findViewById in View class returns null whereas works in Activity class

Question

i am relatively new to Android programming. i have gone through a lot of threads on this topic but none of the solutions mentioned work for me. Here is what i am trying. i am trying to create a layout resource with an XML file and in my MainActivity, i do a setContentView(R.layout.activity_main). i then have a custom view class that extends View in which i try to modify the default layout defined in the activity_main.xml file. However, any time i try to obtain a View Id with the findViewById(R.id.) (from the layout file activity_main.xml) in the Custom view implemented in a separate file, i always get null. If i try to get the id in the MainActivity class with the findViewById(...), i get a proper value.
What am i doing wrong here ?

Here are all the code snippets

thanks

Here is the file "MainActivity.java"

public class MainActivity extends Activity {

@Override
public void onCreate(Bundle savedInstanceState) {


    super.onCreate(savedInstanceState);



    setContentView(R.layout.activity_main);

    MyView myView = new MyView(this, null);

      ....

Here is the file MyView.java

public class MyView extends View {

public static final String DEBUG_TAG = "MYVIEW" ;

public MyView(Context context, AttributeSet attrs) {
    super(context, attrs);


    View tablerow1 = (TableRow) findViewById (R.id.tableRow) ;

    Log.v(DEBUG_TAG," tablerow1 = " + tablerow1 + "\n");



}

And here is the file activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/top_layout"  
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"  >
<TableLayout  
android:id="@+id/tableLayout1"  
android:layout_width="match_parent"  
android:layout_height="0dp"
android:layout_weight="2"
android:background="@color/red"
android:shrinkColumns="*"  
android:stretchColumns="*" >  
<TableRow
    android:id="@+id/tableRow1"  
    android:layout_height="wrap_content"  
    android:layout_width="match_parent"  
    android:gravity="center_horizontal">  

    <Button  
        android:id="@+id/button_00_1"  
        android:text="@string/button_1"
        android:textStyle="bold"
        android:background="@color/green"
        android:layout_margin="0.5dp"
        android:layout_width="0dp"
        android:layout_weight="0.5"           
        android:typeface="serif"></Button> 
    </TableRow>  
  </TableLayout>
</LinearLayout>

The findViewById(...) call in the MyView.java always returns null, whereas if i include it in the MainActivity.java file, i get proper non-null values.

Can someone point out what is wrong here ?

thanks

Ethan Hunt Ethan Hunt · Accepted Answer · 2012-11-06T07:51:17

You can easily access any view/layout from the layout xml of your activity. The only thing you are missing in your code is that you are trying to execute findViewById() in the MyView and trying to access the Activity's other views. This call will work only on the view that are enclosed in your MyView and not outside that. You have to call that activity's findViewByID() of which you are trying to access the views. For example:-

((Activity)context).findViewById(R.id.abc).setVisibility(View.VISIBLE);

In your case you can do it like this:-

public class MyView extends View {
   public static final String DEBUG_TAG = "MYVIEW" ;

   public MyView(Context context, AttributeSet attrs) {
   super(context, attrs);
   View tablerow1 = (TableRow) ((Activity)context).findViewById (R.id.tableRow) ;
   Log.v(DEBUG_TAG," tablerow1 = " + tablerow1 + "\n");
}

findViewById in View class returns null whereas works in Activity class

1 Answers