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

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Fields

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value

The current value of the property. You can both read and assign to this field. 
local health = vm:getNumber("health")

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

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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
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: 
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

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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.
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