Android ViewPager,屏幕旋转后标签不保持状态

Question

我执行与标签类似,由谷歌提供的实施ViewPager 这里.

我的应用有以下行为.我的ViewPager有3个"页面"(fragA,fragB,fragC),我在ViewPager上面实现了3个标签.谷歌的实现和我的实现之间的区别在于标签不用于在片段之间导航.例如,按下我的一个选项卡,在可见片段上加载一组不同的数据.我可以选择tab3选择fragA,选择tab2选择fragB,选择tab1选择fragC.

问题是当我旋转屏幕时.如果我FragmentA选择了第三个选项卡,当我旋转屏幕时,我会保持相同的片段,但现在,第一个选项卡被选中.我希望在旋转屏幕时保持选择相同的选项卡.

这是我管理标签的类:

public class ManagerTabs extends Fragment {

    private TabHost.TabContentFactory mFactory = new TabHost.TabContentFactory() {

        @Override
        public View createTabContent(String tag) {
            View v = new View(getActivity());
            v.setMinimumWidth(0);
            v.setMinimumHeight(0);
            return v;
        }
    };

    public static ManagerTabs newManager() {
        return new ManagerTabs();
    }


    @Override
    public View onCreateView(LayoutInflater inflater,  ViewGroup container,
     Bundle savedInstanceState) {
        view = inflater.inflate(R.layout.datatabs, container, false);
        createTabs();
        createPager();
        return view;
    }

    @Override
    public void onViewCreated(View view, Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        if (savedInstanceState != null) {
            currentPage = savedInstanceState.getInt("currentPage", currentPage);
            currentTab = savedInstanceState.getInt("currentTab", currentTab);
            mTabHost.setCurrentTab(currentTab);
        }
        mPager.setCurrentItem(currentPage);
    }

    private void createPager() {
        mPager = (ViewPager) view.findViewById(R.id.datapager);
        pagerAdapter = new PagerAdapter(getChildFragmentManager());
        mPager.setAdapter(pagerAdapter);

        IconPageIndicator indicator = (IconPageIndicator) rootView.findViewById(R.id.indicator);
        indicator.setViewPager(pager);
        indicator.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
            @Override
            public void onPageSelected(int position) {
                currentTab = mTabHost.getCurrentTab();

                currentPage = mPager.getCurrentItem();

                switch (currentPage) {
                case 0:
                    ((FragmentA) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                case 1:
                    ((FragmentB) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                case 2:
                    ((FragmentC) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                }
            }
        });
    }

    private void createTabs() {

        mTabHost = (TabHost) view.findViewById(android.R.id.tabhost);
        mTabHost.setup();

        mTabHost.addTab(mTabHost.newTabSpec(getString(R.string.day))setIndicator(getString(R.string.day)).setContent(mFactory));
        mTabHost.addTab(mTabHost.newTabSpec(getString(R.string.month)).setIndicator(getString(R.string.month)).setContent(mFactory));
        mTabHost.addTab(mTabHost.newTabSpec(getString(R.string.year)).setIndicator(getString(R.string.year)).setContent(mFactory));

        mTabHost.setOnTabChangedListener(new TabHost.OnTabChangeListener() {
            public void onTabChanged(String tag) {
                currentTab = mTabHost.getCurrentTab();
                currentPage = mPager.getCurrentItem();
                switch (currentPage) {
                case 0:
                    ((FragmentA) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                case 1:
                    ((FragmentB) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                case 2:
                    ((FragmentC) pagerAdapter.getCurrentFragment(currentPage)).getData(currentTab);
                    break;
                }
            }
        });
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("currentPage", mPager.getCurrentItem());
        outState.putInt("currentTab", mTabHost.getCurrentTab());
    }
}

这是创建片段的适配器:

public class PagerAdapter extends FragmentPagerAdapter implements IconPagerAdapter{

    private Fragment[] currentFragment = new Fragment[3];

