Property wrappers allow property declaration to state what kind of property wrapper is used for implementing the property. We can use it for implementing transformations on properties like validating if string is email or not. This is what we will do: creating a property wrapper for email properties and validating emails using NSDataDetector. If value being set is email, we store it and if it is not, we set the property to nil instead.
Creating property wrapper
Property wrappers are types annotated with @propertyWrapper. The type needs to implement one property: wrappedValue. Emails are represented with strings, therefore our wrappedValue property is optional string. Optional is required, because string can contain invalid email and in that case we set the property to nil. Whenever we would like to use this property wrapper, we just need to add @EmailValidated in front of the property definition.
Validating email using NSDataDetector
Validating emails using regular expressions is not easy. Fortunately Apple provides API exactly for this: NSDataDetector. We can create an instance of NSDataDetector with specifying link as detected types. When matching emails, we use anchored option as we expect the string to only include email, nothing else. Anchored will tell the data detector to match starting with the first character. As firstMatch(in:options:range:) uses NSRange, we need to convert Swift’s range to NSRange because those ranges do not have one-to-one match. For this, we can use special NSRange initialiser taking Swift string and its range.
NSDataDetector represents links with URL, therefore we will see if match contains an URL and if URL’s scheme is mailto. If it is, we can extract the matched email and return it.
Using EmailValidated property wrapper
For using the created property wrapper, all we need to do is to annotate property with @EmailValidated.
We created a simple property wrapper for validating emails. We saw that creating a property wrapper for validating email with NSDataDetector requires only a little bit of code.
EmailPropertyWrapper playground (Xcode 11b5)