使用底部导航时保存片段的状态

Question

我使用bottom navigation drawer在片段之间切换，问题是每次切换回片段时都会重新创建它。

我如何保存碎片的状态，并在切换回它时恢复它？

bottomNavigationBar.setTabSelectedListener(new BottomNavigationBar.OnTabSelectedListener(){
@Override
public void onTabSelected(int position) {
if (position==0){
loadFragment(new Daily());
}
if (position==1){
loadFragment(new Trending());
}
if (position==2){
loadFragment(new Random());
}
}
@Override
public void onTabUnselected(int position) {
}
@Override
public void onTabReselected(int position) {
}
});
private void loadFragment(Fragment fragment) {
// load fragment
FragmentTransaction transaction = 
getSupportFragmentManager().beginTransaction();
transaction.attach( fragment);
transaction.addToBackStack(null);
transaction.commit();

}

Answer 1

问题是您总是在任何onTabSelected上创建一个新的Fragment。因此，为了修复它，您需要使用FragmentManager。

可能的解决方案：使用add和show/hide方法。

示例：

private static final String DAILY_TAG = BuildConfig.APPLICATION_ID + ".DAILY_TAG";
private static final String TRENDING_TAG = BuildConfig.APPLICATION_ID + ".TRENDING_TAG";
private static final String RANDOM_TAG = BuildConfig.APPLICATION_ID + ".RANDOM_TAG";
public void onTabSelected(int position) {
FragmentManager fragmentManager = getSupportFragmentManager();
if (position == 0) {
hideFragment(TRENDING_TAG)
hideFragment(RANDOM_TAG)
Fragment fragment = fragmentManager.findFragmentByTag(DAILY_TAG);
FragmentTransaction transaction = fragmentManager.beginTransaction()
if (fragment != null) {
transaction.show(fragment)
} else {
transaction.add(content.id, new Daily(), DAILY_TAG)
}
transaction.commitNow()
} else if (position == 1) {
hideFragment(DAILY_TAG)
hideFragment(RANDOM_TAG)
Fragment fragment = fragmentManager.findFragmentByTag(TRENDING_TAG);
FragmentTransaction transaction = fragmentManager.beginTransaction()
if (fragment != null) {
transaction.show(fragment)
} else {
transaction.add(content.id, new Trending(), TRENDING_TAG)
}
transaction.commitNow()
} else {
hideFragment(TRENDING_TAG)
hideFragment(DAILY_TAG)
Fragment fragment = fragmentManager.findFragmentByTag(RANDOM_TAG);
FragmentTransaction transaction = fragmentManager.beginTransaction()
if (fragment != null) {
transaction.show(fragment)
} else {
transaction.add(content.id, new Random(), RANDOM_TAG)
}
transaction.commitNow()
} 
fragments.put(position, fragment);
loadFragment(fragment);
}
private void hideFragment(String tag) {
FragmentManager fragmentManager = getSupportFragmentManager()
Fragment currentFragment = fragmentManager.findFragmentByTag(tag)
if (currentFragment != null) {
fragmentManager.beginTransaction().hide(currentFragment).commitNow()
}
}

PS-代码可以进行优化。