Custom Attributes in Swift AttributedString and NSAttributedString
In a reasonably sized iOS app, it can often be convenient to add custom attributes to NSAttributedStrings to pass additional metadata for whatever reason associated with a specific part of a string. (Previously in my AttributedString adventures)
This is straightforward with NSAttributedString since its attributes are a dictionary that can hold anything (I’m ignoring serialization here). You add your own key to NSAttributedString.Key, and go:
extension NSAttributedString.Key {
static let userID = NSAttributedString.Key("UserID")
}
let nsAttributedString = NSAttributedString(
string: "Good morning!",
attributes: [.userID: "12345",
.foregroundColor: UIColor.orange,
.link: "https://mastodon.social"])
print(nsAttributedString)
/* Prints
Good morning!{
NSColor = "UIExtendedSRGBColorSpace 1 0.5 0 1";
NSLink = "https://mastodon.social";
UserID = 12345;
}
*/
However, if you need to convert this to the newer Swift-native AttributedString struct for whatever reason (hint: AttributedString is Sendable!), your custom attributes will get lost.
let swiftAttributedString = AttributedString(nsAttributedString)
print(swiftAttributedString)
/* Prints
Good morning! {
NSLink = https://mastodon.social
NSColor = UIExtendedSRGBColorSpace 1 0.5 0 1
}
*/
There is a way to add custom attributes to Swift’s AttributedString, and credit to @toomasvahter for one of the few posts I found that goes in to detail on doing this.
The equivalent to my original example for Swift AttributedString is … this.
It’s pretty verbose, but it’s actually really clever. AttributedString uses a Swift feature called dynamic member lookup to let developers extend its own API by adding new “properties” to it. Notice also how the new properties are strongly typed: the value for link has to be a URL now, when previously I incorrectly used a String:
enum UserIDAttribute: AttributedStringKey {
typealias Value = String
static let name = "UserID"
}
extension AttributeScopes {
public struct MyAttributedStringAttributes: AttributeScope {
let userID: UserIDAttribute
}
var myAttributes: MyAttributedStringAttributes.Type { MyAttributedStringAttributes.self }
}
extension AttributeDynamicLookup {
subscript<T: AttributedStringKey>(dynamicMember keyPath: KeyPath<AttributeScopes.MyAttributedStringAttributes, T>) -> T {
self[T.self]
}
}
var myAttributedString = AttributedString("Good morning Swift!")
myAttributedString.userID = "12345"
myAttributedString.link = URL(string: "https://mastodon.social")
print(myAttributedString)
/* Prints
Good morning Swift! {
UserID = 12345
NSLink = https://mastodon.social
}
*/
Unfortunately, even after all this work, converting between NSAttributedString and AttributedString will lose the custom attribute moving in either direction:
var myAttributedString = AttributedString("Good morning Swift!")
myAttributedString.userID = "12345"
myAttributedString.link = URL(string: "https://mastodon.social")
let newNSAttributedString = NSAttributedString(myAttributedString)
print(newNSAttributedString)
/* Prints
Good morning Swift!{
NSLink = "https://mastodon.social";
}
*/
let nsAttributedString = NSAttributedString(
string: "Good morning!",
attributes: [.userID: "12345",
.link: URL(string: "https://mastodon.social")])
let newSwiftAttributedString = AttributedString(nsAttributedString)
print(newSwiftAttributedString)
/* Prints
Good morning! {
NSLink = https://mastodon.social
}
*/
Here’s the key. Look closely at the AttributedString(_: NSAttributedString) initializer documentation:
This initializer includes all attribute scopes defined by the SDK, such as
AttributeScopes.FoundationAttributes,AttributeScopes.SwiftUIAttributes, andAttributeScopes.AccessibilityAttributes. To use third-party attribute scopes, use the initializersinit(_:including:)orinit(_:including:).
Ahh, I have to pass my attribute scope explicitly!!
Let’s try it:
let nsAttributedString = NSAttributedString(
string: "Good morning!",
attributes: [.userID: "12345",
.link: URL(string: "https://mastodon.social")])
let newSwiftAttributedString = try AttributedString(nsAttributedString,
including: \.myAttributes)
print(newSwiftAttributedString)
/* Prints
Good morning! {
UserID = 12345
}
*/
Well shit. What happened to my link attribute?
Well, it’s not part of the scope I passed. The only thing in my scope is “userID”.
scope
A key path that identifies the attribute scope of the attributes in nsStr. This can be a nested scope that contains several scopes.
What is a nested scope? What does that mean? Finally I track down some notes from WWDC 2021, thanks @mackuba!
However, attribute scopes can be nested in one another, so you can include e.g. a scope of all SwiftUI attributes inside your scope (which in turn includes Foundation attributes)
OK, looking at the example, I literally just include the existing Foundation or UIKit attribute scopes in my own. This seems super weird, but fine.
extension AttributeScopes {
public struct MyAttributedStringAttributes: AttributeScope {
let userID: UserIDAttribute
let uiKit: UIKitAttributes
let foundation: FoundationAttributes
}
var myAttributes: MyAttributedStringAttributes.Type { MyAttributedStringAttributes.self }
}
let nsAttributedString = NSAttributedString(
string: "Good morning NextStep!",
attributes: [.userID: "12345",
.link: URL(string: "https://mastodon.social")!])
let newSwiftAttributedString = try AttributedString(nsAttributedString,
including: \.myAttributes)
print(newSwiftAttributedString)
/* Prints
Good morning NextStep! {
NSLink = https://mastodon.social
UserID = 12345
}
*/
let nsAttributedString2 = try NSAttributedString(newSwiftAttributedString,
including: \.myAttributes)
nsAttributedString2 == nsAttributedString // True!
print(nsAttributedString2)
/* Prints
Good morning NextStep!{
NSLink = "https://mastodon.social";
UserID = 12345;
}
*/
It works! My custom key AND the existing link key transfer in both directions, and the attributed strings come out equal after the conversion.
This was originally posted as a thread on Mastodon, please chime in if you have any feedback!