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

    @Override
    public Fragment getItem(int index) {
        Fragment fragment;
        switch (index) {
        case 0:
            fragment = FragmentA.newInstance();
            break;
        case 1:
            fragment = FragmentB.newInstance();
            break;
        case 2:
            fragment = FragmentC.newInstance();
            break;
        default:
            fragment = null;
            break;
        }

        currentFragment[index] = fragment;

        return fragment;
    }

    @Override
    public int getIconResId(int index) {

        switch (index) {
        case 0:
            return R.drawable.fraga;
        case 1:
            return R.drawable.fragb;
        case 2:
            return R.drawable.fragc;
        default:
            return -1;
        }
    }

    @Override
    public int getCount() {
        return 3;
    }

    public Fragment getCurrentFragment(int currentFrag) {
        return currentFragment[currentFrag];
    }

    public Fragment[] getCurrentFragment() {
        return currentFragment;
    }
}

最后,片段(FragmentA,FragmentB和FragmentC)几乎完全相同:

public class FragmentA extends Fragment {

    public static FragmentA newInstance() {
        return new FragmentA();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        rootView = inflater.inflate(R.layout.datafragment, container, false);

        mTabHost = (TabHost) ((View) container.getParent().getParent()).findViewById(android.R.id.tabhost);

        if (savedInstanceState != null) {
            currentTab = savedInstanceState.getInt("currentTab");
        }
        populateLayout(currentTab);

        return rootView;
    }

    @Override
    public void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("currentTab", currentTab);
    }
}

问题是我onCreateView的所有人总是被叫两次,第二次被叫,信息就丢失了.我已经调试并看到了这种行为,

当我旋转屏幕时,例如在FragmentA中并选择第三个选项卡时,会发生以下情况:

01  ManagerTabs -> onSaveInstanceState
02  FragmentA -> onSaveInstanceState
03  ManagerTabs -> onCreateView
04  ManagerTabs -> onViewCreated

在04调用(ManagerTabs - > onViewCreated)中,mTabHost.setCurrentTab(currentTab);调用但在内部onTabChanged,我得到一个java.lang.NullPointerException因为适配器内的所有片段都是null.

我该怎么办,如果我旋转屏幕,我可以保持当前选定的标签？

Answer 1

屏幕旋转后,FragmentPagerAdapter将创建新实例.但是它ViewPager恢复了它包含的所有片段的状态和状态.ViewPager不调用适配器getView()方法.因此仅Fragment[] currentFragment包含null值.

您可以通过其他方式获取当前片段.

如果使用FragmentPagerAdapter:

FragmentPagerAdapter在它传递给的片段上设置标签FragmentManager.此id取决于页面索引.如何生成此id,您可以在最新的支持库的源代码中看到.

要实现此目的,只需在ManagerTabs片段中添加以下方法:

public Fragment getCurrentPagerFragment(int position) {
    String fragmentTag = "android:switcher:" + mPager.getId() + ":" + position;
    return getChildFragmentManager().findFragmentByTag(fragmentTag);
}

如果使用FragmentStatePagerAdapter:

它不会在片段上设置标签.随着FragmentStatePagerAdapter我们似乎可以得到通过,使用其instantiateItem(ViewGroup container, int position)电话.它返回对位置的片段的引用position.如果FragmentStatePagerAdapter已经存在对有问题的片段的引用,instantiateItem则只返回对该片段的引用,并且不再调用getItem()它来实例化它.在这种情况下,我们的方法将是:

public Fragment getCurrentPagerFragment(int position) {
    FragmentStatePagerAdapter a = (FragmentStatePagerAdapter) mPager.getAdapter();
    return (Fragment) a.instantiateItem(pager, position);
}

最后:

现在使用上面的方法来获取当前片段.在你的情况下:

mTabHost.setOnTabChangedListener(new TabHost.OnTabChangeListener() {
    public void onTabChanged(String tag) {
        currentTab = mTabHost.getCurrentTab();
        currentPage = mPager.getCurrentItem();
        Fragment currentFragment = getCurrentPagerFragment(currentPage);
        switch (currentPage) {
            case 0:
                ((FragmentA) currentFragment).getData(currentTab);
                break;
            case 1:
                ((FragmentB) currentFragment).getData(currentTab);
                break;
            case 2:
                ((FragmentC) currentFragment).getData(currentTab);
                break;
        }
    }
});