Fragment事务管理源码分析

概述

在Fragment使用中,有时候需要对Fragment进行addremoveshowhidereplace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

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/**
* 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

2019-9-2-11-19-20.png
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

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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:

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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

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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

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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

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

终于找到FragmentManager的真身FragmentManagerImpl

FragmentManagerImpl#beginTransaction

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@Override
public 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:
2019-9-2-11-20-42.png

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

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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tag
if (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mFragmentId
fragment.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:

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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_ATTACH
Fragment 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。

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public int commit() {
return commitInternal(false);
}

int commitInternal(boolean allowStateLoss) {
//防止重复commit
if (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true
if (mAddToBackStack) {
mIndex = mManager.allocBackStackIndex(this);
} else {
mIndex = -1;
}
//插入事物队列
mManager.enqueueAction(this, allowStateLoss);
return mIndex;
}

FragmentManagerImpl#enqueueAction

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/**
* 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

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final Handler mHandler = new Handler();

再看看mExecCommit中做了什么操作:

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Runnable mExecCommit = new Runnable() {
@Override
public 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

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

... ...
}

兜兜转转,我们又回到了BackStackRecord

BackStackRecord#run

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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

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