Fragment重叠问题引发的思考

谁践踏了优雅 2022-06-06 09:42 249阅读 0赞

参考：

> [http://stackoverflow.com/questions/16189088/overlapping-hidden-fragments-after-application-gets-killed-and-restored][http_stackoverflow.com_questions_16189088_overlapping-hidden-fragments-after-application-gets-killed-and-restored]
> 
> [http://stackoverflow.com/questions/18274732/android-fragments-overlapping-issue?answertab=active\#tab-top][http_stackoverflow.com_questions_18274732_android-fragments-overlapping-issue_answertab_active_tab-top]

Fragment重叠问题相信很多开发者都遇到个这个问题，也解决个这个问题，前段时间偶然发现，公司项目偶然出现了Fragment重叠的Bug，心里不由一紧，赶紧去stackoverflow搜索了一番，找到了好几种解决方案，最终问题是解决了，不过心里留下了很多疑问（为什么会出现重叠？为什么这么处理之后可以解决问题？这样写会不会引发其他问题？），带着我决定写个Demo去分析下每种解决方法的原理以及可能带来的负面影响。

# 一、问题重现 #

Demo是常见的用Fragment实现的Tab切换，拿了网上现成的Demo改了一下，先给个Fragment重叠的效果图：

![这里写图片描述][20160618205244338]

在Fragment切换时，采用的show/hide的方式，原理是显示某个Fragment时，先把其他几个Fragment先隐藏掉：

private void setTabSelection(int index) {
            clearSelection();
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            hideFragments(transaction);
            switch (index) {
                case 0:
                    messageImage.setImageResource(R.drawable.message_selected);
                    messageText.setTextColor(Color.WHITE);
                    if (messageFragment == null) {
                        messageFragment = new NormalListFragment();
                        transaction.add(R.id.content, messageFragment);
                    } else {
                        transaction.show(messageFragment);
                    }
                    break;
                case 1:
                    contactsImage.setImageResource(R.drawable.contacts_selected);
                    contactsText.setTextColor(Color.WHITE);
                    if (contactsFragment == null) {
                        contactsFragment = new ContactsFragment();
                        transaction.add(R.id.content, contactsFragment);
                    } else {
                        transaction.show(contactsFragment);
                    }
                    break;
                case 2:
                    newsImage.setImageResource(R.drawable.news_selected);
                    newsText.setTextColor(Color.WHITE);
                    if (newsFragment == null) {
                        newsFragment = new NewsFragment();
                        transaction.add(R.id.content, newsFragment);
                    } else {
                        transaction.show(newsFragment);
                    }
                    break;
                case 3:
                default:
                    settingImage.setImageResource(R.drawable.setting_selected);
                    settingText.setTextColor(Color.WHITE);
                    if (settingFragment == null) {
                        settingFragment = new SettingFragment();
                        transaction.add(R.id.content, settingFragment);
                    } else {
                        transaction.show(settingFragment);
                    }
                    break;
            }
            transaction.commit();
        }
    
     
      
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由于Fragment重叠问题是发生在某种特定的情况下，所以在常规环境下很难复现，所以需要在Android 手机开发者选项中把不保留活动这个选项打开，这样每次进入新的Activity，旧的Activity就会马上销毁。

![这里写图片描述][90bd89ffgw1f4znt3es2ej20u01hcq7t.jpg]

# 问题分析 #

假设现在处在第一个Tab 图片列表(NormalListFragment)，然后点击某个Item进入详情页，由于不保留活动，Fragment所在的Activity会销毁掉。然后，我们从详情页返回到图片列表，Activity会重建，Fragment会重新绑定, 整个过程Activity和Fragment的生命周期方法调用Log如下：

