Home > Articles > Programming > General Programming/Other Languages

This chapter is from the book

Classes

Now that you have the beginnings of a robust vector type, let’s put it to work. Your 2D physics simulation will consist of two classes: Particle, which represents a single moving object within the simulation, and Simulation, which contains an array of Particle instances.

Classes are very similar to structures. They have a lot of the same features: initializers, properties, computed properties, and methods. They have a significant difference, however, which we will discuss once the simulation is up and running.

Start by defining the Particle class in your playground. The position is not important, as long as it is above or below (but not inside!) the Vector structure. A Particle has three Vector properties: position, velocity, and acceleration.

struct Vector {
    ...
}

class Particle {

    var position: Vector
    var velocity: Vector
    var acceleration: Vector

}

Classes and structures differ significantly in terms of initializers. Most noticeably, classes do not have automatic initializers, so you will see a compiler error: Class 'Particle' has no initializers.

Fix this by adding an initializer to Particle:

class Particle {

    var position: Vector
    var velocity: Vector
    var acceleration: Vector

    init(position: Vector) {
        self.position = position
        self.velocity = Vector()
        self.acceleration = Vector()
    }
}

You do not need to provide a parameter for every property in a class like you did in Vector’s init(x:y:). You just need to initialize everything. As with initializers for structures, a class’s initializer must initialize all of its properties before returning or performing any other tasks. By requiring this of initializers the Swift compiler guarantees that every instance is fully initialized before it is put to work.

Another approach is to give properties default values:

class Particle {

    var position: Vector
    var velocity: Vector = Vector()
    var acceleration: Vector = Vector()

    init(position: Vector) {
        self.position = position
    }
}

In a simple case like this, there is not a clear benefit to either approach.

Designated and convenience initializers

Like structures, classes can have multiple initializers. At least one of them will be the designated initializer. Remember how you refactored Vector’s init() to call init(x:y:)? A designated initializer is an initializer which other, non-designated initializers – convenience initializers – must call. The rule of thumb with designated initializers is that they are typically the one with the most parameters. Most classes will only have one designated initializer.

The init(position:) initializer is the Particle class’s designated initializer. Add a convenience initializer:

class Particle {
    ...

    init(position: Vector) {
        self.position = position
        self.velocity = Vector()
        self.acceleration = Vector()
    }

    convenience init() {
        self.init(position: Vector())
    }

}

There is an exception to these designated initializer rules: required initializers, which you will see in Chapter 12.

Add an instance method

A particle has a position, velocity, and acceleration. It should also know a little about particle dynamics – specifically, how to update its position and velocity over time. Add an instance method, tick(_:), to perform these calculations.

class Particle {
    ...

    convenience init() {
        self.init(position: Vector())
    }

    func tick(dt: NSTimeInterval) {
        velocity = velocity + acceleration * dt
        position = position + velocity * dt
        position.y = max(0, position.y)
    }

}

The tick(_:) method takes an NSTimeInterval parameter, dt, the number of seconds to simulate. NSTimeInterval is an alias for Double.

Below the definition of Particle, define the Simulation class, which will have an array of Particle objects and its own tick(_:) method:

class Particle {
    ...
}

class Simulation {

    var particles: [Particle] = []
    var time: NSTimeInterval = 0.0

    func addParticle(particle: Particle) {
        particles.append(particle)
    }

    func tick(dt: NSTimeInterval) {
        for particle in particles {
            particle.acceleration = particle.acceleration + gravity
            particle.tick(dt)
            particle.acceleration = Vector()
        }
        time += dt
    }

}

The Simulation class has no initializers defined since all of its properties have default values. The for-in loop iterates over the contents of the particles property. The tick(_:) method applies constant acceleration due to gravity to each of the particles before simulating them for the time interval.

Before you warm up the simulator and add a particle, add a line to evaluate particle.position.y. You will use this shortly with the playground’s Value History. Additionally, add some code to remove particles once they drop below y = 0:

class Simulation {
    ...

    func tick(dt: NSTimeInterval) {
        for particle in particles {
            particle.acceleration = particle.acceleration + gravity
            particle.tick(dt)
            particle.acceleration = Vector()
            particle.position.y
        }
        time += dt
        particles = particles.filter { particle in
            let live = particle.position.y > 0.0
            if !live {
                println("Particle terminated at time \(self.time)")
            }
            return live
        }
    }

}

The last chunk of code filters the particles array, removing any particles that have fallen to the ground. This is a closure, and it is OK if you do not understand it at this point. You will learn more about closures in Chapter 15.

