IOS Swift language 2nd tutorial

IPhone Application Development
Week 2

Swift
• Constants and Variables
• Constants and variables associate a name
(such as maximumNumberOfLoginAttempts or
welcomeMessage) with a value of a particular
type (such as the number 10 or the string
"Hello").
• The value of a constant cannot be changed
once it is set, whereas a variable can be set to
a different value in the future.

Swift
• Declaring Constants and Variables
• Constants and variables must be declared
before they are used. You declare constants
with the let keyword and variables with the
var keyword.
• Here’s an example of how constants and
variables can be used to track the number of
login attempts a user has made:

Swift
let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0
• This code can be read as:
• “Declare a new constant called
maximumNumberOfLoginAttempts, and give it a
value of 10.
• Then, declare a new variable called
currentLoginAttempt, and give it an initial value
of 0.”

Swift
• In this example, the maximum number of
allowed login attempts is declared as a
constant, because the maximum value never
changes.
• The current login attempt counter is declared
as a variable, because this value must be
incremented after each failed login attempt.

Swift
• You can declare multiple constants or multiple
variables on a single line, separated by
commas:
• var x = 0.0, y = 0.0, z = 0.0

Swift
• Type Annotations
• You can provide a type annotation when you
declare a constant or variable, to be clear
about the kind of values the constant or
variable can store.
• Write a type annotation by placing a colon
after the constant or variable name, followed
by a space, followed by the name of the type
to use.

Swift
• This example provides a type annotation for a
variable called welcomeMessage, to indicate that
the variable can store String values:
• var welcomeMessage: String
• The colon in the declaration means “…of type…,”
so the code above can be read as:
• “Declare a variable called welcomeMessage that
is of type String.”

Swift
• The phrase “of type String” means “can store
any String value.” Think of it as meaning “the
type of thing” (or “the kind of thing”) that can
be stored.
• The welcomeMessage variable can now be set
to any string value without error:
• welcomeMessage = "Hello”

Swift
• You can define multiple related variables of
the same type on a single line, separated by
commas, with a single type annotation after
the final variable name:
• var red, green, blue: Double

Swift
• Naming Constants and Variables
• Constant and variable names can contain
almost any character, including Unicode
characters:
• let π = 3.14159
• let 你好 = "你好世界"
• let ?? = "dogcow"

Swift
• Constant and variable names cannot contain
whitespace characters, mathematical symbols,
arrows, private-use (or invalid) Unicode code
points, or line- and box-drawing characters.
• Nor can they begin with a number, although
numbers may be included elsewhere within
the name.

Swift
• Once you’ve declared a constant or variable of
a certain type, you can’t redeclare it again
with the same name, or change it to store
values of a different type.
• Nor can you change a constant into a variable
or a variable into a constant.

Swift
• You can change the value of an existing variable
to another value of a compatible type.
• In this example, the value of friendlyWelcome is
changed from "Hello!" to "Bonjour!":
• var friendlyWelcome = "Hello!"
• friendlyWelcome = "Bonjour!"
• // friendlyWelcome is now "Bonjour!"

Swift
• Unlike a variable, the value of a constant
cannot be changed once it is set. Attempting
to do so is reported as an error when your
code is compiled:
• let languageName = "Swift"
• languageName = "Swift++"
• // this is a compile-time error -
languageName cannot be changed

Swift
• Printing Constants and Variables
• You can print the current value of a constant or variable
with the print(_:separator:terminator:) function:
• print(friendlyWelcome)
• // prints "Bonjour!"
• The print(_:separator:terminator:) function is a global
function that prints one or more values to an appropriate
output.
• In Xcode, for example, the print(_:separator:terminator:)
function prints its output in Xcode’s “console” pane. The
separator and terminator parameter have default values, so
you can omit them when you call this function.

Swift
• By default, the function terminates the line it
prints by adding a line break.
• To print a value without a line break after it,
pass an empty string as the terminator—for
example, print(someValue, terminator: "").

Swift
• Swift uses string interpolation to include the name of a
constant or variable as a placeholder in a longer string,
and to prompt Swift to replace it with the current value
of that constant or variable.
• Wrap the name in parentheses and escape it with a
backslash before the opening parenthesis:
• print("The current value of friendlyWelcome is (friendlyWelcome)")
• // prints "The current value of friendlyWelcome is
Bonjour!"

