Stored Properties

Properties

Access stored and computed values that are part of an instance or type.

Properties associate values with a particular class, structure, or enumeration. Stored properties store constant and variable values as part of an instance, whereas computed properties calculate (rather than store) a value. Computed properties are provided by classes, structures, and enumerations. Stored properties are provided only by classes and structures.

Stored and computed properties are usually associated with instances of a particular type. However, properties can also be associated with the type itself. Such properties are known as type properties.

In addition, you can define property observers to monitor changes in a property's value, which you can respond to with custom actions. Property observers can be added to stored properties you define yourself, and also to properties that a subclass inherits from its superclass.

You can also use a property wrapper to reuse code in the getter and setter of multiple properties.

Stored Properties

In its simplest form, a stored property is a constant or variable that's stored as part of an instance of a particular class or structure. Stored properties can be either variable stored properties (introduced by the var keyword) or constant stored properties (introduced by the let keyword).

You can provide a default value for a stored property as part of its definition, as described in doc:Initialization#Default-Property-Values. You can also set and modify the initial value for a stored property during initialization. This is true even for constant stored properties, as described in doc:Initialization#Assigning-Constant-Properties-During-Initialization.

The example below defines a structure called FixedLengthRange, which describes a range of integers whose range length can't be changed after it's created:

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

Instances of FixedLengthRange have a variable stored property called firstValue and a constant stored property called length. In the example above, length is initialized when the new range is created and can't be changed thereafter, because it's a constant property.

Stored Properties of Constant Structure Instances

If you create an instance of a structure and assign that instance to a constant, you can't modify the instance's properties, even if they were declared as variable properties:

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

Because rangeOfFourItems is declared as a constant (with the let keyword), it isn't possible to change its firstValue property, even though firstValue is a variable property.

This behavior is due to structures being value types. When an instance of a value type is marked as a constant, so are all of its properties.

The same isn't true for classes, which are reference types. If you assign an instance of a reference type to a constant, you can still change that instance's variable properties.

Lazy Stored Properties

A lazy stored property is a property whose initial value isn't calculated until the first time it's used. You indicate a lazy stored property by writing the lazy modifier before its declaration.

Note: You must always declare a lazy property as a variable (with the var keyword), because its initial value might not be retrieved until after instance initialization completes. Constant properties must always have a value before initialization completes, and therefore can't be declared as lazy.

Lazy properties are useful when the initial value for a property is dependent on outside factors whose values aren't known until after an instance's initialization is complete. Lazy properties are also useful when the initial value for a property requires complex or computationally expensive setup that shouldn't be performed unless or until it's needed.

The example below uses a lazy stored property to avoid unnecessary initialization of a complex class. This example defines two classes called DataImporter and DataManager, neither of which is shown in full:

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 hasn't yet been created

The DataManager class has a stored property called data, which is initialized with a new, empty array of String values. Although the rest of its functionality isn't shown, the purpose of this DataManager class is to manage and provide access to this array of String data.

Part of the functionality of the DataManager class is the ability to import data from a file. This functionality is provided by the DataImporter class, which is assumed to take a nontrivial amount of time to initialize. This might be because a DataImporter instance needs to open a file and read its contents into memory when the DataImporter instance is initialized.

Because it's possible for a DataManager instance to manage its data without ever importing data from a file, DataManager doesn't create a new DataImporter instance when the DataManager itself is created. Instead, it makes more sense to create the DataImporter instance if and when it's first used.

Because it's marked with the lazy modifier, the DataImporter instance for the importer property is only created when the importer property is first accessed, such as when its filename property is queried:

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

Note: If a property marked with the lazy modifier is accessed by multiple threads simultaneously and the property hasn't yet been initialized, there's no guarantee that the property will be initialized only once.

Stored Properties and Instance Variables

If you have experience with Objective-C, you may know that it provides two ways to store values and references as part of a class instance. In addition to properties, you can use instance variables as a backing store for the values stored in a property.

Swift unifies these concepts into a single property declaration. A Swift property doesn't have a corresponding instance variable, and the backing store for a property isn't accessed directly. This approach avoids confusion about how the value is accessed in different contexts and simplifies the property's declaration into a single, definitive statement. All information about the property --- including its name, type, and memory management characteristics --- is defined in a single location as part of the type's definition.

