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Watchers

A watcher reacts to one specific signal or computed value.

It is similar to connecting to a Roblox signal with :Connect(), except the thing being watched is state, not an event.

Key Point

Watchers are useful for reacting to one signal or computed value.

Unlike effect, a watcher does not automatically track everything it reads. You choose exactly what value it should watch.

local coins, setCoins = Reheaven.signal(100)

local stop = Reheaven.watch(coins, function(value, previous)
    print("Coins changed:", previous, "->", value)
end)

Watch Does Not Run Immediately​

watch does not call the callback when it is created.

It waits until the watched value changes.

local coins, setCoins = Reheaven.signal(100)

Reheaven.watch(coins, function(value, previous)
    print(value, previous)
end)

-- Nothing prints yet.

setCoins(150)

-- 150 100

Previous Value​

The callback receives the new value first, then the previous value.

Reheaven.watch(coins, function(value, previous)
    print("New:", value)
    print("Old:", previous)
end)

Watching Computeds​

Watchers can also observe computed values.

local canBuyItem = Reheaven.computed(function()
    return coins() >= 50
end)

local stop = Reheaven.watch(canBuyItem, function(value, previous)
    print("Can buy changed:", previous, "->", value)
end)

Stopping a Watcher​

watch returns a cleanup function.

Call it when you no longer want updates.

local stop = Reheaven.watch(coins, function(value)
    print(value)
end)

stop()

Watch vs Effect​

Use watch when you want to react to one specific signal or computed value.

Use effect when the function reads state directly and you want Reheaven to track it automatically.

Simple Rule

Use watch when you already know what to watch.

Use effect when you want Reheaven to figure it out from the values being read.

Next Steps​

Next, learn how batch groups multiple updates together.