Pass previous value to function when calling :OnUpdate()

[MattSchrubb]

Using :OnUpdate() is great to check when a "key"'s value is changed, but what if we want to know what the previous value was? ie. To check how much the value changed when using Increment.

DataStore2("Gold", player):OnUpdate(function(newVal, prevVal)
    if newVal - prevVal > 10 then
         print("Now that's a lot of gold")
    end
end)

I'm not sure if there is a way to do this, but if not, then adding a way to access previously stored data easily would be the next step.

[MattSchrubb]

I actually did it myself. Only updating the :Set() functions and the :_Update() functions.

function DataStore:_Update(dontCallOnUpdate, prevVal)
    if not dontCallOnUpdate then
        for _,callback in pairs(self.callbacks) do
            callback(self.value, prevVal, self)
        end
    end

    self.haveValue = true
    self.valueUpdated = true
end

function DataStore:Set(value, _dontCallOnUpdate)
    local prevValue = self.value
    self.value = clone(value)
    self:_Update(_dontCallOnUpdate, prevValue)
end

function DataStore:Update(updateFunc)
        local prevVal = self.value
    self.value = updateFunc(self.value)
    self:_Update(nil, prevVal)
end

function CombinedDataStore:Set(value, dontCallOnUpdate)
    local tableResult = self.combinedStore:GetTable({})
    local prevVal = tableResult[self.combinedName]
    tableResult[self.combinedName] = value
    self.combinedStore:Set(tableResult, dontCallOnUpdate)
    self:_Update(dontCallOnUpdate, prevVal)
end

function CombinedDataStore:Update(updateFunc)
    local prevVal = self:Get()
    self:Set(updateFunc(self:Get()))
    self:_Update(nil, prevVal)
end

function CombinedDataStore:_Update(dontCallOnUpdate, prevVal)
    if not dontCallOnUpdate then
        for _, callback in pairs(self.onUpdateCallbacks or {}) do
            callback(self:Get(), prevVal, self)
        end
    end

    self.combinedStore:_Update(true)
end

[Kampfkarren]

Putting the previous value as the second argument is the most ergonomic, but it's a breaking change--we pass the data store as the second parameter. That seems like a mistake in hindsight, but I wonder if anyone relies on that behavior?

[MattSchrubb]

I haven't found a need to use the data store passed, but indeed it would be a breaking change for any game that uses that functionality.

[MattSchrubb]

Putting the previous value as the second argument is the most ergonomic, but it's a breaking change--we pass the data store as the second parameter. That seems like a mistake in hindsight, but I wonder if anyone relies on that behavior?

Can you add a function that would be able to do this? Or add a way to get the previous value?

[Kampfkarren]

I don't think it's that much of an issue to just have it as a last parameter, and if you don't need to use the data store argument to just _ it out.

dataStore:OnUpdate(function(newValue, _, oldValue)

[MattSchrubb]

Is this something that is planned on being added or are we supposed to just put it in ourselves? And if we have to put it in ourselves, is there a way that we can override the functions instead of needing to edit the code for every update?

[Kampfkarren]

You can submit a pull request :)