How to reference one control from another during inflation?

Question

I am trying to reference a sibling control through XML.

To declare an attribute to reference an id from MyTextView:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <declare-styleable name="MyTextView">
        <attr name="valueTextViewId" format="reference" />
    </declare-styleable>
</resources>

fragment_example.xml - How to use custom attribute:

<!-- Declare a "Title" text view that references a "Value" -->
<com.example.MyTextView
    android:id="@+id/foo"
    example:valueTextViewId="@id/bar"
    ... />

<!-- Depending on the "text" attribute of this "Value" textview -->
<!-- Do something within "Title" textview -->
<com.example.MyTextView android:id="@+id/bar" />

MyFragment.java - Inflating the controls

public View onCreateView(LayoutInflater inflater, ViewGroup container, 
        Bundle savedInstanceState) {
        // calls MyTextView Ctor
    View v = inflater.inflate(R.layout.fragment_example, container, false);
}

MyTextView class constructor - During inflation do something with referenced textview:

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

    TypedArray a = ctx.obtainStyledAttributes(attrs, R.styleable.MyTextView);
    int refId = a.getResourceId(R.styleable.MyTextView_valueTextViewId);

    // Updated to use context
    if (refId > -1 && context instanceof Activity) {
        Activity a = (Activity)context;
        View v = a.findViewById(refId);

        // THE PROBLEM: v is null
        if (v != null) {
            // In my case, I want to check if the "Value" textview
            // is empty. If so I will set "this" textColor to gray
        }
    }
}

in this example v is always null. I assume because during Layout Inflation, the controls are not added yet. Another thing to note is that this is in a Fragment, therefore that might be the reason I cant find the view in the parent activity.

Is it possible to reference a control from another like this?

Answer 1

Is it possible to reference a control from another like this?

It is possible to reference another View from a View.

But to do property checking in the constructor of a View is not advised.
There is no guarantee that any particular View is instantiated before any other during View inflation.

Compare these two layouts:
first_layout.xml

<com.example.MyTextView
    ...
    android:id="@+id/foo"
    example:valueTextViewId="@+id/bar" />

<com.example.MyTextView
    ...
    android:id="@+id/bar" />

second_layout.xml

<com.example.MyTextView
    ...
    android:id="@+id/bar" />

<com.example.MyTextView
    ...
    android:id="@+id/foo"
    example:valueTextViewId="@+id/bar" />

In this example, it's clear that property checking from the constructor will not work in one of these layouts.

I agree that it is possible to store the reference to another View within a View:

public MyTextView(Context context, AttributeSet attrs) {
    super(context, attrs);
    TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.MyTextView);
    mReferenceId = a.getResourceId(R.styleable.MyTextView_valueTextViewId);
    ...
}

private int mReferenceId;

public View getReferenceViewFromActivity() {
    if (getContext() instanceof Activity) {
        return ((Activity)getContext()).findViewById(mReferenceId);
    return null;
}

public View getReferenceView(View view) {
    return view.findViewById(mReferenceId);
}

But you should definitely do any and all property checking within the Activity or Fragment:

@Override
public void onViewCreated(View view, Bundle savedInstanceState) {
    super.onViewCreated(view, savedInstanceState);
    MyTextView myTextView = (MyTextView)view.findViewById(R.id.foo);
    MyReferenceView refView = (MyReferenceView)myTextView.getReferenceView(view);
    //
    // do property checking
    //
}

Answer 2

if a textview with the id of bar is available within the id of bar, you can do something like this.

<com.example.MyTextView
    android:id="@+id/foo"
    ...
    android:tag="bar" />

<com.example.MyTextView android:id="@+id/bar" />

and

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

      int barId = getResources().getIdentifier(getTag(), "id", packageName);
      TextView bar = mActivity.findViewById(barId);

    if (bar.getText() == "") {
        // Gray out this "title" textview
        setColor(android.R.color.gray);
    }

    // maybe set a text change listener to bar to make it future-proof
}

i would just pass the id as a tag on your MyTextView so you dont need to create a new attribute.