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findViewById() returns null for an inflated layout

I have an Activity and its layout. Now I need to add a LinearLayout from another layout, menu_layout.xml.
LayoutInflater inflater;
inflater = (LayoutInflater) this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
LinearLayout layout = (LinearLayout) inflater.inflate(R.layout.menu_layout, null);
After this, findViewById() returns null. Is there is any solution for that?
Note: I can't put both XML in one place, and using <include> also is not working.

Explanation
When you inflate a layout, the layout is not in the UI yet, meaning the user will not be able to see it until it's been added. To do this, you must get a hold of a ViewGroup (LinearLayout,RelativeLayout,etc) and add the inflated View to it. Once they're added, you can work on them as you would with any other views including the findViewById method, adding listeners, changing properties, etc
Code
//Inside onCreate for example
setContentView(R.layout.main); //Sets the content of your activity
View otherLayout = LayoutInflater.from(this).inflate(R.layout.other,null);
//You can access them here, before adding `otherLayout` to your activity
TextView example = (TextView) otherLayout.findViewById(R.id.exampleTextView);
//This container needs to be inside main.xml
LinearLayout container = (LinearLayout)findViewById(R.id.container);
//Add the inflated view to the container
container.addView(otherLayout);
//Or access them once they're added
TextView example2 = (TextView) findViewById(R.id.exampleTextView);
//For example, adding a listener to the new layout
otherLayout.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
//Your thing
}
});
Assuming
main.xml contains a LinearLayout with the id container
other.xml is a layout file in your project
other.xml contains a TextView with the id exampleTextView

Try using
layoutObject.findViewById();

Dynamically add a button to a view from a listview Adapter GetView method

As a little eperiment, I'm trying to do the following.
I have an AXML describing a vertical linear layout which contains a listview (only filling 200dp of the vertical linear layout ). The AXML is inflated when the activity starts with SetContentView. Then the listview is correctly populated with values using its Adapter.
In the GetView method of the listview Adapter, I am trying to also dynamically create a button and add it to the linear layout, but for some reason the button is not added.
If I try to add the button in the constructor method of the Adapter instead, it is correctly added.
Can you tell me what could be possibly going wrong?
Let me add some code:
class TracksAdapter : BaseAdapter<string> {
Activity context;
List<Dictionary<string,string>> trackList;
// constructor
public TracksAdapter (Activity context, List<Dictionary<string,string>> trackList) {
this.context = context;
this.trackList = trackList;
// Just as a little test, if I create the button from here it will be correctly added to linear layout:
var ll = context.FindViewById<LinearLayout>(Resource.Id.linLayForResultsActivity);
Button b1 = new Button(context);
b1.Text = "Btn";
ll.AddView(b1);
}
public override View GetView(int position, View oldView, ViewGroup parent) {
// if I create the button from here it will not be added to the layout
var ll = context.FindViewById<LinearLayout>(Resource.Id.linLayForResultsActivity);
Button b1 = new Button(context);
b1.Text = "Btn";
ll.AddView(b1);
// this other code is working
View view = context.LayoutInflater.Inflate(Resource.Layout.ResultItem, null);
var artistLabel = view.FindViewById<TextView>(Resource.Id.resultArtistNameTextView);
artistLabel.Text = trackList[position]["trackArtistName"];
return view;
}
}
Update: adding some more context information because I know this can be a bit weird to understand without it:
In GetView, I don't need to return the new button I am trying to create there. GetView only need to return a listview view item, but, along its execution, GetView also has to create and add a button to the linear layout containing the listview.
The real code is much more complex than that. I have simplified it in the question. In the real code, the listview items are made of text and a button. The GetView also attaches event handlers to the buttons. Then what I need is, when a user clicks a button in any of the listview items, another button is added below the listview. So I need the code for adding another button to be in GetView, and the button needs to be added outside of the listview, ie. to the linear layout containing the listview.

