Register changes when an array is mutated

[josegl]

Hi guys.
I would like to be able to detect the modifications that happens when an array is updated using the array methods like splice, pop and so on. This methods mutates the actual array and I'm struggling to be able to detect this changes in the interception of the markModify function through a proxy, because as this functions mutate the array, the current value and the "new" value are the same, so any delta can be detected.

Do you have any idea on how this could be achieved?

[vkarpov15]

This is a bit tricky. Do you need the state of the array before any changes are made?

[josegl]

It would be nice if somehow it would work as the set or $set functions where the arguments are the path being changed and the new value for the given path before applying the new value. However I understand that in this case where the array is mutated, it is very difficult to achieve.

I did a fork where in the implementation of the splice function in /lib/types/array/methods/index.js I included a call to this._markModify() just the line before this one: ret = [].splice.apply(arr, vals); I hoped that doing this the call to markModified would bubble up through the model tree, and eventually It would reach my proxy where I intercept the call and I can mark the given path to check for differences. And this is what happens. However, the problem is that when I check the current and the "new" value the array has not updated yet, so there are not any differences to register. If I mark the path to check later, taking the current value as old_value, and then I wait to the changes to actually apply, then when I compare the old_value with the current value for the given path, the values are the same, because the old_value is a pointer to the array that has been actually changed, so they are the same array, and any change can be register.

However as I write this lines I have had an idea: maybe I could try to create a copy instead of a simple assignation for the given path for the old_value, and mark it to check it later, so in this case when the array is actually modified, then I could compare different objects.