AdapterDelegates

Read the motivation for this project in my blog post.

Dependencies

This library is available on maven central:

compile 'com.hannesdorfmann:adapterdelegates:1.2.0'

Idea

The idea of this library is to build your adapters by composing reusable components.

Favor composition over inheritance

The idea is that you define an AdapterDelegate for each view type. This delegate is responsible for creating ViewHolder and binding ViewHolder for a certain viewtype. An AdapterDelegate get added to an AdapterDelegatesManager. This manager is the man in the middle between RecyclerView.Adapter and each AdapterDelegate.

For example:

public class CatAdapterDelegate extends AbsAdapterDelegate<List<Animal>> {

  private LayoutInflater inflater;

  public CatAdapterDelegate(Activity activity, int viewType) {
    super(viewType);
    inflater = activity.getLayoutInflater();
  }

  @Override public boolean isForViewType(@NonNull List<Animal> items, int position) {
    return items.get(position) instanceof Cat;
  }

  @NonNull @Override public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent) {
    return new CatViewHolder(inflater.inflate(R.layout.item_cat, parent, false));
  }

  @Override public void onBindViewHolder(@NonNull List<Animal> items, int position,
      @NonNull RecyclerView.ViewHolder holder) {

    CatViewHolder vh = (CatViewHolder) holder;
    Cat cat = (Cat) items.get(position);

    vh.name.setText(cat.getName());
  }

  static class CatViewHolder extends RecyclerView.ViewHolder {

    public TextView name;

    public CatViewHolder(View itemView) {
      super(itemView);
      name = (TextView) itemView.findViewById(R.id.name);
    }
  }
}

Then an AnimalAdapter could look like this:

public class AnimalAdapter extends RecyclerView.Adapter {

  private AdapterDelegatesManager<List<Animal>> delegatesManager;
  private List<Animal> items;

  public AnimalAdapter(Activity activity, List<Animal> items) {
    this.items = items;

    // Delegates
    delegatesManager = new AdapterDelegatesManager<>();
    delegatesManager.addDelegate(new CatAdapterDelegate(activity, 0));
    delegatesManager.addDelegate(new DogAdapterDelegate(activity, 1));
    delegatesManager.addDelegate(new GeckoAdapterDelegate(activity, 2));
    delegatesManager.addDelegate(new SnakeAdapterDelegate(activity, 3));

  }

  @Override public int getItemViewType(int position) {
    return delegatesManager.getItemViewType(items, position);
  }

  @Override public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    return delegatesManager.onCreateViewHolder(parent, viewType);
  }

  @Override public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {
    delegatesManager.onBindViewHolder(items, position, holder);
  }

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

Reducing boilerplate code

As you have seen in the code snipped above this may require to write the same boiler plate code again and again to hook in AdapterDelegatesManager to Adapter. This can be reduced by extending either from ListDelegationAdapter if the data source the adapter displays is java.util.List<?> or AbsDelegationAdapter which is a more general one (not limited to java.util.List)

ListDelegationAdapter

In example the same AnimalAdapter from above could be simplified as follows by extending from ListDelegationAdapter:

public class AnimalAdapter extends ListDelegationAdapter<List<Animal>> {

  public AnimalAdapter(Activity activity, List<Animal> items) {

    // DelegatesManager is a protected Field in ListDelegationAdapter
    delegatesManager.addDelegate(new CatAdapterDelegate(activity, 0));
    delegatesManager.addDelegate(new DogAdapterDelegate(activity, 1));
    delegatesManager.addDelegate(new GeckoAdapterDelegate(activity, 2));
    delegatesManager.addDelegate(new SnakeAdapterDelegate(activity, 3));

    // Set the items from super class.
    setItems(items);
  }
}

AbsListItemAdapterDelegate

Also you may have noticed that you often have to write boilerplate code to cast items and ViewHolders when working with list of items as adapters dataset source. AbsListItemAdapterDelegate can help you here. Let's take this class to create a CatListItemAdapterDelegate similar to the CatAdapterDelegate from top of this page but without the code for casting items.

public class CatListItemAdapterDelegate extends AbsListItemAdapterDelegate<Cat, Animal, CatViewHolder> {

  private LayoutInflater inflater;

  public CatAdapterDelegate(Activity activity, int viewType) {
    super(viewType);
    inflater = activity.getLayoutInflater();
  }

  @Override public boolean isForViewType(Cat item, List<Animal> items, int position) {
    return item instanceof Cat;
  }

  @Override public CatViewHolder onCreateViewHolder(ViewGroup parent) {
    return new CatViewHolder(inflater.inflate(R.layout.item_cat, parent, false));
  }

  @Override public void onBindViewHolder(Cat item, CatViewHolder vh) {
    vh.name.setText(item.getName());
  }

  static class CatViewHolder extends RecyclerView.ViewHolder {

    public TextView name;

    public CatViewHolder(View itemView) {
      super(itemView);
      name = (TextView) itemView.findViewById(R.id.name);
    }
  }
}

As you see, instead of writing code that casts list item to cat we can use AbsListItemAdapterDelegate to do the same job (by declaring generic types).

Fallback AdapterDelegate

What if your adapter's data source contains a certain element you don't have registered an AdapterDelegate for? In this case the AdapterDelegateManager will throw an exception at runtime. However, this is not always what you want. You can specify a fallback AdapterDelegate that will be used if no other AdapterDelegate has been found to handle a certain AdapterDelegate.

AdapterDelegate fallbackDelegate = ...;
adapterDelegateManager.setFallbackDelegate( fallbackDelegate );

Please note that the fallback delegate must return an integer value from fallbackDelegate.getItemViewType() that doesn't conflict with any other AdapterDelegate added by adapterDelegateManager.addDelegate( fooDelegate ). The manager will check for conflicts at runtime. To minimize the risk of conflicts you can use AbsFallbackAdapterDelegate as base class for your fallback implementation. AbsFallbackAdapterDelegate uses Integer.MAX_VALUE - 1 internally to avoid conflicts with other adapter delegates. However, you are free to write your own adapter delegate fallback that doesn't extend from AbsFallbackAdapterDelegate. Note also that boolean return type of isForViewType() of a fallback delegate will be ignored (will not be take into account).

License

Copyright 2015 Hannes Dorfmann

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

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