Use the LayoutInflator to create a view based on your layout template, and then inject it into the view where you need it.
LayoutInflater vi = (LayoutInflater) getApplicationContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View v = vi.inflate(R.layout.your_layout, null);
// fill in any details dynamically here
TextView textView = (TextView) v.findViewById(R.id.a_text_view);
textView.setText("your text");
// insert into main view
ViewGroup insertPoint = (ViewGroup) findViewById(R.id.insert_point);
insertPoint.addView(v, 0, new ViewGroup.LayoutParams(ViewGroup.LayoutParams.FILL_PARENT, ViewGroup.LayoutParams.FILL_PARENT));

I looked in you code, you are returning view, while you add the button to ll, you should return ll
what you return in getView() is what you see in the list item layout, since you're adding the button to ll and returning view, the button won't appear.
you can add the button to view as you implementation
Also check this:
Try using boolean addViewInLayout (View child, int index, ViewGroup.LayoutParams params)
http://developer.android.com/reference/android/view/ViewGroup.html#addViewInLayout(android.view.View, int, android.view.ViewGroup.LayoutParams)

It's working... Without making any changes now it's working as it should... ! Ugh!
I really don't know what I was doing wrong here... probably it was because of some sort of caching of older version of the installed APK.. ? I know this sort of stuff can happen, and that's why I've always been uninstalling the app before deplyoing the new version to the device... but still...!

I've a linear layout in a fragment. How to dynamically add a view to it from the activity?

I'm posting data from one fragment and updating the other fragment by adding a view to it. I'm not using listView. I am using LinearLayout.
I've a very simple doubt. How do I get the view and update it by adding another view to it?
Basically I want to inflate the linearlayout that already has the data and add a view to it in the main activity that adds the fragment.
Edit:
LinearLayout ll = (LinearLayout)fragment.getView().findViewById(R.id.commentFragmentLayout);
ViewGroup view = (ViewGroup) ll.getParent();
View customView = view;
TextView commentContext = (TextView) customView.findViewById(R.id.commentText);
commentContext.setText("hello");
view.addView(customView);
I tried to do that in the main activity but my view gave null pointer exception.
Edit 2:
FragmentManager fm = getSupportFragmentManager();
// You can find Fragments just like you would with a View by using FragmentManager.
android.support.v4.app.Fragment fragment = fm.findFragmentById(R.id.fragmentCommentContent);
LayoutInflater inflater = (LayoutInflater) getApplicationContext().getSystemService(getApplicationContext().LAYOUT_INFLATER_SERVICE);
View child = inflater.inflate(R.layout.comments_list_item, null);
LinearLayout ll = (LinearLayout)fragment.getView().findViewById(R.id.commentFragmentLayout);
TextView newText = (TextView) child.findViewById(R.id.commentText);
newText.setText("Important text");
ll.addView(newText);
I tried that and got : The specified child already has a parent. Call removeView() on child's parent first. The problem is, if I call remove view, I lose the previous data that I've inserted in the layout.

The code you posted doesn't make any sense. You get the parent of the LinearLayout with the id R.id.commentFragment from the fragment, in that parent you search for a TextView to set some text and then you add the parent to itself.
I've a very simple doubt. How do I get the view and update it by
adding another view to it?
You get the View of a fragment with getView(). You search for the desired component on the view returned by getView() and then add the desired view to that container:
LinearLayout ll = (LinearLayout)fragment.getView().findViewById(R.id.commentFragmentLayout);
TextView newText = new TextView(context);
newText.setText("Important text");
ll.addView(newText);
Basically I want to inflate the linearlayout that already has the data
and add a view to it in the main activity that adds the fragment.
You inflate a layout file not a view that is already present. I really don't understand what you want here try to explain better.

LayoutParams params =new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT);
LinearLayout layout = new LinearLayout(this);
layout.setOrientation(LinearLayout.VERTICAL);

lay=(LinearLayout)rootView.findViewById(R.id.Lay);
lay.addView(yourView);

findViewById in specific layout?

Instead of prefixing id's in xml, is it possible to specify the particular layout in code? For example, if I have 3 layouts, each with a button that has an id of "btn". Is it possible to specify which layout to findViewById(R.id.btn) in?

