# Methods

> Version: *Swift 5.6*\
> Source: [*swift-book: Methods*](https://docs.swift.org/swift-book/LanguageGuide/Methods.html)\
> Digest Date: *March 1, 2022*

* [Methods](#methods)
  * [Overview](#overview)
  * [Instance Methods](#instance-methods)
    * [The self Property](#the-self-property)
    * [Modifying Value Types from Within Instance Methods](#modifying-value-types-from-within-instance-methods)
    * [Assigning to self Within a Mutating Method](#assigning-to-self-within-a-mutating-method)
  * [Type Methods](#type-methods)

## Overview

*Methods* are functions that are associated with a particular type.

* *Classes*, *structures*, and *enumerations* can all define *instance methods*, which encapsulate specific tasks and functionality for working with an instance of a given type.
* *Classes*, *structures*, and *enumerations* can also define *type methods*, which are associated with the type itself. Type methods are similar to *class methods* in Objective-C.

The fact that *structures* and *enumerations* can define methods in Swift is a major difference from C and Objective-C.

* In Objective-C, *classes* are the only types that can define methods.
* In Swift, you can choose whether to define a *class*, *structure*, or *enumeration*, and still have the flexibility to define methods on the type you create.

## Instance Methods

*Instance methods* are functions that belong to instances of a particular *class*, *structure*, or *enumeration*. They support the functionality of those instances, either by providing ways to access and modify *instance properties*, or by providing functionality related to the instance’s purpose. Instance methods have exactly the same syntax as functions, as described in [Functions](https://docs.swift.org/swift-book/LanguageGuide/Functions.html).

* An instance method has implicit access to all other instance methods and properties of that type.
* An instance method can be called only on a specific instance of the type it belongs to. It can’t be called in isolation without an existing instance.

Here’s an example that defines a simple `Counter` class, which can be used to count the number of times an action occurs:

```swift
class Counter {
    var count = 0
    func increment() {
        count += 1
    }
    func increment(by amount: Int) {
        count += amount
    }
    func reset() {
        count = 0
    }
}
```

You call instance methods with the same *dot syntax* as properties:

```swift
let counter = Counter()
// the initial counter value is 0
counter.increment()
// the counter's value is now 1
counter.increment(by: 5)
// the counter's value is now 6
counter.reset()
// the counter's value is now 0
```

*Function parameters* can have both

* a *name* (for use within the function’s body) and
* an *argument label* (for use when calling the function),

as described in [Function Argument Labels and Parameter Names](https://docs.swift.org/swift-book/LanguageGuide/Functions.html#ID166). The same is true for *method parameters*, because **methods are just functions that are associated with a type**.

### The self Property

Every instance of a type has an *implicit property* called `self`, which is exactly equivalent to the instance itself. You use the `self` property to refer to the current instance within its own instance methods.

The `increment()` method in the example above could have been written like this:

```swift
func increment() {
    self.count += 1
}
```

In practice, you don’t need to write `self` in your code very often. If you don’t explicitly write `self`, Swift assumes that you are referring to a *property* or *method* of the current instance whenever you use a known property or method name within a method. This assumption is demonstrated by the use of count (rather than `self.count`) inside the three instance methods for `Counter`.

The main exception to this rule occurs when a parameter name for an instance method has the *same name* as a property of that instance. In this situation, **the parameter name takes precedence**, and it becomes necessary to refer to the property in a more qualified way. You use the `self` property to distinguish between the parameter name and the property name.

Here, `self` disambiguates between a method parameter called `x` and an instance property that’s also called `x`:

```swift
struct Point {
    var x = 0.0, y = 0.0
    func isToTheRightOf(x: Double) -> Bool {
        return self.x > x
    }
}
let somePoint = Point(x: 4.0, y: 5.0)
if somePoint.isToTheRightOf(x: 1.0) {
    print("This point is to the right of the line where x == 1.0")
}
// Prints "This point is to the right of the line where x == 1.0"
```

Without the `self` prefix, Swift would assume that both uses of `x` referred to the *method parameter* called `x`.

### Modifying Value Types from Within Instance Methods

*Structures* and *enumerations* are *value types*. By default, the properties of a value type can’t be modified from within its instance methods.

However, if you need to modify the properties of your *structure* or *enumeration* within a particular method, you can opt in to *mutating* behavior for that method. The method can then mutate (that is, change) its properties from within the method, and any changes that it makes are written back to the original structure when the method ends. The method can also assign a completely new instance to its implicit `self` property, and this new instance will replace the existing one when the method ends.

You can opt in to this behavior by placing the `mutating` keyword before the `func` keyword for that method:

```swift
struct Point {
    var x = 0.0, y = 0.0
    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        x += deltaX
        y += deltaY
    }
}
var somePoint = Point(x: 1.0, y: 1.0)
somePoint.moveBy(x: 2.0, y: 3.0)
print("The point is now at (\(somePoint.x), \(somePoint.y))")
// Prints "The point is now at (3.0, 4.0)"
```

The `Point` structure above defines a mutating `moveBy(x:y:)` method, which moves a `Point` instance by a certain amount. Instead of returning a new point, this method actually modifies the point on which it’s called. The `mutating` keyword is added to its definition to enable it to modify its properties.

Note that you can’t call a mutating method on a *constant* of structure type, because its properties can’t be changed, even if they’re variable properties, as described in [Stored Properties of Constant Structure Instances](https://docs.swift.org/swift-book/LanguageGuide/Properties.html#ID256):

```swift
let fixedPoint = Point(x: 3.0, y: 3.0)
fixedPoint.moveBy(x: 2.0, y: 3.0)
// this will report an error
```

### Assigning to self Within a Mutating Method

Mutating methods can assign an entirely new instance to the implicit `self` property. The `Point` example shown above could have been written in the following way instead:

```swift
struct Point {
    var x = 0.0, y = 0.0
    mutating func moveBy(x deltaX: Double, y deltaY: Double) {
        self = Point(x: x + deltaX, y: y + deltaY)
    }
}
```

This version of the mutating `moveBy(x:y:)` method creates a new structure whose `x` and `y` values are set to the target location. The end result of calling this alternative version of the method will be exactly the same as for calling the earlier version.

