Creating a custom activity indicator in Swift allows you to tailor the appearance and behavior of your loading spinner to fit the style of your app. Here's a step-by-step guide to creating a simple custom activity indicator using UIView Step 1: Create a New Swift File for the Custom Activity Indicator Create a new Swift file and name it RotatingCirclesView.swift . Add the following code to define a custom UIView subclass for your activity indicator: // // RotatingCirclesView.swift // Welcome In // // Created by Praveen Kumar on 05/09/24. // import UIKit class RotatingCirclesView : UIView { let circle1 = UIView ( frame : CGRect ( x : 20 , y : 20 , width : 60 , height : 60 )) let circle2 = UIView ( frame : CGRect ( x : 120 , y : 20 , width : 60 , height : 60 )) let position : [ CGRect ] = [ CGRect ( x : 30 , y : 20 , width : 60 , height : 60 ), CGRect ( x : 60 , y : 15 ,...
Swift 4.0 Tutorial
Tradition suggests that the first program in
a new language should print the words “Hello, world!” on the screen. In Swift,
this can be done in a single line:
print("Hello, world!")
If you have written code in C or Objective-C,
this syntax looks familiar to you—in Swift, this line of code is a complete
program. You don’t need to import a separate library for functionality like
input/output or string handling. Code written at global scope is used as the
entry point for the program, so you don’t need a main() function. You also
don’t need to write semicolons at the end of every statement.
This tour gives you enough information to
start writing code in Swift by showing you how to accomplish a variety of
programming tasks. Don’t worry if you don’t understand something—everything
introduced in this tour is explained in detail in the rest of this book.
NOTE
For the best experience, open this chapter as
a playground in Xcode. Playgrounds allow you to edit the code listings and see
the result immediately.
Simple Values
Use let to make a constant and var to make a
variable. The value of a constant doesn’t need to be known at compile time, but
you must assign it a value exactly once. This means you can use constants to
name a value that you determine once but use in many places.
var myVariable = 42
myVariable = 50
let myConstant = 42
A constant or variable must have the same
type as the value you want to assign to it. However, you don’t always have to
write the type explicitly. Providing a value when you create a constant or
variable lets the compiler infer its type. In the example above, the compiler
infers that myVariable is an integer because its initial value is an integer.
If the initial value doesn’t provide enough
information (or if there is no initial value), specify the type by writing it
after the variable, separated by a colon.
let implicitInteger = 70
let implicitDouble = 70.0
let explicitDouble: Double = 70
EXPERIMENT
Create a constant with an explicit type of
Float and a value of 4.
Values are never implicitly converted to
another type. If you need to convert a value to a different type, explicitly
make an instance of the desired type.
let label = "The width is "
let width = 94
let widthLabel = label + String(width)
EXPERIMENT
Try removing the conversion to String from
the last line. What error do you get?
There’s an even simpler way to include values
in strings: Write the value in parentheses, and write a backslash (\) before
the parentheses. For example:
let apples = 3
let oranges = 5
let appleSummary = "I have \(apples)
apples."
let fruitSummary = "I have \(apples +
oranges) pieces of fruit."
EXPERIMENT
Use \() to include a floating-point
calculation in a string and to include someone’s name in a greeting.
Use three double quotes (""")
for strings that take up multiple lines. Indentation at the start of each
quoted line is removed, as long as it matches the indentation of the closing
quote. For example:
let quotation = """
I said "I have \(apples) apples."
And then I said "I have \(apples +
oranges) pieces of fruit."
"""
Create arrays and dictionaries using brackets
([]), and access their elements by writing the index or key in brackets. A
comma is allowed after the last element.
var shoppingList = ["catfish",
"water", "tulips", "blue paint"]
shoppingList[1] = "bottle of water"
var occupations = [
"Malcolm": "Captain",
"Kaylee": "Mechanic",
]
occupations["Jayne"] = "Public
Relations"
To create an empty array or dictionary, use
the initializer syntax.
let emptyArray = [String]()
let emptyDictionary = [String: Float]()
If type information can be inferred, you can
write an empty array as [] and an empty dictionary as [:]—for example, when you
set a new value for a variable or pass an argument to a function.
shoppingList = []
occupations = [:]
Control Flow
Use if and switch to make conditionals, and
use for-in, while, and repeat-while to make loops. Parentheses around the
condition or loop variable are optional. Braces around the body are required.
let individualScores = [75, 43, 103, 87, 12]
var teamScore = 0
for score in individualScores {
if
score > 50 {
teamScore += 3
}
else {
teamScore += 1
}
}
print(teamScore)
In an if statement, the conditional must be a
Boolean expression—this means that code such as if score { ... } is an error,
not an implicit comparison to zero.
