Attributes

The Attribute observer is triggered when an attribute is set to a non-nil value.

Observers.observeAttribute(myInstance, "MyAttribute", function(value)
    print("Attribute is", value)

    return function()
        print("Attribute is no longer", value)
    end
end)

Typing

Technically, the value of an attribute is not known (it's not necessarily unknown, as we know it's some sort of attribute value, of which there is a limited amount of types). However, to make the API easier to use, observeAttribute uses a generic value to allow developers to denote what type they expect. For instance, a string:

Observers.observeAttribute(myInstance, "MyAttribute", function(value: string) ... end)

However, be aware that this does not enforce the attribute to be a specific type. Especially in client-facing code, it is good to ensure that the type of the attribute matches what is expected. For instance, if a string is expected:

Observers.observeAttribute(myInstance, "MyAttribute", function(value: string)
    assert(typeof(value) == "string", "expected string for MyAttribute; got " .. typeof(value))
    ...
end)

Guards

A guard is an optional predicate function that can be used to control if the observer function should be triggered for a given value. For instance, this could be used to type-check the value at runtime.

Observers.observeAttribute(
    myInstance,
    "MyAttribute",
    function(value: string)
        print("value is a string", value)
    end,
    function(value: unknown)
        -- Guard function ensures the value is a string.
        -- The observer will only trigger if this returns a truthy value.
        return typeof(value) == "string"
    end
)