在FragmentStatePagerAdapter管理的选定ViewPager Fragment上,UserVisibleHint为false

Question

我在Android应用中遇到了一个非常难以诊断的问题.getUserVisibleHint()返回false当前选定的片段,ViewPager它应该返回true(因为它是可见的并且被选中).

我已经将我看到这种行为的实例描述如下:

• 片段已选中并且当前显示在a中 ViewPager
• ViewPager由a管理 FragmentStatePagerAdapter
• 之前选择了片段,其状态已保存,之后由其恢复 PagerAdapter
  • viewpager中至少有3个选项卡
  • 用户导航到选项卡3,然后导航到选项卡1,然后返回选项卡3.
• 应用程序使用支持库版本24.0.0或更高版本

Answer 1

调试显示FragmentStatePagerAdapter实际上正确设置了所选选项卡的状态,setPrimaryItem(ViewGroup container, int position, Object object)但稍后将其设置为falseFragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)

//from FragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)
f.mUserVisibleHint = f.mSavedFragmentState.getBoolean(
        FragmentManagerImpl.USER_VISIBLE_HINT_TAG, true);

f.mSavedFragmentState上面已经保存了可见状态,false因为当片段不再出现在屏幕上时它被保存了.

所以这里的问题是国家损失; 正在设置可见状态FragmentStatePagerAdapter#setPrimaryItem但在onResume调用片段方法之前丢失了一段时间.

修复

在库中修复此错误之前,请覆盖setPrimaryItem您的PagerAdapter并强制首先提交任何挂起的事务.

public static class SectionsPagerAdapter extends FragmentStatePagerAdapter {
    public SectionsPagerAdapter(FragmentManager fm) {
        super(fm);
    }

    @Override
    public void setPrimaryItem(ViewGroup container, int position, Object object) {
        //Force any pending transactions to save before we set an item as primary
        finishUpdate(null);
        super.setPrimaryItem(container, position, object);
    }

    @Override
    public Fragment getItem(int position) {
        Fragment fragment = new DummySectionFragment();
        Bundle args = new Bundle();
        args.putInt(DummySectionFragment.ARG_SECTION_NUMBER, position + 1);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public int getCount() {
        return 4;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        return "Page " + (position + 1);
    }
}

要解决此问题,FragmentStatePagerAdapter必须在设置用户可见提示之前提交任何片段事务.

FragmentStatePagerAdapter

只是为了展示里面发生的事情 FragmentStatePagerAdapter

@Override
public Object instantiateItem(ViewGroup container, int position) {
    // If we already have this item instantiated, there is nothing
    // to do.  This can happen when we are restoring the entire pager
    // from its saved state, where the fragment manager has already
    // taken care of restoring the fragments we previously had instantiated.
    if (mFragments.size() > position) {
        Fragment f = mFragments.get(position);
        if (f != null) {
            return f;
        }
    }

    if (mCurTransaction == null) {
        mCurTransaction = mFragmentManager.beginTransaction();
    }

    Fragment fragment = getItem(position);
    if (DEBUG) Log.v(TAG, "Adding item #" + position + ": f=" + fragment);
    if (mSavedState.size() > position) {
        Fragment.SavedState fss = mSavedState.get(position);
        if (fss != null) {
            fragment.setInitialSavedState(fss);
        }
    }
    while (mFragments.size() <= position) {
        mFragments.add(null);
    }
    fragment.setMenuVisibility(false);
    fragment.setUserVisibleHint(false);
    mFragments.set(position, fragment);
    mCurTransaction.add(container.getId(), fragment);

    return fragment;
}

@Override
public void setPrimaryItem(ViewGroup container, int position, Object object) {
    Fragment fragment = (Fragment)object;
    if (fragment != mCurrentPrimaryItem) {
        if (mCurrentPrimaryItem != null) {
            mCurrentPrimaryItem.setMenuVisibility(false);
            mCurrentPrimaryItem.setUserVisibleHint(false);
        }
        if (fragment != null) {
            fragment.setMenuVisibility(true);
            fragment.setUserVisibleHint(true);
        }
        mCurrentPrimaryItem = fragment;
    }
}

@Override
public void finishUpdate(ViewGroup container) {
    if (mCurTransaction != null) {
        mCurTransaction.commitNowAllowingStateLoss();
        mCurTransaction = null;
    }
}