Issue

Currently, each child of the LazyColumnFor composable is implicitly wrapped with a key containing the index of the item. Because of this if the order of the items is changed, the children, that got reordered, are needlessly recomposed (this is because the value of the keys have changed). In the extreme case, when an item is inserted at the beginning of the list, the entire subtree of LazyColumnFor get recomposed.

Motivation

Oftentimes the items that are rendered in these lazy composables, have unique ids. Thus, it would be useful if there was a way of specifying the id of the item as a key.

Possible solution

I have thought of several solutions:

Remove the keys and encourage users to wrap each child with a key.
Add an optional function parameter that would compute the value of the key for each item.

Option 1 would function similarly to how it does in React. Not specifying a key could be a warning.

Personally, I like option 2 better. I am not sure how this parameter could be named, but its type could simply be:

(Int, T) -> Any

where Int is the index of the item
T is the type of the item
the return value would be supplied to the key

The default value for the parameter could be defined as { i, _ -> i }. This way, when the parameter is not specified LazyColumnFor (or LazyRowFor) would function exactly as it does now.

Also maybe a better type signature could be T.(Int) -> Any, this way if the user wants to use an id of the item as a key, the function would be as simple as { id } instead of { _, item -> item.id }.

Issue 164901852

Links (4)

Issue metadata