How to pass view reference to android custom view?

Question

I did as follows

1) Creating a styleable

<declare-styleable name="Viewee">
    <attr name="linkedView" format="reference"/>
</declare-styleable>

2) defining custom view layout

<LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:background="#ffc0">
    <TextView
            android:id="@+id/custom_text"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="[text]"
            />
</LinearLayout>

3) Creating required class

public class Viewee extends LinearLayout
{
public Viewee(Context context, AttributeSet attributeSet)
{
    super(context, attributeSet);
    View.inflate(context, R.layout.viewee, this);
    TextView textView = (TextView) findViewById(R.id.custom_text);
    TypedArray typedArray = context.obtainStyledAttributes(attributeSet, R.styleable.Viewee);
    int id = typedArray.getResourceId(R.styleable.Viewee_linkedView, 0);
    if (id != 0)
    {
        View view = findViewById(id);
        textView.setText(((TextView) view).getText().toString());
    }

    typedArray.recycle();
}
}

and finally in an activity like below

<LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res/com.ns"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:orientation="vertical">
    <TextView
            android:id="@+id/tvTest"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text="android"/>
    <com.ns.Viewee
            android:layout_width="fill_parent"
            android:layout_height="fill_parent"
            app:linkedView="@+id/tvTest"
            />
</LinearLayout>

now although I recieve a non zero id in Viewee constractor, findViewById(id) retuns null and NullPointerException occures.

what am I missing?

I did it as described here

Answer 1

I found the answer!

The issue was with findViewById(id) and where I called it. findViewById only looks for a child view not a view exist on upper hierarchy level as documentation says . So I have to call something like getRootView().findViewById(id) but that also returns null becase where I called it was not corrent.

In Viewee constractor Viewee itself has not attached to its root yet so that call causes NullPointerException.

So If I call to getRootView().findViewById(id) somewhere else after constraction, it works fine and both "@+id/tvTest" and "@id/tvTest" are correct. I've tested it!

the answer is as follows

public class Viewee extends LinearLayout
{
    public Viewee(Context context, AttributeSet a)
    {
        super(context, attributeSet);
        View.inflate(context, R.layout.main6, this);
        TextView textView = (TextView) findViewById(R.id.custom_text);
        TypedArray t = context.obtainStyledAttributes(a, R.styleable.Viewee);
        int id = t.getResourceId(R.styleable.Viewee_linkedView, 0);
        if (id != 0)
        {
            _id = id;
        }

        t.recycle();
    }

    private int _id;

    public void Foo()
    {
        TextView textView = (TextView) findViewById(R.id.custom_text);
        View view = getRootView().findViewById(_id);
        textView.setText(((TextView) view).getText().toString());
    }
}

and Foo is called when it is required to process the attached view via its reference id somewhere else in your activity and the like.

The credit completely goes to those guys contributed in this post. I had not seen that post before submitting the question.

Answer 2

I know this is an old question, but I thought I would add another way of doing this as I wanted to encapsulate everything into my custom view.

Instead of calling from the outside, another way of getting a view higher up in the hierarchy, I've hooked into onAttachedToWindow() instead:

public class MyCustomView extends LinearLayout {
    private int siblingResourceId;
    private View siblingView;

    public MyCustomView(Context context, AttributeSet a) {
        super(context, attributeSet);
        inflate(context, R.layout.main6, this);
        TextView textView = (TextView) findViewById(R.id.custom_text);
        TypedArray t = context.obtainStyledAttributes(a, R.styleable.Viewee);
        siblingResourceId = t.getResourceId(R.styleable.MyCustomView_siblingResourceId, NO_ID);
        t.recycle();
    }

    @Override
    public void onAttachedToWindow() {
        super.onAttachedToWindow();
        if (siblingResourceId != NO_ID) {
            siblingView = ((View) getParent()).findViewById(siblingResourceId);
        }
    }
}

onAttachedToWindow is called quite early, but apparently late enough for the whole view hierarchy to have settled. It works flawless for my needs at least and is a little bit more controlled and doesn't need interaction from outside to work ;-)

EDIT: Kotlin code added

class MyCustomView(context: Context, attributeSet: AttributeSet) : LinearLayout(context, attributeSet) {
    private val siblingResourceId: Int
    private lateinit var siblingView: View

    // All other constructors left out for brevity.

    init {
        inflate(context, R.layout.main6, this)
        val textView = findViewById<TextView>(R.id.custom_text)
        val t = context.obtainStyledAttributes(a, R.styleable.Viewee)
        siblingResourceId = t.getResourceId(R.styleable.MyCustomView_siblingResourceId, NO_ID)
        t.recycle()
    }

    override fun onAttachedToWindow() {
        super.onAttachedToWindow()
        if (siblingResourceId != NO_ID) {
            siblingView = (parent as View).findViewById(siblingResourceId) 
        }
    }
}

Note: We assume that the parent of this custom View is a View itself.

Answer 3

Your described android:id is set to app:linkedView="@+id/tvTest. However, @+id/tvTest is used to create a new id with name "tvTest". What you want to do is use app:linkedView="@id/tvTest.