Coordination

In Elements, sources can declare internal dependencies with respect to other sources, similarly to what CoordinatorLayout.Behaviors do. This is done by overriding the dependsOn() function:

override fun dependsOn(other: Source<*>): Boolean {
    return other is ContactsSource
}

This has a few consequences:

• the source receives all the dependencies objects in the onPageOpened. This also means that the callback is only fired when the dependency has provided items for that page.
• the source receives the onPageChanged callback, when the dependency has changed the page content.
• the source receives the insertBefore() and insertAfter() callbacks, for relative ordering

When a source declares dependencies, we give this source the opportunity to choose how and where to insert its elements. If source A depends on source B and C, you can imagine, for each actual page, an imaginary page made of items from B and C for that page.

public open fun insertBefore(page: Page, dependencies: List<Element<*>>, element: Element<*>, position: Int, available: Int): Int {
    return available // Insert all our A items before B and C items
    return if (element.data == "foo") 1 else 0 // Add a header to "foo" objects
    return if (position % 4 == 0) 1 else 0 // Add an item each 4 items
}

Same goes for insertAfter(). Once the source has inserted all its available items, the methods are not called anymore.