vide/docs/api/reactivity-flow.md

Reactivity API: Control Flow

show()

Shows one of two components depending on an input source.

• Type

  function show<T>(source: () -> unknown, component: () -> T): () -> T?
  function show<T, U>(source: () -> unknown, component: () -> T, fallback: () -> U): () -> T | U
  

• Details

  Returns a source holding an instance of the currently shown component.

  When the input source changes from a falsey to a truthy value, the component will be reran under a new reactive scope. If it changes from a truthy to falsey value, the reactive scope the component was created in will be destroyed, and the returned source will output nil, or a fallback component if given.

  The fallback component is also ran under a new reactive scope, and destroyed when the input source switches back to truthy.

switch()

Shows one of a set of components depending on an input source and a mapping table.

• Type

  function switch<K, V>(source: () -> K): (map: Map<K, () -> V>) -> V?
  

• Details

  Returns a source holding an instance of the currently shown component.

  When the input source changes, the new value will be used to lookup a given mapping table to get a component, which will be ran under a new reactive scope. If the input source changes, the reactive scope the component was created in will be destroyed, and a new component created under a new reactive scope. If no component is found for an input value, the switch will output nil.

• Example

  local logged = source(false)
  
  local button = switch(logged) {
      [true] = function()
          return Button { Text = "Log out", Toggle = logged }
      end,
  
      [false] = function()
          return Button { Text = "Log in",  Toggle = logged }
      end
  }
  

indexes()

Maps each index in a table source to an object.

• Type

  function indexes<KI, VI, VO>(
      source: () -> Map<KI, VI>,
      transform: (value: () -> VI, index: KI) -> VO
  ): Array<VO>
  
  

• Details

  Returns a source holding an array of instances currently shown.

  When the input source changes, each index in the new table is compared with the last input table.

  • For any new index, the transform function is ran under a new reactive scope to produce a new instance.
  • For any removed index, the reactive scope for that index is destroyed.
  • Unchanged indexes are untouched.

  The transform function is called only ever once for each index in the source table.

  1. First argument is a source containing the index's value.
  2. Second argument is the index itself.

  Anytime an existing index's value changes, the transform function is not rerun, instead the source value for that index will update, causing anything depending on it to update too.

• Example

  The intended purpose of this function is to map each index in a table to a UI element.

  type Item = {
      name: string,
      icon: number
  }
  
  local items = source {} :: () -> Array<Item>
  
  local displays = indexes(items, function(item, i)
      return ItemDisplay {
          Name = function()
              return i .. ": " .. item().name
          end,
  
          Image = function()
              return "rbxassetid://" .. item().icon
          end,
      }
  end)
  

values()

Maps each value in a table source to an object.

• Type

  function values<KI, VI, VO>(
      source: () -> Map<KI, VI>,
      transform: (value: VI, index: () -> KI) -> VO
  ): Array<VO>
  
  

• Details

  Returns a source holding an array of instances currently shown.

  When the input source changes, each value in the new table is compared with the last input table. Similar to indexes() but for values instead of indexes.

  • For any new value, the transform function is ran under a new reactive scope to produce a new instance.
  • For any removed value, the reactive scope for that value is destroyed.
  • Unchanged values are untouched.

  The transform function is only ever called once for each value in the source table.

  1. First argument is the value itself.
  2. Second argument is a source containing the value's index.

  Anytime an existing value's index changes, the transform function is not rerun, instead the source index for that value will update, causing anything depending on it to update too.

  ::: warning Having primitive values in the input source table can cause unexpected behavior, as duplicate values can result in multiple tranforms being ran for a single value, meaning there can be multiple source indexes bound to the same UI element. Strict mode has checks for this. :::

• Example

  The intended purpose of this function is to map each value in a table to a UI element.

  type Item = {
      name: string,
      icon: number
  }
  
  local items = source {} :: () -> Array<Item>
  
  local displays = values(items, function(item, i)
      return ItemDisplay {
          Name = function()
              return i() .. ": " .. item.Name
          end
  
          Image = "rbxassetid://" .. item.icon,
      }
  end)
  

• Extra

  When should you use indexes() and values()?

  values() should be used when you have a fixed set of objects where the same objects can be re-arranged in the source table. It maps a value to a UI element.

  e.g.

  • List of all players.
  • Inventory of items.
  • Chat message history.
  • Toast notifications.

  indexes() should be used in other cases, especially when your source table has primitive value. It maps an index to a UI element.

  e.g.

  • List of character or weapon stats.

  In most cases, both functions will appear to have the same behavior. The main difference is performance, picking the right function to use can result in less property updates and less re-renders. One case to note is that values() works nicely when animating re-ordering of instances, since the value is not destroyed when indexes are changed, and the source index can easily be put through a spring.

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