You can use if and let together to work with
values that might be missing. These values are represented as optionals. An
optional value either contains a value or contains nil to indicate that a value
is missing. Write a question mark (?) after the type of a value to mark the
value as optional.
var optionalString: String? =
"Hello"
print(optionalString == nil)
var optionalName: String? = "John
Appleseed"
var greeting = "Hello!"
if let name = optionalName {
greeting = "Hello, \(name)"
}
EXPERIMENT
Change optionalName to nil. What greeting do
you get? Add an else clause that sets a different greeting if optionalName is
nil.
If the optional value is nil, the conditional
is false and the code in braces is skipped. Otherwise, the optional value is
unwrapped and assigned to the constant after let, which makes the unwrapped
value available inside the block of code.
Another way to handle optional values is to
provide a default value using the ?? operator. If the optional value is
missing, the default value is used instead.
let nickName: String? = nil
let fullName: String = "John
Appleseed"
let informalGreeting = "Hi \(nickName ??
fullName)"
Switches support any kind of data and a wide
variety of comparison operations—they aren’t limited to integers and tests for
equality.
let vegetable = "red pepper"
switch vegetable {
case "celery":
print("Add some raisins and make ants on a log.")
case "cucumber",
"watercress":
print("That would make a good tea sandwich.")
case let x where
x.hasSuffix("pepper"):
print("Is it a spicy \(x)?")
default:
print("Everything tastes good in soup.")
}
EXPERIMENT
Try removing the default case. What error do
you get?
Notice how let can be used in a pattern to
assign the value that matched the pattern to a constant.
After executing the code inside the switch
case that matched, the program exits from the switch statement. Execution
doesn’t continue to the next case, so there is no need to explicitly break out
of the switch at the end of each case’s code.
You use for-in to iterate over items in a
dictionary by providing a pair of names to use for each key-value pair.
Dictionaries are an unordered collection, so their keys and values are iterated
over in an arbitrary order.
let interestingNumbers = [
"Prime": [2, 3, 5, 7, 11, 13],
"Fibonacci": [1, 1, 2, 3, 5, 8],
"Square": [1, 4, 9, 16, 25],
]
var largest = 0
for (kind, numbers) in interestingNumbers {
for
number in numbers {
if number > largest {
largest = number
}
}
}
print(largest)
EXPERIMENT
Add another variable to keep track of which
kind of number was the largest, as well as what that largest number was.
Use while to repeat a block of code until a
condition changes. The condition of a loop can be at the end instead, ensuring
that the loop is run at least once.
var n = 2
while n < 100 {
n
*= 2
}
print(n)
var m = 2
repeat {
m
*= 2
} while m < 100
print(m)
You can keep an index in a loop by using
..< to make a range of indexes.
var total = 0
for i in 0..<4 {
total += i
}
print(total)
Use ..< to make a range that omits its
upper value, and use ... to make a range that includes both values.
Functions and Closures
Use func to declare a function. Call a
function by following its name with a list of arguments in parentheses. Use
-> to separate the parameter names and types from the function’s return
type.
func greet(person: String, day: String) ->
String {
return "Hello \(person), today is \(day)."
}
greet(person: "Bob", day:
"Tuesday")
EXPERIMENT
Remove the day parameter. Add a parameter to
include today’s lunch special in the greeting.
By default, functions use their parameter
names as labels for their arguments. Write a custom argument label before the
parameter name, or write _ to use no argument label.
func greet(_ person: String, on day: String)
-> String {
return "Hello \(person), today is \(day)."
