"Favor composition over inheritance" for RecyclerView Adapters
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Latest commit db15b92 Jan 7, 2017 @sockeqwe Releasing Version 3.0.1

README.md

AdapterDelegates

Read the motivation for this project in my blog post.

Dependencies

This library is available on maven central:

compile 'com.hannesdorfmann:adapterdelegates3:3.0.1'

Build Status

Please note that since 3.0 the group id has been changed to adapterdelegates3.

Snapshot

compile 'com.hannesdorfmann:adapterdelegates3:3.0.2-SNAPSHOT'

You also have to add the url to the snapshot repository:

allprojects {
  repositories {
    ...

    maven { url "https://oss.sonatype.org/content/repositories/snapshots/" }
}

Changelog

See releases section

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 AdapterDelegate<List<Animal>> {

  private LayoutInflater inflater;

  public CatAdapterDelegate(Activity activity) {
    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, @Nullable List<Object> payloads) {

    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);
    }
  }
}

Please note that onBindViewHolder() last parameter payloads is null unless you use from adpater.notify. There are more methods like onViewRecycled(ViewHolder), onFailedToRecycleView(ViewHolder), onViewAttachedToWindow(ViewHolder) and onViewDetachedFromWindow(ViewHolder) you can override in your AdapterDelegate class.

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;

    delegatesManager = new AdapterDelegatesManager<>();

    // AdapterDelegatesManager internally assigns view types integers
    delegatesManager.addDelegate(new CatAdapterDelegate(activity))
                    .addDelegate(new DogAdapterDelegate(activity))
                    .addDelegate(new GeckoAdapterDelegate(activity));

    // You can explicitly assign integer view type if you want to
    delegatesManager.addDelegate(23, new SnakeAdapterDelegate(activity));
  }

  @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

For 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))
                    .addDelegate(new DogAdapterDelegate(activity))
                    .addDelegate(new GeckoAdapterDelegate(activity))
                    .addDelegate(23, new SnakeAdapterDelegate(activity));

    // 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) {
    inflater = activity.getLayoutInflater();
  }

  @Override public boolean isForViewType(Animal 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, @Nullable List<Object> payloads) {
    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 view type.

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

Note also that boolean return type of isForViewType() of a fallback delegate will be ignored (will not be take into account). So it doesn't matter if you return true or false. You can use AbsFallbackAdapterDelegate that already implements isForViewType() so that you only have to override onCreateViewHolder() and onBindViewHolder() for your fallback adapter delegate.

Migrating from 2.x to 3.0

In contrast to 2.x in 3.0 AdapterDelegate is no longer a interface, but rather an abstract class. To keep version backward compatible with project that are already using 1.x or 2.x the package has been renamed to com.hannesdorfmann.adapterdelegates3 and also the artifact id has been renamed to adapterdelegates3.

See the releases section for more information about changes.

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.