SWIFTUI How to capture VStack height with geometry (after the VStack was displayed)?

ONLINECAST

2 min read

·

06-10-2024

7

0

Share

SwiftUI: Measuring the Height of a VStack After It's Displayed

Problem: You're working on a SwiftUI app and need to know the exact height of a VStack after it's been rendered on the screen. This is crucial for dynamic layouts, positioning elements based on calculated heights, or even implementing animations based on the VStack's final size. However, SwiftUI doesn't provide an easy way to directly access a VStack's height post-rendering.

Solution: We can leverage the GeometryReader view in SwiftUI to measure the VStack's dimensions after it's displayed. This allows us to access its height and use it dynamically within our layout.

Scenario: Imagine you have a VStack that contains a list of items, and you want to display a scroll bar only if the VStack's content exceeds the available screen space. We can accomplish this by using GeometryReader to capture the VStack's height and compare it to the available screen height.

Original Code:

struct ContentView: View {
    var body: some View {
        VStack {
            Text("Item 1")
            Text("Item 2")
            Text("Item 3")
            // ... more items
        }
        .padding() // Add padding for better visual separation
    }
}

Enhanced Code with GeometryReader:

struct ContentView: View {
    @State private var vStackHeight: CGFloat = 0

    var body: some View {
        VStack {
            Text("Item 1")
            Text("Item 2")
            Text("Item 3")
            // ... more items
        }
        .padding()
        .background(
            GeometryReader { geometry in
                Color.clear // Invisible background
                    .onAppear {
                        VStackHeight = geometry.size.height
                    }
            }
        )
        // Now you can conditionally show the scrollbar based on vStackHeight
        .overlay(
            if vStackHeight > UIScreen.main.bounds.height {
                // Display a scrollbar here
            }
        )
    }
}

Explanation:

1. We create a @State variable vStackHeight to store the calculated height of the VStack.
2. We wrap our VStack with GeometryReader.
3. We use onAppear within GeometryReader to ensure the height is captured after the VStack is rendered.
4. geometry.size.height provides the height of the VStack within the GeometryReader.
5. The VStackHeight variable is updated with this calculated height.
6. We can now use the vStackHeight variable to conditionally display a scrollbar or perform any other action based on the VStack's size.

Additional Notes:

• We use Color.clear as the background color of the GeometryReader to ensure it doesn't interfere with the VStack's appearance.
• You can adjust the onAppear modifier to onPreferenceChange if you want to dynamically update the vStackHeight whenever the VStack's content changes.
• The GeometryReader approach is versatile and can be adapted to measure the dimensions of other SwiftUI views.

In conclusion: Using GeometryReader to capture a VStack's height after it's been rendered opens up possibilities for dynamic layout adjustments and interactive elements in your SwiftUI app. This approach gives you precise control over the layout and allows you to create more responsive user interfaces.