Develop Reference

Why is fragment calling OnCreate / OnCreateView when using fragment transactions with FrameLayout?

The main page of my application has a FrameLayout.
I'm instantiating two fragments when the activity starts, and I'm trying to use a menu button to swap between the fragment.
scanHistoryFrag = new HistoryFragment();
scanFrag = new ScanFragment();
I never replace these objects - I use the same ones throughout the lifecycle of the application. However, when I swap them in my FrameLayout...
private void ChangeFragment(Android.Support.V4.App.Fragment fragment)
{
Android.Support.V4.App.FragmentTransaction ft = SupportFragmentManager.BeginTransaction();
ft.Replace(Resource.Id.fragmentContainer, fragment);
ft.Commit();
}
OnCreate and OnCreateView are called on the Fragment again... which means any adjustments I made post creation on that fragment are overwritten with initial values again. I can't seem to find any explanation for why this is happening or how I might avoid it.
The ChangeFragment method is being called by OnOptionsItemSelected, as I'm using a menu button to toggle them.

I never replace these objects - I use the same ones throughout the lifecycle of the application.
Initialization of a subclass of Fragment just create a instance of this class object, the constructor of this class will be called, but it will not go through the lifecycle of Fragment unless this Fragment is added, for more information, you can refer to Fragments. To understand it easier, I personal think the instance saves the data state of this Fragment class, but the events of lifecycle handle the view state of this Fragment.
which means any adjustments I made post creation on that fragment are overwritten with initial values again.
Yes, you're right. To avoid overwritting with initial values again, we can cache the fragment's view in OnCreateView for example like this:
private View rootView;
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
// Use this to return your custom view for this Fragment
// return inflater.Inflate(Resource.Layout.YourFragment, container, false);
if (rootView == null)
{
//first time creating this fragment view
rootView = inflater.Inflate(Resource.Layout.fragmentlayout1, container, false);
//Initialization
//TODO:
}
else
{
//not first time creating this fragment view
ViewGroup parent = (ViewGroup)rootView.Parent;
if (parent != null)
{
parent.RemoveView(rootView);
}
}
return rootView;
}

WebView prevent reloading in RetainInstance fragment

I am having a problem preventing a WebView from reloading when inside a fragment with retainInstance set to true.
The routes I have tried:
SaveInstanceState (Doesn't pass a bundle on rotation because of retainInstance)
A custom bundle which is saved on onSaveInstanceState and restored in create
Keeping the WebView reference and attempt to add it back to the view hierarchy
There must be an easier way?

You can do this pretty easily. Since you're already retaining the instance, keep a reference to the WebView in the Fragment and detach it from the parent when the Fragment's onDestroyView() is called. If it's non-null, just return that in onCreateView(). For example:
public class RetainedWebviewFragment extends Fragment {
private WebView mWebView;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
if (mWebView == null) {
mWebView = new WebView(getActivity());
}
return mWebView;
}
#Override
public void onDestroyView() {
super.onDestroyView();
if (getRetainInstance() && mWebView.getParent() instanceof ViewGroup) {
((ViewGroup) mWebView.getParent()).removeView(mWebView);
}
}
}
However, you will leak one reference to the first Activity on the first orientation change -- something I've not yet figured out how to avoid. tThere's no way to set the context on a View to my knowledge. This does work, though, if you're okay with that one memory leak.
EDIT: Alternatively, if you're instantiating it programmatically as I'm doing in this example, you could just use getActivity().getApplicationContext() to avoid leaking the activity. If you use the provided LayoutInflater, it will give the application's context to the Views when inflating.
Don't use the application context for a WebView -- this will cause unexpected crashes when showing dropdown views and potentially other problems.

How to prevent the reload of a webView in a fragment when another fragment in the top was closed

I'm making an app with webviews.
I just have a single activity containing
a FrameLayout to dynamically add fragments.
Every fragment contains a webView.
Everything works fine, but when I remove a fragment
from the stack, the webview of the fragment in the
top of the stack is reloaded so the content inside the
script of the html is called again.
The restoreState() method is not working.
This is the onCreateView of the fragment:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
view = inflater.inflate(R.layout.wv_container, container, false);
webView = (WebView)view.findViewById(R.id.webviewcontainer);
if(webViewBundle == null)
webView.loadUrl("file:///android_asset/www/main.html");
else
webView.restoreState(webViewBundle);
return view;
}
This is the onPause() method:
#Override
public void onPause() {
super.onPause();
webViewBundle = new Bundle();
webViewContent.saveState(webViewBundle);
}
How should I prevent the page reloading for my case?
I don't want to hack the html with flags or something like that.
Thanks in advance!

