A view modifier that sets the pivot position when the view is used as a ViewAttachmentEntity in a RealityView

View+AttachmentPivot.swift

//
//  View+AttachmentPivot.swift
//
//  Created by Drew Olbrich on 4/11/24.
//  Copyright © 2024 Lunar Skydiving LLC. All rights reserved.
//
//  MIT License
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

import SwiftUI

extension View {
    
    /// Sets the pivot position when this view is used as a `ViewAttachmentEntity` in a
    /// `RealityView`.
    ///
    /// For example, if `.bottom` is specified, then the view attachment entity's
    /// `position` property will specify the position in space corresponding to the
    /// bottom center of the view.
    ///
    /// In the absence of this view modifier, the default behavior for `RealityView`
    /// attachments is the equivalent of `attachmentPivot(.center)`.
    ///
    /// ## Example Usage:
    ///  
    /// ```
    /// RealityView { content, attachments in
    ///     ...
    /// } attachments: {
    ///     Attachment(id: "my-attachment-id") {
    ///         MyAttachment()
    ///             .attachmentPivot(.bottom)
    ///     }
    /// }
    /// ```
    func attachmentPivot(_ pivotAlignment: Alignment) -> some View {
        return modifier(AttachmentPivotViewModifier(pivotAlignment: pivotAlignment))
    }
    
}

private struct AttachmentPivotViewModifier: ViewModifier {
    
    let pivotAlignment: Alignment

    @Environment(\.layoutDirection) private var layoutDirection
    
    func body(content: Content) -> some View {
        GeometryReader { geometryProxy in
            let offset = offset(for: pivotAlignment, geometryProxy: geometryProxy)
            HStack {
                if pivotAlignment.horizontal != .leading {
                    Spacer(minLength: 0)
                }
                VStack {
                    if pivotAlignment.vertical != .top {
                        Spacer(minLength: 0)
                    }
                    content
                    if pivotAlignment.vertical != .bottom {
                        Spacer(minLength: 0)
                    }
                }
                if pivotAlignment.horizontal != .trailing {
                    Spacer(minLength: 0)
                }
            }
            .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .center)
            .offset(x: offset.x, y: offset.y)
        }
    }
    
    private func offset(for alignment: Alignment, geometryProxy: GeometryProxy) -> CGPoint {
        let x: CGFloat
        let y: CGFloat

        switch alignment.horizontal {
        case .leading:
            switch layoutDirection {
            case .leftToRight:
                x = geometryProxy.size.width/2
            case .rightToLeft:
                x = -geometryProxy.size.width/2
            @unknown default:
                assertionFailure("Unsupported layout direction: \(layoutDirection)")
                x = 0
            }
        case .trailing:
            switch layoutDirection {
            case .leftToRight:
                x = -geometryProxy.size.width/2
            case .rightToLeft:
                x = geometryProxy.size.width/2
            @unknown default:
                assertionFailure("Unsupported layout direction: \(layoutDirection)")
                x = 0
            }
        case .center:
            x = 0
        default:
            assertionFailure("Unsupported horizontal alignment: \(alignment.horizontal)")
            x = 0
        }
        
        switch alignment.vertical {
        case .top:
            y = geometryProxy.size.height/2
        case .bottom:
            y = -geometryProxy.size.height/2
        case .center:
            y = 0
        default:
            assertionFailure("Unsupported vertical alignment: \(alignment.horizontal)")
            y = 0
        }
        
        return CGPoint(x: x, y: y)
    }
    
}