android - 在由FragmentStatePagerAdapter管理的选定ViewPager Fragment上，UserVisibleHint为false

我在Android应用中遇到了非常难以诊断的问题。当getUserVisibleHint()应该返回true(因为它是可见的并且被选中)时，它会在false中返回当前所选 fragment 上的ViewPager。

我对实例的特征进行了描述，如下所示:

fragment 已选择，当前显示在ViewPager

中

ViewPager由FragmentStatePagerAdapter

管理

fragment 是先前选择的，其状态已保存，后来由PagerAdapter恢复

viewpager中至少3个标签页

用户导航至选项卡3，然后导航至选项卡1，然后返回至选项卡3。

应用程序使用支持库版本24.0.0或更高版本

最佳答案

调试显示FragmentStatePagerAdapter实际上在setPrimaryItem(ViewGroup container, int position, Object object)中正确设置了所选选项卡的状态，但后来在FragmentManager#moveToState(Fragment f, int newState, int transit, int transitionStyle, boolean keepActive)中将其设置为false

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

上面的f.mSavedFragmentState已将可见状态另存为false，因为在 fragment 不再出现在屏幕上时已将其保存。
因此，这里的问题是状态丢失；可见状态已在FragmentStatePagerAdapter#setPrimaryItem中设置，但在调用 fragment 的onResume方法之前已丢失了一段时间。
修复
在库中修复此错误之前，请在setPrimaryItem中覆盖PagerAdapter并强制所有未决事务首先提交。

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 fragment 事务。FragmentStatePagerAdapter只是为了显示FragmentStatePagerAdapter内部发生的事情

@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;
    }
}

关于android - 在由FragmentStatePagerAdapter管理的选定ViewPager Fragment上，UserVisibleHint为false，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/44052437/