Use DataBinding library to set background color resource or null

Question

I would like to set background color or null on my view using DataBinding library but I get an exception trying to run it.

java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.Integer.intValue()' on a null object reference

This is how I do it:

android:background="@{article.sponsored ? @color/sponsored_article_background : null}"

I also tried setting conversion but it didn't work.

@BindingConversion
public static ColorDrawable convertColorToDrawable(int color) {
   return new ColorDrawable(color);
}

Eventually, I resolved it with workaround using @BindingAdapter but I would like to know how to do it properly.

Answer 1

Reason:

First thing to know is that DataBinding library already provides a convertColorToDrawable binding converter located in android.databinding.adapters.Converters.convertColorToDrawable(int).

Using android:background should "theoretically" work, because it has a corresponding setBackground(Drawable) method. The problem is that it sees that you try to pass a color as a first argument so it tried to launch this converter before applying it to setBackground(Drawable) method. If databinding decides to use a converter it will use it on both arguments, so also on null, right before applying a final result to a setter.
Because null cannot be castes to int (and you cannot invoke intValue() on it) it throws NullPointerException.

There is a mention about the fact that mixed argument types are not supported in official Data Binding Guide.

Here are two solutions for this problem. Although you can use any of these two solutions, the first one is much easier.

Solutions:

1. As drawable

If you define your color not as a color but as a drawable in your resources (it can be in our colors.xml file:

<drawable name="sponsored_article_background">#your_color</drawable>

or

<drawable name="sponsored_article_background">@color/sponsored_article_background</drawable>

then you should be able to use android:background like you originally wanted to but providing drawable instead of color:

android:background="@{article.sponsored ? @drawable/sponsored_article_background : null}"

Here arguments has compatible types: first is Drawable and second is null so it can also be cast to a Drawable.

2. As resource id

app:backgroundResource="@{article.sponsored ? R.color.sponsored_article_background : 0}"

but it will also require to add your R class import in data section:

<data>
    <import type="com.example.package.R" />
    <variable ... />
</data>

Passing 0 as a "null resource id" is safe because setBackgroundResource method of View checks whether resid is different than 0 and sets null as a background drawable otherwise. No unnecessary transparent drawable objects are created there.

public void setBackgroundResource(int resid) {
    if (resid != 0 && resid == mBackgroundResource) {
        return;
    }

    Drawable d= null;
    if (resid != 0) {
        d = mResources.getDrawable(resid);
    }
    setBackgroundDrawable(d);

    mBackgroundResource = resid;
}