06-18 21:35:36.479 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onPause
    06-18 21:35:36.479 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onPause
    06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onSaveInstanceState   Bundle[{android:viewHierarchyState=Bundle[{android:views={
       16908290=android.view.AbsSavedState$1@6962f3e, 2131624006=android.view.AbsSavedState$1@6962f3e, 2131624007=android.view.AbsSavedState$1@6962f3e, 2131624008=android.support.v7.widget.Toolbar$SavedState@bda98cc, 2131624009=android.view.AbsSavedState$1@6962f3e, 2131624052=android.view.AbsSavedState$1@6962f3e, 2131624053=android.view.AbsSavedState$1@6962f3e, 2131624054=android.view.AbsSavedState$1@6962f3e, 2131624055=android.view.AbsSavedState$1@6962f3e, 2131624056=android.view.AbsSavedState$1@6962f3e, 2131624057=android.view.AbsSavedState$1@6962f3e, 2131624058=android.view.AbsSavedState$1@6962f3e, 2131624059=android.view.AbsSavedState$1@6962f3e, 2131624060=android.view.AbsSavedState$1@6962f3e, 2131624061=android.view.AbsSavedState$1@6962f3e, 2131624062=android.view.AbsSavedState$1@6962f3e, 2131624063=android.view.AbsSavedState$1@6962f3e, 2131624064=android.view.AbsSavedState$1@6962f3e}, android:focusedViewId=2131624102}], android:support:fragments=android.support.v4.app.FragmentManagerState@696715}]
    06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onStop
    06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onStop
    06-18 21:35:36.910 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onDestroyView
    06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onDestroy
    06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onDetach
    06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onDestroy
    06-18 21:35:39.527 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onAttach
    06-18 21:35:39.527 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onCreate
    06-18 21:35:39.527 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity   onCreate   Bundle[{android:viewHierarchyState=Bundle[mParcelledData.dataSize=1512], android:support:fragments=android.support.v4.app.FragmentManagerState@e7e75b8}]
    06-18 21:35:39.666 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment   onCreateView   Bundle[{android:view_state={
       2131624102=AbsListView.SavedState{
       402bf7e selectedId=-9223372036854775808 firstId=-1 viewTop=0 position=0 height=1557 filter=null checkState=null}}}]
    06-18 21:35:39.672 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment   onActivityCreated   Bundle[{android:view_state={
       2131624102=AbsListView.SavedState{
       402bf7e selectedId=-9223372036854775808 firstId=-1 viewTop=0 position=0 height=1557 filter=null checkState=null}}}]
    06-18 21:35:39.672 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onAttach
    06-18 21:35:39.672 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onCreate
    06-18 21:35:39.673 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment   onCreateView   null
    06-18 21:35:39.675 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment   onActivityCreated   null
    06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onStart
    06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onStart
    06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onStart
    06-18 21:35:39.679 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onRestoreInstanceState  Bundle[{android:viewHierarchyState=Bundle[{android:views={
       16908290=android.view.AbsSavedState$1@6962f3e, 2131624006=android.view.AbsSavedState$1@6962f3e, 2131624007=android.view.AbsSavedState$1@6962f3e, 2131624008=android.support.v7.widget.Toolbar$SavedState@80d4056, 2131624009=android.view.AbsSavedState$1@6962f3e, 2131624052=android.view.AbsSavedState$1@6962f3e, 2131624053=android.view.AbsSavedState$1@6962f3e, 2131624054=android.view.AbsSavedState$1@6962f3e, 2131624055=android.view.AbsSavedState$1@6962f3e, 2131624056=android.view.AbsSavedState$1@6962f3e, 2131624057=android.view.AbsSavedState$1@6962f3e, 2131624058=android.view.AbsSavedState$1@6962f3e, 2131624059=android.view.AbsSavedState$1@6962f3e, 2131624060=android.view.AbsSavedState$1@6962f3e, 2131624061=android.view.AbsSavedState$1@6962f3e, 2131624062=android.view.AbsSavedState$1@6962f3e, 2131624063=android.view.AbsSavedState$1@6962f3e, 2131624064=android.view.AbsSavedState$1@6962f3e}, android:focusedViewId=2131624102}], android:support:fragments=android.support.v4.app.FragmentManagerState@e7e75b8}]
    06-18 21:35:39.680 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onResume
    06-18 21:35:39.680 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onResume
    06-18 21:35:39.680 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment  onResume
    
     
      
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从上面的Log可以发现，重新创建Activity时，NormalListFragment每个周期方法都走了两遍。这意味着同时创建了两个NormalListFragment实例，这个两个NormalListFragment一个是我代码里面主动创建的，另外一个则是上次Activity异常销毁时保存的，因为恢复的这个Fragment没有拿到引用，所以无法去做操作的（隐藏显示），这意味着我切换到其他tab时，这个Fragment会一直显示，这正是Fragment重叠问题的根源所在。

下面从源码角度证实Activity在异常情况下销毁时，会保存Fragment。

从Log第三行打印的Bundle的值我们可以发现，Activity在异常销毁时会调用onSaveInstanceState方法，系统会默认保存一些数据，包括Fragment