Computed Properties

In addition to stored properties, classes, structures, and enumerations can define computed properties, which don't actually store a value. Instead, they provide a getter and an optional setter to retrieve and set other properties and values indirectly.

struct Point {
    var x = 0.0, y = 0.0
}
struct Size {
    var width = 0.0, height = 0.0
}
struct Rect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        }
        set(newCenter) {
            origin.x = newCenter.x - (size.width / 2)
            origin.y = newCenter.y - (size.height / 2)
        }
    }
}
var square = Rect(origin: Point(x: 0.0, y: 0.0),
    size: Size(width: 10.0, height: 10.0))
let initialSquareCenter = square.center
// initialSquareCenter is at (5.0, 5.0)
square.center = Point(x: 15.0, y: 15.0)
print("square.origin is now at (\(square.origin.x), \(square.origin.y))")
// Prints "square.origin is now at (10.0, 10.0)".

This example defines three structures for working with geometric shapes:

• Point encapsulates the x- and y-coordinate of a point.
• Size encapsulates a width and a height.
• Rect defines a rectangle by an origin point and a size.

The Rect structure also provides a computed property called center. The current center position of a Rect can always be determined from its origin and size, and so you don't need to store the center point as an explicit Point value. Instead, Rect defines a custom getter and setter for a computed variable called center, to enable you to work with the rectangle's center as if it were a real stored property.

The example above creates a new Rect variable called square. The square variable is initialized with an origin point of (0, 0), and a width and height of 10. This square is represented by the light green square in the diagram below.

The square variable's center property is then accessed through dot syntax (square.center), which causes the getter for center to be called, to retrieve the current property value. Rather than returning an existing value, the getter actually calculates and returns a new Point to represent the center of the square. As can be seen above, the getter correctly returns a center point of (5, 5).

The center property is then set to a new value of (15, 15), which moves the square up and to the right, to the new position shown by the dark green square in the diagram below. Setting the center property calls the setter for center, which modifies the x and y values of the stored origin property, and moves the square to its new position.

Shorthand Setter Declaration

If a computed property's setter doesn't define a name for the new value to be set, a default name of newValue is used. Here's an alternative version of the Rect structure that takes advantage of this shorthand notation:

struct AlternativeRect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            let centerX = origin.x + (size.width / 2)
            let centerY = origin.y + (size.height / 2)
            return Point(x: centerX, y: centerY)
        }
        set {
            origin.x = newValue.x - (size.width / 2)
            origin.y = newValue.y - (size.height / 2)
        }
    }
}

Shorthand Getter Declaration

If the entire body of a getter is a single expression, the getter implicitly returns that expression. Here's another version of the Rect structure that takes advantage of this shorthand notation and the shorthand notation for setters:

struct CompactRect {
    var origin = Point()
    var size = Size()
    var center: Point {
        get {
            Point(x: origin.x + (size.width / 2),
                  y: origin.y + (size.height / 2))
        }
        set {
            origin.x = newValue.x - (size.width / 2)
            origin.y = newValue.y - (size.height / 2)
        }
    }
}

Omitting the return from a getter follows the same rules as omitting return from a function, as described in doc:Functions#Functions-With-an-Implicit-Return.

Read-Only Computed Properties

A computed property with a getter but no setter is known as a read-only computed property. A read-only computed property always returns a value, and can be accessed through dot syntax, but can't be set to a different value.

Note: You must declare computed properties --- including read-only computed properties --- as variable properties with the var keyword, because their value isn't fixed. The let keyword is only used for constant properties, to indicate that their values can't be changed once they're set as part of instance initialization.

You can simplify the declaration of a read-only computed property by removing the get keyword and its braces:

struct Cuboid {
    var width = 0.0, height = 0.0, depth = 0.0
    var volume: Double {
        return width * height * depth
    }
}
let fourByFiveByTwo = Cuboid(width: 4.0, height: 5.0, depth: 2.0)
print("the volume of fourByFiveByTwo is \(fourByFiveByTwo.volume)")
// Prints "the volume of fourByFiveByTwo is 40.0".

