安卓使用Fragment出现重叠现象解决办法
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转载自:http://blog.csdn.net/jiangxuqaz/article/details/51707031 
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参考：
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
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：
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(); }- 1
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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。
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 onPause06-18 21:35:36.479 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onPause06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onSaveInstanceState Bundle[{android:viewHierarchyState=Bundle[{android:views={16908290=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624006</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624007=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624008</span>=android<span class="hljs-preprocessor">.support</span><span class="hljs-preprocessor">.v</span>7<span class="hljs-preprocessor">.widget</span><span class="hljs-preprocessor">.Toolbar</span>$SavedState@bda98cc, 2131624009=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624052</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624053=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624054</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624055=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624056</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624057=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624058</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624059=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624060</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624061=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624062</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624063=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624064</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$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 onStop06-18 21:35:36.893 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onStop06-18 21:35:36.910 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onDestroyView06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onDestroy06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onDetach06-18 21:35:36.914 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onDestroy06-18 21:35:39.527 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onAttach06-18 21:35:39.527 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onCreate06-18 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 onAttach06-18 21:35:39.672 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onCreate06-18 21:35:39.673 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onCreateView null06-18 21:35:39.675 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onActivityCreated null06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onStart06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onStart06-18 21:35:39.676 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onStart06-18 21:35:39.679 3004-3004/com.jx.androiddemos I/FragmentLearn: FragmentMainActivity onRestoreInstanceState Bundle[{android:viewHierarchyState=Bundle[{android:views={16908290=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624006</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624007=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624008</span>=android<span class="hljs-preprocessor">.support</span><span class="hljs-preprocessor">.v</span>7<span class="hljs-preprocessor">.widget</span><span class="hljs-preprocessor">.Toolbar</span>$SavedState@80d4056, 2131624009=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624052</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624053=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624054</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624055=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624056</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624057=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624058</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624059=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624060</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624061=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624062</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624063=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624064</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$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 onResume06-18 21:35:39.680 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onResume06-18 21:35:39.680 3004-3004/com.jx.androiddemos I/FragmentLearn: NormalListFragment onResume- 1
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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<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624006</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624007=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624008</span>=android<span class="hljs-preprocessor">.support</span><span class="hljs-preprocessor">.v</span>7<span class="hljs-preprocessor">.widget</span><span class="hljs-preprocessor">.Toolbar</span>$SavedState@bda98cc, 2131624009=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624052</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624053=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624054</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624055=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624056</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624057=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624058</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624059=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624060</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624061=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624062</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e, 2131624063=android.view.AbsSavedState$1<span class="hljs-localvars">@6962</span>f3e, <span class="hljs-number">2131624064</span>=android<span class="hljs-preprocessor">.view</span><span class="hljs-preprocessor">.AbsSavedState</span>$1@6962f3e}, android:focusedViewId=2131624102}], android:support:fragments=android.support.v4.app.FragmentManagerState@696715}]- 1
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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); }- 1
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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";- 1
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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}- 1
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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; }- 1
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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();- 1
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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
@Overrideprotected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); ...... if (savedInstanceState != null) { mCustomVariable = savedInstanceState.getInt("variable", 0); }}@Overrideprotected void onSaveInstanceState(Bundle outState) { //super.onSaveInstanceState(outState); outState.putInt("variable", mCustomVariable);}- 1
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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"- 1
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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(); }- 1
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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;- 1
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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。
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