Swift Properties

swift-properties.swift

/*
 Referenced from Stanford CS193p
 
 #: important
 ##: very important
 
 Properties
 - Stored Properties
 - Property Observers
 - Global and Local Variables
 - #Type Properties
 
 Reference: https://docs.swift.org/swift-book/LanguageGuide/Properties.html
 */

/*
 Stored Properties
 */
struct FixedLengthRange {
    var firstValue: Int
    let length: Int
}
var rangeOfThreeItems = FixedLengthRange(firstValue: 0, length: 3)
// the range represents integer values 0, 1, and 2
rangeOfThreeItems.firstValue = 6
// the range now represents integer values 6, 7, and 8

// Stored Properties of Constant Structure Instances
let rangeOfFourItems = FixedLengthRange(firstValue: 0, length: 4)
// this range represents integer values 0, 1, 2, and 3
// rangeOfFourItems.firstValue = 6
// this will report an error, even though firstValue is a variable property

// Lazy Stored Properties
class DataImporter {
    /*
     DataImporter is a class to import data from an external file.
     The class is assumed to take a nontrivial amount of time to initialize.
     */
    var filename = "data.txt"
    // the DataImporter class would provide data importing functionality here
}

class DataManager {
    lazy var importer = DataImporter()
    var data = [String]()
    // the DataManager class would provide data management functionality here
}

let manager = DataManager()
manager.data.append("Some data")
manager.data.append("Some more data")
// the DataImporter instance for the importer property has not yet been created

print(manager.importer.filename)
// the DataImporter instance for the importer property has now been created
// Prints "data.txt"

// If a property marked with the lazy modifier is accessed by multiple threads simultaneously and the property has not yet been initialized, there is no guarantee that the property will be initialized only once.

/*
 Property Observers
 */
class StepCounter {
    var totalSteps: Int = 0 {
        willSet(newTotalSteps) {
            print("About to set totalSteps to \(newTotalSteps)")
        }
        didSet {
            if totalSteps > oldValue  {
                print("Added \(totalSteps - oldValue) steps")
            }
        }
    }
}
let stepCounter = StepCounter()
stepCounter.totalSteps = 200
// About to set totalSteps to 200
// Added 200 steps
stepCounter.totalSteps = 360
// About to set totalSteps to 360
// Added 160 steps
stepCounter.totalSteps = 896
// About to set totalSteps to 896
// Added 536 steps

/*
 #Type Properties
 */

// Type Property Syntax
struct SomeStructure {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 1
    }
}
class SomeClass {
    static var storedTypeProperty = "Some value."
    static var computedTypeProperty: Int {
        return 27
    }
    class var overrideableComputedTypeProperty: Int {
        return 107
    }
}

// Querying and Setting Type Properties
print(SomeStructure.storedTypeProperty)
// Prints "Some value."
SomeStructure.storedTypeProperty = "Another value."
print(SomeStructure.storedTypeProperty)
// Prints "Another value."
print(SomeClass.computedTypeProperty)
// Prints "27"