Menu toggle
latest: v0.5.0 GitHub

In Elements, sources have the responsibility of providing model data. They can be extremely simple and extremely complex if needed, through a dependency mechanism.

For now, let’s look at the main callbacks and APIs:

Callback / API Description
onPageOpened(page: Page, dependencies: List<Element<*>>) The given page was opened. You are required to provide results using one of the postResult methods. If the source declared some dependencies, this method will be called once the dependencies are resolved, and will be passed a list of the elements provided by the dependencies.
onPageChanged(page: Page, dependencies: List<Element<*>>) The given page was changed. This is only called if the source declares some dependencies. At this point it can process the list of dependencies elements and, if needed, update its results, as above, using one of the postResult methods.
postResult(page: Page, data: Collection<T>) Posts results for the given page. Can be called at any time, from any thread.
postResult(page: Page, error: Exception) Posts an error for the given page. This means we have no results for this page.
postResult(page: Page, liveData: LiveData<List<T>>) Binds this page to the given LiveData object. This means that anytime this LiveData provides results, we will post them to the given page.

When posting results, the list of T data is converted to a list of Element<T> object. The source can declare the element type for each T data in the getElementType(T) callback.

State restoration

Sources hold strong references to pages and model data. This has a pleasant consequence:

If you hold sources in your architecture ViewModel, and pass them to a new adapter, the adapter will restore pages and data seamlessly during configuration changes.

The approach should be the following:

override fun onViewCreated(view: View, savedInstanceState: Bundle?) {

Overriding results

Source subclasses have the opportunity to change the results that were posted. See, for example, this code that replaces empty lists with a this list is empty Element:

override fun onPostResult(page: Page, result: Result<T>): List<Element<T>> {
    if (result.error != null) {
        return listOf(createEmptyElement(ELEMENT_TYPE_ERROR, result.error))
    } else if (result.values.isEmpty()) {
        return listOf(createEmptyElement(ELEMENT_TYPE_EMPTY))
    } else {
        return result.values

At this point, you will need a presenter for the ELEMENT_TYPE_ERROR and ELEMENT_TYPE_EMPTY types. In fact, this is already available in the extensions provided by the library.


Like the Google’s Paging library, the adapter will use DiffUtil to compute what changed and call the appropriate notify* methods on the adapter.

The computation is performed

  • in a background thread
  • at the page level. This means that it will deal with less elements and be fast
  • anytime something in the page has changed

For this to work better, please provide an implementation of the equality callbacks:

class ContactsSource : Source<Contact> {

    // Whether two items point to the same model entity (that might be changed). 
    override fun areItemsTheSame(first: Contact, second: Contact): Boolean {
        return first.id() == second.id()

    // Used to detect whether the model entity has changed. Will call notifyItemChanged().
    // Defaults to full equality.
    override fun areContentsTheSame(first: Contact, second: Contact): Boolean {
        return first == second