Types of Visibility Range

The framework currently supports six types of Visibility Event:

  • Visible Event: this event is triggered when at least one pixel of the Component is visible.
  • Invisible Event: this event is triggered when the Component no longer has any pixels on the screen.
  • Focused Visible Event: this event is triggered when either the Component occupies at least half of the viewport, or, if the Component is smaller than half the viewport, when it is fully visible.
  • Unfocused Visible Event: this event is triggered when the Component is no longer focused, i.e. it is not fully visible and does not occupy at least half the viewport.
  • Full Impression Visible Event: this event is triggered when the entire Component has passed through the viewport at some point.
  • VisibilityChangedEvent: this event is triggered when the visibility of the Component on the screen changes.

Usage

IMPORTANT: Visibility ranges require incremental mount to be enabled on the relevant Component.

To register visibility event handlers for a component you can follow the same steps as for setting any other event handler.

Here is an example:

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@LayoutSpec
class MyLayoutSpec {

  @OnCreateLayout
  static Component onCreateLayout(ComponentContext c) {

    return Column.create(c)
        .alignItems(YogaAlign.STRETCH)
        .child(Text.create(c)
            .text("This is MY layout spec")
            .visibleHandler(MyLayout.onTitleVisible(c))
            .invisibleHandler(MyLayout.onTitleInvisible(c))
            .focusedHandler(MyLayout.onComponentFocused(c, "someStringParam"))
            .fullImpressionHandler(MyLayout.onComponentFullImpression(c)))
            .visibilityChangedhandler(MyLayout.onComponentVisibilityChanged(c))
        .build();
  }

  @OnEvent(VisibleEvent.class)
  static void onTitleVisible(ComponentContext c) {
    Log.d("VisibilityRanges", "The title entered the Visible Range");
  }

  @OnEvent(InvisibleEvent.class)
  static void onTitleInvisible(ComponentContext c) {
    Log.d("VisibilityRanges", "The title is no longer visible");
  }

  @OnEvent(FocusedVisibleEvent.class)
  static void onComponentFocused(
      ComponentContext c,
      @Param String stringParam) {
    Log.d(
        "VisibilityRanges",
        "The component is focused with param: " + stringParam);
  }

  @OnEvent(FullImpressionVisibleEvent.class)
  static void onComponentFullImpression(ComponentContext c) {
    Log.d("VisibilityRanges", "The component has logged a full impression");
  }

  @OnEvent(VisibilityChangedEvent.class)
  static void onComponentVisibilityChanged(
      ComponentContext c,
      @FromEvent int visibleHeight,
      @FromEvent int visibleWidth,
      @FromEvent float percentVisibleHeight,
      @FromEvent float percentVisibleWidth) {
      Log.d("VisibilityRanges", "The component visible size is " + visibleHeight + "h" + visibleWidth + "w");
  }

}

VisibilityChangedEvents should be used with particular care since they will be dispatched on every frame while scrolling. No heavy work should be done inside the VisibilityChangedEvents handlers. Visible, Invisible, Focused, Unfocused and Full Impression events are the recommended over VisibilityChanged events whenever possible.

Changing LithoView visibility

There are cases when you need to trigger visibility events on the LithoView components because the UI visibility changed, but the UI did not receive any callback to inform it of this change. An example is when a new activity is added to the back stack, covering the UI. In such cases you can call setVisibilityHint on the LithoView to tell the UI whether it is visible or not. You may want to do this when Fragment#setUserVisibleHint or onResume/onPause are called. An example is when Fragment#setUserVisibleHint or onResume/onPaused are called.

Example usage:

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LithoView.setVisibilityHint(true); // This will dispatch visible/focused events as necessary on all components inside this LithoView
LithoView.setVisibilityHint(false); // This will dispatch invisible/unfocused events as necessary on all components inside this LithoView

Troubleshooting

If you are not seeing your visibility event fired when you expect it to be, you can take the following steps:

  1. Verify that incremental mount is enabled for your Component. It is enabled by default, but if you turned it off, then visibility events will not be fired.
  2. Verify that you actually set the event that you defined in your spec on your Component (i.e. by calling visibleHandler(MyLayout.onTitleVisible(c)) or similar.
  3. Visibility handlers are fired in MountState.processVisibilityOutputs(). You can step through this method and see why the event that you expect to be fired is not being fired.

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