循环FragmentPagerAdapter

Question

我有一个正常工作的FragmentPagerAdapter，但我想知道的是，如果FragmentPagerAdapter在最后一个视图上，我如何才能让它循环回到第一个视图并继续正常运行

这是我正在使用的示例代码

public class FragmentPager extends FragmentActivity {
MyAdapter mAdapter;
ViewPager mPager;
static final int NUM_ITEMS = 5;

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

    mAdapter = new MyAdapter(getSupportFragmentManager());

    mPager = (ViewPager)findViewById(R.id.pager);
    mPager.setAdapter(mAdapter);
}

public static class MyAdapter extends FragmentPagerAdapter {
    public MyAdapter(FragmentManager fm) {
        super(fm);
    }

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

    @Override
    public Fragment getItem(int position) {
        return MyFragment.newInstance(position);
    }
}

public static class MyFragment extends ListFragment{
    int mNum;

    /**
     * Create a new instance of CountingFragment, providing "num"
     * as an argument.
     */
    static MyFragment newInstance(int num) {
        MyFragment f = new MyFragment();

        // Supply num input as an argument.
        Bundle args = new Bundle();
        args.putInt("num", num);
        f.setArguments(args);

        return f;
    }

    /**
     * When creating, retrieve this instance's number from its arguments.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mNum = getArguments() != null ? getArguments().getInt("num") : 1;
    }

    /**
     * The Fragment's UI is just a simple text view showing its
     * instance number.
     */
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_pager_list, container, false);
        View tv = v.findViewById(R.id.text);
        ((TextView)tv).setText("Fragment #" + mNum);
        return v;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        setListAdapter(new ArrayAdapter<String>(getActivity(),
                android.R.layout.simple_list_item_1, new String[] {"test1","test2","test3","test4","Test5"}));
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        Log.i("FragmentList", "Item clicked: " + id);
    }
}

}

基本上，当它到达末尾时，我如何让它像显示任何其他片段一样显示第一个片段？

即。A->B、B->C、C->A和反向A->C、C->B、B->A

Answer 1

这是我的解决方案，我有"n“个屏幕(在本例中是5个)，因为我想回到列表末尾的第一个屏幕，所以我”添加另一个屏幕“，这只是最后一个屏幕的精确副本，也是反向的。基本上我现在有"n“个屏幕+2 (在本例中是7个)。屏幕是这样设置的：copy of C、A、B、C、copy of A，然后我设置一个手势监听器来观察某些页面上的滑动。

如果在列表的末尾(“A的副本”)，我检测到向左滑动页面，我将移动到> a certain number of pixels A，并继续以正常的页面旋转方式向后滑动。

向相反方向滑动也是如此。

问题

当从一个结束页面移动到模拟循环时，它确实会变得有点慢，因为ViewPager加载了您当前所在页面之前和之后的页面，但由于您位于页面列表的末尾或开头，因此它不会像通常那样加载下一个页面，因为从技术上讲，没有下一个页面。只有在每个列表行都有复杂的自定义视图时，才会发生这种情况

您不能将它与用于页面滚动的动画一起使用，因为它看起来不是无缝的，并且当您从复制的页面移动到实际页面时(即A到A的副本)，它将具有动画效果。

此外，它似乎有时会卡住在页面的末尾，我不知道为什么

这是我的完整演示应用程序，基于android开发人员站点上的viewpager示例

public class FragmentPager extends FragmentActivity{
MyAdapter mAdapter;
public static ViewPager mPager;
static int currentPage = 1;
static final int NUM_ITEMS = 7;
public static View vContainer;
float dxInitial=0;

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

    mAdapter = new MyAdapter(getSupportFragmentManager());

    mPager = (ViewPager)findViewById(R.id.pager);
    mPager.setAdapter(mAdapter);
    mPager.setCurrentItem(1,false);

    mPager.setOnTouchListener(new OnTouchListener(){

        @Override
        public boolean onTouch(View v, MotionEvent event) {


            if(event.getAction() == MotionEvent.ACTION_DOWN){
                dxInitial=event.getX();
                Log.d("INITIAL X", String.valueOf(dxInitial));
            }else if(event.getAction() == MotionEvent.ACTION_MOVE){
                if(event.getX() - dxInitial > 100 && currentPage==1){
                    mPager.setCurrentItem(6,false);
                }else if(dxInitial - event.getX() > 100 && currentPage==5){
                    mPager.setCurrentItem(0,false);
                }else if(dxInitial - event.getX() > 50 && currentPage==6){
                    mPager.setCurrentItem(1,false);
                }else if(event.getX() - dxInitial > 50 && currentPage==0){
                    mPager.setCurrentItem(5,false);
                }
            }

            /*if(Math.abs(dx) > Math.abs(dy)) {
                if(dx>75){ directiion = "right";
                }else if{
                    direction = "left";
                }
            } else {
                if(dy>0) direction = "down";
                else direction = "up";
            }*/

            return false;
        }

    });
    mPager.setOnPageChangeListener(new OnPageChangeListener(){

        @Override
        public void onPageScrollStateChanged(int state) {
            // TODO Auto-generated method stub
            if(state == ViewPager.SCROLL_STATE_IDLE){

