{"id":9136,"date":"2022-10-10T20:45:25","date_gmt":"2022-10-10T20:45:25","guid":{"rendered":"http:\/\/putridparrot.com\/blog\/?p=9136"},"modified":"2022-10-10T20:45:25","modified_gmt":"2022-10-10T20:45:25","slug":"properties-binding-property-wrappers-and-mvvm-with-swift","status":"publish","type":"post","link":"https:\/\/putridparrot.com\/blog\/properties-binding-property-wrappers-and-mvvm-with-swift\/","title":{"rendered":"Properties, Binding, Property Wrappers and MVVM with Swift"},"content":{"rendered":"

Swift includes properties within the language (see Properties<\/a>), but also supports (what we’d call in C#) property or data binding mechanisms.<\/p>\n

Before we get started, I’m writing this post as a learning exercise, I’d suggest you checkout the references at the bottom of the post for a much more in depth look at this subject.<\/p>\n

If you’re still reading, then let’s get started…<\/p>\n

To begin with we’ll look at how we declare properties in Swift first. For those coming from C# or similar languages, then the simplest stored property looks just like a field<\/p>\n

Property Declarations<\/strong><\/p>\n

\r\nstruct MyModel {\r\n    var id: String;\r\n}\r\n<\/pre>\n
The above declaration is known as a stored property, in that we can store values with the specific instance of the MyModel<\/em> type, this is really the equivalent of having a property with a default getter and setter.<\/p>\n
We can also declare properties as readonly like this<\/p>\n
\r\nstruct MyModel {\r\n    var id: String {\r\n        return "SomeId"\r\n    }\r\n}\r\n<\/pre>\n
Both of the above examples are basically properties using “shorthand syntax”, like C# we can also create properties with get <\/em> and set<\/em> keywords – of course these are useful if the setting of the property (for example) does more than just assign a value, hence we have a form (notice you can use return<\/em> or in these example we’re using implicit return<\/em><\/p>\n
\r\nstruct MyModel {\r\n    private var _id: String = "SomeId"\r\n        \r\n    var id: String {\r\n        get {\r\n            _id\r\n        }\r\n        set {\r\n            _id = newValue\r\n        }\r\n    }\r\n}\r\n<\/pre>\n
Property Observer<\/strong><\/p>\n
Property observers allow us to monitor a property change. For example if you have a struct\/class with the shorthand syntax for a property then we can willSet<\/em> and\/or didSet<\/em> to our property, for example<\/p>\n
\r\nstruct MyModel {\r\n    var id: String = "" {\r\n        willSet(newTitle) {\r\n            print("About to set value")\r\n        }\r\n        didSet {\r\n            if title != oldValue {\r\n                print("Set Value")\r\n            }\r\n        }\r\n    }\r\n}\r\n<\/pre>\n
Now when the property is about to be set, the willSet<\/em> code is called and when a value changes didSet<\/em> will be called. Obviously you wouldn’t want to do anything within these methods that could affect performance.<\/p>\n
Property Wrapper<\/strong><\/p>\n
A Property Wrapper allows us to (well as the name suggests) wrap a property within a type that we can separate the storage of the property from functionality that acts upon that property. For example if our title<\/em> has a length constraint, we might create a property wrapper to implement this functionality<\/p>\n
\r\n@propertyWrapper\r\nstruct LengthConstraint {\r\n    private var value = ""\r\n    var wrappedValue: String {\r\n        get { value }\r\n        set { value = String(newValue.prefix(5)) }\r\n    }\r\n}\r\n<\/pre>\n
Property Binding<\/strong><\/p>\n
So what’s this got to do with binding\/data binding\/property binding? Well we’ve now seen how we declare propeties and can add property wrappers to the property. Within SwiftUI we can use the @Published<\/em> property wrapper to our property which gets observed by a UI element and when the value changes updates the UI or in the case of TextField, when changes are made to the text field it’s placed into our property.<\/p>\n
There are several different property wrappers for this type of functionality<\/p>\n