This example defines a new structure called Cuboid, which represents a 3D rectangular box with width, height, and depth properties. This structure also has a read-only computed property called volume, which calculates and returns the current volume of the cuboid. It doesn't make sense for volume to be settable, because it would be ambiguous as to which values of width, height, and depth should be used for a particular volume value. Nonetheless, it's useful for a Cuboid to provide a read-only computed property to enable external users to discover its current calculated volume.

Property Observers

Property observers observe and respond to changes in a property's value. Property observers are called every time a property's value is set, even if the new value is the same as the property's current value.

You can add property observers in the following places:

• Stored properties that you define
• Stored properties that you inherit
• Computed properties that you inherit

For an inherited property, you add a property observer by overriding that property in a subclass. For a computed property that you define, use the property's setter to observe and respond to value changes, instead of trying to create an observer. Overriding properties is described in doc:Inheritance#Overriding.

You have the option to define either or both of these observers on a property:

• willSet is called just before the value is stored.
• didSet is called immediately after the new value is stored.

If you implement a willSet observer, it's passed the new property value as a constant parameter. You can specify a name for this parameter as part of your willSet implementation. If you don't write the parameter name and parentheses within your implementation, the parameter is made available with a default parameter name of newValue.

Similarly, if you implement a didSet observer, it's passed a constant parameter containing the old property value. You can name the parameter or use the default parameter name of oldValue. If you assign a value to a property within its own didSet observer, the new value that you assign replaces the one that was just set.

Note: The willSet and didSet observers of superclass properties are called when a property is set in a subclass initializer, after the superclass initializer has been called. They aren't called while a class is setting its own properties in the body of the initializer.

For more information about initializer delegation, see doc:Initialization#Initializer-Delegation-for-Value-Types and doc:Initialization#Initializer-Delegation-for-Class-Types.

Here's an example of willSet and didSet in action. The example below defines a new class called StepCounter, which tracks the total number of steps that a person takes while walking. This class might be used with input data from a pedometer or other step counter to keep track of a person's exercise during their daily routine.

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

The StepCounter class declares a totalSteps property of type Int. This is a stored property with willSet and didSet observers.

The willSet and didSet observers for totalSteps are called whenever the property is assigned a new value. This is true even if the new value is the same as the current value.

This example's willSet observer uses a custom parameter name of newTotalSteps for the upcoming new value. In this example, it simply prints out the value that's about to be set.

The didSet observer is called after the value of totalSteps is updated. It compares the new value of totalSteps against the old value. If the total number of steps has increased, a message is printed to indicate how many new steps have been taken. The didSet observer doesn't provide a custom parameter name for the old value, and the default name of oldValue is used instead.

Note: If you pass a property that has observers to a function as an in-out parameter, the willSet and didSet observers are always called. This is because of the copy-in copy-out memory model for in-out parameters: The value is always written back to the property at the end of the function. For a detailed discussion of the behavior of in-out parameters, see doc:Declarations#In-Out-Parameters.

Property Wrappers

A property wrapper adds a layer of separation between code that manages how a property is stored and the code that defines a property. For example, if you have properties that provide thread-safety checks or store their underlying data in a database, you have to write that code on every property. When you use a property wrapper, you write the management code once when you define the wrapper, and then reuse that management code by applying it to multiple properties.

To define a property wrapper, you make a structure, enumeration, or class that defines a wrappedValue property. In the code below, the TwelveOrLess structure ensures that the value it wraps always contains a number less than or equal to 12. If you ask it to store a larger number, it stores 12 instead.

@propertyWrapper
struct TwelveOrLess {
    private var number = 0
    var wrappedValue: Int {
        get { return number }
        set { number = min(newValue, 12) }
    }
}

The setter ensures that new values are less than or equal to 12, and the getter returns the stored value.

Note: The declaration for number in the example above marks the variable as private, which ensures number is used only in the implementation of TwelveOrLess. Code that's written anywhere else accesses the value using the getter and setter for wrappedValue, and can't use number directly. For information about private, see doc:AccessControl.