            }
        }

        @Override
        public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {

        }

        @Override
        public void onPageSelected(int arg0) {
            currentPage=arg0;

        }

    });
}

public static class MyAdapter extends FragmentPagerAdapter{
    public MyAdapter(FragmentManager fm) {
        super(fm);
    }

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

    @Override
    public Fragment getItem(int position) {
        return MyFragment.newInstance(position);
    }

    @Override
    public Object instantiateItem(View container, int position) {
        vContainer=container;
        return super.instantiateItem(container, position);

    }
}

public static class MyFragment extends ListFragment{
    int mNum;

    /**
     * Create a new instance of CountingFragment, providing "num"
     * as an argument.
     */
    static MyFragment newInstance(int num) {
        MyFragment f = new MyFragment();

        // Supply num input as an argument.
        Bundle args = new Bundle();
        args.putInt("num", num);
        f.setArguments(args);

        return f;
    }

    /**
     * When creating, retrieve this instance's number from its arguments.
     */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mNum = getArguments() != null ? getArguments().getInt("num") : 1;
    }

    /**
     * The Fragment's UI is just a simple text view showing its
     * instance number.
     */
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_pager_list, container, false);
        View tv = v.findViewById(R.id.text);
        if(mNum == 6){
            mNum=1;
        }else if(mNum==7){
            mNum=5;
        }else if(mNum==0){
            mNum=5;
        }
        ((TextView)tv).setText("Fragment #" + mNum);
        return v;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        setListAdapter(new ArrayAdapter<String>(getActivity(),
                android.R.layout.simple_list_item_1, new String[] {"test1","test2","test3","test4","Test5"}));
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        Log.i("FragmentList", "Item clicked: " + id);
    }
}

}

Answer 2

这是一个相当不错的技巧，但对我来说，它工作得很好：

@Override
public int getCount() {
    return NUM_ITEMS * 1000;
}

这将使您的页面“循环”1000x不要忘记在getItem上使用%操作，如下所示

@Override
public Fragment getItem(int position) {
    position = position % 1000;
    return MyFragment.newInstance(position);
}

Answer 3

我根据tyczj的回答做了一个Kotlin解决方案，它处理在适配器创建后加载carousel项的时间(例如，从服务器加载项)，也不会获取触摸侦听器，而是等待ViewPager2的空闲状态。在我的例子中，我有一个用户配置文件的旋转木马。

根据tycjz的回答，如果我们有一组A B C D项，那么ViewPager2函数以C A B C D A结尾，但是我们从ViewPager2 @ position 1而不是0开始，所以它以A结尾。

inner class UserProfileSlidePagerAdapter(fa: FragmentActivity) : FragmentStateAdapter(fa) {

    var profileToJumpTo: Int? = null

    override fun getItemCount(): Int {
        if (userIdsForLocalUsers.size > 0) {
            return userIdsForLocalUsers.size + 2
        }
        return userIdsForLocalUsers.size
    }

    override fun createFragment(position: Int): Fragment {
        var positionToUse = position - 1
        if (position > userIdsForLocalUsers.size)
        {
            positionToUse = 0
        }
        if (positionToUse < 0)
        {
            positionToUse = userIdsForLocalUsers.size - 1
        }
        val userId = userIdsForLocalUsers[positionToUse]
        this@ProfileViewModel.availableUserProfilesHash[userId].let {userProfile ->
            return CarouselUserProfileFragment.newInstance(userProfile!!)
        }
    }
}

如下所示，在管理了PageAdapter的"off by 1“定位问题之后，您可以确定当ViewPager2 onPageSelected事件发生时，您是在循环的开始还是结束，然后选择当页面处于onPageScrollStateChanged的空闲状态时跳转到哪个页面。所以在ViewPager2所在的Fragment中：

this.profiles_view_pager.registerOnPageChangeCallback(object: ViewPager2.OnPageChangeCallback() {
    override fun onPageScrollStateChanged(state: Int) {
        if (state == ViewPager2.SCROLL_STATE_IDLE) {
            val adapter = this@CarouselFragment.profiles_view_pager.adapter as ProfileViewModel.UserProfileSlidePagerAdapter
            val profileToJumpTo = adapter.profileToJumpTo
            profileToJumpTo?.let {
                this@CarouselFragment.profiles_view_pager.setCurrentItem(profileToJumpTo, false)
                adapter.profileToJumpTo = null
            }
        }
        super.onPageScrollStateChanged(state)
    }
    override fun onPageSelected(position: Int) {
        val adapter = this@CarouselFragment.profiles_view_pager.adapter as ProfileViewModel.UserProfileSlidePagerAdapter
        adapter.itemCount.let { itemCount ->
            if (position == 0) {
                adapter.profileToJumpTo = itemCount - 2
            }
            if (position == itemCount - 1) {
                adapter.profileToJumpTo = 1
            }
        }
        super.onPageSelected(position)
    }
})

我还可以通过LiveData将当前项设置为"+1“。

this.profileViewModel.loadUserProfilesResult.observe(viewLifecycleOwner, Observer {     
    this.profiles_view_pager.setCurrentItem(1, false)
 })