}
greet("John", on:
"Wednesday")
Use a tuple to make a compound value—for
example, to return multiple values from a function. The elements of a tuple can
be referred to either by name or by number.
func calculateStatistics(scores: [Int]) ->
(min: Int, max: Int, sum: Int) {
var
min = scores[0]
var
max = scores[0]
var
sum = 0
for
score in scores {
if score > max {
max = score
} else if score < min {
min = score
}
sum += score
}
return (min, max, sum)
}
let statistics = calculateStatistics(scores:
[5, 3, 100, 3, 9])
print(statistics.sum)
print(statistics.2)
Functions can be nested. Nested functions
have access to variables that were declared in the outer function. You can use
nested functions to organize the code in a function that is long or complex.
func returnFifteen() -> Int {
var
y = 10
func add() {
y += 5
}
add()
return y
}
returnFifteen()
Functions are a first-class type. This means
that a function can return another function as its value.
func makeIncrementer() -> ((Int) ->
Int) {
func addOne(number: Int) -> Int {
return 1 + number
}
return addOne
}
var increment = makeIncrementer()
increment(7)
A function can take another function as one
of its arguments.
func hasAnyMatches(list: [Int], condition:
(Int) -> Bool) -> Bool {
for
item in list {
if condition(item) {
return true
}
}
return false
}
func lessThanTen(number: Int) -> Bool {
return number < 10
}
var numbers = [20, 19, 7, 12]
hasAnyMatches(list: numbers, condition:
lessThanTen)
Functions are actually a special case of
closures: blocks of code that can be called later. The code in a closure has
access to things like variables and functions that were available in the scope
where the closure was created, even if the closure is in a different scope when
it is executed—you saw an example of this already with nested functions. You
can write a closure without a name by surrounding code with braces ({}). Use in
to separate the arguments and return type from the body.
numbers.map({ (number: Int) -> Int in
let
result = 3 * number
return result
})
EXPERIMENT
Rewrite the closure to return zero for all odd
numbers.
You have several options for writing closures
more concisely. When a closure’s type is already known, such as the callback
for a delegate, you can omit the type of its parameters, its return type, or
both. Single statement closures implicitly return the value of their only
statement.
let mappedNumbers = numbers.map({ number in 3
* number })
print(mappedNumbers)
You can refer to parameters by number instead
of by name—this approach is especially useful in very short closures. A closure
passed as the last argument to a function can appear immediately after the
parentheses. When a closure is the only argument to a function, you can omit
the parentheses entirely.
let sortedNumbers = numbers.sorted { $0 >
$1 }
print(sortedNumbers)
Objects and Classes
Use class followed by the class’s name to
create a class. A property declaration in a class is written the same way as a
constant or variable declaration, except that it is in the context of a class.
Likewise, method and function declarations are written the same way.
class Shape {
var
numberOfSides = 0
func simpleDescription() -> String {
return "A shape with \(numberOfSides) sides."
}
}
EXPERIMENT
Add a constant property with let, and add
another method that takes an argument.
Create an instance of a class by putting
parentheses after the class name. Use dot syntax to access the properties and
methods of the instance.
var shape = Shape()
shape.numberOfSides = 7
var shapeDescription =
shape.simpleDescription()
This version of the Shape class is missing
something important: an initializer to set up the class when an instance is
created. Use init to create one.
class NamedShape {
var
numberOfSides: Int = 0
var
name: String
init(name: String) {
self.name = name
}
func simpleDescription() -> String {
return "A shape with \(numberOfSides) sides."
}
}
Notice how self is used to distinguish the
name property from the name argument to the initializer. The arguments to the
initializer are passed like a function call when you create an instance of the
class. Every property needs a value assigned—either in its declaration (as with
numberOfSides) or in the initializer (as with name).
Use deinit to create a deinitializer if you
need to perform some cleanup before the object is deallocated.
Subclasses include their superclass name
after their class name, separated by a colon. There is no requirement for
classes to subclass any standard root class, so you can include or omit a
superclass as needed.
Methods on a subclass that override the
superclass’s implementation are marked with override—overriding a method by
accident, without override, is detected by the compiler as an error. The
compiler also detects methods with override that don’t actually override any
method in the superclass.
class Square: NamedShape {
var
sideLength: Double
init(sideLength: Double, name: String) {
self.sideLength = sideLength
super.init(name: name)
numberOfSides = 4
}
func area() -> Double {
return sideLength * sideLength
}
override func simpleDescription() -> String {
return "A square with sides of length \(sideLength)."
