Add PagingMappingAdapter and convert GiphyMp4Adapter.

app/src/main/java/org/thoughtcrime/securesms/util/adapter/mapping/MappingAdapter.java

@@ -44,9 +44,9 @@ import kotlin.jvm.functions.Function1;
  */
 public class MappingAdapter extends ListAdapter<MappingModel<?>, MappingViewHolder<?>> {
 
-  private final Map<Integer, Factory<?>> factories;
-  private final Map<Class<?>, Integer>   itemTypes;
-  private       int                      typeCount;
+  final Map<Integer, Factory<?>> factories;
+  final Map<Class<?>, Integer>   itemTypes;
+        int                      typeCount;
 
   public MappingAdapter() {
     super(new MappingDiffCallback());

app/src/main/java/org/thoughtcrime/securesms/util/adapter/mapping/PagingMappingAdapter.java

@@ -0,0 +1,69 @@
+package org.thoughtcrime.securesms.util.adapter.mapping;
+
+import android.view.View;
+import android.widget.FrameLayout;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.Nullable;
+
+import org.signal.paging.PagingController;
+import org.thoughtcrime.securesms.util.ViewUtil;
+
+/**
+ * A specialized {@link MappingAdapter} backed by a {@link PagingController}.
+ */
+public class PagingMappingAdapter<Key> extends MappingAdapter {
+
+  private PagingController<Key> pagingController;
+
+  public PagingMappingAdapter() {
+    this(1, ViewUtil.dpToPx(100));
+  }
+
+  public PagingMappingAdapter(int placeHolderWidth, int placeHolderHeight) {
+    registerFactory(Placeholder.class, parent -> {
+      View view = new FrameLayout(parent.getContext());
+      view.setLayoutParams(new FrameLayout.LayoutParams(placeHolderWidth, placeHolderHeight));
+      return new MappingViewHolder.SimpleViewHolder<>(view);
+    });
+  }
+
+  public void setPagingController(@Nullable PagingController<Key> pagingController) {
+    this.pagingController = pagingController;
+  }
+
+  @Override
+  protected @Nullable MappingModel<?> getItem(int position) {
+    if (pagingController != null) {
+      pagingController.onDataNeededAroundIndex(position);
+    }
+    return super.getItem(position);
+  }
+
+  @Override
+  public int getItemViewType(int position) {
+    MappingModel<?> item = getItem(position);
+    if (item == null) {
+      //noinspection ConstantConditions
+      return itemTypes.get(Placeholder.class);
+    }
+
+    Integer type = itemTypes.get(item.getClass());
+    if (type != null) {
+      return type;
+    }
+    throw new AssertionError("No view holder factory for type: " + item.getClass());
+  }
+
+  private static class Placeholder implements MappingModel<Placeholder> {
+    @Override
+    public boolean areItemsTheSame(@NonNull Placeholder newItem) {
+      return false;
+    }
+
+    @Override
+    public boolean areContentsTheSame(@NonNull Placeholder newItem) {
+      return false;
+    }
+  }
+}