06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity  onSaveInstanceState   Bundle[{android:viewHierarchyState=Bundle[{android:views={
       16908290=android.view.AbsSavedState$1@6962f3e, 2131624006=android.view.AbsSavedState$1@6962f3e, 2131624007=android.view.AbsSavedState$1@6962f3e, 2131624008=android.support.v7.widget.Toolbar$SavedState@bda98cc, 2131624009=android.view.AbsSavedState$1@6962f3e, 2131624052=android.view.AbsSavedState$1@6962f3e, 2131624053=android.view.AbsSavedState$1@6962f3e, 2131624054=android.view.AbsSavedState$1@6962f3e, 2131624055=android.view.AbsSavedState$1@6962f3e, 2131624056=android.view.AbsSavedState$1@6962f3e, 2131624057=android.view.AbsSavedState$1@6962f3e, 2131624058=android.view.AbsSavedState$1@6962f3e, 2131624059=android.view.AbsSavedState$1@6962f3e, 2131624060=android.view.AbsSavedState$1@6962f3e, 2131624061=android.view.AbsSavedState$1@6962f3e, 2131624062=android.view.AbsSavedState$1@6962f3e, 2131624063=android.view.AbsSavedState$1@6962f3e, 2131624064=android.view.AbsSavedState$1@6962f3e}, android:focusedViewId=2131624102}], android:support:fragments=android.support.v4.app.FragmentManagerState@696715}]
    
     
      
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这个我们从Activity源码中的onSaveInstanceState方法也能够确认：

protected void onSaveInstanceState(Bundle outState) {
            outState.putBundle(WINDOW_HIERARCHY_TAG, mWindow.saveHierarchyState());
            Parcelable p = mFragments.saveAllState();
            if (p != null) {
                outState.putParcelable(FRAGMENTS_TAG, p);
            }
            getApplication().dispatchActivitySaveInstanceState(this, outState);
        }
     
      
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在onSaveInstanceState方法中，首先调用mFragments（FragmentManager）的saveAllState方法把Fragment数据保存到Parcelable 变量中，然后通过调用outState.putParcelable(FRAGMENTS\_TAG, p);  
保存到Bundle 中，FRAGMENTS\_TAG这个常量

static final String FRAGMENTS_TAG = "android:support:fragments";
     
      
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也正是上面Log Bundle 数据中 键值对的一个键：

android:support:fragments=android.support.v4.app.FragmentManagerState@696715}
     
      
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Activity异常销毁时保存Fragment已经可以确认，那么这个保存Fragment在重新创建Activity时，怎么恢复的了？这就需要研究Activity的onCreate方法了

protected void onCreate(@Nullable Bundle savedInstanceState) {
            if (DEBUG_LIFECYCLE) Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);
            if (mLastNonConfigurationInstances != null) {
                mAllLoaderManagers = mLastNonConfigurationInstances.loaders;
            }
            if (mActivityInfo.parentActivityName != null) {
                if (mActionBar == null) {
                    mEnableDefaultActionBarUp = true;
                } else {
                    mActionBar.setDefaultDisplayHomeAsUpEnabled(true);
                }
            }
            if (savedInstanceState != null) {
                Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
                mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
                        ? mLastNonConfigurationInstances.fragments : null);
            }
            mFragments.dispatchCreate();
            getApplication().dispatchActivityCreated(this, savedInstanceState);
            if (mVoiceInteractor != null) {
                mVoiceInteractor.attachActivity(this);
            }
            mCalled = true;
        }
    
     
      
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ActivityonCreate方法中和Fragment相关的应该是这几句：

if (savedInstanceState != null) {
                Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
                mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
                        ? mLastNonConfigurationInstances.fragments : null);
            }
            mFragments.dispatchCreate();
     
      
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当savedInstanceState 不为空时，意味着Activity上次销毁时保存了数据，会掉用FragmentManager 的 restoreAllState 方法，这个方法比较长，就不贴出来了，这个方法主要作用就是从savedInstanceState 把保存的Fragment都取出来，实例化，绑定到当前Activity。  
通过上面的分析，对Fragment的保存和恢复应该有了比较清楚的理解，也找到的Fragment重叠的根源所在，那么下一步就是如何解决问题。

# 解决方法 #

我在网上找到3中比较有代表性的解决方法，当然，实际可能有更多的解决方法，但每种方法的核心思想都是一样的，就是如何处理Activity异常销毁时保存的Fragment。

## 方法一、重写 onSaveInstanceState方法 ##

@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ......
    
        if (savedInstanceState != null) {
            mCustomVariable = savedInstanceState.getInt("variable", 0);
        }
    }
    
    @Override
    protected void onSaveInstanceState(Bundle outState) {
        //super.onSaveInstanceState(outState);
        outState.putInt("variable", mCustomVariable);
    }
     
      
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这样重写之后，相当于不会调用Activity的onSaveInstanceState的方法保存系统默认数据，只保存自己需要的数据。Activity异常销毁时不会保存Fragment，当然也就不会再有重叠的问题出现。  
不过这样处理是可能出现问题的，Activity的onSaveInstanceState方法不仅仅只是保存Fragment，还会保存获取焦点的View的状态，ActionBar，以及调用View的onSaveInstanceState 保存View的相关数据。

