Fragment重叠问题引发的思考

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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:

这里写图片描述

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();
    }

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

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

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}]

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);
    }

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";

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}

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;
    }

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();

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);
}

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"

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();
    }

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;

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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