Use custom View in a RecyclerView Adapter?

Question

I have a basic custom View which looks like this:

public class CustomView extends RelativeLayout {

    private User user;

    private ImageView profilePicture;

    public CustomView(Context context) {
        super(context);
        init();
    }

    public CustomView(Context context, AttributeSet attrs) {
        super(context, attrs);
        init();
    }

    private void init() {
        inflate(getContext(), R.layout.custom_layout, this);

        profilePicture = (ImageView) findViewById(R.id.profilePicture);

        // ACCESS USER MODEL HERE
        // e.g. user.getUsername()
    }

}

As you can see, I'd like to access user data in the View (i.e.: user.getUsername()).

I also need to be able to use the custom View in a RecyclerView Adapter.

Here is what my Adapter currently looks like:

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {

    private Context context;

    private List<User> userData;

    public MyAdapter(Context context, List<User> userData) {
        this.context = context;
        this.userData = userData;
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        public ViewHolder(View v) {
            super(v);
        }
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater inflater = LayoutInflater.from(context);

        // HOW TO INFLATE THE CUSTOM VIEW?

        // ViewHolder viewHolder = new ViewHolder(customView);

        return viewHolder;
    }

    @Override
    public void onBindViewHolder(final ViewHolder holder, int position) {
        // ANYTHING HERE?
    }

    @Override
    public int getItemCount() {
        return userData.size();
    }

}

How can I inflate the custom View in the Adapter?
Also, should I put anything in onBindViewHolder()?

Note: I must use a custom View, as I use this View under different Adapters (i.e.: not just this RecyclerView Adapter).

Solution 1

Assuming a CustomView class that looks something like this:

public class CustomView extends RelativeLayout {
    private User user;
    private ImageView profilePicture;

    // override all constructors to ensure custom logic runs in all cases
    public CustomView(Context context) {
        this(context, null);
    }
    public CustomView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }
    public CustomView(Context context, AttributeSet attrs, int defStyleAttr) {
        this(context, attrs, defStyleAttr, 0);
    }
    public CustomView(
            Context context,
            AttributeSet attrs,
            int defStyleAttr,
            int defStyleRes
    ) {
        super(context, attrs, defStyleAttr, defStyleRes);

        // put all custom logic in this constructor, which always runs
        inflate(getContext(), R.layout.custom_layout, this);
        profilePicture = (ImageView) findViewById(R.id.profilePicture);
    }

    public void setUser(User newUser) {
        user = newUser;
        // ACCESS USER MODEL HERE
        // e.g. user.getUsername()
    }
}

Your RecyclerView.Adapter and RecyclerView.ViewHolder could look something like this:

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {
    // no Context reference needed—can get it from a ViewGroup parameter
    private List<User> userData;

    public MyAdapter(List<User> userData) {
        // make own copy of the list so it can't be edited externally
        this.userData = new ArrayList<User>(userData);
    }

    @Override
    public int getItemCount() {
        return userData.size();
    }

    @Override
    public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        // no need for a LayoutInflater instance—
        // the custom view inflates itself
        CustomView itemView = new CustomView(parent.getContext());
        // manually set the CustomView's size
        itemView.setLayoutParams(new ViewGroup.LayoutParams(
                ViewGroup.LayoutParams.MATCH_PARENT,
                ViewGroup.LayoutParams.WRAP_CONTENT
        ));
        return new ViewHolder(itemView);
    }

    @Override
    public void onBindViewHolder(final ViewHolder holder, int position) {
        holder.getCustomView().setUser(userData.get(position));
    }

    public class ViewHolder extends RecyclerView.ViewHolder {
        private CustomView customView;

        public ViewHolder(View v) {
            super(v);
            customView = (CustomView) v;
        }

        public CustomView getCustomView() {
            return customView;
        }
    }
}

• The CustomView manages its own setup, which occurs in its own constructor and in this case uses inflation of an XML file. (Alternatively, it could set up its child views programmatically.)
• Because of this, the RecyclerView.Adapter doesn't need to perform any inflation—it just creates a new CustomView instance, and lets the CustomView worry about its own setup.
• The CustomView can't get a User instance until its setUser method is called, so user access cannot occur in the constructor. In any case, over one CustomView lifetime, a RecyclerView could ask it to show information for many different users at different times. The CustomView needs to be able to do this. Therefore, a setUser method is introduced.
• Because the CustomView is instantiated by code instead of by XML, attributes for size can't be defined in XML. Therefore, sizing is done programmatically after instantation.
• onBindViewHolder simply calls setUser on the CustomView to link the CustomView with the correct User instance.
• The ViewHolder class is now just a link between a RecyclerView item and a CustomView.