The saveState method in your onPause returns a WebBackForwardList, which contains a list of WebHistory items. These are basically the titles, urls and favicons from recently visited pages. It does not cache the contents of the downloaded url.
Try looking at the method saveWebArchive instead.

Android FragmentTab host and Fragments inside Fragments

I have an app with hierarchy like this:
FragmentTabHost (Main Activity)
- Fragment (tab 1 content - splitter view)
- Fragment (lhs, list)
- Framment (rhs, content view)
- Fragment (tab 2 content)
- Fragment (tab 2 content)
All fragment views are being inflated from resources.
When the app starts everything appears and looks fine. When I switch from the first tab to another tab and back again I get inflate exceptions trying to recreate tab 1's views.
Digging a little deeper, this is what's happening:
On the first load, inflating the splitter view causes its two child fragments to be added to the fragment manager.
On switching away from the first tab, it's view is destroyed but it's child fragments are left in the fragment manager
On switching back to the first tab, the view is re-inflated and since the old child fragments are still in the fragment manager an exception is thrown when the new child fragments are instantiated (by inflation)
I've worked around this by removing the child fragments from the fragment manager (I'm using Mono) and now I can switch tabs without the exception.
public override void OnDestroyView()
{
var ft = FragmentManager.BeginTransaction();
ft.Remove(FragmentManager.FindFragmentById(Resource.Id.ListFragment));
ft.Remove(FragmentManager.FindFragmentById(Resource.Id.ContentFragment));
ft.Commit();
base.OnDestroyView();
}
So I have a few questions:
Is the above the correct way to do this?
If not, how should I be doing it?
Either way, how does saving instance state tie into all of this so that I don't lose view state when switching tabs?

I'm not sure how to do this in Mono, but to add child fragments to another fragment, you can't use the FragmentManager of the Activity. Instead, you have to use the ChildFragmentManager of the hosting Fragment:
http://developer.android.com/reference/android/app/Fragment.html#getChildFragmentManager()
http://developer.android.com/reference/android/support/v4/app/Fragment.html#getChildFragmentManager()
The main FragmentManager of the Activity handles your tabs.
The ChildFragmentManager of tab1 handles the split views.

OK, I finally figured this out:
As suggested above, first I changed the fragment creation to be done programatically and had them added to the child fragment manager, like so:
public override View OnCreateView(LayoutInflater inflater, ViewGroup viewGroup, Bundle savedInstance)
{
var view = inflater.Inflate(Resource.Layout.MyView, viewGroup, false);
// Add fragments to the child fragment manager
// DONT DO THIS, SEE BELOW
var tx = ChildFragmentManager.BeginTransaction();
tx.Add(Resource.Id.lhs_fragment_frame, new LhsFragment());
tx.Add(Resource.Id.rhs_fragment_frame, new RhsFragment());
tx.Commit();
return view;
}
As expected, each time I switch tabs, an extra instance of Lhs/RhsFragment would be created, but I noticed that the old Lhs/RhsFragment's OnCreateView would also get called. So after each tab switch, there would be one more call to OnCreateView. Switch tabs 10 times = 11 calls to OnCreateView. This is obviously wrong.
Looking at the source code for FragmentTabHost, I can see that it simply detaches and re-attaches the tab's content fragment when switching tabs. It seems the parent Fragment's ChildFragmentManager is keeping the child fragments around and automatically recreating their views when the parent fragment is re-attached.
So, I moved the creation of fragments to OnCreate, and only if we're not loading from saved state:
public override void OnCreate(Bundle savedInstanceState)
{
base.OnCreate(savedInstanceState);
if (savedInstanceState == null)
{
var tx = ChildFragmentManager.BeginTransaction();
tx.Add(Resource.Id.lhs_fragment_frame, new LhsFragment());
tx.Add(Resource.Id.rhs_fragment_frame, new RhsFragment());
tx.Commit();
}
}
public override View OnCreateView(LayoutInflater inflater, ViewGroup viewGroup, Bundle savedInstance)
{
// Don't instatiate child fragments here
return inflater.Inflate(Resource.Layout.MyView, viewGroup, false);
}
This fixed the creation of the additional views and switching tab's basically worked now.
The next question was saving and restoring view state. In the child fragments I need to save and restore the currently selected item. Originally I had something like this (this is the child fragment's OnCreateView)
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance)
{
var view = inflater.Inflate(Resource.Layout.CentresList, container, false);
// ... other code ommitted ...
// DONT DO THIS, SEE BELOW
if (savedInstance != null)
{
// Restore selection
_selection = savedInstance.GetString(KEY_SELECTION);
}
else
{
// Select first item
_selection =_items[0];
}
return view;
}
The problem with this is that the tab host doesn't call OnSaveInstanceState when switching tabs. Rather the child fragment is kept alive and it's _selection variable can be just left alone.
So I moved the code to manage selection to OnCreate:
public override void OnCreate(Bundle savedInstance)
{
base.OnCreate(savedInstance);
if (savedInstance != null)
{
// Restore Selection
_selection = savedInstance.GetString(BK_SELECTION);
}
else
{
// Select first item
_selection = _items[0];
}
}
public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstance)
{
// Don't restore/init _selection here
return inflater.Inflate(Resource.Layout.CentresList, container, false);
}
Now it all seems to be working perfectly, both when switching tabs and changing orientation.