Swift
• Comments
• Use comments to include non-executable text
in your code, as a note or reminder to
yourself. Comments are ignored by the Swift
compiler when your code is compiled.
• Comments in Swift are very similar to
comments in C. Single-line comments begin
with two forward-slashes (//):
• // this is a comment

Swift
• Comments
• Multiline comments start with a forward-slash
followed by an asterisk (/*) and end with an
asterisk followed by a forward-slash (*/):
• /* this is also a comment,
• but written over multiple lines */

Swift
• Comments
• Unlike multiline comments in C, multiline comments in
Swift can be nested inside other multiline comments. You
write nested comments by starting a multiline comment
block and then starting a second multiline comment within
the first block. The second block is then closed, followed by
the first block:
• /* this is the start of the first multiline comment
• /* this is the second, nested multiline comment */
• this is the end of the first multiline comment */
• Nested multiline comments enable you to comment out
large blocks of code quickly and easily, even if the code
already contains multiline comments.

Swift
• Semicolons
• Unlike many other languages, Swift does not
require you to write a semicolon (;) after each
statement in your code, although you can do so if
you wish.
• However, semicolons are required if you want to
write multiple separate statements on a single
line:
• let cat = "?"; print(cat)
• // prints "?"

Swift
• Integers
• Integers are whole numbers with no fractional
component, such as 42 and -23. Integers are either
signed (positive, zero, or negative) or unsigned
(positive or zero).
• Swift provides signed and unsigned integers in 8, 16,
32, and 64 bit forms. These integers follow a naming
convention similar to C, in that an 8-bit unsigned
integer is of type UInt8, and a 32-bit signed integer is
of type Int32.
• Like all types in Swift, these integer types have
capitalized names.

Swift
• Integers
• Integer Bounds
• You can access the minimum and maximum values of
each integer type with its min and max properties:
• let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8
• let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8
• The values of these properties are of the appropriate-
sized number type (such as UInt8 in the example
above) and can therefore be used in expressions
alongside other values of the same type.

Swift
• Integers
• Int
• In most cases, you don’t need to pick a specific size of integer to
use in your code. Swift provides an additional integer type, Int,
which has the same size as the current platform’s native word size:
• On a 32-bit platform, Int is the same size as Int32.
• On a 64-bit platform, Int is the same size as Int64.
• Unless you need to work with a specific size of integer, always use
Int for integer values in your code. This aids code consistency and
interoperability. Even on 32-bit platforms, Int can store any value
between -2,147,483,648 and 2,147,483,647, and is large enough for
many integer ranges.

Swift
• Integers
• UInt
• Swift also provides an unsigned integer type,
UInt, which has the same size as the current
platform’s native word size:
• On a 32-bit platform, UInt is the same size as
UInt32.
• On a 64-bit platform, UInt is the same size as
UInt64.

Swift
• Floating-Point Numbers
• Floating-point numbers are numbers with a fractional
component, such as 3.14159, 0.1, and -273.15.
• Floating-point types can represent a much wider range
of values than integer types, and can store numbers
that are much larger or smaller than can be stored in
an Int.
• Swift provides two signed floating-point number types:
• Double represents a 64-bit floating-point number.
• Float represents a 32-bit floating-point number.

Swift
• Floating-Point Numbers
• Double has a precision of at least 15 decimal
digits, whereas the precision of Float can be as
little as 6 decimal digits.
• The appropriate floating-point type to use
depends on the nature and range of values
you need to work with in your code. In
situations where either type would be
appropriate, Double is preferred.

Swift
• Type Safety and Type Inference
• Swift is a type safe language. A type safe language
encourages you to be clear about the types of
values your code can work with. If part of your
code expects a String, you can’t pass it an Int by
mistake.
• Because Swift is type safe, it performs type checks
when compiling your code and flags any
mismatched types as errors. This enables you to
catch and fix errors as early as possible in the
development process.

Swift
• Type-checking helps you avoid errors when you’re
working with different types of values.
• However, this doesn’t mean that you have to specify
the type of every constant and variable that you
declare.
• If you don’t specify the type of value you need, Swift
uses type inference to work out the appropriate type.
• Type inference enables a compiler to deduce the type
of a particular expression automatically when it
compiles your code, simply by examining the values
you provide.

Swift
• Because of type inference, Swift requires far
fewer type declarations than languages such
as C or Objective-C.
• Constants and variables are still explicitly
typed, but much of the work of specifying
their type is done for you.