You apply a wrapper to a property by writing the wrapper's name before the property as an attribute. Here's a structure that stores a rectangle that uses the TwelveOrLess property wrapper to ensure its dimensions are always 12 or less:

struct SmallRectangle {
    @TwelveOrLess var height: Int
    @TwelveOrLess var width: Int
}

var rectangle = SmallRectangle()
print(rectangle.height)
// Prints "0".

rectangle.height = 10
print(rectangle.height)
// Prints "10".

rectangle.height = 24
print(rectangle.height)
// Prints "12".

The height and width properties get their initial values from the definition of TwelveOrLess, which sets TwelveOrLess.number to zero. The setter in TwelveOrLess treats 10 as a valid value so storing the number 10 in rectangle.height proceeds as written. However, 24 is larger than TwelveOrLess allows, so trying to store 24 end up setting rectangle.height to 12 instead, the largest allowed value.

When you apply a wrapper to a property, the compiler synthesizes code that provides storage for the wrapper and code that provides access to the property through the wrapper. (The property wrapper is responsible for storing the wrapped value, so there's no synthesized code for that.) You could write code that uses the behavior of a property wrapper, without taking advantage of the special attribute syntax. For example, here's a version of SmallRectangle from the previous code listing that wraps its properties in the TwelveOrLess structure explicitly, instead of writing @TwelveOrLess as an attribute:

struct SmallRectangle {
    private var _height = TwelveOrLess()
    private var _width = TwelveOrLess()
    var height: Int {
        get { return _height.wrappedValue }
        set { _height.wrappedValue = newValue }
    }
    var width: Int {
        get { return _width.wrappedValue }
        set { _width.wrappedValue = newValue }
    }
}

The _height and _width properties store an instance of the property wrapper, TwelveOrLess. The getter and setter for height and width wrap access to the wrappedValue property.

Setting Initial Values for Wrapped Properties

The code in the examples above sets the initial value for the wrapped property by giving number an initial value in the definition of TwelveOrLess. Code that uses this property wrapper can't specify a different initial value for a property that's wrapped by TwelveOrLess --- for example, the definition of SmallRectangle can't give height or width initial values. To support setting an initial value or other customization, the property wrapper needs to add an initializer. Here's an expanded version of TwelveOrLess called SmallNumber that defines initializers that set the wrapped and maximum value:

@propertyWrapper
struct SmallNumber {
    private var maximum: Int
    private var number: Int

    var wrappedValue: Int {
        get { return number }
        set { number = min(newValue, maximum) }
    }

    init() {
        maximum = 12
        number = 0
    }
    init(wrappedValue: Int) {
        maximum = 12
        number = min(wrappedValue, maximum)
    }
    init(wrappedValue: Int, maximum: Int) {
        self.maximum = maximum
        number = min(wrappedValue, maximum)
    }
}

The definition of SmallNumber includes three initializers --- init(), init(wrappedValue:), and init(wrappedValue:maximum:) --- which the examples below use to set the wrapped value and the maximum value. For information about initialization and initializer syntax, see doc:Initialization.

When you apply a wrapper to a property and you don't specify an initial value, Swift uses the init() initializer to set up the wrapper. For example:

struct ZeroRectangle {
    @SmallNumber var height: Int
    @SmallNumber var width: Int
}

var zeroRectangle = ZeroRectangle()
print(zeroRectangle.height, zeroRectangle.width)
// Prints "0 0".

The instances of SmallNumber that wrap height and width are created by calling SmallNumber(). The code inside that initializer sets the initial wrapped value and the initial maximum value, using the default values of zero and 12. The property wrapper still provides all of the initial values, like the earlier example that used TwelveOrLess in SmallRectangle. Unlike that example, SmallNumber also supports writing those initial values as part of declaring the property.

When you specify an initial value for the property, Swift uses the init(wrappedValue:) initializer to set up the wrapper. For example:

struct UnitRectangle {
    @SmallNumber var height: Int = 1
    @SmallNumber var width: Int = 1
}

var unitRectangle = UnitRectangle()
print(unitRectangle.height, unitRectangle.width)
// Prints "1 1".