はじめに

SwiftUIでシェア機能を実装する際はUIActivityViewControllerを使用します。
ここにかなり深い落とし穴があるので落ちないように気をつけましょう。
私は落っこちて原因に気づかず長い間、クラッシュを生み出していました笑
みなさんはそうならない様に気をつけましょう。

結論

iPadで普通にUIActivityViewControllerを使うとクラッシュします

パターン1(おすすめ)

import SwiftUI

struct ContentView: View {
    @State var ShowSharePopover: Bool = false
    var body: some View {
        Button("シェア") {
            Share()
        }
        .popover(isPresented: $ShowSharePopover) {
            ShareView()
        }
    }
    // iPhone: ハーフモーダル
    // iPad: ポップアップ
    func Share() {
        if UIDevice.current.userInterfaceIdiom == .pad {
            ShowSharePopover = true
        } else {
            let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
            let activityViewController = UIActivityViewController(activityItems: [link], applicationActivities: nil)
            let scene = UIApplication.shared.connectedScenes.first(where: { $0.activationState == .foregroundActive }) as? UIWindowScene
            let viewController = scene?.keyWindow?.rootViewController
            viewController?.present(activityViewController, animated: true, completion: nil)
        }
    }
}

struct ShareView: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> UIActivityViewController {
        let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
        let activityViewController = UIActivityViewController( activityItems: [link], applicationActivities: nil)
        return activityViewController
    }
    func updateUIViewController(_ vc: UIActivityViewController, context: Context) {
    }
}

iPhone	iPad

パターン2

import SwiftUI

struct ContentView: View {
    @State var ShowShareSheet: Bool = false
    var body: some View {
        Button("シェア") {
            share()
        }
        .sheet(isPresented: $ShowShareSheet) {
            ShareView()
        }
    }
    // iPhone: ハーフモーダル
    // iPad: 中央ポップアップ
    func share() {
        if UIDevice.current.userInterfaceIdiom == .pad {
            ShowShareSheet = true
        } else {
            let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
            let activityViewController = UIActivityViewController(activityItems: [link], applicationActivities: nil)
            let scene = UIApplication.shared.connectedScenes.first(where: { $0.activationState == .foregroundActive }) as? UIWindowScene
            let viewController = scene?.keyWindow?.rootViewController
            viewController?.present(activityViewController, animated: true, completion: nil)
        }
    }
}

struct ShareView: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> UIActivityViewController {
        let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
        let activityViewController = UIActivityViewController( activityItems: [link], applicationActivities: nil)
        return activityViewController
    }
    func updateUIViewController(_ vc: UIActivityViewController, context: Context) {
    }
}

iPhone	iPad

パターン3(使用してはいけない)

import SwiftUI

struct ContentView: View {
    var body: some View {
        Button("シェア") {
            share()
        }
    }
    // iPhone: ハーフモーダル
    // iPad: クラッシュ
    func share() {
        let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
        let activityViewController = UIActivityViewController(activityItems: [link], applicationActivities: nil)
        let scene = UIApplication.shared.connectedScenes.first(where: { $0.activationState == .foregroundActive }) as? UIWindowScene
        let viewController = scene?.keyWindow?.rootViewController
        viewController?.present(activityViewController, animated: true, completion: nil)
    }
}

iPhone	iPad
	クラッシュ

パターン4

import SwiftUI

struct ContentView: View {
    @State var ShowSharePopover: Bool = false
    var body: some View {
        Button("シェア") {
            Share()
        }
        .popover(isPresented: $ShowSharePopover) {
            ShareView()
        }
    }
    // iPhone: モーダル
    // iPad: ポップアップ
    func Share() {
        ShowSharePopover = true
    }
}

struct ShareView: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> UIActivityViewController {
        let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
        let activityViewController = UIActivityViewController( activityItems: [link], applicationActivities: nil)
        return activityViewController
    }
    func updateUIViewController(_ vc: UIActivityViewController, context: Context) {
    }
}

iPhone	iPad

パターン5

import SwiftUI

struct ContentView: View {
    @State var ShowShareSheet: Bool = false
    var body: some View {
        Button("シェア") {
            Share()
        }
        .sheet(isPresented: $ShowShareSheet) {
            ShareView()
        }
    }
    // iPhone: モーダル
    // iPad: 中央ポップアップ
    func Share() {
        ShowShareSheet = true
    }
}

struct ShareView: UIViewControllerRepresentable {
    func makeUIViewController(context: Context) -> UIActivityViewController {
        let link = URL(string: "https://qiita.com/SNQ-2001/items/86646b661ccc4a7a9034")!
        let activityViewController = UIActivityViewController( activityItems: [link], applicationActivities: nil)
        return activityViewController
    }
    func updateUIViewController(_ vc: UIActivityViewController, context: Context) {
    }
}

iPhone	iPad

おわり

今回作成したプロジェクトを置いとくので参考にしてください。