Mutating methods for *enumerations* can set the implicit `self` parameter to be a different case from the same enumeration:

```swift
enum TriStateSwitch {
    case off, low, high
    mutating func next() {
        switch self {
        case .off:
            self = .low
        case .low:
            self = .high
        case .high:
            self = .off
        }
    }
}
var ovenLight = TriStateSwitch.low
ovenLight.next()
// ovenLight is now equal to .high
ovenLight.next()
// ovenLight is now equal to .off
```

This example defines an enumeration for a three-state switch. The switch cycles between three different power states (`off`, `low` and `high`) every time its `next()` method is called.

## Type Methods

Instance methods, as described above, are methods that you call on an instance of a particular type. You can also define methods that are called on the type itself.

These kinds of methods are called *type methods*. You indicate type methods by writing the `static` keyword before the method’s `func` keyword. **Classes can use the `class` keyword instead, to allow subclasses to override the superclass’s implementation of that method.**

> **NOTE**: In Objective-C, you can define type-level methods only for Objective-C classes. In Swift, you can define type-level methods for all *classes*, *structures*, and *enumerations*. Each type method is explicitly scoped to the type it supports.

Type methods are called with dot syntax, like instance methods. However, you call type methods on the type, not on an instance of that type. Here’s how you call a type method on a class called `SomeClass`:

```swift
class SomeClass {
    class func someTypeMethod() {
        // type method implementation goes here
    }
}
SomeClass.someTypeMethod()
```

Within the body of a *type method*, the implicit `self` property refers to the type itself, rather than an instance of that type. This means that you can use `self` to disambiguate between *type properties* and *type method parameters*, just as you do for *instance properties* and *instance method parameters*.

More generally, any *unqualified method and property names* that you use within the body of a *type method* will refer to other type-level methods and properties. A *type method* can call another *type method* with the other method’s name, without needing to prefix it with the type name. Similarly, *type methods* on structures and enumerations can access type properties by using the type property’s name without a type name prefix.（本段是说，在类方法中使用本类的其他类方法或类属性时，不需要指定类名，可直接调用）

The example below defines a structure called `LevelTracker`, which tracks a player’s progress through the different levels or stages of a game. It’s a single-player game, but can store information for multiple players on a single device.

All of the game’s levels (apart from level one) are locked when the game is first played. Every time a player finishes a level, that level is unlocked for all players on the device.

The LevelTracker structure uses `type properties and methods` to keep track of which levels of the game have been unlocked. It also tracks the current level for an individual player.

```swift
struct LevelTracker {
    static var highestUnlockedLevel = 1
    var currentLevel = 1

    static func unlock(_ level: Int) {
        if level > highestUnlockedLevel { highestUnlockedLevel = level }
    }

    static func isUnlocked(_ level: Int) -> Bool {
        return level <= highestUnlockedLevel
    }

    @discardableResult
    mutating func advance(to level: Int) -> Bool {
        if LevelTracker.isUnlocked(level) {
            currentLevel = level
            return true
        } else {
            return false
        }
    }
}
```

The `LevelTracker` structure keeps track of the highest level that any player has unlocked. This value is stored in a *type property* called `highestUnlockedLevel`.

`LevelTracker` also defines two *type functions* to work with the `highestUnlockedLevel` property.

* The first is a *type function* called `unlock(_:)`, which updates the value of `highestUnlockedLevel` whenever a new level is unlocked.
* The second is a convenience *type function* called `isUnlocked(_:)`, which returns true if a particular level number is already unlocked.

(Note that these *type methods* can access the `highestUnlockedLevel` type property *without* your needing to write it as `LevelTracker.highestUnlockedLevel`.)

In addition to its *type property* and *type methods*, LevelTracker tracks an individual player’s progress through the game. It uses an instance property called `currentLevel` to track the level that a player is currently playing.

The `advance(to:)` method returns a Boolean value to indicate whether or not it was actually able to set `currentLevel`. Because it’s not necessarily a mistake for code that calls the `advance(to:)` method to ignore the return value, this function is marked with the `@discardableResult` attribute. For more information about this attribute, see [Attributes](https://docs.swift.org/swift-book/ReferenceManual/Attributes.html).

The `LevelTracker` structure is used with the `Player` class, shown below, to track and update the progress of an individual player:

```swift
class Player {
    var tracker = LevelTracker()
    let playerName: String
    func complete(level: Int) {
        LevelTracker.unlock(level + 1)
        tracker.advance(to: level + 1)
    }
    init(name: String) {
        playerName = name
    }
}
```

The `Player` class creates a new instance of `LevelTracker` to track that player’s progress. It also provides a method called `complete(level:)`, which is called whenever a player completes a particular level. This method unlocks the next level for all players and updates the player’s progress to move them to the next level.

You can create an instance of the `Player` class for a new player, and see what happens when the player completes level one:

```swift
var player = Player(name: "Argyrios")
player.complete(level: 1)
print("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)")
// Prints "highest unlocked level is now 2"
```

If you create a second player, whom you try to move to a level that’s not yet unlocked by any player in the game, the attempt to set the player’s current level fails:

```swift
player = Player(name: "Beto")
if player.tracker.advance(to: 6) {
    print("player is now on level 6")
} else {
    print("level 6 hasn't yet been unlocked")
}
// Prints "level 6 hasn't yet been unlocked"
```