# Methods
---
*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](206_Functions.html "Functions").

You write an instance method within the opening and closing braces of the type it belongs to. 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 cannot 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++
    }
    func incrementBy(amount: Int) {
        count += amount
    }
    func reset() {
        count = 0
    }
}
```

The `Counter` class defines three instance methods:

- `increment` increments the counter by `1`.
- `incrementBy(amount: Int)` increments the counter by an specified integer amount.
- `reset` resets the counter to zero.

The `Counter` class also declares a variable property, `count`, to keep track of the current counter value.

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.incrementBy(5)
// the counter's value is now 6
counter.reset()
// the counter's value is now 0
```

### Local and External Parameter Names for Methods

Function parameters can have both a local name (for use within the function’s body) and an external name (for use when calling the function), as described in [External Parameter Names](206_Functions.html#external-parameter-names "External Parameter Names"). The same is true for method parameters, because methods are just functions that are associated with a type. However, the default behavior of local names and external names is different for functions and methods.

Methods in Swift are very similar to their counterparts in Objective-C. As in Objective-C, the name of a method in Swift typically refers to the method’s first parameter using a preposition such as `with`, `for`, or `by`, as seen in the `incrementBy` method from the preceding `Counter` class example. The use of a preposition enables the method to be read as a sentence when it is called. Swift makes this established method naming convention easy to write by using a different default approach for method parameters than it uses for function parameters.

Specifically, Swift gives the *first* parameter name in a method a local parameter name by default, and gives the second and subsequent parameter names both local *and* external parameter names by default. This convention matches the typical naming and calling convention you will be familiar with from writing Objective-C methods, and makes for expressive method calls without the need to qualify your parameter names.

Consider this alternative version of the `Counter` class, which defines a more complex form of the `incrementBy` method:

```swfit
class Counter {
    var count: Int = 0
    func incrementBy(amount: Int, numberOfTimes: Int) {
        count += amount * numberOfTimes
    }
}
```

This `incrementBy` method has two parameters—`amount` and `numberOfTimes`. By default, Swift treats `amount` as a local name only, but treats `numberOfTimes` as both a local *and* an external name. You call the method as follows:

```swfit
let counter = Counter()
counter.incrementBy(5, numberOfTimes: 3)
// counter value is now 15
```

You don’t need to define an external parameter name for the first argument value, because its purpose is clear from the function name `incrementBy`. The second argument, however, is qualified by an external parameter name to make its purpose clear when the method is called.

This default behavior effectively treats the method as if you had written a hash symbol (#) before the `numberOfTimes` parameter:

```swfit
func incrementBy(amount: Int, #numberOfTimes: Int) {
    count += amount * numberOfTimes
}
```

The default behavior described above mean that method definitions in Swift are written with the same grammatical style as Objective-C, and are called in a natural, expressive way.

### Modifying External Parameter Name Behavior for Methods

Sometimes it’s useful to provide an external parameter name for a method’s first parameter, even though this is not the default behavior. You can either add an explicit external name yourself, or you can prefix the first parameter’s name with a hash symbol to use the local name as an external name too.

Conversely, if you do not want to provide an external name for the second or subsequent parameter of a method, override the default behavior by using an underscore character (`_`) as an explicit external parameter name for that parameter.

### The self Property

Every instance of a type has an implicit property called `self`, which is exactly equivalent to the instance itself. You use this implicit `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:

```swfit
func increment() {
    self.count++
}
```

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 implicit `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 is also called `x`:

```swfit
struct Point {
    var x = 0.0, y = 0.0
    func isToTheRightOfX(x: Double) -> Bool {
        return self.x > x
    }
}
let somePoint = Point(x: 4.0, y: 5.0)
if somePoint.isToTheRightOfX(1.0) {
    println("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 cannot 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:

```swfit
struct Point {
    var x = 0.0, y = 0.0
    mutating func moveByX(deltaX: Double, y deltaY: Double) {
        x += deltaX
        y += deltaY
    }
}
var somePoint = Point(x: 1.0, y: 1.0)
somePoint.moveByX(2.0, y: 3.0)
println("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 `moveByX` 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 is called. The `mutating` keyword is added to its definition to enable it to modify its properties.

Note that you cannot call a mutating method on a constant of structure type, because its properties cannot be changed, even if they are variable properties, as described in [Stored Properties of Constant Structure Instances](210_Properties.html#stored-properties-of-constant-structure-instances "Stored Properties of Constant Structure Instances"):

```swfit
let fixedPoint = Point(x: 3.0, y: 3.0)
fixedPoint.moveByX(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:

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

This version of the mutating `moveByX` method creates a brand 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 member from the same enumeration:

```swfit
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 are called 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 for classes by writing the keyword `class` before the method’s `func` keyword, and type methods for structures and enumerations by writing the keyword `static` before the method’s `func` keyword.

> **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`:

```swfit
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. For structures and enumerations, this means that you can use `self` to disambiguate between static properties and static 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 static properties by using the static property’s name without a type name prefix.

The example below defines a structure called `LevelTracker`, which tracks a player’s progress through the different levels or stages of a game. It is 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 static 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.

```swfit
struct LevelTracker {
    static var highestUnlockedLevel = 1
    static func unlockLevel(level: Int) {
        if level > highestUnlockedLevel { highestUnlockedLevel = level }
    }
    static func levelIsUnlocked(level: Int) -> Bool {
        return level <= highestUnlockedLevel
    }
    var currentLevel = 1
    mutating func advanceToLevel(level: Int) -> Bool {
        if LevelTracker.levelIsUnlocked(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 static property called `highestUnlockedLevel`.

`LevelTracker` also defines two type functions to work with the `highestUnlockedLevel` property. The first is a type function called `unlockLevel`, which updates the value of `highestUnlockedLevel` whenever a new level is unlocked. The second is a convenience type function called `levelIsUnlocked`, which returns `true` if a particular level number is already unlocked. (Note that these type methods can access the `highestUnlockedLevel` static property without your needing to write it as `LevelTracker.highestUnlockedLevel`.)

In addition to its static 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.

To help manage the `currentLevel` property, `LevelTracker` defines an instance method called `advanceToLevel`. Before updating `currentLevel`, this method checks whether the requested new level is already unlocked. The `advanceToLevel` method returns a Boolean value to indicate whether or not it was actually able to set `currentLevel`.

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

```swfit
class Player {
    var tracker = LevelTracker()
    let playerName: String
    func completedLevel(level: Int) {
        LevelTracker.unlockLevel(level + 1)
        tracker.advanceToLevel(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 `completedLevel`, 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. (The Boolean return value of `advanceToLevel` is ignored, because the level is known to have been unlocked by the call to `LevelTracker.unlockLevel` on the previous line.)

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

```swfit
var player = Player(name: "Argyrios")
player.completedLevel(1)
println("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 is not yet unlocked by any player in the game, the attempt to set the player’s current level fails:

```swfit
player = Player(name: "Beto")
if player.tracker.advanceToLevel(6) {
    println("player is now on level 6")
} else {
    println("level 6 has not yet been unlocked")
}
// prints "level 6 has not yet been unlocked"
```