I asked a question about how to add a Fragment that contained something drawn using OpenGL ES
here. Someone was kind enough to answer that for me, but unfortunately today I encountered another problem. As I mentioned in my other question, my purpose is to add other Fragments next to the one that contains OpenGL and because I am a beginner in Android development I don't seem to understand how this is done.
Here's what I want: right now, my code is exactly the one from my other question. I also have this Fragment:
public class TextFragment extends Fragment
{
private TextView textview;
#Override
public View onCreateView(LayoutInflater inflater,
ViewGroup container, Bundle savedInstanceState)
{
View view = inflater.inflate(R.layout.text_fragment,
container, false);
textview = (TextView) view.findViewById(R.id.textView1);
return view;
}
}
together with its layout:
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="#+id/frag2">
<TextView
android:id="#+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true"
android:text="Fragment Two"
android:textAppearance="?android:attr/textAppearanceLarge" />
</RelativeLayout>
and I want to add this to my main activity, where right now I only have the OpenGL Fragment. Here's my main activity:
public class FragmentExampleActivity extends FragmentActivity implements ToolbarFragment.ToolbarListener
{
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
getSupportFragmentManager().addOnBackStackChangedListener(new FragmentManager.OnBackStackChangedListener()
{
public void onBackStackChanged()
{
int backCount = getSupportFragmentManager().getBackStackEntryCount();
if (backCount == 0)
{
finish();
}
}
});
if (savedInstanceState == null)
{
getSupportFragmentManager()
.beginTransaction()
.add(R.id.main_container, new OpenGLES20ActivityFrag())
.addToBackStack(null)
.commit();
}
}
}
and the Fragment that has OpenGL in it and that I have already added to the main activity:
public class OpenGLES20ActivityFrag extends Fragment
{
private GLSurfaceView mGLView;
public OpenGLES20ActivityFrag()
{
super();
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
mGLView = new MyGLSurfaceView(this.getActivity());
return mGLView;
}
}
What I tried and failed: using another call to the .add method inside getSupportFragmentManager() or adapting this bit of code for my second Fragment
getSupportFragmentManager()
.beginTransaction()
.add(R.id.frag2, TextFragment)
.addToBackStack(null)
.commit();
that gave me an 'expression expected' error in the add method. I tried adding this constructor to my second Fragment
public TextFragment()
{
super();
}
and then inside the add method I put .add(R.id.frag2, new TextFragment())
which still didn't work.
In order to dynamically add a Fragment to a layout, what you need is a container (like in your case, it was R.id.main_container). Thus, if you want to add multiple fragments, what you need is multiple containers, like so:
<LinearLayout android:orientation="vertical" android:layout_height="fill_parent" android:layout_width="fill_parent">
<FrameLayout android:id="#+id/main_container_1" android:layout_weight="1" android:layout_height="fill_parent" android:layout_width="fill_parent"/>
<FrameLayout android:id="#+id/main_container_2" android:layout_weight="1" android:layout_height="fill_parent" android:layout_width="fill_parent"/>
</LinearLayout>
(this snippet is from How to split the screen with two equal LinearLayouts? )
And then you would need to add the two Fragments:
if (savedInstanceState == null)
{
getSupportFragmentManager()
.beginTransaction()
.add(R.id.main_container_1, new OpenGLES20ActivityFrag())
.commit();
getSupportFragmentManager()
.beginTransaction()
.add(R.id.main_container_2, new TextFragment())
.commit();
}
Please note that with multiple Fragments on a single Activity, it's better not to add them to the backstack, because then you'd have to press Back as many times as there are Fragments, and in this case it's more reasonable to navigate between the "views" or states of the application with Activities, and not by replacing the Fragments.
(considering the backstack doesn't change, I don't think the backstack listener needs to be removed, but that's done so that if you press Back, you don't end the Activity, but the Fragments within it first if you have them added to the backstack. But the Activity doesn't end when it contains no fragments, and you'd have an "empty view", hence why that was added.)
Please also check if the rotation works and data is maintained even after the activity reconstruction, because there's a chance you need to set the retain instance state to true explicitly on the Fragments for that to work.
This is a canonical question for a problem frequently posted on StackOverflow.
I'm following a tutorial. I've created a new activity using a wizard. I get NullPointerException when attempting to call a method on Views obtained with findViewById() in my activity onCreate().
