> ## Documentation Index
> Fetch the complete documentation index at: https://rive.app/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Property

Represents a mutable property that stores a value and supports change listeners.

## Fields

### `value`

The current value of the property.

You can both read and assign to this field.

```lua highlight={4,5} theme={null}
local health = vm:getNumber("health")

if health then
  print(health.value) -- read
  health.value = 100  -- write
end
```

### `addListener`

Registers a listener that is called whenever the property's value changes.

This function has two signatures:

* `addListener(callback)` — Simple form, just pass the callback function.
* `addListener(anchor, callback)` — Pass an anchor object and a callback.
  The anchor is stored alongside the listener and passed to the callback
  when invoked. This is useful for preventing garbage collection of objects
  you need to keep alive while the listener is active.

**Important:** If you obtain a ViewModel or property using a local variable
(e.g., `local vm = context:viewModel()`) and add listeners without storing
the ViewModel elsewhere, it may be garbage collected after the function
returns. To prevent this, either:

* Store the ViewModel on `self` (e.g., `self.vm = context:viewModel()`)
* Pass the ViewModel as the anchor parameter to `addListener`

```lua highlight={8,10} theme={null}
local health = vm:getNumber("health")

if health then
  local function onHealthChanged(prop)
    print("New health:", prop.value)
  end

  health:addListener(onHealthChanged)

  health.value = 50 -- Triggers listener
end
```

Using the anchor parameter to keep the ViewModel alive:

```lua highlight={5} theme={null}
local vm = context:viewModel()
local health = vm:getNumber("health")

if health then
  health:addListener(vm, function(anchor)
    -- 'anchor' is the vm we passed, kept alive by the listener
    print("Health changed!")
  end)
end
```

### `removeListener`

Removes a previously registered listener.

This function has two signatures:

* `removeListener(callback)` — Pass the callback function to remove.
* `removeListener(anchor, callback)` — Pass an anchor and the callback.
  The anchor parameter is accepted for API symmetry with `addListener`,
  but only the callback is used for matching.

```lua highlight={6} theme={null}
local health = vm:getNumber("health")

if health then
  local function onHealthChanged(prop)
    print("New health:", prop.value)
    health:removeListener(onHealthChanged)
  end

  health:addListener(onHealthChanged)

  health.value = 50 -- Triggers listener
end
```