Swift: array[0] vs array.first. Why they behawe differently?

Question

I have ScrollView in LazyVGrid. Inside there is forEach cycle create views that appear. User can create new item in this scrollView. When new view is created using ScrollViewReader I observe change of custom Bool property that toggled and scroll view to top using

.onChange(of: boolProperty) { _ in
                    withAnimation {
                        reader.scrollTo(viewModel.items.[0].id, anchor: .top)
                    }
                }

and everything works fine, but if i change [0] to .first like this

.onChange(of: boolProperty) { _ in
                    withAnimation {
                        reader.scrollTo(viewModel.items.first?.id, anchor: .top)
                    }
                }

behavior isn't the same and view scrolls only a little bit and not to the top

Sweeper · Accepted Answer · 2023-12-15 10:07:51Z

3

first returns an optional type (returns nil when the array is empty), so viewModel.items.first?.id is of type ID?, where ID is the type of id.

On the other hand, viewModel.items.[0].id is of the non-optional type ID. The subscript returns a non-optional type, and crashes when the array is empty.

scrollTo scrolls to a view in the scroll view, that has the id you specified. Your views probably has non-optional IDs, so passing a value of an optional type will never match any view you have, and therefore doesn't scroll to the desired view.

answered Dec 15, 2023 at 10:07

Sweeper

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3 Comments

JanMensch Over a year ago

Small addition, just to be on the very safe side: when using items[0] you should wrap the code in if !items.isEmpty { ... }. Using .first (as it's an Optional) will make sure the code will not crash. If you blindly get items[0], your app will crash if the items array is empty.

Alejandro Iván Over a year ago

What JanMensch could be extended to any index really, for example you could define extension Array { func element(at index: Index) -> Element? { indices.contains(index) ? self[index] : nil } } or similar. An actual Element if the index exists, or nil if it doesn't, effectively working similar to first, last, etc.

Alejandro Iván Over a year ago

Or using a subscript like this one.

sonle · Accepted Answer · 2023-12-15 10:05:24Z

1

It's different, items[0].id is non-optinal, that's, i.e, an Int. And items.first?.id is opposite, Optional<Int>. Basically, it won't match that's why scroll does not work. Try this:

withAnimation {
    if let firstID = viewModel.items.first?.id {
        reader.scrollTo(firstID, anchor: .top)
    }
}

answered Dec 15, 2023 at 10:05

sonle

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2 Comments

synecdochenoire Over a year ago

thank you, solution works but i was looking to more clarifying answer like answer above from Sweeper

sonle Over a year ago

It's ok, you got the idea.

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