Activity onCreate():
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
Layout XML (fragment_main.xml):
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="packagename.MainActivity$PlaceholderFragment" >
<View
android:layout_width="100dp"
android:layout_height="100dp"
android:id="#+id/something" />
</RelativeLayout>
The tutorial is probably outdated, attempting to create an activity-based UI instead of the fragment-based UI preferred by wizard-generated code.
The view is in the fragment layout (fragment_main.xml) and not in the activity layout (activity_main.xml). onCreate() is too early in the lifecycle to find it in the activity view hierarchy, and a null is returned. Invoking a method on null causes the NPE.
The preferred solution is to move the code to the fragment onCreateView(), calling findViewById() on the inflated fragment layout rootView:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
View something = rootView.findViewById(R.id.something); // not activity findViewById()
something.setOnClickListener(new View.OnClickListener() { ... });
return rootView;
}
As a side note, the fragment layout will eventually be a part of the activity view hierarchy and discoverable with activity findViewById() but only after the fragment transaction has been run. Pending fragment transactions get executed in super.onStart() after onCreate().
Try OnStart() method and just use
View view = getView().findViewById(R.id.something);
or Declare any View using getView().findViewById method in onStart()
Declare click listener on view by anyView.setOnClickListener(this);
Try to shift your accessing views to the onViewCreated method of fragment because sometimes when you try to access the views in onCreate method they are not rendered at the time resulting null pointer exception.
#Override
public void onViewCreated(View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
Agreed, this is a typical error because people often don't really understand how Fragments work when they begin working on Android development. To alleviate confusion, I created a simple example code that I originally posted on Application is stopped in android emulator , but I posted it here as well.
An example is the following:
public class ContainerActivity extends FragmentActivity implements ExampleFragment.Callback
{
#Override
public void onCreate(Bundle saveInstanceState)
{
super.onCreate(saveInstanceState);
this.setContentView(R.layout.activity_container);
if (saveInstanceState == null)
{
getSupportFragmentManager().beginTransaction()
.add(R.id.activity_container_container, new ExampleFragment())
.addToBackStack(null)
.commit();
}
getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
{
public void onBackStackChanged()
{
int backCount = getSupportFragmentManager().getBackStackEntryCount();
if (backCount == 0)
{
finish();
}
}
});
}
#Override
public void exampleFragmentCallback()
{
Toast.makeText(this, "Hello!", Toast.LENGTH_LONG).show();
}
}
activity_container.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<FrameLayout
android:id="#+id/activity_container_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
ExampleFragment:
public class ExampleFragment extends Fragment implements View.OnClickListener
{
public static interface Callback
{
void exampleFragmentCallback();
}
private Button btnOne;
private Button btnTwo;
private Button btnThree;
private Callback callback;
#Override
public void onAttach(Activity activity)
{
super.onAttach(activity);
try
{
this.callback = (Callback) activity;
}
catch (ClassCastException e)
{
Log.e(this.getClass().getSimpleName(), "Activity must implement Callback interface.", e);
throw e;
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
View rootView = inflater.inflate(R.layout.fragment_example, container, false);
btnOne = (Button) rootView.findViewById(R.id.example_button_one);
btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
btnThree = (Button) rootView.findViewById(R.id.example_button_three);
btnOne.setOnClickListener(this);
btnTwo.setOnClickListener(this);
btnThree.setOnClickListener(this);
return rootView;
}
#Override
public void onClick(View v)
{
if (btnOne == v)
{
Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
}
else if (btnTwo == v)
{
Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
}
else if (btnThree == v)
{
callback.exampleFragmentCallback();
}
}
}
fragment_example.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:id="#+id/example_button_one"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="30dp"
android:text="#string/hello"
android:layout_marginLeft="20dp"
android:layout_marginRight="20dp"/>
<Button
android:id="#+id/example_button_two"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/example_button_one"
android:layout_alignRight="#+id/example_button_one"
android:layout_below="#+id/example_button_one"
android:layout_marginTop="30dp"
android:text="#string/hello" />
<Button
android:id="#+id/example_button_three"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/example_button_two"
android:layout_alignRight="#+id/example_button_two"
android:layout_below="#+id/example_button_two"
android:layout_marginTop="30dp"
android:text="#string/hello" />
</RelativeLayout>
And that should be a valid example, it shows how you can use an Activity to display a Fragment, and handle events in that Fragment. And also how to communicate with the containing Activity.
The view "something" is in fragment and not in activity, so instead of accessing it in activity you must access it in the fragment class like
In PlaceholderFragment.class
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View root = inflater.inflate(R.layout.fragment_main, container,
false);
View something = root .findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... });
return root;
}
You are trying to access UI elements in the onCreate() but , it is too early to access them , since in fragment views can be created in onCreateView() method.
