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docs/api/reactivity-core.md

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+# Reactivity API: Core
+
+<br/>
+
+## source()
+
+Creates a new source state with the given value.
+
+- **Type**
+
+    ```lua
+    function source<T>(value: T): (T?) -> T
+    ```
+
+- **Details**
+
+    Calling the returned state with no arguments will return its stored value,
+    calling with arguments will set a new value.
+
+    Reading from the state from within any reactive scope will cause changes
+    to that state to be tracked and anything depending on it to update.
+
+- **Example**
+
+    ```lua
+    local count = source(0)
+
+    count() -- 0
+
+    count(count() + 1) -- 1
+    ```
+
+## watch()
+
+Runs a callback on state change.
+
+- **Type**
+
+    ```lua
+    function watch(callback: () -> ()): Unwatch
+
+    type Unwatch = () -> ()
+    ```
+
+- **Details**
+
+    The callback is ran immediately to determine what states are referenced.
+
+    Any time a state referenced in the callback is changed, the callback will be
+    reran.
+
+    Also returns a function that when called, stops the watcher immediately.
+
+    ::: warning
+    `callback()` cannot yield.
+    :::
+
+- **Example**
+
+    ```lua
+    local state = wrap(1)
+
+    watch(function()
+        print(state.Value)
+    end)
+
+    -- prints 1
+
+    state.Value += 1
+
+    -- prints 2
+    ```
+
+## derive()
+
+Derives a new state from existing states.
+
+- **Type**
+
+    ```lua
+    function derive<T>(source: () -> T): () -> T
+    ```
+
+- **Details**
+
+    The derived state will have its value recalculated when any source state it
+    derives from is updated.
+
+    Anytime its value is recalculated it is also cached, subsequent calls will
+    retun this cached value until it recalculates again.
+
+    Takes a callback that is immediately run to determine what states are being
+    referenced.
+
+    ::: warning
+    `source()` cannot yield.
+    :::
+
+- **Example**
+
+    ```lua
+    local count = wrap(0)
+    local text = derive(function() return `count: {count()}` end)
+
+    text() -- "count: 0"
+
+    count(1)
+
+    text() -- "count: 1"
+    ```
+
+## indexes()
+
+Maps each index in a table to an object.
+
+- **Type**
+
+    ```lua
+    function indexes<KI, VI, VO>(
+        source: () -> Map<KI, VI>,
+        transform: (value: () -> VI, index: KI) -> VO
+    ): Array<VO>
+
+- **Details**
+
+    The transform function is called only ever *once* for each index in the
+    source table. The first argument is a state containing the index's value and
+    the second argument is just the index.
+
+    Anytime a new index is added, the transform function will be called again for
+    that new index.
+
+    Anytime an existing index value changes, the transform function is not rerun,
+    instead the passed state for that index will update, causing anything
+    depending on it to update too.
+
+    Returns a state containing an array of all objects returned by the transform.
+
+    ::: warning
+    `transform()` cannot yield.
+    :::
+
+- **Example**
+
+    The intended purpose of this function is to map each index in a table to
+    a UI element.
+
+    ```lua
+    type Item = {
+        name: string,
+        icon: number
+    }
+
+    local items = source {} :: () -> Array<Item>
+
+    local displays = indexes(numbers, function(item, i)
+        return ItemDisplay {
+            Name = function()
+                return item().name
+            end,
+
+            Image = function()
+                return "rbxassetid://" .. item().icon
+            end,
+
+            LayoutOrder = i
+        }
+    end)
+    ```
+
+## values()
+
+Maps each value in a table to an object.
+
+- **Type**
+
+    ```lua
+    function values<KI, VI, VO>(
+        source: () -> Map<KI, VI>,
+        transform: (value: VI, index: () -> KI) -> VO
+    ): Array<VO>
+
+- **Details**
+
+    The transform function is called only ever *once* for each value in the
+    source table. The first argument is the index's value and
+    the second argument is a state containing the index.
+
+    Anytime a new value is added, the transform function will be called again
+    for that new value.
+
+    Anytime an existing value's index changes, the transform function is not
+    rerun, instead the passed state for that value will update, causing anything
+    depending on it to update too.
+
+    Returns a state containing an array of all objects returned by the transform.
+
+    ::: warning
+    `transform()` cannot yield.
+    :::
+
+- **Example**
+
+    The intended purpose of this function is to map each value in a table to
+    a UI element.
+
+    ```lua
+    type Item = {
+        name: string,
+        icon: number
+    }
+
+    local items = source {} :: () -> Array<Item>
+
+    local displays = values(numbers, function(item, i)
+        return ItemDisplay {
+            Name = item.Name
+
+            Image = "rbxassetid://" .. item.icon,
+
+            LayoutOrder = i
+        }
+    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.
+
+--------------------------------------------------------------------------------