}
}
let test = Square(sideLength: 5.2, name:
"my test square")
test.area()
test.simpleDescription()
EXPERIMENT
Make another subclass of NamedShape called
Circle that takes a radius and a name as arguments to its initializer.
Implement an area() and a simpleDescription() method on the Circle class.
In addition to simple properties that are
stored, properties can have a getter and a setter.
class EquilateralTriangle: NamedShape {
var
sideLength: Double = 0.0
init(sideLength: Double, name: String) {
self.sideLength = sideLength
super.init(name: name)
numberOfSides = 3
}
var
perimeter: Double {
get {
return 3.0 * sideLength
}
set {
sideLength = newValue / 3.0
}
}
override func simpleDescription() -> String {
return "An equilateral triangle with sides of length
\(sideLength)."
}
}
var triangle =
EquilateralTriangle(sideLength: 3.1, name: "a triangle")
print(triangle.perimeter)
triangle.perimeter = 9.9
print(triangle.sideLength)
In the setter for perimeter, the new value
has the implicit name newValue. You can provide an explicit name in parentheses
after set.
Notice that the initializer for the
EquilateralTriangle class has three different steps:
Setting the value of properties that the
subclass declares.
Calling the superclass’s initializer.
Changing the value of properties defined by
the superclass. Any additional setup work that uses methods, getters, or
setters can also be done at this point.
If you don’t need to compute the property but
still need to provide code that is run before and after setting a new value,
use willSet and didSet. The code you provide is run any time the value changes
outside of an initializer. For example, the class below ensures that the side
length of its triangle is always the same as the side length of its square.
class TriangleAndSquare {
var
triangle: EquilateralTriangle {
willSet {
square.sideLength = newValue.sideLength
}
}
var
square: Square {
willSet {
triangle.sideLength = newValue.sideLength
}
}
init(size: Double, name: String) {
square = Square(sideLength: size, name: name)
triangle = EquilateralTriangle(sideLength: size, name: name)
}
}
var triangleAndSquare =
TriangleAndSquare(size: 10, name: "another test shape")
print(triangleAndSquare.square.sideLength)
print(triangleAndSquare.triangle.sideLength)
triangleAndSquare.square = Square(sideLength:
50, name: "larger square")
print(triangleAndSquare.triangle.sideLength)
When working with optional values, you can
write ? before operations like methods, properties, and subscripting. If the
value before the ? is nil, everything after the ? is ignored and the value of
the whole expression is nil. Otherwise, the optional value is unwrapped, and
everything after the ? acts on the unwrapped value. In both cases, the value of
the whole expression is an optional value.
let optionalSquare: Square? =
Square(sideLength: 2.5, name: "optional square")
let sideLength = optionalSquare?.sideLength
Enumerations and Structures
Use enum to create an enumeration. Like
classes and all other named types, enumerations can have methods associated
with them.
enum Rank: Int {
case ace = 1
case two, three, four, five, six, seven, eight, nine, ten
case jack, queen, king
func simpleDescription() -> String {
switch self {
case .ace:
return "ace"
case .jack:
return "jack"
case .queen:
return "queen"
case .king:
return "king"
default:
return String(self.rawValue)
}
}
}
let ace = Rank.ace
let aceRawValue = ace.rawValue
EXPERIMENT
Write a function that compares two Rank values
by comparing their raw values.
By default, Swift assigns the raw values
starting at zero and incrementing by one each time, but you can change this
behavior by explicitly specifying values. In the example above, Ace is
explicitly given a raw value of 1, and the rest of the raw values are assigned
in order. You can also use strings or floating-point numbers as the raw type of
an enumeration. Use the rawValue property to access the raw value of an
enumeration case.
