CURRENT SCENARIO
In my app I am having navigation drawer with fragments. Everything works fine in portrait mode.
PROBLEM
Suppose when in portrait mode I select second item from navigation drawer. It loads perfectly but when I rotate my phone to landscape mode, first fragment from the navigation menu gets loaded instead of second.
I know I have to save instance for fragment but I don't know how to do it and should I do it in main activity or in fragment itself
You should do that in your Fragment.
Just follow these links:
Android - save/restore fragment state
Or perhaps:
Once for all, how to correctly save instance state of Fragments in back stack?
Also, let's mention about onRestoreInstanceState, Fragment's doesn't have that method.So, you should use onActivityCreated which receives a bundle with the saved instance state (or null).
Take a look at the docs:
I answered the same question in this thread:
I tried to explain why the solution I give works, so if you're interested check it out.
To solve the problem I simply put the code that inflates the initial fragment inside an if (in the OnCreate of the navigation drawer activity):
super.onCreate(savedInstanceState);
if(savedInstanceState==null){
FragmentManager fM = getSupportFragmentManager();
fM.beginTransaction().replace(R.id.NavDrawContent,new home_fragment()).commit();
}
so that it doesn't inflate the first fragment when we change the orientation while in the second
I found a better solution using the new tool called ViewModel class. When the screen rotates, it automatically destroys de activity and creates it again, so we lost the focus of the last fragment selected. ViewModel helps us keeping the data and it's so useful for forms too.
MainActivityViewModel.ktPut this code inside:
class MainActivityViewModel : ViewModel() { var menuItem : Int = R.id.nav_inicio }
I'm choosing R.id.nav_inicio as default menu item, modify it depending of your activity_main_drawer.xml (Default name for android studio)
3. Then, declare it into your MainActivity with the navigation drawer:
lateinit var vm : MainActivityViewModel
vm = ViewModelProviders.of(this).get(MainActivityViewModel::class.java)Then create a function that loads the default Fragment:
fun loadDefaultFragment() {
when (vm.menuItem) {
R.id.nav_inicio -> {
fab.hide()
val fragment = InicioFragment()
val manager = supportFragmentManager
manager.beginTransaction().replace(R.id.content_main, fragment, fragment.getTag()).commit()
}
R.id.nav_puntosventa -> {
fab.show()
val fragment = ListaPuntosFragment()
val manager = supportFragmentManager
manager.beginTransaction().replace(R.id.content_main, fragment, fragment.getTag()).commit()
}
else -> {
// default fragment if you consider necessary
}
}
}
this function must be called into your onCreate method after the initialization of the ViewModel.
6. Finally, we have to update the viewModel value each time we select a new menu item, so modify the override function onNavigationItemSelected adding it at the beginning:
override fun onNavigationItemSelected(item: MenuItem): Boolean {
// Handle navigation view item clicks here.
vm.menuItem = item.itemId
// rest of the code ...
}
Now , when you rotate your screen, de viewmodel value save the position, you can use it with forms maybe.
There is a problem with nav controller backstack after changed config
In your Activity with fragment: try saving the nav controller state before changing the configuration. After changing the configuration, add it back to the nav controller again. Like:
override fun onSaveInstanceState(savedInstanceState: Bundle) {
super.onSaveInstanceState(savedInstanceState)
savedInstanceState.putBundle("nav_state", fragment.findNavController().saveState())}
override fun onRestoreInstanceState(savedInstanceState: Bundle) {
super.onRestoreInstanceState(savedInstanceState)
fragment.findNavController().restoreState(savedInstanceState.getBundle("nav_state"))}
Thats my case
The new Android Architecture components introduces a ViewModel class which helps create lifecycle aware components. It solves most of these issues regarding lifecycle management. If you are using Java code in your project, you could introduce a ViewModel class. Follow these steps:
add this to your build.gradle(app module)
implementation 'androidx.lifecycle:lifecycle-extensions:2.2.0'
add this to your build.gradle(project module)
dependencies {
classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.72"
}
Create a new MainActivityViewModel.kt class with the code below
class MainActivityViewModel : ViewModel() {
var menuItem: Int = R.id.nav_home
}
In your MainActivity.java add the following at the top of the class
private MainActivityViewModel vm;
and in your MainActivity.java onCreate method
vm = new ViewModelProvider(this).get(MainActivityViewModel.class);
loadDefaultFragment();
Now create a method loadFragment() in MainActivity.java with the code below
private void loadFragment(Fragment fragment) {
// load fragment
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();
transaction.replace(R.id.contentMainLayout, fragment);
transaction.addToBackStack(null);
transaction.commit();
}
Create another method loadDefaultFragment() in your MainActivity.java
private void loadDefaultFragment(){
if(vm.getMenuItem()==R.id.nav_home){
fragment = new HomeFragment();
loadFragment(fragment);
}else if(vm.getMenuItem()==R.id.nav_another_fragment){
fragment = new AnotherFragment();
loadFragment(fragment);
}
//you can add or exclude other fragments here depending on your requirements
}
Finally, we have to update the viewModel value each time we select a new menu item, so modify the override method onNavigationItemSelected adding it at the beginning:
// Handle navigation view item clicks here.
vm.setMenuItem(item.getItemId());