## 方法二、给每个Fragment根布局设置背景，拦截点击事件 ##

android:background="@android:color/white"
     android:clickable="true"
     
      
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Fragment背景默认是透明的，所以我们能看到两个Fragment重叠在一起。当我们为每个Fragment添加背景之后，即使两个Fragment叠加在一起，我们也只看到一个。至于为什么要设置clickable=”true”，是因为两个Fragment叠加在一起，虽然我们只能看到上面那个，但是下面那个仍然能接收到事件。设置clickable=”true”时，上面的Fragment会拦截掉所有事件。  
这样处理，能从视觉上解决问题，但是Activity异常销毁时，同一个Fragment同时出现两个实例的客观事实没有改变。有时你会发现Fragment中的某个网络接口明明应该只调用一次，Log却打印调用两次，其实是Fragment创建了两个实例。

## 方法三、用replace替代show/hide ##

用replace实现Tab切换的写法：

private void setTabSelection2(int index) {
            clearSelection();
            FragmentTransaction transaction = fragmentManager.beginTransaction();
            switch (index) {
                case 0:
                    messageImage.setImageResource(R.drawable.message_selected);
                    messageText.setTextColor(Color.WHITE);
                    if (messageFragment == null) {
                        messageFragment = new NormalListFragment();
                    }
                    transaction.replace(R.id.content, messageFragment);
                    break;
                case 1:
                    contactsImage.setImageResource(R.drawable.contacts_selected);
                    contactsText.setTextColor(Color.WHITE);
                    if (contactsFragment == null) {
                        contactsFragment = new ContactsFragment();
                    }
                    transaction.replace(R.id.content, contactsFragment);
                    break;
                case 2:
                    newsImage.setImageResource(R.drawable.news_selected);
                    newsText.setTextColor(Color.WHITE);
                    if (newsFragment == null) {
                        newsFragment = new NewsFragment();
                    }
                    transaction.replace(R.id.content, newsFragment);
                    break;
                case 3:
                default:
                    settingImage.setImageResource(R.drawable.setting_selected);
                    settingText.setTextColor(Color.WHITE);
                    if (settingFragment == null) {
                        settingFragment = new SettingFragment();
                    }
                    transaction.replace(R.id.content, settingFragment);
                    break;
            }
            transaction.commit();
        }
     
      
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replace的做法是每次把Activity(准确的说是Activity添加Fragment的布局)的Fragment先全部移除掉，再添加新的Fragment，这样操作确保Activity的布局容器每次只会存在一个Fragment。当然不会出现重叠问题。  
replace Fragment操作的源码，很容易看出是先移除容器包含的Fragment，然后再添加：

case OP_REPLACE: {
                        Fragment f = op.fragment;
                        if (mManager.mAdded != null) {
                            for (int i = 0; i < mManager.mAdded.size(); i++) {
        
                                Fragment old = mManager.mAdded.get(i);
                                if (FragmentManagerImpl.DEBUG) {
                                    Log.v(TAG,
                                            "OP_REPLACE: adding=" + f + " old=" + old);
                                }
                                if (f == null || old.mContainerId == f.mContainerId) {
                                    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;
                                            if (FragmentManagerImpl.DEBUG) {
                                                Log.v(TAG, "Bump nesting of "
                                                        + old + " to " + old.mBackStackNesting);
                                            }
                                        }
                                        mManager.removeFragment(old, mTransition, mTransitionStyle);
                                    }
                                }
                            }
                        }
                        if (f != null) {
                            f.mNextAnim = op.enterAnim;
                            mManager.addFragment(f, false);
                        }
                    }
                    break;
     
      
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当然，这样处理后每次点击Tab之后，每个Fragment都要重新创建实例，走周期方法，加载数据。具体能不能这样做，还要看业务上允不允许每次重新加载数据。

转自：http://blog.csdn.net/jiangxuqaz/article/details/51707031

[http_stackoverflow.com_questions_16189088_overlapping-hidden-fragments-after-application-gets-killed-and-restored]: http://stackoverflow.com/questions/16189088/overlapping-hidden-fragments-after-application-gets-killed-and-restored
[http_stackoverflow.com_questions_18274732_android-fragments-overlapping-issue_answertab_active_tab-top]: http://stackoverflow.com/questions/18274732/android-fragments-overlapping-issue?answertab=active#tab-top
[20160618205244338]: /images/20220606/634d8687d0244f7aa0e5401bcd77c401.png
[90bd89ffgw1f4znt3es2ej20u01hcq7t.jpg]: /images/20220606/3d9c6fbe90c14c8084256b867ed73af1.png