RecyclerView Adapter & ItemDecoration extensions that make it easy to build and customize an expandable RecyclerView.

This implementation includes support for header, sub header, and content views for each section in your RecyclerView. Sub header views can be shown and hidden at will for customizability.

The Adapter keeps track of each section’s expansion state (Minimized, Expanded, Loading, or Loading Error). Using the loading and error states, lazy loading of data for each section is easy to implement.

Add to root build.gradle

allprojects {
    repositories {
	...
	maven { url 'https://jitpack.io' }
    }
}

Add to app build.gradle

dependencies {
    compile 'com.github.chriselder:expandable-recycler-view:v1.0.0'
}

Create an adapter that extends ExpandableRecyclerViewAdapter.

You must provide 6 types to the ExpandableRecyclerViewAdapter. This is annoying, but once its done, binding view holders for each section becomes very strait forward to implement.

1. Header: Model class for your section headers
2. SubHeader: Model class for your section sub headers
3. Content: Model class for the content rows in each section
4. HeaderViewHolder: ViewHolder class for your header rows
5. SubHeaderViewHolder: ViewHolder class for your sub header rows
6. ContentViewHolder: ViewHolder class for the content rows in each section

You must also call the super(...) constructor and provide the class values for each of the ViewHolders types.

public class MyExpandableRecyclerViewAdapter
        extends ExpandableRecyclerViewAdapter<Header, SubHeader, Content, HeaderViewHolder, SubHeaderViewHolder, ContentViewHolder>
{
    public MyExpandableRecyclerViewAdapter()
    {
        super(HeaderViewHolder.class, SubHeaderViewHolder.class, ContentViewHolder.class);
    }
}

By design, the ExpandableRecyclerViewAdapter forces your subclass to implement methods of the Expandable Interface. These methods mimic the workflow for a regular RecyclerView and make it simple to handle header, sub header, and content views.

You provide the ExpandableRecyclerView with the number of sections as well as the model objects for each section. A reference to the model objects you provide here is kept internally and used to populate the RecyclerView.

int getNumberOfSections();                                // return the number of sections in your view
Header getHeaderForSection(int sectionIndex);             // provide the Header object for a section
SubHeader getSubHeaderForSection(int sectionIndex);       // provide the SubHeader model object for a section
List<Content> getContentForSection(int sectionIndex);     // provide an ArrayList() of Content model objects for each section
int getDefaultExpansionStateForSection(int sectionIndex); // provide the default expansion state for each section

A specific method is provided to create each type of ViewHolder (header, sub header and content).

HeaderViewHolder createSectionHeaderViewHolder(ViewGroup parent);
SubHeaderViewHolder createSectionSubHeaderViewHolder(ViewGroup parent);
ContentViewHolder createSectionContentViewHolder(ViewGroup parent);

A specific method is provided to bind each type of ViewHolder. The holder, model object corresponding to the row, index of the section and expansion state are provided.

void bindSectionHeaderViewHolder(HeaderViewHolder holder, 
                                     Header header,
                                     int sectionIndex,
                                     int expansionState);
void bindSectionSubHeaderViewHolder(…);
void bindSectionContentViewHolder(…);

This method is called every time the RecyclerView updates. Use it to provide logic to either show or hide a sub header for a specific section.

boolean shouldShowSectionSubHeader(int sectionIndex, int expansionState);

This method is used to specify what state you want each section to save as when the RecyclerView is destroyed. Ex. From a loading state, you may wish to return to a minimized state if your loading operation is destroyed aswell.

int getSavedStateForSection(int sectionIndex, int expansionState);

It is very simple to implement responding to touch events on a row or child views of a row, just implement the corresponding interface described below. ExpandableRecyclerViewAdapter implements OnClickListener and delegates to its own custom interfaces. To make it dead easy to perform useful actions on touch events, the interface methods provide you with the Model object corresponding to the touched row, the section index of the section the row was in, and the expansion state of that section.

For example:

void sectionHeaderClicked(Header headerModelObject, int sectionIndex, int expansionState);

To respond to a touch event on a row, implement the ExpandableRowOnClickListener interface, and register the implementing class with the ExpandableRecyclerViewAdapter. The interface methods will then automatically be called when each specific type of view is clicked (header, sub header, content).

public class MyExpandableRecyclerViewAdapter
        extends ExpandableRecyclerViewAdapter<Header, SubHeader, Content, HeaderViewHolder, SubHeaderViewHolder, ContentViewHolder>
        implements ExpandableRowOnClickListener<Header, SubHeader, Content>
{
    public MyExpandableRecyclerViewAdapter()
    {
        ...
        this.expandableRowOnClickListener = this;
    }
}

To respond to touch events on a child view in a row, implement the expandableRowSubViewOnClickListener interface, and register the implementing class with the ExpandableRecyclerViewAdapter. Then when binding the ViewHolder, set the child view's onClickListener to be the ExpandableRecyclerViewAdapter.

public class MyExpandableRecyclerViewAdapter
        extends ExpandableRecyclerViewAdapter<Header, SubHeader, Content, HeaderViewHolder, SubHeaderViewHolder, ContentViewHolder>
        implements ExpandableRowSubViewOnClickListener<Header, SubHeader, Content>
{
    public MyExpandableRecyclerViewAdapter()
    {
        ...
        this.expandableRowSubViewOnClickListener = this;
    }
    
    public void bindSectionContentViewHolder(ContentViewHolder holder, ...)
    {
        holder.childView.setOnClickListener(this);
    }
}

To change expansion state of a specific section call one of the four provided methods. You must call these methods yourself as ExpandableRecyclerViewAdapter does not do it automatically. It is recommended to change expansion state in respnse to a touch event (described above).

setExpansionStateMinimized(int sectionIndex);
setExpansionStateExpanded(int sectionIndex);
setExpansionStateLoading(int sectionIndex);
setExpansionStateError(int sectionIndex);

ExpandableRecyclerViewAdapter provides its own update methods, do not call the standard RecyclerView update methods (Ex. adapter.notifyDataSetChanged())

void notifyExpandableDataSetChanged();
void notifySectionHeaderChanged(int sectionIndex);
void notifySectionSubHeaderChanged(int sectionIndex);
void notifySectionContentChanged(int sectionIndex);
void notifySectionChanged(int sectionIndex)

To save the state of the RecyclerView, call the ExpandableRecyclerViewAdapter’s onSaveInstanceState & onRestoreInstance methods from the corresponding methods in your activity.

public class MainActivity
{
    @Override
    protected void onSaveInstanceState(Bundle outState)
    {
        super.onSaveInstanceState(outState);
        adapter.onSaveInstanceState(outState);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState)
    {
        super.onRestoreInstanceState(savedInstanceState);
        adapter.onRestoreInstanceState(savedInstanceState);
    }
}

To provide item decorations for specific rows, create an ItemDecoration class that extends ExpandableRecyclerViewItemDecoration. Implement the abstract methods provided:

void getHeaderOffsets(…);
void getSubHeaderOffsets(…);
void getContentRowOffsets(…);
void onDrawSectionHeader(…);
void onDrawSectionSubHeader(…);
void onDrawContent(…);

1. Fork it!
2. Create your feature branch: git checkout -b my-new-feature
3. Commit your changes: git commit -am 'Add some feature'
4. Push to the branch: git push origin my-new-feature
5. Submit a pull request :D

Published under the MIT License.