當用戶到達ViewPager的末尾時，如何動態添加片段以啟動ViewPager？

Question

我目前有一個基本的ViewPager ，它利用FragmentStatePagerAdapter來初始化片段。

此ViewPager從最后開始向后滾動（即向左滾動）。

當用戶接近ViewPager列表的開頭時，我希望以編程方式將更多項添加到適配器列表的開頭，實質上創建一個無限可滾動的ViewPager （向左）而不會破壞用戶的體驗（即不重新加載他們的當前片段作為ViewPager添加了片段）。 但是，我無法做到這一點。

目前，我正在檢測我的適配器的getItem()函數何時傳遞位置0，填充適配器使用更多項的列表的開頭，然后調用notifyDataSetChanged() 。 但是，發生的情況是位置1處的片段（一旦它進入焦點就會觸發getItem()調用）並且0保留在ViewPager的開頭，並且新片段全部無序地加載。

這是我的FragmentStatePagerAdapter ：

class WeekyMoodFragmentAdapter extends FragmentStatePagerAdapter {

    private final int NUM_WEEKS_TO_INITIALIZE = 4;
    private final List<Long> timestampList = new ArrayList<>();
    private final ViewPager mViewPager;

    WeekyMoodFragmentAdapter(FragmentManager fragmentManager, ViewPager viewPager) {
        super(fragmentManager);

        mViewPager = viewPager;
        initializeTimestampList();
    }

    private void initializeTimestampList() {

        DateTime mondayOfThisWeek = new DateTime(System.currentTimeMillis());
        while (mondayOfThisWeek.getDayOfWeek() != DateTimeConstants.MONDAY) {
            mondayOfThisWeek = new DateTime(mondayOfThisWeek.getMillis()
                    - TimeUnit.HOURS.toMillis(DateTimeConstants.HOURS_PER_DAY));
        }

        long weekStartMS = mondayOfThisWeek.withTimeAtStartOfDay().getMillis()
                - TimeUnit.DAYS.toMillis(DateTimeConstants.DAYS_PER_WEEK * NUM_WEEKS_TO_INITIALIZE);

        addTimestampsToList(weekStartMS);
    }

    private void addTimestampsToList(long weekStartMS) {
        for (int i = 0; i < NUM_WEEKS_TO_INITIALIZE; ++i) {
            weekStartMS += TimeUnit.DAYS.toMillis(DateTimeConstants.DAYS_PER_WEEK);

            timestampList.add(i, weekStartMS);
        }
        notifyDataSetChanged();
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = WeeklyMoodFragment.newInstance(timestampList.get(position));
        if (position == 0) {
            addTimestampsToList(timestampList.get(position)
                    - TimeUnit.DAYS.toMillis
                    (DateTimeConstants.DAYS_PER_WEEK * (NUM_WEEKS_TO_INITIALIZE+1)));
        }
        return fragment;
    }

    @Override
    public int getCount() {
        return timestampList.size();
    }
}

以及我如何初始化我的ViewPager ：

ViewPager moodViewPager = (ViewPager) view.findViewById(R.id.weeklyMoodViewPager);

moodViewPager.setAdapter(new WeekyMoodFragmentAdapter(getChildFragmentManager());
moodViewPager.setCurrentItem(moodViewPager.getAdapter().getCount());

Answer 1

你有沒有考慮過getCount()返回從timestampList.get(position%timestampList.size())創建的Integer.MAX_VALUE和newInstance getCount()有點hacky的方法？ 並將初始頁面setCurrentItem設置為Integer.MAX_VALUE/2或最近的appriopiate

@Override
public Fragment getItem(int position) {
    return WeeklyMoodFragment.newInstance(
        timestampList.get(position % timestampList.size()));
        //                         ^  here important
}

@Override
public int getCount() {
    return Integer.MAX_VALUE;
}

    ...

moodViewPager.setCurrentItem(Integer.MAX_VALUE/2);
//or 
WeekyMoodFragmentAdapteradapter = new WeekyMoodFragmentAdapter(getChildFragmentManager());
moodViewPager.setAdapter(adapter);
//dividing integers will round
int roundedCount = (Integer.MAX_VALUE/2) / adapter.getCount();
moodViewPager.setCurrentItem(roundedCount * adapter.getCount())