And onActivityCreated() method is reliable to handle any actions on them, since activity is fully loaded in this state.
Add the following in your activity_main.xml
<fragment
android:id="#+id/myFragment"
android:name="packagename.MainActivity$PlaceholderFragment"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
</fragment>
Since you have declared your View in the fragment_main.xml,move that piece of code where you get the NPE in the onCreateView() method of the fragment.
This should solve the issue.
in the posted code above in the question there is a problem :
you are using R.layout.activity_main in oncreate method, but the xml files name is "fragment_main.xml" , means you are trying to get the view of fragment_main.xml file which is not being shown so it gives null pointer exception. change the code like :
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_main);// your xml layout ,where the views are
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
You have to remember important thing is :
NullPointerException occurs when you have declared your variable and trying to retreive its value before assigning value to it.
Use onViewCreated() Method whenever using or calling views from fragments.
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
View v = view.findViewById(R.id.whatever)
}
I've got the same NullPointerException initializing a listener after calling findViewById() onCreate() and onCreateView() methods.
But when I've used the onActivityCreated(Bundle savedInstanceState) {...} it works. So, I could access the GroupView and set my listener.
I hope it be helpful.
Most popular library for finding views which is used by almost every developer.
ButterKnife
As I can their are enough answers explaining finding views with proper methodology. But if you are android developer and code frequently on daily basis then you can use butter-knife which saves a lot time in finding views and you don't have write code for it, With in 2-3 steps you can find views in milliseconds.
Add dependency in app level gradle:
implementation 'com.jakewharton:butterknife:8.8.1'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.8.1'
Add plugin for butter knife:
File -> Settings -> plugins->
Then search for Android ButterKnife Zelezny and install plugin and restart your studio and you are done with it.
Now just go to Oncreate method of your activity and right click on your layout_name and tap on generate button and select butterknife injection option and your views references will be automatically created like mention below:
#BindView(R.id.rv_featured_artist)
ViewPager rvFeaturedArtist;
#BindView(R.id.indicator)
PageIndicator indicator;
#BindView(R.id.rv_artist)
RecyclerView rvArtist;
#BindView(R.id.nsv)
NestedScrollingView nsv;
#BindView(R.id.btn_filter)
Button btnFilter;
I've got a listfragment inside a fragment. I want to get the list inside the parent fragment, in the onActivityCreated but i can't figure out how to do so.
I want to do that because the list is a custom listview with 2 textview and a checkbox and i need to see which items are checked.
My parent fragment :
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
getChildFragmentManager()
.beginTransaction()
.replace(R.id.list_topgain_frame, new ListFragmentTopGain())
.commit();
return inflater.inflate(R.layout.parier3, container, false);
}
#Override
public void onActivityCreated(Bundle savedInstanceState) {
super.onActivityCreated(savedInstanceState);
btnGainValider = (Button) getActivity().findViewById(R.id.btnGainValider);
btnGainValider.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View v) {
}
});
}
The frame layout that i replace with the ListFragment :
<FrameLayout
android:id="#+id/list_choixamis_frame"
android:layout_width="match_parent"
android:layout_height="150dp"
android:layout_below="#id/txvChoixAmis"
android:layout_marginBottom="20dp" />
I found a little trick cause i still can't find how to get the list in the parent fragment..
Anyway, if someone else had this issue, I bypassed by using a listener in my customAdapter. then i pass the information : customAdapter -> ListFragment -> ParentFragment.
I am trying to use Android fragments in a very simple way, similar to the tutorial on the Android developer website.
I have an Activity (MediaInfoActivity) with the following code:
public class MediaInfoActivity extends FragmentActivity {
private final String TAG = "MediaInfoActivity";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.i(TAG, "onCreate()");
setContentView(R.layout.media_info_activity_layout);
}
}
Here is the code for the media_info_activity_layout.xml file:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<fragment class="com.hawkforce.test.MediaInfoFragment"
android:id="#+id/mediaInfoFragment"
android:layout_weight="1"
android:layout_width="match_parent"
android:layout_height="0dp" />
<FrameLayout android:id="#+id/mediaPlayerBarPanel"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<fragment class="com.hawkforce.test.MediaPlayerBarFragment"
android:id="#+id/mediaPlayerBar"
android:layout_width="match_parent"
android:layout_height="wrap_content" />
</FrameLayout>
And finally here is the code for MediaInfoFragment:
public class MediaInfoFragment extends Fragment {
private final static String TAG = "MediaInfoFragment";
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Log.i(TAG, "onCreate()");
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
Log.i(TAG, "onCreateView()");
if (container == null) {
Log.i(TAG, "onCreateView(): container = null");
}
return inflater.inflate(R.layout.media_info_fragment_layout, container, false);
}
}
Here is my problem : the container passed in the onCreateView() method of the MediaInfoFragment is null. As I understood, this should only be the case for non-UI Fragments. However, my Fragment has a UI, which is displayed OK on the screen when I launch MediaInfoActivity. It causes problems because no style declared in the xml layout file of the fragment is applied.
