LastAtBottomVStack.swift

//
//  LastAtBottomVStack.swift
//
//
//  Created by Satish Vekariya on 21/04/2023.
//

import SwiftUI

// MARK: - LastAtBottomVStack
/// A view that arranges its subviews in a vertical line and place last view at bottom or at last position based on given `viewPortSize`.
///
/// Recommendation: Embed this layout inside `GeometryReader` & `ScrollView` like:
///
/// ```
/// GeometryReader { geo in
///    ScrollView {
///        LastAtBottomVStack(viewPortSize: geo.size) {
///            Rectangle().fill(.red)
///                .frame(height: 150)
///            Rectangle().fill(.blue)
///                .frame(height: 130)
///            Rectangle().fill(.gray) /// Will stay at bottom or last
///                .frame(height: 200)
///        }
///    }
/// }
/// ```
///
public struct LastAtBottomVStack: Layout {
    public let viewPortSize: CGSize

    public init(viewPortSize: CGSize) {
        self.viewPortSize = viewPortSize
    }

    public func sizeThatFits(
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout ()
    )
        -> CGSize {
        .init(
            /// View's width
            width: viewPortSize.width,
            /// Calculate total height required for subviews and pick max value from total heigh and view port height
            height: max(viewPortSize.height, subviews.reduce(0) { $0 + $1.dimensions(in: proposal).height })
        )
    }

    public func placeSubviews(
        in bounds: CGRect,
        proposal: ProposedViewSize,
        subviews: Subviews,
        cache: inout ()
    ) {
        var occupiedHeight = bounds.origin.y // Set initial value

        for view in subviews {
            /// This view height
            let thisViewHeight = view.dimensions(in: proposal).height
            /// Check, can place view at bottom within bound
            let canPlaceLastViewAtBottom = thisViewHeight + occupiedHeight >= bounds.height

            let yPosition: CGFloat

            if subviews.count > 1 && view == subviews.last && !canPlaceLastViewAtBottom {
                /// Set bottom position within bounds
                yPosition = bounds.height - thisViewHeight
            } else {
                /// Set last position irrespective to bounds
                yPosition = occupiedHeight
            }
            /// Set view's position
            view.place(at: .init(x: bounds.midX, y: yPosition), anchor: .top, proposal: proposal)

            /// Increment `occupiedHeight` value
            occupiedHeight += thisViewHeight
        }
    }
}

// MARK: - LastAtBottomVStack_Previews
struct LastAtBottomVStack_Previews: PreviewProvider {
    static var previews: some View {
        GeometryReader { geo in
            ScrollView {
                LastAtBottomVStack(viewPortSize: geo.size) {
                    Rectangle().fill(.red)
                        .frame(height: 150)
                    Rectangle().fill(.blue)
                        .frame(height: 130)
                    Rectangle().fill(.gray) /// Will stay at bottom or last
                        .frame(height: 200)
                }
            }
        }
    }
}