Handling orientation changes with Fragments

I'm currently testing my app with a multipane Fragment-ised view using the HC compatibility package, and having a lot of difficultly handling orientation changes.
My Host activity has 2 panes in landscape (menuFrame and contentFrame), and only menuFrame in portrait, to which appropriate fragments are loaded. If I have something in both panes, but then change the orientation to portrait I get a NPE as it tries to load views in the fragment which would be in the (non-existent) contentFrame. Using the setRetainState() method in the content fragment didn't work. How can I sort this out to prevent the system loading a fragment that won't be shown?
Many thanks!

It seems that the onCreateViewMethod was causing issues; it must return null if the container is null:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (container == null) // must put this in
return null;
return inflater.inflate(R.layout.<layout>, container, false);
}

Probably not the ideal answer but if you have contentFrame for portrait and in your activity only load up the menuFrame when the savedInstanceState is null then your content frame fragments will be shown on an orientation change.
Not ideal though as then if you hit the back button (as many times as necessary) then you'll never see the menu fragment as it wasn't loaded into contentFrame.
It is a shame that the FragmentLayout API demos doesn't preserve the right fragment state across an orientation change. Regardless, having thought about this problem a fair bit, and tried out various things, I'm not sure that there is a straightforward answer. The best answer that I have come up with so far (not tested) is to have the same layout in portrait and landscape but hide the menuFrame when there is something in the detailsFrame. Similarly show it, and hide frameLayout when the latter is empty.

Create new Instance only for First Time.
This does the trick:
Create a new Instance of Fragment when the
activity start for the first time else reuse the old fragment.
How can you do this?
FragmentManager is the key
Here is the code snippet:
if(savedInstanceState==null) {
userFragment = UserNameFragment.newInstance();
fragmentManager.beginTransaction().add(R.id.profile, userFragment, "TAG").commit();
}
else {
userFragment = fragmentManager.findFragmentByTag("TAG");
}
Save data on the fragment side
If your fragment has EditText, TextViews or any other class variables
which you want to save while orientation change. Save it
onSaveInstanceState() and Retrieve them in onCreateView() method
Here is the code snippet:
// Saving State
#Override
public void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
outState.putString("USER_NAME", username.getText().toString());
outState.putString("PASSWORD", password.getText().toString());
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.user_name_fragment, parent, false);
username = (EditText) view.findViewById(R.id.username);
password = (EditText) view.findViewById(R.id.password);
// Retriving value
if (savedInstanceState != null) {
username.setText(savedInstanceState.getString("USER_NAME"));
password.setText(savedInstanceState.getString("PASSWORD"));
}
return view;
}
You can see the full working code HERE