Now you are ready to run the simulator. Create an instance of the simulator and a particle, add the particle to the simulation, and see what happens.

class Simulation {
    ...
}

let simulation = Simulation()

let ball = Particle()
ball.acceleration = Vector(x: 0, y: 100)
simulation.addParticle(ball)

while simulation.particles.count > 0 && simulation.time < 500 {
    simulation.tick(1.0)
}

You should see the playground tally up (20 times) on a number of lines. If the playground runs the simulation continuously, you can stop it by commenting out the while loop. Select the three lines and hit Command-/ to toggle the comment marks:

// while simulation.particles.count > 0 && simulation.time < 500 {
//    simulation.tick(1.0)
// }

Double-check your code against the listings above, in particular the lines that filter the particles array and the line that increments time.

Once you have the simulation running as expected, click the Variables View circle in the playground sidebar on the line that reads particle.position.y, as shown in Figure 3.1. A graph will appear, showing the Y values of the particle over time. The X axis on this graph represents iterations over time and not the X coordinate of the particle.

Figure 3.1

Figure 3.1 Graph data history of particle.position.y

Inheritance

Suppose you wanted to simulate a particle that had different behavior than the Particle class you have already implemented: a rocket that propels itself with thrust over a certain period of time. Since Particle already knows about physics, it would be natural to extend and modify its behavior through subclassing.

Define the Rocket class as a subclass of Particle.

class Rocket: Particle {

    let thrust: Double
    var thrustTimeRemaining: NSTimeInterval
    let direction = Vector(x: 0, y: 1)

    convenience init(thrust: Double, thrustTime: NSTimeInterval) {
        self.init(position: Vector(), thrust: thrust, thrustTime: thrustTime)
    }

    init(position: Vector, thrust: Double, thrustTime: NSTimeInterval) {
        self.thrust = thrust
        self.thrustTimeRemaining = thrustTime
        super.init(position: position)
    }

}

The thrust property represents the magnitude of the rocket’s thrust. thrustTimeRemaining is the number of seconds that the thrust will be applied for. direction is the direction that the thrust will be applied in.

Take a minute to go through the initializers you just typed in. Which is the designated initializer? (Remember the rule of thumb about designated initializers?)

In order to guarantee that a class’s properties are initialized, initializers are only inherited if a subclass does not add any properties needing initialization. Thus, Rocket provides its own initializers and calls the superclass’s designated initializer.

Next you will override the tick(_:) method, which will do a little math to calculate the acceleration due to thrust and apply it before calling the superclass’s – Particle’s – tick(_:) method.

class Rocket: Particle {
    ...

    init(position: Vector, thrust: Double, thrustTime: NSTimeInterval) {
        self.thrust = thrust
        self.thrustTimeRemaining = thrustTime
        super.init(position: position)
    }

    override func tick(dt: NSTimeInterval) {
        if thrustTimeRemaining > 0.0 {
            let thrustTime = min(dt, thrustTimeRemaining)
            let thrustToApply = thrust * thrustTime
            let thrustForce = direction * thrustToApply
            acceleration = acceleration + thrustForce
            thrustTimeRemaining -= thrustTime
        }
        super.tick(dt)
    }

}

Finally, create an instance of Rocket and add it to the simulation in place of the ball:

let simulation = Simulation()

let ball = Particle()
ball.acceleration = Vector(x: 0, y: 100)
simulation.addParticle(ball)
// let ball = Particle()
// ball.acceleration = Vector(x: 0, y: 100)
// simulation.addParticle(ball)

let rocket = Rocket(thrust: 10.0, thrustTime: 60.0)
simulation.addParticle(rocket)

The simulation will run for 70 “seconds” with these parameters. The Value History shows quite a different profile! (Figure 3.2)

Figure 3.2

Figure 3.2 The rocket’s Y position over time

Note that inheritance is one key differentiator between classes and structures: structures do not support inheritance.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020