The basic context is defined via setContentView(R.lyaout.my_layout). If you inflate another layout using LayoutInflater.inflate() you get a layout object, lets call it buttonLayout. You can now differ between this.findViewById(R.id.button) and buttonLayout.findViewById(R.id.button) and both will give you different button references.

findViewById is a method of the View class. You can specify where the view should be searched for like that
final View container = new View(context);
container.findViewById(R.id.btn);

If your content view is a complex hierarchy that has several views with id btn, you will need to navigate to a subtree of the hierarchy and search from there. Suppose you have three LinearLayout views, each with a btn view somewhere in it. If you can first select the correct LinearLayout (by id, tag, position, or some other means), you can then find the correct btn view. If the relevant LinearLayout has id of branch1, for instance:
View parent = findViewById(R.id.branch1); // Activity method
View btn = parent.findViewById(R.id.btn); // View method

if you have your btns inside different Viewgroups, it is possible, but requires to give ViewGroups a different name! Easyiest will be for that purpose to define the Layout of the Button inside its own XML (i.e button_layout.xml)
inside your Activity you can do this:
public MyActivity extends Activity{
Button btn1, btn2, btn3;
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
LayoutInflater inflater = (LayoutInflater)getSystemService(LAYOUT_INFLATER_SERVICE);
LinearLayout ll = new LinearLayout(this);
setContentView(ll);
btn1 = (Button)inflater.inflate(R.layout.button_layout, ll);
btn2 = (Button)inflater.inflate(R.layout.button_layout, ll);
btn3 = (Button)inflater.inflate(R.layout.button_layout, ll);
}
}

How to inflate one view with a layout

I have a layout defined in XML. It contains also:
<RelativeLayout
android:id="#+id/item"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
/>
I would like to inflate this RelativeView with other XML layout file. I may use different layouts depending on a situation. How should I do it? I was trying different variations of
RelativeLayout item = (RelativeLayout) findViewById(R.id.item);
item.inflate(...)
But none of them worked fine.

I'm not sure I have followed your question- are you trying to attach a child view to the RelativeLayout? If so you want to do something along the lines of:
RelativeLayout item = (RelativeLayout)findViewById(R.id.item);
View child = getLayoutInflater().inflate(R.layout.child, null);
item.addView(child);

You inflate an XML resource. See the LayoutInflater doc .
If your layout is in a mylayout.xml, you would do something like:
View view;
LayoutInflater inflater = (LayoutInflater) getContext().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
view = inflater.inflate(R.layout.mylayout, null);
RelativeLayout item = (RelativeLayout) view.findViewById(R.id.item);

Though late answer,
but would like to add that one way to get this
LayoutInflater layoutInflater = (LayoutInflater)this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = layoutInflater.inflate(R.layout.mylayout, item );
where item is the parent layout where you want to add a child layout.

It's helpful to add to this, even though it's an old post, that if the child view that is being inflated from xml is to be added to a viewgroup layout, you need to call inflate with a clue of what type of viewgroup it is going to be added to. Like:
View child = getLayoutInflater().inflate(R.layout.child, item, false);
The inflate method is quite overloaded and describes this part of the usage in the docs. I had a problem where a single view inflated from xml wasn't aligning in the parent properly until I made this type of change.

Even simpler way is to use
View child = View.inflate(context, R.layout.child, null)
item.addChild(child) //attach to your item

Try this code :
If you just want to inflate your layout :
View view = LayoutInflater.from(context).inflate(R.layout.your_xml_layout,null); // Code for inflating xml layout
RelativeLayout item = view.findViewById(R.id.item);
If you want to inflate your layout in container(parent layout) :
LinearLayout parent = findViewById(R.id.container); //parent layout.
View view = LayoutInflater.from(context).inflate(R.layout.your_xml_layout,parent,false);
RelativeLayout item = view.findViewById(R.id.item); //initialize layout & By this you can also perform any event.
parent.addView(view); //adding your inflated layout in parent layout.

