RecyclerView getLayoutParams returns null (sometimes)

Question

I have a RecyclerView, for which I have a custom Adapter, and I have set the LayoutManager to be a FlexboxLayoutManager. For every child, I want to set FlexGrow to be 1.

In Google's example, demo-cat-gallery (https://github.com/google/flexbox-layout), they do this in the ViewHolder:

void bindTo(Drawable drawable) {
    mImageView.setImageDrawable(drawable);
    ViewGroup.LayoutParams lp = mImageView.getLayoutParams();
    if (lp instanceof FlexboxLayoutManager.LayoutParams) {
        FlexboxLayoutManager.LayoutParams flexboxLp = (FlexboxLayoutManager.LayoutParams) lp;
        flexboxLp.setFlexGrow(1.0f);
    }
}

This is then called by the RecyclerView.Adapter in onBindViewHolder. This works fine, but when I did the same in my app, it would only setFlexGrow for some items, and never the ones at the top. I realised that for some items (seemingly randomly), getLayoutParams() was returning null, but for others it was returning the correct FlexboxLayoutManager.LayoutParams.

I realised the difference was that in the Cat-gallery example, onCreateViewHolder was doing

public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.viewholder_cat, parent, false);
    return new CatViewHolder(view);
}

while in my app I was doing

public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType)
{
    return new MyViewHolder(new MyCustomView(getContext()));
}

It seems that inflating with a reference to the parent means that getLayoutParams() is never null, so changed my code to

View v = new MyCustomView(getContext());
parent.addView(v);
return new MyViewHolder(v);

And it now works correctly, with setFlexGrow() always being set. However, this feels wrong - I know you're not meant to explicitly add views to the RecyclerView parent.

So my question is:

1 - Why were LayoutParams randomly null for some items, but fine for others?

2 - How can I get LayoutParams to always be set, without doing something horrible like 'parent.addView(v);', or is it actually alright to do this?

Thanks :)

RecyclerView getLayoutParams returns null (sometimes)

1 Answers