Use the init?(rawValue:) initializer to make
an instance of an enumeration from a raw value. It returns either the
enumeration case matching the raw value or nil if there is no matching Rank.
if let convertedRank = Rank(rawValue: 3) {
let
threeDescription = convertedRank.simpleDescription()
}
The case values of an enumeration are actual
values, not just another way of writing their raw values. In fact, in cases
where there isn’t a meaningful raw value, you don’t have to provide one.
enum Suit {
case spades, hearts, diamonds, clubs
func
simpleDescription() -> String {
switch self {
case .spades:
return "spades"
case .hearts:
return "hearts"
case .diamonds:
return "diamonds"
case .clubs:
return "clubs"
}
}
}
let hearts = Suit.hearts
let heartsDescription =
hearts.simpleDescription()
EXPERIMENT
Add a color() method to Suit that returns
“black” for spades and clubs, and returns “red” for hearts and diamonds.
Notice the two ways that the hearts case of
the enumeration is referred to above: When assigning a value to the hearts
constant, the enumeration case Suit.hearts is referred to by its full name
because the constant doesn’t have an explicit type specified. Inside the
switch, the enumeration case is referred to by the abbreviated form .hearts
because the value of self is already known to be a suit. You can use the
abbreviated form anytime the value’s type is already known.
If an enumeration has raw values, those values
are determined as part of the declaration, which means every instance of a
particular enumeration case always has the same raw value. Another choice for
enumeration cases is to have values associated with the case—these values are
determined when you make the instance, and they can be different for each
instance of an enumeration case. You can think of the associated values as
behaving like stored properties of the enumeration case instance. For example,
consider the case of requesting the sunrise and sunset times from a server. The
server either responds with the requested information, or it responds with a
description of what went wrong.
enum ServerResponse {
case result(String, String)
case failure(String)
}
let success = ServerResponse.result("6:00
am", "8:09 pm")
let failure =
ServerResponse.failure("Out of cheese.")
switch success {
case let .result(sunrise, sunset):
print("Sunrise is at \(sunrise) and sunset is at \(sunset).")
case let .failure(message):
print("Failure...
\(message)")
}
EXPERIMENT
Add a third case to ServerResponse and to the
switch.
Notice how the sunrise and sunset times are
extracted from the ServerResponse value as part of matching the value against
the switch cases.
Use struct to create a structure. Structures
support many of the same behaviors as classes, including methods and
initializers. One of the most important differences between structures and
classes is that structures are always copied when they are passed around in your
code, but classes are passed by reference.
struct Card {
var
rank: Rank
var
suit: Suit
func simpleDescription() -> String {
return "The \(rank.simpleDescription()) of
\(suit.simpleDescription())"
}
}
let threeOfSpades = Card(rank: .three, suit:
.spades)
let threeOfSpadesDescription =
threeOfSpades.simpleDescription()
EXPERIMENT
Add a method to Card that creates a full deck
of cards, with one card of each combination of rank and suit.
Protocols and Extensions
Use protocol to declare a protocol.
protocol ExampleProtocol {
var
simpleDescription: String { get }
mutating func adjust()
}
Classes, enumerations, and structs can all
adopt protocols.
class SimpleClass: ExampleProtocol {
var
simpleDescription: String = "A very simple class."
var
anotherProperty: Int = 69105
func adjust() {
simpleDescription += " Now
100% adjusted."
}
}
var a = SimpleClass()
a.adjust()
let aDescription = a.simpleDescription
struct SimpleStructure: ExampleProtocol {
var
simpleDescription: String = "A simple structure"
mutating func adjust() {
simpleDescription += " (adjusted)"
}
}
var b = SimpleStructure()
b.adjust()
let bDescription = b.simpleDescription
EXPERIMENT
Write an enumeration that conforms to this
protocol.
Notice the use of the mutating keyword in the
declaration of SimpleStructure to mark a method that modifies the structure.
The declaration of SimpleClass doesn’t need any of its methods marked as mutating
because methods on a class can always modify the class.
Use extension to add functionality to an
existing type, such as new methods and computed properties. You can use an
extension to add protocol conformance to a type that is declared elsewhere, or
even to a type that you imported from a library or framework.
extension Int: ExampleProtocol {
var
simpleDescription: String {
return "The number \(self)"
}
mutating func adjust() {
self += 42
}
}
print(7.simpleDescription)
EXPERIMENT
Write an extension for the Double type that
adds an absoluteValue property.
