabled(Log.VERBOSE)) {
Log.v(FragmentManager.TAG, "Removing view " + view + " for "
+ "fragment " + f + " from container " + container);
}
// If the local container is different from the fragment
// container, that means onAnimationEnd was called, onDestroyView
// was dispatched and the fragment was already moved to state, so
// we should early return here instead of attempting to move to
// state again.
if (container != f.mContainer) {
return;
}
}
}
// If a fragment has an exit animation (or transition), do not destroy
// its view immediately and set the state after animating
if (mExitAnimationCancellationSignals.get(f) == null) {
fragmentStateManager.destroyFragmentView();
}
}
// fall through
case Fragment.CREATED:
if (newState < Fragment.CREATED) {
if (mExitAnimationCancellationSignals.get(f) != null) {
// We are waiting for the fragment's view to finish animating away.
newState = Fragment.CREATED;
} else {
fragmentStateManager.destroy();
}
}
// fall through
case Fragment.ATTACHED:
if (newState < Fragment.ATTACHED) {
fragmentStateManager.detach();
}
}
}
if (f.mState != newState) {
if (isLoggingEnabled(Log.DEBUG)) {
Log.d(TAG, "moveToState: Fragment state for " + f + " not updated inline; "
+ "expected state " + newState + " found " + f.mState);
}
f.mState = newState;
}
}
该方法是整个Fragment框架中的核心方法，它会根据目标state和Fragment当前的state一步一步的升级或降级Fragment的State。最终回调到Fragment的相关生命周期方法。 至此整个commit方法的调用链条就分析完毕了。
由于篇幅有限，mManager.moveToState(Fragment f, int newState)我将新写一篇文章，专门图解一番。其实LifeCycle组件的State变化也是类似的，一步一步升级或降级。
最后
小编分享一些 Android 开发相关的学习文档、面试题、Android 核心笔记等等文档，希望能帮助到大家学习提升，如有需要参考的可以直接去我 CodeChina地址：https://codechina.csdn.net/u012165769/Android-T3 访问查阅。