layout inflation
View view = null;
LayoutInflater inflater = (LayoutInflater)getSystemService(Context.LAYOUT_INFLATER_SERVICE);
view = inflater.inflate(R.layout.mylayout, null);
main.addView(view);

If you're not in an activity you can use the static from() method from the LayoutInflater class to get a LayoutInflater, or request the service from the context method getSystemService() too :
LayoutInflater i;
Context x; //Assuming here that x is a valid context, not null
i = (LayoutInflater) x.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
//OR
i = LayoutInflater.from(x);
(I know it's almost 4 years ago but still worth mentioning)

AttachToRoot Set to True
Just think we specified a button in an XML layout file with its layout width and layout height set to match_parent.
<Button xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="#+id/custom_button">
</Button>
On This Buttons Click Event We Can Set Following Code to Inflate Layout on This Activity.
LayoutInflater inflater = LayoutInflater.from(getContext());
inflater.inflate(R.layout.yourlayoutname, this);
Hope this solution works for you.!

If you want to add a single view multiple time then you have to use
layoutInflaterForButton = getActivity().getLayoutInflater();
for (int noOfButton = 0; noOfButton < 5; noOfButton++) {
FrameLayout btnView = (FrameLayout) layoutInflaterForButton.inflate(R.layout.poll_button, null);
btnContainer.addView(btnView);
}
If you do like
layoutInflaterForButton = getActivity().getLayoutInflater();
FrameLayout btnView = (FrameLayout) layoutInflaterForButton.inflate(R.layout.poll_button, null);
and
for (int noOfButton = 0; noOfButton < 5; noOfButton++) {
btnContainer.addView(btnView);
}
then it will throw exception of all ready added view.

If you are you trying to attach a child view to the RelativeLayout? you can do by following
RelativeLayout item = (RelativeLayout)findViewById(R.id.item);
View child = getLayoutInflater().inflate(R.layout.child, item, true);

I had the hardest time with this error, because of my unique circumstances, but finally found a solution.
My situation: I am using a separate view (XML) which holds a WebView, then opens in an AlertDialog when I click a button in my main activity view. But somehow or another the WebView belonged to the main activity view (probably because I pull the resource from here), so right before I assigned it to my AlertDialog (as a view), I had to get the parent of my WebView, put it into a ViewGroup, then remove all the views on that ViewGroup. This worked, and my error went away.
// set up Alert Dialog box
AlertDialog.Builder alert = new AlertDialog.Builder(this);
// inflate other xml where WebView is
LayoutInflater layoutInflater = (LayoutInflater)this.getSystemService
(Context.LAYOUT_INFLATER_SERVICE);
View v = layoutInflater.inflate(R.layout.your_webview_layout, null);
final WebView webView = (WebView) v.findViewById(R.id.your_webview_id);
// more code...
.... later on after I loaded my WebView ....
// first, remove the parent of WebView from it's old parent so can be assigned a new one.
ViewGroup vg = (ViewGroup) webView.getParent();
vg.removeAllViews();
// put WebView in Dialog box
alert.setView(webView);
alert.show();

With Kotlin, you can use:
val content = LayoutInflater.from(context).inflate(R.layout.[custom_layout_name], null)
[your_main_layout].apply {
//..
addView(content)
}

When add a layout to an Activity in Kotlin, see these steps:
Add just in Activity - One layout as a parent
Xml file of new
Layout Give the value of R.
val parent: LinearLayout =findViewById(R.id.stars)
val view =
LayoutInflater.from(applicationContext).inflate(R.layout.another, parent,false)
Where parent is not necessary, can be null But warning message will be appear
view.findViewById<ImageView>(R.id.ivTimer).setImageResource(R.drawable.t2)
Any view must be set value in this way, finally add
parent.apply { addView(view)}

I had used below snippet of code for this and it worked for me.
LinearLayout linearLayout = (LinearLayout)findViewById(R.id.item);
View child = getLayoutInflater().inflate(R.layout.child, null);
linearLayout.addView(child);