android - Magic with obtainStyledAttributes method

Question

Code:

public class CustomLayoutWithText extends LinearLayout {

    private Context context;
    private AttributeSet attrs;

    public CustomLayoutWithText(Context context, AttributeSet attrs) {
        super(context, attrs);
        this.context = context;
        this.attrs = attrs;
        fooAttrs();
    }

    @Override
    protected void onFinishInflate() {
        super.onFinishInflate();
        fooAttrs();
    }

    private void fooAttrs() {
        int[] set = {
            android.R.attr.text        // idx 0
        };
        TypedArray a = context.obtainStyledAttributes(attrs, set);
        Log.d(null, a.getString(0));
    }
}

and XML:

<com.korovyansk.android.views.CustomLayoutWithText
     xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="wrap_content"
     android:text="Some text"/>

Reasonable to expect that output will be:
Some text
Some text

But it's:
Some text
null

Why second time it appears null? And how to avoid it?

Answer 1

I think all data in attrs gets recycled. In fact in the examples everywhere they recommend to explicitly recycle all obtained typed arrays from the AttributeSet in constructor of the View. I think you should follow the best practice too.

    TypedArray a = context.obtainStyledAttributes(attrs, set);
    try {
        // read attributes from a
        // ...
    } finally {
        attributes.recycle();
    }

My guess is that when onFinishInflate() is called, attrs is already empty or unavailable.

In fact, according to the code of LayoutInflater, the AttributeSet that is passed into View constructor during layout inflation is actually based on XmlPullParser. The onFinishInflate() is called on the parent view after inflating all of its children.

It looks like at this point in 'onFinishInflate()' you cannot obtain the values of your attribute from the attrs. Especially if you have children inside your layout - the attrs will reference to the same parser, but the parser's position will point to the attributes of the last parsed child.