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Fragment 源码解析add()和replace()方法

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09/15
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1.有问题的代码：

MainActivity

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private HomeFragment mHomeFragment;
private FindFragment mFindFragment;
private NewFragment mNewFragment;
private MessageFragment mMessageFragment; @Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); findViewById(R.id.home_rb).setOnClickListener(this);
findViewById(R.id.find_rb).setOnClickListener(this);
findViewById(R.id.new_rb).setOnClickListener(this);
findViewById(R.id.message_rb).setOnClickListener(this); // 默认一进入页面就添加主Fragment
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
mHomeFragment = new HomeFragment();
fragmentTransaction.add(R.id.main_tab_fl, mHomeFragment);
// 最后记得提交
fragmentTransaction.commit();
} @Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.home_rb:
homeRbClick();
break;
case R.id.find_rb:
findRbClick();
break;
case R.id.new_rb:
newRbClick();
break;
case R.id.message_rb:
messageRbClick();
break;
}
} private void homeRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
// 替换成当前页面
fragmentTransaction.replace(R.id.main_tab_fl, mHomeFragment);
fragmentTransaction.commit();
} private void findRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
if (mFindFragment == null) {
mFindFragment = new FindFragment();
}
fragmentTransaction.replace(R.id.main_tab_fl, mFindFragment);
fragmentTransaction.commit();
} private void newRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
if (mNewFragment == null) {
mNewFragment = new NewFragment();
}
fragmentTransaction.replace(R.id.main_tab_fl, mNewFragment);
fragmentTransaction.commit();
} private void messageRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
if (mMessageFragment == null) {
mMessageFragment = new MessageFragment();
}
fragmentTransaction.replace(R.id.main_tab_fl, mMessageFragment);
fragmentTransaction.commit();
}
}

目前的效果是这个样子的，看似没有任何的问题，这个也是最简单的方式

别的思路问题：一般的思路我们会换实现方法，当然其他方式肯定也可以实现如ViewPager+Fragment但是我们需要预加载要不然也会出问题，一旦预加载就需要去访问网络，即使用户可能不切换Fragment就退出App了这个时候其实加载了所有Fragment的数据，而且主页一旦复杂有可能会崩溃或造成内存溢出的问题。

2.源码分析：

add方法其实只是设置了一些必要参数，并没有做任何的处理，这也是说google为什么一定要我们不要忘记commit()的原因：

final class BackStackRecord extends FragmentTransaction implements
FragmentManager.BackStackEntry, FragmentManagerImpl.OpGenerator {

public FragmentTransaction add(int containerViewId, Fragment fragment) {
doAddOp(containerViewId, fragment, null, OP_ADD);
return this;
} private void doAddOp(int containerViewId, Fragment fragment, String tag, int opcmd) {
fragment.mFragmentManager = mManager; // tag可以说是唯一标识我们可以通过它从FragmentManager中找到对应的Fragment
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 != ) {
if (fragment.mFragmentId != && fragment.mFragmentId != containerViewId) {
throw new IllegalStateException("Can't change container ID of fragment "
+ fragment + ": was " + fragment.mFragmentId
+ " now " + containerViewId);
}
// 把Fragment的ContainerId和FragmentId指定为我们传递过来的布局中的ViewGroup的id。
fragment.mContainerId = fragment.mFragmentId = containerViewId;
} // 见名思意 Op是什么？就当是一些基本参数吧
Op op = new Op();
op.cmd = opcmd;
op.fragment = fragment;
addOp(op);
} 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++;
}

既然add方法只是设置了一些参数而已，那么肯定就在commit()中做了些什么，找啊找啊找啊找，找到这么个方法（有些代码我就省略）：

void moveToState(Fragment f, int newState, int transit,
int transitionStyle, boolean keepActive){
// ... 省略部分代码
f.onAttach(mHost.getContext());
// 这个方法一调用就会执行Fragment的onAttach(Activity activity)这个生命周期方法
if (f.mParentFragment == null) {
mHost.onAttachFragment(f);
} if (!f.mRetaining) {
f.performCreate(f.mSavedFragmentState);
// 执行生命周期onCreate(savedInstanceState);
}
f.mRetaining = false;
if (f.mFromLayout) {
ViewGroup container = null;
if (f.mContainerId != ) {
//从activity中找到我们需要存放Fragment的ViewGroup布局
container = (ViewGroup)mContainer.onFindViewById(f.mContainerId);
if (container == null && !f.mRestored) {
throwException(new IllegalArgumentException(
"No view found for id 0x"
+ Integer.toHexString(f.mContainerId) + " ("
+ f.getResources().getResourceName(f.mContainerId)
+ ") for fragment " + f));
}
}
// 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(
f.mSavedFragmentState), null, f.mSavedFragmentState);
// 这个方法过后会执行onCreateView()生命周期且f.mView就是我们自己覆盖Fragment返回的View
if (f.mView != null) {
f.mInnerView = f.mView;
// v4包兼容11以下的版本我还是没说错啊
if (Build.VERSION.SDK_INT >= ) {
ViewCompat.setSaveFromParentEnabled(f.mView, false);
} else {
f.mView = NoSaveStateFrameLayout.wrap(f.mView);
} if (container != null) {
Animation anim = loadAnimation(f, transit, true,
transitionStyle);
if (anim != null) {
setHWLayerAnimListenerIfAlpha(f.mView, anim);
f.mView.startAnimation(anim);
}
// 如果ViewGroup不等于null就把从onCreateView()生命周期中获得的View添加到该布局中
// 最主要的就是这个方法，其实我们可以把Fragment理解成一个自定义的类
// 通过onCreateView()获取的到View添加到一个FragmentActivity的一个ViewGroup中
// 只不过它有自己的生命周期而已......
container.addView(f.mView);
}
// 如果是隐藏那就设置为不可见
if (f.mHidden) f.mView.setVisibility(View.GONE);
// 执行onViewCreated()生命周期方法
f.onViewCreated(f.mView, f.mSavedFragmentState);
} else {
f.mInnerView = null;
} f.performActivityCreated(f.mSavedFragmentState);
if (f.mView != null) {
f.restoreViewState(f.mSavedFragmentState);
}
f.mSavedFragmentState = null;
}
// 代码省略......
}
}
// 后面的我们就不看了，这上面的代码我自己做了一些整合，把它连贯起来了
// 因为我们把add方法写在了Activity中的onCreate()方法中所以做了一些处理......

