在 ViewPager 中保存 Fragment 状态

Question

我尝试了the sample code from the API，但它并没有真正起作用，所以我实现了自己的：

FragmentPagerSupport

public class FragmentPagerSupport extends FragmentActivity {

static final int NUM_ITEMS = 10;
MyAdapter mAdapter;
ViewPager mPager;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    mAdapter = new MyAdapter(getSupportFragmentManager());
    Log.i("Pager", "mAdapter = " + mAdapter.toString());

    mPager = (ViewPager)findViewById(R.id.pager);
    if (mPager == null)
        Log.i("Pager", "mPager = null");
    else 
        Log.i("Pager", "mPager = " + mPager.toString());

    Log.i("Pager", "Setting Pager Adapter");
    mPager.setAdapter(mAdapter);
}

public static class MyAdapter extends FragmentPagerAdapter {
    public MyAdapter(FragmentManager fm) {
        super(fm);
        Log.i("Pager", "MyAdapter constructor");
    }

    @Override
    public int getCount() {
        Log.i("Pager", "MyAdapter.getCount()");
        return NUM_ITEMS;            
    }

    @Override
    public Fragment getItem(int position) {
        Log.i("Pager", "MyAdapter.getItem()");

        return TestFragment.newInstance(position);
    }
}

public static class TestFragment extends Fragment {

    public static TestFragment newInstance(int position) {
        Log.i("Pager", "TestFragment.newInstance()");

        TestFragment fragment = new TestFragment();

        Bundle args = new Bundle();
        args.putInt("position", position);
        fragment.setArguments(args);

        return fragment;
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // TODO Auto-generated method stub
        Log.i("Pager", "TestFragment.onCreateView()");

        LinearLayout layout = (LinearLayout)inflater.inflate(R.layout.fragment_item, null);
        int position = getArguments().getInt("position");

        TextView tv = (TextView)layout.findViewById(R.id.text);
        tv.setText("Fragment # " + position);
        tv.setTextColor(Color.WHITE);
        tv.setTextSize(30);

        return layout;
    }

}

}

ma​​in.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <android.support.v4.view.ViewPager
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    />
</LinearLayout>

fragment_item.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >
    <TextView  
    android:id="@+id/text"
    android:layout_width="match_parent" 
    android:layout_height="match_parent"
    android:gravity="center" 
    android:text="@string/hello"
    />
</LinearLayout>

我的问题是，不是每次用户左右滑动时都创建一个新的Fragment 实例，而是如何保存片段的状态（到某些数据结构）然后恢复它？

API demo 似乎根本没有任何状态信息保存代码。

Answer 1

在FragmentActivity 中使用ViewPager.setOffscreenPageLimit()

ViewPager pager = (ViewPager) findViewById(R.id.viewPager);
pager.setOffscreenPageLimit(2);

在How do I tell my custom FragmentPagerAdapter to stop destroying my fragments?查看更多信息

Answer 2

只需在 FragmentpagerAdapter 中重写此方法

@Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            // TODO Auto-generated method stub
            super.destroyItem(ViewGroup container, int position, Object object);
        }

删除super.destroyItem(ViewGroup container, int position, Object object);

来自您的代码

Answer 3

更新： 在 MyAdapter 的 getItem 中，您返回 Fragment 的新实例。 Android 不会在每次滑动时调用 getItem。它只会调用 getItem 来获取新实例，但只要现有实例可用，就会重用它们。

关于演示的状态部分。我帮不了你。我认为在 Fragments/Activity 中恢复状态的常规技术适用于此，因此在 ViewPager 中加载它时没有什么特别的（但我可能错了）。

Answer 4

如果你覆盖

public void destroyItem(View container, int position, Object object)

不调用super，fragment不会被销毁，会被复用。

Answer 5

适配器应该从 FragmentStatePagerAdapter 而不是 FragmentPageAdapter 扩展，并且适配器将保持从位置到片段项的引用。在activity或fragment parent上，为PageIndicator设置监听OnPageChangeListener，检测fragment item的位置是否被激活，并更新相关数据的状态。

关于片段项的数据状态，我认为应该从 Activity 或 Fragment 父级保存/恢复状态。

适配器可以添加如下代码：

public static class MyAdapter extends FragmentStatePagerAdapter {
    private SparseArray<TestFragment> mPageReferenceMap 
                              = new SparseArray<TestFragment>();
    ...

    @Override 
    public Object instantiateItem(ViewGroup viewGroup, int position) {
        Object obj = super.instantiateItem(viewGroup, position);

        //Add the reference when fragment has been create or restore
        if (obj instanceof TestFragment) {
                TestFragment f= (TestFragment)obj;
                mPageReferenceMap.put(position, f);
        }

        return obj;
    }

    @Override
    public void destroyItem(ViewGroup container, int position, Object object) {
        //Remove the reference when destroy it
        mPageReferenceMap.remove(position);

        super.destroyItem(container, position, object);
    }

    public TestFragment getFragment(int key) {

        return mPageReferenceMap.get(key);
    }
    ...
}

希望对您有所帮助。

Answer 6

我在类似的地方绊倒了... 我想将我的片段保存在我的适配器中以避免无用的重新加载，因为我在 FragmentPagerAdapter 中只有 5 个片段。

我基本上必须创建一个数组并覆盖getItem(position) 方法：

private StoryListFragment[] fragmentsArray
        = new StoryListFragment[getCount()];

public SectionsAdapter(FragmentManager fm) {
    super(fm);
}


@Override
public Fragment getItem(int position) {

    if(fragmentsArray[position]!=null)
        return fragmentsArray[position];
    StoryListFragment storyListFragment = null;
    switch(position){
        case(0):
            storyListFragment = StoryListFragment.newInstance(StoriesBank.NEWS);
            break;
        case(1):
            storyListFragment = StoryListFragment.newInstance(StoriesBank.FEATURES);
            break;
        case(2):
             storyListFragment=  StoryListFragment.newInstance(StoriesBank.ARTS);
            break;
    }

    fragmentsArray[position] =  storyListFragment;
    return storyListFragment;
}

@Override
public Object instantiateItem(ViewGroup container, int position) {
    StoryListFragment fragment = (StoryListFragment) super.instantiateItem(container, position);
    fragmentsArray[position] = fragment;
    return fragment;
}