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Value Observers

Observers allow you to detect when values are updated. They receive the new value and the previous value as parameters. A value can have any number of parameters.

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local value = Proton.require("value")
local observer = Proton.require("observer")


local health = value.new(100, "number", "Health")


observer.new(health, function(newHealth, previousHealth)

    if newHealth < previousHealth then
        print("Ow! I took " .. (previousHealth - newHealth) .. " damage!")
    else
        print("I was healed by " .. (newHealth - previousHealth) .. " points!")
    end

end)

health:increment(-10)   -- Ow! I took 10 damage!
health:increment(5)     -- I was healed by 5 points!
health:increment(0)

Observers will only function when the value is changed to a new, unique value. This means that, in both of the following code snippets, the observer will never function.

local name = value.new("John")
name:set("John")

local age = value.new(20, "number")
age:increment(0)

Observer Validation

By default, observers will function any time the value is changed to a new value, regardless of what that value is. If we want to set up a validation check so that we can choose whether or not the observer should function based on the new and previous value, we can pass a third argument to observer.new as a function that returns true when the observer should be triggered.

For instance, here's an example of a two-player game system that notifies the player when it's their turn:

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local turnNumber = value.new(0)


observer.new(
    turnNumber,
    function(turn, prevTurn)
        print("It's your turn!")
    end,
    function(turn, prevTurn)
        return (turn % 2 == 0)
    end
)

turnNumber:increment(1)
turnNumber:increment(1) -- It's your turn!
turnNumber:increment(1)
turnNumber:increment(1) -- It's your turn!
turnNumber:increment(1)
turnNumber:increment(1) -- It's your turn!

Disconnecting Observers

Observers can be disconnected by calling :disconnect().

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local name = value.new("John")
local obs = observer.new(name, function(newName, previousName)
    print("My name changed to " .. newName .. "!")
end)

name:set("Jonas")   -- My name changed to Jonas!

obs:disconnect()

name:set("John")
print(name:get())   -- John

-- The value of "name" was still set to "John", but since the observer has been disconnected, it won't be printed.