AdapterDelegates

Read the motivation for this project in my blog post.

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. Then you can compose your RecyclerView Adapter by registering the AdapterDelegates that you really need.

Changelog

See releases section

Quickstart: Kotlin DSL

There are 2 artifacts for kotlin users that allow you to write Adapter Delegates more convenient by providing a DSL:

Dependencies

implementation 'com.hannesdorfmann:adapterdelegates4-kotlin-dsl:4.3.1'

// If you use Kotlin Android Extensions and synthetic properties (alternative to findViewById())
implementation 'com.hannesdorfmann:adapterdelegates4-kotlin-dsl-layoutcontainer:4.3.1'

// If you use ViewBinding
implementation 'com.hannesdorfmann:adapterdelegates4-kotlin-dsl-viewbinding:4.3.1'

How to use it

fun catAdapterDelegate(itemClickedListener : (Cat) -> Unit) = adapterDelegate<Cat, Animal>(R.layout.item_cat) {

    // This is the initializer block where you initialize the ViewHolder.
    // Its called one time only in onCreateViewHolder.
    // this is where you can call findViewById() and setup click listeners etc.

    val name : TextView = findViewById(R.id.name)
    name.setClickListener { itemClickedListener(item) } // Item is automatically set for you. It's set lazily though (set in onBindViewHolder()). So only use it for deferred calls like clickListeners.

    bind { diffPayloads -> // diffPayloads is a List<Any> containing the Payload from your DiffUtils
        // This is called anytime onBindViewHolder() is called
        name.text = item.name // Item is of type Cat and is the current bound item.
    }
}

In case you want to use kotlin android extensions and synthetic properties (as alternative to findViewById()) use adapterDelegateLayoutContainer instead of adapterDelegate like this:

fun catAdapterDelegate(itemClickedListener : (Cat) -> Unit) = adapterDelegateLayoutContainer<Cat, Animal>(R.layout.item_cat) {

    name.setClickListener { itemClickedListener(item) } // no need for findViewById(). Name is imported as synthetic property from kotlinx.android.synthetic.main.item_cat

    bind { diffPayloads ->
        name.text = item.name
    }
}

If you use adapterDelegateLayoutContainer() don't forget to add

androidExtensions {
    experimental = true
}

to your build.gradle to enable LayoutContainer.

In case you want to use ViewBinding\DataBinding use adapterDelegateViewBinding instead of adapterDelegate like this:

fun cat2AdapterDelegate(itemClickedListener : (Cat) -> Unit) = adapterDelegateViewBinding<Cat, DisplayableItem, ItemCatBinding>(
    { layoutInflater, root -> ItemCatBinding.inflate(layoutInflater, root, false) }
) {
    binding.name.setOnClickListener {
        itemClickedListener(item)
    }
    bind {
        binding.name.text = item.name
    }
}

You have to specify if a specific AdapterDelegate is responsible for a specific item. Per default this is done with an instanceof check like item instanceof Cat. You can override this if you want to handle it in a custom way by setting the on lambda and return true or false:

adapterDelegate<Cat, Animal> (
    layout = R.layout.item_cat,
    on = { item: Animal, items: List, position: Int ->
        if (item is Cat && position == 0)
            true // return true: this adapterDelegate handles it
        else
            false // return false
    }
){
    ...
    bind { ... }
}

The same on parameter is available for adapterDelegateLayoutContainer() and adapterDelegateViewBinding() DSL.

Compose your Adapter

Finally, you can compose your RecyclerView Adapter by registering your AdapterDelegates like this:

val adapter = ListDelegationAdapter<List<Animal>>(
    catAdapterDelegate(...),
    dogAdapterDelegate(),
    snakeAdapterDelegate()
)

fun vs. val

You could define your AdapterDelegate also as TopLevel val like this:

// top level property inside CatDelegate.kt
val catDelegate = adapterDelegate<Cat, Animal> {
    ...
    bind { ... }
}

but a top level val is a static field at the end so that this adapter delegate will be kept for the lifetime of your application in memory. Therefore, we would recommend to prefer write your AdapterDelegate as a function and call this function to actually instantiate the AdapterDelegate. Then the AdapterDelegate can be garbage collected as soon as the user leaves the screen the AdapterDelegate is used in.

// top level function inside CatDelegate.kt
fun catAdapterDelegate() = adapterDelegate<Cat, Animal> {
   ...
   bind { ... }
}

Dependencies if you don't use Kotlin DSL

This library is available on maven central:

implementation 'com.hannesdorfmann:adapterdelegates4:4.3.1'

Please note that since 4.0 the group id has been changed to adapterdelegates4.

Snapshot

implementation 'com.hannesdorfmann:adapterdelegates4:4.3.2-SNAPSHOT'

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

allprojects {
  repositories {
    ...

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

How to use it in Java

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 adapter.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 snippet 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).

DiffUtil & ListAdapter = AsyncListDifferDelegationAdapter

Support library 27.0.1 introduced ListAdapter - the new extension of RecyclerView.Adapter that uses AsyncListDiffer internally. It does calculating diff in the background thread by default and does all particular animations for you items collection. Hence you don't need carry about notify* methods, AsyncListDiffer does all the job for you. And AdapterDelegates supports it too.

This library offers the equivalent to ListAdapter which is called AsyncListDifferDelegationAdapter that can be used together with any regular AdapterDelegate.

public class DiffAdapter extends AsyncListDifferDelegationAdapter<Animal> {
    public DiffAdapter() {
        super(DIFF_CALLBACK) // Your diff callback for diff utils
        delegatesManager
            .addDelegate(new DogAdapterDelegate());
            .addDelegate(new CatAdapterDelegate());
    }
}

Pagination

There is an additional artifact for the pagination library:

implementation 'com.hannesdorfmann:adapterdelegates4-pagination:4.3.1'

Use PagedListDelegationAdapter.

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.

Version 3.x to 4.0 migration

AdapterDelegates3 uses com.android.support:recyclerview-v7:x.y.z whereas AdapterDelegates4 uses androidx.recyclerview:recyclerview:1.0.0. Migration should be easy. Just use IntelliJ IDE or Android Studio 'Replace in Path' (can be found inside Edit main menu then Find submenu): Replace com.hannesdorfmann.adapterdelegates3 with com.hannesdorfmann.adapterdelegates4. You might also have to replace android.support.v7.widget.RecyclerView with androidx.recyclerview.widget.RecyclerView and android.support.annotation.NonNull with androidx.annotation.NonNull.

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.