> ## 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.

# PropertyTrigger

Represents a trigger property that can fire events and notify listeners.
Unlike other [Property](/scripting/api-reference/data-value/property) types, a trigger does not store a persistent value. It emits an event when fire() is called.

## Fields

### `addListener`

Registers a listener that is invoked when the trigger fires.

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={5} theme={null}
local vmi = context:viewModel()
if vmi then
  local cannon = vmi:getTrigger('cannon')
  if cannon then
    cannon:addListener(function()
      print("cannon fired!")
    end)

    cannon:fire()
  end
end
```

Using the anchor parameter to keep the ViewModel alive:

```lua highlight={5} theme={null}
local vm = context:viewModel()
if vm then
  local cannon = vm:getTrigger('cannon')
  if cannon then
    cannon:addListener(vm, function(anchor)
      -- 'anchor' is the vm we passed, kept alive by the listener
      print("cannon fired!")
    end)
  end
end
```

### `removeListener`

Removes a previously registered listener.
Always remove listeners when they are no longer needed to avoid leaks.

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={17} theme={null}
function init(self: MyNode, context: Context): boolean
  local vmi = context:viewModel()
  if vmi then
    local cannon = vmi:getTrigger('cannon')
    if cannon then
      self.cannon = cannon
      self.onCannonFired = onCannonFired
      cannon:addListener(onCannonFired)
    end
  end

  return true
end

function removeCannonListener(self: MyNode)
  if self.cannon then
    self.cannon:removeListener(self.onCannonFired)
  end
end
```

## Methods

### `fire`

<div class="signature">
  ```lua theme={null}
  fire() -> ()
  ```
</div>

Fires the trigger and notifies all registered listeners.

```lua highlight={9} theme={null}
local vmi = context:viewModel()
if vmi then
  local cannon = vmi:getTrigger('cannon')
  if cannon then
    cannon:addListener(function()
      print("cannon fired!")
    end)

    cannon:fire()
  end
end
```