Here is my Log:
I/MediaInfoActivity: onCreate()
I/MediaInfoFragment: onCreate()
I/MediaInfoFragment: onCreateView()
I/MediaInfoFragment: onCreateView(): container = null
Am I missing anything obvious here ?
You just have to create a inflater like bellow in your fragment.
View rootView;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (rootView == null) {
rootView = inflater.inflate(R.layout.activity_my_cart, null);
} else {
((ViewGroup) container.getParent()).removeView(rootView);
}
return rootView;
}
I hope it will work as per your question.
I am not sure since I don't have the code of the SDK in front of me but I think that the life-cycle of calling Fragment "onCreateView" function is different between the two cases:
1. Static settings of fragment in layout
2. Loading pragmatically with FragmentManager.
In the first case the debugger get into Fragment.onCreateView() method immediately upon adding the content view to the parent activity as part of onCreate() code:
When calling: setContentView(R.layout.some_layoue);
You will see the debugger get into Fragment.onCreateView() before going to next line
In the second case the Fragment.onCreateView() is being invoked only after the onCreate() of the activity is finished.
This looks like design bug for me but possibly as design feature.
Anyway the container is null when adding fragment statically because the related object was not yet created.
In fact the difference between the two situations is much deeper. In the case of static fragments toggling between fragments will not create the view hierarchy correctly.
For example if you will add button-A to fragment A and button-B to Fragment-B and toggle the fragments with a code looks like this (highlighting only the relevant code):
public void turnOnFragment() {
FragmentManager manager = getFragmentManager();
if (manager != null) {
manager.beginTransaction()
.setCustomAnimations(android.R.animator.fade_in, android.R.animator.fade_out)
.attach(this)
.commit();
}
}
public void turnOffFragment() {
FragmentManager manager = getFragmentManager();
if (manager != null) {
manager.popBackStackImmediate(null, FragmentManager.POP_BACK_STACK_INCLUSIVE);
manager.beginTransaction()
.setCustomAnimations(android.R.animator.fade_in, android.R.animator.fade_out)
.detach(this)
.commit();
}
}
You will see that in the case of static fragments the buttons from both fragments are presented although turning on and off. If however fragments are added programatically the toggle works fine and view hierarchy is cleaned and show only button from relevant fragment.
This is based of my experience with version 4.4.2
Let's say I have this button:
<Button
android:id="#+id/idone"
android:layout_width="0dip"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="D2"
android:onClick="myMeth"/>
I have several times used this to call methods from a layout xml as it calls the method from the activity that inflated such view.
Recently with DialogFragments, well it does not work at all. I keep getting an error telling me that such method does not exist. Where is it then looking for such method? I have added it to the DialogFragment class:
public class myActivity extends DialogFragment {
public DiceDialog() {
// empty constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.myDialog, container);
getDialog().setTitle("Hello");
return view;
}
public void myMeth(View view) {
//...
}
As well as in the activity that instantiates the FragmentManager and calls the dialog:
public Class MainActiviry Extends FragmentActivity {
//...
public void onCreate(Bundle savedInstanceState) {
// ..
FragmentManager fm = getSupportFragmentManager();
MyActivity dialog = new AddDiceDialog();
dialog.show(fm, "tag");
}
public void myMeth(View view){
//...
}
And still the messag is that MyMeth is not found.
I have already read that using interfaces and listeners is the correct way to communicate between activity and dialog fragments, but what I am trying to figure out here is where that myMeth call is being made, because well,it is called.
You can implement public myMeth(View view) in your Activity, which will then check for the currently visible Fragment, and call its method.
If you want to use more then one callable method in your Fragment, you can utilize the id's of the calling views and implement a switch, calling a different fragment method according to the id of the View.