到这里应该能够了解Fragment的工作流程了吧，接下来我们看replace方法中究竟做了？其实和add差不多只是把int opcmd变成了OP_REPLACE替换操作：

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

这个时候去commit会调用mManager.removeFragment(old, transition, transitionStyle)方法把原来的移除，然后把当前的Fragment添加进去，那岂不是每点击一个上一就被销毁了，那之前动画到哪里来了做了写什么事都被干掉重新创建了

if (mManager.mAdded != null) {
for (int i = mManager.mAdded.size() - ; i >= ; 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 = exitAnim;
if (mAddToBackStack) {
old.mBackStackNesting += ;
}
mManager.removeFragment(old, transition, transitionStyle);
}
}
}
}
if (f != null) {
f.mNextAnim = enterAnim;
mManager.addFragment(f, false);
}

 

3.优化代码：

到这里源码就以完毕有兴趣的小伙伴可以自己仔细去看看源码，接下来我们就来解决问题，我们肯定在调用replace方法的时候希望它不要移除原来的，那怎么办改Android的底层源码吗？那就只能换方法了，思路就是如果该Fragment不存在FragmentManager中我们就去添加，否则我们把之前的隐藏而不是替换移除，把当前的显示即可，最后代码就是：

private void homeRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
List<Fragment> fragments = fragmentManager.getFragments();
for (Fragment fragment : fragments) {
fragmentTransaction.hide(fragment);
} fragmentTransaction.show(mHomeFragment);
fragmentTransaction.commit();
} @OnClick(R.id.find_rb)
private void findRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
List<Fragment> fragments = fragmentManager.getFragments();
for (Fragment fragment : fragments) {
fragmentTransaction.hide(fragment);
} if(mFindFragment == null){
mFindFragment = new FindFragment();
fragmentTransaction.add(R.id.main_tab_fl,mFindFragment);
}else {
fragmentTransaction.show(mFindFragment);
} fragmentTransaction.commit();
} @OnClick(R.id.new_rb)
private void newRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
List<Fragment> fragments = fragmentManager.getFragments();
for (Fragment fragment : fragments) {
fragmentTransaction.hide(fragment);
} if(mNewFragment == null){
mNewFragment = new NewFragment();
fragmentTransaction.add(R.id.main_tab_fl,mNewFragment);
}else {
fragmentTransaction.show(mNewFragment);
} fragmentTransaction.commit();
} @OnClick(R.id.message_rb)
private void messageRbClick() {
FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
List<Fragment> fragments = fragmentManager.getFragments();
for (Fragment fragment : fragments) {
fragmentTransaction.hide(fragment);
} if(mMessageFragment == null){
mMessageFragment = new MessageFragment();
fragmentTransaction.add(R.id.main_tab_fl,mMessageFragment);
}else {
fragmentTransaction.show(mMessageFragment);
} fragmentTransaction.commit();
}

封装

public class FragmentManagerHelper {
// 管理类FragmentManager
private FragmentManager mFragmentManager;
// 容器布局id containerViewId
private int mContainerViewId; /**
* 构造函数
* @param fragmentManager 管理类FragmentManager
* @param containerViewId 容器布局id containerViewId
*/
public FragmentManagerHelper(@Nullable FragmentManager fragmentManager, @IdRes int containerViewId) {
this.mFragmentManager = fragmentManager;
this.mContainerViewId = containerViewId;
} /**
* 添加Fragment
*/
public void add(Fragment fragment){
// 开启事物
FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction();
// 第一个参数是Fragment的容器id，需要添加的Fragment
fragmentTransaction.add(mContainerViewId, fragment);
// 一定要commit
fragmentTransaction.commit();
} /**
* 切换显示Fragment
*/
public void switchFragment(Fragment fragment){
// 开启事物
FragmentTransaction fragmentTransaction = mFragmentManager.beginTransaction(); // 1.先隐藏当前所有的Fragment
List<Fragment> childFragments = mFragmentManager.getFragments();
for (Fragment childFragment : childFragments) {
fragmentTransaction.hide(childFragment);
} // 2.如果容器里面没有我们就添加，否则显示
if(!childFragments.contains(fragment)){
fragmentTransaction.add(mContainerViewId,fragment);
}else{
fragmentTransaction.show(fragment);
} // 替换Fragment
// fragmentTransaction.replace(R.id.main_tab_fl,mHomeFragment);
// 一定要commit
fragmentTransaction.commit();
}
}