Object Interaction

Learning Goals

• Explain the difference between a class and an object
• Write a program where 2 or more classes interact

Technical Vocabulary

• class
• object
• initializer
• property
• action

Creating Objects with Classes

Up until now, we’ve only written classes to the extent that we can print details about an object created from one of those classes. When we build more complex programs, we typically have many classes, and the objects from those classes interact in some way.

Let’s look at an example; step by step. Today we will be talking about items in a pantry, with The Perfect 10 in mind 🍪💗

With your partner:

• Explain what is happening on each line of the following code snippet
• Predict what will print to the console for the three print statements at the bottom

// Pantry.swift

class Pantry {

  var walkIn = false
  var temperature = 0

  init(pantryWalkIn : Bool, pantryTemp : Int) {
    walkIn = pantryWalkIn
    temperature = pantryTemp
  }

}

// main.swift

var pantry = Pantry(pantryWalkIn: true, pantryTemp: 62)

print(pantry.walkIn)
print(pantry.temperature)

As of now, our pantry can’t do much. It just sits there once it’s created with its properties. Let’s add a property where the Pantry can store foods (let’s call it contents), then an action that enables the Pantry to add new foods.

Update Pantry

Let’s update our pantry with that contents property:

class Pantry {

  var walkIn = false
  var temperature = 0
  var contents = [String]()

  init(pantryWalkIn : Bool, pantryTemp : Int) {
    walkIn = pantryWalkIn
    temperature = pantryTemp
  }

}

Since we are starting with an empty array, we know that at some point we are going to add things to it. Think back to our Arrays lesson, we learned about the append method. For our pantry action, it might look like this:

func addFood(food : String) {
  contents.append(food)
}

Build a Second Class

We have a bit going on with our pantry, but no interaction yet. Let’s make a food class, and then we’ll come back and edit our pantry.

class Food {

  var name = ""
  var calories = 0

  init(foodName : String, numberCalories : Int) {
    name = foodName
    calories = numberCalories
  }

}

Object Interaction

Now that we have Food objects with a little more information than a string, we can start making our Pantry and Food objects interact. In the addFood action on our Pantry class, we will actually create a food object with that string that’s passed in. This means we also need to pass in an Int to addFood for the calories. Let’s update that action:

func addFood(food : String, calories : Int) {
    var newFood = Food(foodName: food, numberCalories: calories)
    contents.append(newFood)
}

Notice that the food object is stored in a variable called newFood. On the next line, we are still using the append method, this time appending a food object instead of the food string.

Now that we are passing in a food object instead of a string, we need to update the contents property in our Pantry class to hold an array of Food objects rather than Strings.

var contents = [Food]()

Object Interaction

There are many ways objects can interact; we will mainly work with code that looks like what we did with the pantry and foods today.

To recap:

• We wrote two classes that had a relationship (a pantry has a collection of foods)
• We wrote an action on the pantry that adds foods
• The action takes in any arguments that are required, then creates a food object. It then appends that food object to the pantry’s contents property.