You can use a protocol name just like any
other named type—for example, to create a collection of objects that have
different types but that all conform to a single protocol. When you work with
values whose type is a protocol type, methods outside the protocol definition
are not available.
let protocolValue: ExampleProtocol = a
print(protocolValue.simpleDescription)
// print(protocolValue.anotherProperty) // Uncomment to see the error
Even though the variable protocolValue has a
runtime type of SimpleClass, the compiler treats it as the given type of
ExampleProtocol. This means that you can’t accidentally access methods or
properties that the class implements in addition to its protocol conformance.
Error Handling
You represent errors using any type that
adopts the Error protocol.
enum PrinterError: Error {
case outOfPaper
case noToner
case onFire
}
Use throw to throw an error and throws to
mark a function that can throw an error. If you throw an error in a function,
the function returns immediately and the code that called the function handles
the error.
func send(job: Int, toPrinter printerName: String)
throws -> String {
if
printerName == "Never Has Toner" {
throw PrinterError.noToner
}
return "Job sent"
}
There are several ways to handle errors. One
way is to use do-catch. Inside the do block, you mark code that can throw an error
by writing try in front of it. Inside the catch block, the error is
automatically given the name error unless you give it a different name.
do {
let
printerResponse = try send(job: 1040, toPrinter: "Bi Sheng")
print(printerResponse)
} catch {
print(error)
}
EXPERIMENT
Change the printer name to "Never Has
Toner", so that the send(job:toPrinter:) function throws an error.
You can provide multiple catch blocks that
handle specific errors. You write a pattern after catch just as you do after
case in a switch.
do {
let
printerResponse = try send(job: 1440, toPrinter: "Gutenberg")
print(printerResponse)
} catch PrinterError.onFire {
print("I'll just put this over here, with the rest of the
fire.")
} catch let printerError as PrinterError {
print("Printer error: \(printerError).")
} catch {
print(error)
}
EXPERIMENT
Add code to throw an error inside the do
block. What kind of error do you need to throw so that the error is handled by
the first catch block? What about the second and third blocks?
Another way to handle errors is to use try?
to convert the result to an optional. If the function throws an error, the
specific error is discarded and the result is nil. Otherwise, the result is an
optional containing the value that the function returned.
let printerSuccess = try? send(job: 1884,
toPrinter: "Mergenthaler")
let printerFailure = try? send(job: 1885,
toPrinter: "Never Has Toner")
Use defer to write a block of code that is
executed after all other code in the function, just before the function
returns. The code is executed regardless of whether the function throws an
error. You can use defer to write setup and cleanup code next to each other,
even though they need to be executed at different times.
var fridgeIsOpen = false
let fridgeContent = ["milk",
"eggs", "leftovers"]
func fridgeContains(_ food: String) ->
Bool {
fridgeIsOpen = true
defer {
fridgeIsOpen = false
}
let
result = fridgeContent.contains(food)
return result
}
fridgeContains("banana")
print(fridgeIsOpen)
Generics
Write a name inside angle brackets to make a
generic function or type.
func makeArray<Item>(repeating item:
Item, numberOfTimes: Int) -> [Item] {
var
result = [Item]()
for
_ in 0..<numberOfTimes {
result.append(item)
}
return result
}
makeArray(repeating: "knock",
numberOfTimes: 4)
You can make generic forms of functions and
methods, as well as classes, enumerations, and structures.
// Reimplement the Swift standard library's
optional type
enum OptionalValue<Wrapped> {
case none
case some(Wrapped)
}
var possibleInteger: OptionalValue<Int>
= .none
possibleInteger = .some(100)
Use where right before the body to specify a
list of requirements—for example, to require the type to implement a protocol,
to require two types to be the same, or to require a class to have a particular
superclass.
func anyCommonElements<T: Sequence, U:
Sequence>(_ lhs: T, _ rhs: U) -> Bool
where T.Iterator.Element: Equatable, T.Iterator.Element ==
U.Iterator.Element {
for lhsItem in lhs {
for rhsItem in rhs {
if lhsItem == rhsItem {
return true
}
}
}
return false
}
anyCommonElements([1, 2, 3], [3])
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