Deleting from a SwiftData query

Paul Hudson    @twostraws    November 19th 2023

We already used @Query to place SwiftData objects into a SwiftUI List, and with only a little more work we can enable both swipe to delete and a dedicated Edit/Done button.

Just as with regular arrays of data, most of the work is done by attaching an onDelete(perform:) modifier to ForEach, but rather than just removing items from an array we instead need to find the requested object in our query then use it to call delete() on our model context. Once all the objects are deleted, SwiftData's autosave system will kick in and apply the changes permanently.

So, start by adding this method to ContentView:

func deleteBooks(at offsets: IndexSet) {
    for offset in offsets {
        // find this book in our query
        let book = books[offset]

        // delete it from the context
        modelContext.delete(book)
    }
}

We can trigger that by adding an onDelete(perform:) modifier to the ForEach of ContentView, but remember: it needs to go on the ForEach and not the List.

Add this modifier now:

.onDelete(perform: deleteBooks)

That gets us swipe to delete, and we can go one better by adding an Edit/Done button too. Find the toolbar() modifier in ContentView, and add another ToolbarItem:

ToolbarItem(placement: .topBarLeading) {
    EditButton()
}

That completes ContentView, so try running the app – you should be able to add and delete books freely now, and can delete by using swipe to delete or using the edit button.

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