Scrolling and communicating with the UI

caution

This page covers the older Java codegen-based Sections API. If using the Kotlin Lazy Collection API, refer to the interactions docs for Lazy Collection for similar relevant content.

Introduction

Data flows through the section hierarchy before they are eventually represented on the UI by components. Section provides a set of functionalities to allow you to respond to the data and interact with the UI.

@OnRefresh

A method annotated with this annotation will be called when the the UI rendered by the section's hierarchy is requesting for a refresh of the content.

API

Call SectionTree#refresh() to propagate your refresh request to all the sections in the hierarchy. Then you can handle it in your section like this:

class SectionSpec {

  @OnRefresh
  static void onRefresh(SectionContext c, @Prop yourProp, @State yourState) {
    // Handle your refresh request
  }
}

@OnViewportChanged

A method annotated with this annotation will be called when there is a change in the visible viewport.

API

Call SectionTree#viewPortChanged() or SectionTree#viewPortChangedFromScrolling() to allow your sections to know that something on the viewport is now different.

class SectionSpec {

  @OnViewportChanged
  static void onViewportChanged(
    SectionContext c,
    int firstVisiblePosition,
    int lastVisiblePosition,
    int totalCount,
    int firstFullyVisibleIndex,
    int lastFullyVisibleIndex,
    @Prop YourProp prop,
    @State YourState state) {
  }
}

1. firstVisiblePosition Position of the first visible components in the viewport. Components are partially hidden from the visible viewport.

2. lastVisiblePosition Position of the last visible components in the viewport. Components are partially hidden from the visible viewport.

3. totalCount Total number of items in the section's hierarchy, with the section that contains the annotated method as its root.

4. firstFullyVisibleIndex Position of the first fully visible components in the viewport.

5. lastFullyVisibleIndex Position of the last fully visible components in the viewport.

Change in Viewport

A viewport change could occur due to any number of the following reasons:

1. Components added to the visible viewport.
2. Components removed from the visible viewport.
3. Scrolling.
4. Components in the visible viewport are updated.
5. Components have moved in or out of the visible viewport.

Positions and Counts

Positions and total count returned are with respect to the number of components this section has. For example:

       Section A
      /         \
 Section B   Section C
    |           |
  10 items    10 items
  (visible)   (hidden)

When the first item of Section C comes into the viewport due to scrolling, firstVisiblePosition of Section C is 0 while the lastVisiblePosition of Section B is 10.

Section A has a total of 20 items while Section B and C have 10 each.

@OnDataBound

A method annotated with this annotation will be called when the data changes corresponding to this section's hierarchy is made available to the SectionTree.Target.

Data changes could occur due to any number of the following:

1. Insertion
2. Update
3. Delete
4. Move

Availability of these data changes do not mean they are visible on the viewport. To detect visibility, use @OnViewportChanged.

class SectionSpec {

  @OnDataBound
  static void OnDataBound(
    SectionContext c,
    @Prop YourProp prop
    @State YourState state) {
    // Handle data changes
  }
}

Scrolling: requestFocus()

Use this method to give focus to one of the components in a section. If the component is hidden from the visible viewport, the section will be scrolled to reveal it, thereby calling the @OnViewportChanged annotated method.

The data that renders the component being requested for focus has to be available before the method can work. Hence, only use requestFocus() after the @OnDataBound annotated method has been called.

API

There a few variations of the requestFocus() method.

SectionLifecycle.requestFocus(SectionContext c, int index)

Calls focus by the index of the Component in the Section scoped by the given SectionContext

info

As with @OnViewportChanged, the index is with respect to the number of components this section has.

SectionLifecycle.requestFocus(SectionContext c, String sectionKey)

Calls focus to the first index of the component in the section represented by the section's key you provided.

SectionLifecycle.requestFocusWithOffset(SectionContext c, int index, int offset)

Same as SectionLifecycle.requestFocus(SectionContext c, int index) but with an offset.

SectionLifecycle.requestFocusWithOffset(SectionContext c, String sectionKey, int offset)

Same as SectionLifecycle.requestFocus(SectionContext c, String sectionKey) but with an offset.

SectionLifecycle.requestFocus(SectionContext c, String sectionKey, FocusType focusType)

FocusType is either:

1. FocusType.START Calls focus to the first index of the component in the section

2. FocusType.END Calls focus to the last index of the component in the section

SectionLifecycle.requestSmoothFocus(SectionContext c, int index, SmoothScrollAlignmentType type)

Use this variation if you need to specify a snapping mode. SmoothScrollAlignmentType is either:

1. SNAP_TO_ANY
2. SNAP_TO_START
3. SNAP_TO_END
4. SNAP_TO_CENTER

For the full list of methods available to request scroll to a position, check out the SectionLifecycle documentation.

@OnEvent(LoadingEvent.class)

Sections should use this annotation to declare a method to receive events about its children loading state.

API

class YourSectionSpec {

  @OnCreateChildren
  protected static Children onCreateChildren(SectionContext c) {
    return Children.create()
        .child(
          ChildSection
            .create(c)
            .loadingEventHandler(YourSection.onLoadingEvent(c)))
        .build();
  }

  @OnEvent(LoadingEvent.class)
  static void onLoadingEvent(
    SectionContext c,
    @FromEvent LoadingState loadingState,
    @FromEvent boolean isEmpty,
    @FromEvent Throwable t) {

    switch (loadingState) {
        case INITIAL_LOAD:
        case LOADING:
          // Handle loading
          break;
        case FAILED:
          // Handle failure
          break;
        case SUCCEEDED:
          // Handle success
          break;
    }

    // Dispatch the same loading event up the hierarchy.
    SectionLifecycle.dispatchLoadingEvent(
        c,
        isEmpty,
        loadingState,
        t);
  }
}

LoadingState loadingState

1. INITIAL_LOAD Loading has started

2. LOADING Loading is still ongoing

3. SUCCEEDED Loading is successful

4. FAILED Loading has failed.

boolean isEmpty

Returns true if the data set is empty after the loading event

Throwable t

Returns the reason for a LOAD_FAILED event

Dispatch up the hierarchy

The loading event will be passed up the hierarchy until there is a section that has chosen to handle it. If your section handles the loading event, it has to dispatch the event up its hierarchy if there are parent sections looking to handle it as well.

SectionLifecycle.dispatchLoadingEvent(c, isEmpty, loadingState, t);