Using pre-built custom views from another class within RecyclerViews (i.e. not inflating XML within the RecyclerView.Adapter) never seems to be discussed. I think it's an excellent idea even when the custom view is exclusively used within a RecyclerView, because it promotes separation of concerns and adherence to the Single Responsibility Principle.

Solution 2

CustomView extends RelativeLayout {

You have a View already (well, a ViewGroup)

HOW TO INFLATE THE CUSTOM VIEW?

You don't need to... The point of a Custom View object is to not need XML, therefore no inflation.

You can create new CustomView(), but you need to set all the layout parameters, which looks cleaner in XML, I think.

---

Most RecyclerView tutorials show inflating via XML though.

View customView = inflater.inflate(...);
ViewHolder viewHolder = new ViewHolder(customView);

That should work because in the class chain, you have CustomView > RelativeLayout > ViewGroup > View

LayoutInflater inflater = LayoutInflater.from(context);

Like I said before, you don't need this if there is no XML file you want to inflate.

You also don't need the context variable.

parent.getContext() is a fine solution.

// ANYTHING HERE?

Well, yeah, you should "bind" the ViewHolder with the data that the ViewHolder should hold.

Again, most, if not all, tutorials cover this.

Comments

[user7638732]

"Note: I must use a custom view, as I use this view under different adapters (i.e. not just this RecyclerView adapter)."

[Alex Peters]

You mention that most/all tutorials cover using an external view class as the view for a ViewHolder. Could you please link one or two? I feel I've looked at a lot of tutorials on the topic of RecyclerViews and ViewHolders and I don't think I've seen this particular thing mentioned ever.

[OneCricketeer]

@Alex guides.codepath.com/android/… & guides.codepath.com/android/…

[Alex Peters]

Okay. The question states that the custom view needs to be used both within and outside a RecyclerView though, so inflating XML directly isn't really an option. Nonetheless, your answer hits the nail on the head as far as instantiating the custom view, passing a Context instance, and needing to manually set layout parameters go.

[Luke Needham]

This is a great answer - and with the last paragraph it becomes a fantastic answer. I've started using custom views in all my RecyclerView Adapters, and the result is so clean

[Alex Peters]

Thanks, Luke—I appreciate your compliments. I'm still amazed that this approach isn't adopted more often in projects I see.

[Michael]

remember to change the RecyclerView.Adapter adapter parameter to your custom view holder

[drmrbrewer]

@AlexPeters you say "Because the CustomView is instantiated by code instead of by XML, attributes for size can't be defined in XML. Therefore, sizing is done programmatically after instantiation." What about this way: stackoverflow.com/a/38690715/4070848 ?

[Alex Peters]

@drmrbrewer, sorry, but I don't understand. Could you please rephrase the question?

[drmrbrewer]

@AlexPeters it was more an observation than a question I suppose. The linked post shows that you can still define your attributes in XML, then read them as an AttributeSet and pass them as the second argument in your custom view constructor. Maybe handy if you already have lots of custom attributes defined in XML... can migrate to your suggested approach (thanks btw) and yet keep the attributes defined via XML.

[Alex Peters]

Okay, I understand now. Sounds like a perfectly good approach if your custom component inflates from XML in the first place, and especially if that component relies on specific size constraints at its topmost level. I don't think I gave too much consideration to what size values are possible/acceptable once wrapped in a ViewHolder. I guess the sizing constraints I put in my answer above were based purely off what I figured might be the most appropriate. I don't know how well things might function if the child isn't as wide as the parent, for example.