Majid's blog about Swift development

Drag and drop in SwiftUI

Another iPadOS feature released in SwiftUI with Xcode 11.4 was the drag and drop. Finally, we can use drag and drop API not only with UIKit but also with SwiftUI. This week we will learn all about drag and drop interactions in SwiftUI.

Basics

Drag and drop interactions allow users to send data between two apps or two scenes of the same app. For example, you can run Safari and Notes apps side-by-side and drag the links from Safari to Notes app. As part of the post, we will build a bookmarking app that uses drag and drop to save or open links stored in the app.

Drag

SwiftUI provides onDrag modifier that allows us to register a closure that will create and return NSItemProvider. NSItemProvider is the class that describes the content and the type of draggable item. Let’s take a look at how we can use onDrag modifier.

struct BookmarksList: View {
    @State private var links: [URL] = [
        URL(string: "https://twitter.com/mecid")!
    ]

    var body: some View {
        List {
            ForEach(links, id: \.self) { url in
                Text(url.absoluteString)
                    .onDrag { NSItemProvider(object: url as NSURL) }
            }
        }
        .navigationBarTitle("Bookmarks")
    }
}

As you can see in the example above, all we need to do is to use onDrag modifier that returns an instance of NSItemProvider. Now you can run the app on the iPad simulator side-by-side with Safari and try to drag and drop the link from our app to Safari.

Drop

Unlike drag operation, drop interaction is a little bit complicated. SwiftUI gave us three overloads of onDrop modifier. The most interesting one is the overload that accepts DropDelegate. DropDelegate is the protocol that we can implement to control drop interaction. Let’s take a look at the quick example.

struct BookmarksDropDelegate: DropDelegate {
    @Binding var bookmarks: [URL]

    func performDrop(info: DropInfo) -> Bool {
        guard info.hasItemsConforming(to: ["public.url"]) else {
            return false
        }

        let items = info.itemProviders(for: ["public.url"])
        for item in items {
            _ = item.loadObject(ofClass: URL.self) { url, _ in
                if let url = url {
                    DispatchQueue.main.async {
                        self.bookmarks.insert(url, at: 0)
                    }
                }
            }
        }

        return true
    }
}

Here we have a BookmarksDropDelegate struct that conforms to DropDelegate protocol. DropDelegate protocol requires us to implement performDrop function. This function should return true whenever the drop succeeded or false if it failed. performDrop function has DropInfo parameter, which provides us the information about items that should be dropped.

Using DropInfo, we can filter items and accept only links. Apple uses Uniform Type Identifiers to identify the type of data. We want to receive only URLs that’s why we use public.link identifier. DropInfo also provides us the location of drop using CGPoint instance.

To learn more about Uniform Type Identifiers, take a look at Apple’s documentation.

DropDelegate also has a few not required functions that we can use to understand when dropping started and ended. Finally, we are ready to register our BookmarksDropDelegate on the List or ForEach to enable drop interaction.

struct BookmarksList: View {
    @State private var links: [URL] = [
        URL(string: "https://twitter.com/mecid")!
    ]

    var body: some View {
        List {
            ForEach(links, id: \.self) { url in
                Text(url.absoluteString)
                    .onDrag { NSItemProvider(object: url as NSURL) }
            }
            .onDrop(
                of: ["public.url"],
                delegate: BookmarksDropDelegate(bookmarks: $links)
            )
        }
        .navigationBarTitle("Bookmarks")
    }

Unfortunately, onDrop modifier doesn’t affect the List or any view that inside a List. It works well with VStack, ScrollView, and other views. But it doesn’t work with the List.

I hope it is a bug, and Apple will fix it soon. We can replace the List with ScrollView to make onDrop modifier working. But in this case, we will lose all the features of the List component.

Insert

Gladly, SwiftUI provides us another modifier that we can use on ForEach view. onInsert modifier looks like a simplified version of onDrop modifier, and it works inside List component. Let’s take a look at the quick usage example of onInsert modifier.

struct BookmarksList: View {
    @State private var links: [URL] = [
        URL(string: "https://twitter.com/mecid")!
    ]

    var body: some View {
        List {
            ForEach(links, id: \.self) { url in
                Text(url.absoluteString)
                    .onDrag { NSItemProvider(object: url as NSURL) }
            }
            .onInsert(of: ["public.url"], perform: drop)
        }
        .navigationBarTitle("Bookmarks")
    }

    private func drop(at index: Int, _ items: [NSItemProvider]) {
        for item in items {
            _ = item.loadObject(ofClass: URL.self) { url, _ in
                DispatchQueue.main.async {
                    url.map { self.links.insert($0, at: index) }
                }
            }
        }
    }
}

As you can see, we use onInsert modifier to accept the insertion of links. We also pass the drop function that handles the process of loading the URL object and insertion.

Conclusion

Today we learned another new feature that released with Xcode 11.4. Please note that all drag and drop related modifiers are available as part of iOS 13.4. I hope you enjoy the post. Feel free to follow me on Twitter and ask your questions related to this post. Thanks for reading, and see you next week!