如何检查项目是否可见 - SwiftUI ScrollView

Question

Trying to programmatically determine when an item is being displayed on screen in a ScrollView in SwiftUI. I understand that a ScrollView renders at one time rather than rendering as items appear (like in List), but I am constrained to using ScrollView as I have.scrollTo actions.

I also understand that in UIKit with UIScrollView, it is possible to use a CGRectIntersectsRect between the item frame and the ScrollView frame in the UIScrollViewDelegate but I would prefer to find a solution in SwiftUI if possible.

Example code looks like this:

ScrollView {
    ScrollViewReader { action in
        ZStack {
            VStack {
                ForEach(//array of chats) { chat in
                    //chat display bubble
                        .onAppear(perform: {chatsOnScreen.append(chat)})
                }.onReceive(interactionHandler.$activeChat, perform: { _ in
                    //scroll to active chat
                })
            }
        }
    }
}

Ideally, when a user scrolls, it would check which items are on screen and zoom the view to fit the largest item on screen.

Answer 1

When you use VStack in ScrollView all content is created at once at build time, so onAppear does not fit your intention, instead you should use LazyVStack , which will create each subview only before it appears on screen, so onAppear will be called when you expect it.

So it should be like

ScrollViewReader { action in
   ScrollView {
        LazyVStack {                              // << this !!
            ForEach(//array of chats) { chat in
                //chat display bubble
                    .onAppear(perform: {chatsOnScreen.append(chat)})
            }.onReceive(interactionHandler.$activeChat, perform: { _ in
                //scroll to active chat
            })
        }
    }
}