Focus Manager on TV platforms

Cell states:

Common cells are available in 2 states:

focused
unfocused

Menu cells are available in 4 states:

unfocused and unselected
focused and unselected
unfocused and selected
focused and selected

In the following diagrams, the components will be marked in blue, and the potential targets of focus movement will be marked in green. See table below.

Normal states	Target states

Directions:

The direction of potential targets areas is calculated from the edge of the component holding the "focused" state. There are 4 directions:

left
up
right
down

Vertical	Horizontal
Up	Right

Down	Left

Tree structure:

All focusable items are kept in a logical tree which makes focus movement between items easier and quicker. Focusable, as well as FocsuableGroups items, are wrapped in nodes. The node contains the following information:

id - for root component, it is root.
component- Focusable or FocusableGroupor null for root.
parent- parent node
children- array of nodes inside of group
lastFocusedItem if this is a group node it might hold id of last selected item in group.

Two nodes inside of the same group will be called siblings. Diagram below shows how components and siblings are structured:

Components in tree	Siblings in tree

Targets:

Targets are siblings of the currently focused component or siblings of the parent in a specified direction. If there are no targets in the current group, the algorithm goes up the tree until it finds a sibling. If a sibling has been found the algorithm will try to find the most accurate focusable item based on distance and priorities.

Siblings found in group:

Siblings found for parent group

No siblings at any level

Focus priorities:

Currently selected item - like a menu item
Previously focused item in the parent group
Preferrable items - items which have been marked as preferable
The first item on the list - if by any accident above priories are not satisfied

Focusable component:

In code, the component which supposes to be focusable will have to be wrapped with Focusable component from package react-native-ui-components and can provide optional selected state. Each focusable component has to have defined unique id as well as it needs to provide groupId. As a developer, you can define your preferable focus by adding preferredFocus. The focused state will be provided as below:

     <Focusable
        id={id}
        groupId={"menu"}
        preferredFocus={index === 0}//optional
        selected={isSelected}>//optional
        {focused => (
            <YourComponent/>
        )}
     </Focusable>

FocusableGroup component:

Each Focusable item has to be placed in FocusableGroup. FocusableGroup can containt either FocusableGroup or Focusable, however all children need to be the same.

     <FocusableGroup
        id={id}
        groupId={"xyz"}//optional
        preferredFocus={index === 0}>//optional
            <Focusable/>
            <Focusable/>
            <Focusable/>
     </FocusableGroup>

Focus events sequence:

Focus events are propagated to two Focusable items, current and next, when moving focus in following sequence:

current.willLoseFocus
if next has been found
- next.willReceiveFocus
- current.blur
- current.hasLostFocus
- next.focus
- next.hasReceivedFocus
else
- current.failedLostFocus

Focus manager on TV platforms