Using ReferenceFileDocument in SwiftUI
Create a SwiftUI document-based app using reference types instead of value types for your document
At WWDC 2020, Apple introduced the creation of document-based apps created entirely in SwiftUI using the new DocumentGroup element. Along with this new addition, Xcode 12 also provides a built-in template for generating a document-based app using SwiftUI using FileDocument. The new template works great out-of-the-box if you are using value types for your document, but what about using reference types?
Luckily, Apple thought of this and also provided us with a protocol for reference typed documents called ReferenceFileDocument but (as of the time of writing this article) have not yet explained its usage. In this article, I will demonstrate how to use ReferenceFileDocument by converting Apple’s provided Xcode template to use reference types.
To start, create a new Xcode project using the Multiplatform -> Document App template type. Open the generated <YourProjectName>Document.swift file and have a look around. As you can see, Xcode has generated a struct for our document that adopts the FileDocument protocol. The first thing we need to do is to change this struct to a class and change the protocol to useReferenceFileDocument:
If you build and run now you will get an error, but we will soon be fixing that problem. To solve the complier issues, next we need to properly conform to the ReferenceFileDocument protocol.
Next, replace the existing write() function with the following:
As you can see the ReferenceFileDocument protocol replaces the write() function with a new version that passes a Snapshot to the function that is provided by our implementation of the snapshot() method. Apple documentation describes Snapshot as:
A type of the document snapshot that can be used for serialization in parallel to the main document being editable.
In simpler terms this is just a “copy” of our current document state that will be written to the file on save. In this example we are simply returning the current value of self.text.
Now there is just one more piece of housekeeping in our document file to update. ReferenceFileDocument conforms to the ObservableObject protocol so in order for our document changes to be observable to SwiftUI we need to add @Published property wrapper to our text variable:
There is just a couple more steps required before we can build and run our project. Navigate over to <YourProjectName>App.swift and replace the body property with the following:
Lastly, we need to change the document variable in ContentView.swift to use @ObservedObject instead of @Binding:
With that last change we are done! Build and run the project and try out your shiny new document-based app using reference types!
Happy coding!
Resources
- Download full source code for this tutorial here.
