Introduction to Object-Oriented Programming

Transcript

In this demo, you’ll create a subclass of MuseumObject. You’ll move primaryImageSmall to this subclass, then learn how to write the subclass’s init and override showImage().

Open the playground in the starter folder. First of all, notice that MuseumObject is a class, not a struct. Inheritance only works with classes.

You’ll start by creating a PublicDomainObject subclass of MuseumObject. The primaryImageSmall property is a string, and it’s non-empty only for public domain art objects, so it makes sense to remove this property from MuseumObject and have it only in PublicDomainObject.

In MuseumObject, comment out or delete these lines:

let primaryImageSmall: String
primaryImageSmall: String,
self.primaryImageSmall = primaryImageSmall

Scroll down to the creation of a MuseumObject, to fix the error. Comment out or delete this line:

primaryImageSmall: "https://images.metmuseum.org/CRDImages/ep/original/DT1567.jpg",

Then, scroll back to showImage() and modify it to always open SFSafariViewController:

guard let url = URL(string: objectURL) else { return }
PlaygroundPage.current.liveView = SFSafariViewController(url: url)

Now, every instance of MuseumObject shows its image by loading its page in Safari.

There’s another error down here in MuseumObjectView, which you’ll fix after creating your subclass.

And you’re ready to create your subclass:

class PublicDomainObject: MuseumObject {
  
}

This is how you subclass MuseumObject: PublicDomainObject: MuseumObject. In other words, PublicDomainObject is a MuseumObject. It inherits all the properties and methods of MuseumObject, and you can add more properties and methods, or override MuseumObject methods and redefine them to do something different.

Next, add the property you removed from MuseumObject:

let primaryImageSmall: String

Because you’ve added a property to this subclass, you need to define its init method. Start by copying and editing the signature of the init in MuseumObject:

init(objectID: Int,
     title: String,
     objectURL: String,
     primaryImageSmall: String, 
     creditLine: String, = tru
     isPublicDomain: Bool = true) { }   

Uncomment or insert primaryImageSmall: String,, then set the default value of isPublicDomain.

Now, in the init body, set your new property, then call MuseumObject’s init:

self.primaryImageSmall = primaryImageSmall
super.init(objectID: objectID,
           title: title,
           objectURL: objectURL,
           creditLine: creditLine,
           isPublicDomain: isPublicDomain)

And override showImage():

override func showImage() {
  PlaygroundPage.current.setLiveView(MuseumObjectView(object: self))
}

Now, every instance of PublicDomainObject shows its image in MuseumObjectView(object:).

Scroll down to MuseumObjectView and change its parameter to be a PublicDomainObject:

let object: PublicDomainObject

Next, create an instance of your new subclass — copy-paste and edit the MuseumObject declaration:

let object_pd =
PublicDomainObject(objectID: 436535,
                   title: "Wheat Field with Cypresses",
                   objectURL: "https://www.metmuseum.org/art/collection/search/436535",
                   primaryImageSmall: "https://images.metmuseum.org/CRDImages/ep/original/DT1567.jpg"
                   creditLine: "Purchase, The Annenberg Foundation Gift, 1993")

Remember: You set a default value for isPublicDomain, so you don’t have to include it here.

Now, you’ve created the same art object as a MuseumObject and as a PublicDomainObject.

First, call showImage() with the MuseumObject instance:

object.showImage()

It displays the art object’s web page because a MuseumObject always calls SFSafariViewController.

Comment out object.showImage() and call showImage() with your PublicDomainObject:

object_pd.showImage()

As expected, you get the MuseumObjectView, because that’s what the PublicDomainObject method calls.

That ends this demo. Continue with the lesson for a summary.