Destruction problems in Fusion


Roblox made two frustrating API decisions that are actively hampering Fusion’s ability to provide a clean way of managing destruction.

  1. Roblox APIs, such as instances and event connections, require manual destruction.
  2. Roblox does not provide a viable way to run code when an object is garbage collected.

Luckily, as many high-profile rants have highlighted, Fusion’s finally gotten around to solving half of this problem. With the introduction of destructors to Fusion 0.2, it’s now possible for state objects to specify how contained values should be destroyed, meaning instances and event connections play nicely with state objects now.

The problem is that we only run that destruction code when values are being replaced. When a state object falls out of scope and becomes completely inaccessible, though, we still don’t destroy the value inside the state object. In fact, the entire object gets garbage collected with the live value still inside. For obvious reasons, this is a management issue; if we can’t guarantee that your destructors will run, then the door is wide open for memory leaks.

Raw, unsandboxed Lua 5.1 actually does have a way of supporting exactly this. If you apply a metatable with a __gc method inside, then that __gc method is invoked when a table is garbage collected. This is the key ingredient that lets you build completely garbage-collected APIs, even if they still fundamentally need to talk to non-garbage-collected APIs.

However, for very good security reasons no doubt, Roblox’s sandbox disables this metamethod. Unfortunately though, it’s disablement without replacement, the absolute worst kind of disablement; quite literally the only way you can detect an object has been cleaned up is by polling a weak table. So unless you want a background process running all of the time, scanning your objects for liveness, we simply can’t provide a garbage collected API at any level, because Fusion absolutely must support instances and event connections, and those things don’t garbage collect themselves. We need to run that destruction code manually.

To be fair, we could absolutely optimise that background process. If a state object has been interacted with recently, we know it’s been in scope recently, so we don’t need to check for liveness unless state objects get left alone for a long time. We also don’t have to poll frequently; based on how quickly state objects are being created, we can easily scale up polling when lots of creation or destruction is occuring, and reduce it down to perhaps zero polling for static or idle interfaces. No matter how it’s sliced though, polling sucks and it is to be avoided where possible.

So, what else is there? We could just tell you to figure it out yourself, essentially just lobbing a destroy method in every state object and providing no extra API support at all. This would obviously change how you write components; take this button component, for example:

local function Button(props)
    local isHovering = Value(false)
    local isPressed = Value(false)

    return New "TextButton" {
        -- ... props ...
    }
end)

Perhaps the simplest pattern would be to store all your state objects in a table, and pass that table to a cleanup key. You could do this today if you wanted to:

local function Button(props)
    local state = {
        isHovering = Value(false)
        isPressed = Value(false)
    }

    return New "TextButton" {
        -- ... props ...

        [Cleanup] = function()
            for _, object in state do
                object:destroy()
            end
        end
    }
end)

This has one fatal downside though; every single state object must be directly inside of that state table. This means no more inline computeds in property tables, and no more nesting state objects inside of each other. That’s of particular relevance for users of this common pattern of creating a computed value for the sole purpose of animating it:

local transparency = Spring(Computed(function()
    return if isHovering:get() then 0.5 else 1
end), 25)

Looking at these examples raises an interesting observation though, and brings into focus why garbage collection really was the idiomatic, correct solution here. The thing that matters in these examples is the owner of the state object.

To put it generally, we need some way to distinguish between obtaining a value we own and must destroy, versus obtaining a value someone else owns and we musn’t touch. We don’t want to destroy a global state object when we assign it to our temporary button, but any inline computed objects do need to be destroyed.

Garbage collection is supposed to take care of that! That’s literally its entire value proposition - that you need not think about that at all!

(also coincidentally, Rust solved this problem without a garbage collector by using a borrow checker to allow for really awesome static memory management with no garbage collector or complex destruction process at all, just scoping.)

Anyway, we need to get pragmatic and realistic. The chances of Roblox changing this are close to zero right now, so we need to build something ourselves. Whatever solution we come up with needs to represent ownership of values in some way. Some work I did on ‘managed types’ a while back might be relevant here.

The nice thing is, if we find a solution for the above, it by definition should also be applicable to the prior issues with instances inside of state objects, so we’re virtually guaranteed a one-size-fits-all solution that could replace our current destructor callbacks on state objects, if we find a solution.


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