Swift
• Type inference is particularly useful when you
declare a constant or variable with an initial
value.
• This is often done by assigning a literal value (or
literal) to the constant or variable at the point
that you declare it. (A literal value is a value that
appears directly in your source code, such as 42
and 3.14159 in the examples below.)

Swift
• For example, if you assign a literal value of 42
to a new constant without saying what type it
is, Swift infers that you want the constant to
be an Int, because you have initialized it with a
number that looks like an integer:
• let meaningOfLife = 42
• // meaningOfLife is inferred to be of type Int

Swift
• Likewise, if you don’t specify a type for a floating-
point literal, Swift infers that you want to create a
Double:
• let pi = 3.14159
• // pi is inferred to be of type Double
• Swift always chooses Double (rather than Float)
when inferring the type of floating-point
numbers.

Swift
• If you combine integer and floating-point literals
in an expression, a type of Double will be inferred
from the context:
• let anotherPi = 3 + 0.14159
• // anotherPi is also inferred to be of type Double
• The literal value of 3 has no explicit type in and of
itself, and so an appropriate output type of
Double is inferred from the presence of a
floating-point literal as part of the addition.

Swift
• Numeric Literals
• Integer literals
• Integer literals can be written as:
• A decimal number, with no prefix
• A binary number, with a 0b prefix
• An octal number, with a 0o prefix
• A hexadecimal number, with a 0x prefix

Swift
• Integer literals
• All of these integer literals have a decimal value of 17:
• let decimalInteger = 17
• let binaryInteger = 0b10001 // 17 in binary notation
• let octalInteger = 0o21 // 17 in octal notation
• let hexadecimalInteger = 0x11 // 17 in hexadecimal
notation

Swift
• Floating-point literals
• Floating-point literals can be decimal (with no
prefix), or hexadecimal (with a 0x prefix).
• They must always have a number (or hexadecimal
number) on both sides of the decimal point.
• Decimal floats can also have an optional
exponent, indicated by an uppercase or lowercase
e; hexadecimal floats must have an exponent,
indicated by an uppercase or lowercase p.

Swift
• For decimal numbers with an exponent of exp,
the base number is multiplied by 10exp:
• 1.25e2 means 1.25 x 102, or 125.0.
• 1.25e-2 means 1.25 x 10-2, or 0.0125.

Swift
• For hexadecimal numbers with an exponent of exp, the
base number is multiplied by 2exp:
• 0xFp2 means 15 x 22, or 60.0.
• 0xFp-2 means 15 x 2-2, or 3.75.
• All of these floating-point literals have a decimal value
of 12.1875:
• let decimalDouble = 12.1875
• let exponentDouble = 1.21875e1
• let hexadecimalDouble = 0xC.3p0

Swift
• Numeric literals can contain extra formatting to
make them easier to read. Both integers and
floats can be padded with extra zeros and can
contain underscores to help with readability.
• Neither type of formatting affects the underlying
value of the literal:
• let paddedDouble = 000123.456
• let oneMillion = 1_000_000
• let justOverOneMillion = 1_000_000.000_000_1

Swift
• Numeric Type Conversion
• Use the Int type for all general-purpose
integer constants and variables in your code,
even if they are known to be non-negative.
• Using the default integer type in everyday
situations means that integer constants and
variables are immediately interoperable in
your code and will match the inferred type for
integer literal values.

Swift
• Numeric Type Conversion
• Use other integer types only when they are
specifically needed for the task at hand, because
of explicitly-sized data from an external source, or
for performance, memory usage, or other
necessary optimization.
• Using explicitly-sized types in these situations
helps to catch any accidental value overflows and
implicitly documents the nature of the data being
used.

Swift
• Integer Conversion
• The range of numbers that can be stored in an
integer constant or variable is different for
each numeric type. An Int8 constant or
variable can store numbers between -128 and
127, whereas a UInt8 constant or variable can
store numbers between 0 and 255.

Swift
• A number that will not fit into a constant or variable of
a sized integer type is reported as an error when your
code is compiled:
• let cannotBeNegative: UInt8 = -1
• // UInt8 cannot store negative numbers, and so
this will report an error
• let tooBig: = Int8.max + 1
• // Int8 cannot store a number larger than its
maximum value,
• // and so this will also report an error

Swift
• Because each numeric type can store a
different range of values, you must opt in to
numeric type conversion on a case-by-case
basis.
• This opt-in approach prevents hidden
conversion errors and helps make type
conversion intentions explicit in your code.

Swift
• To convert one specific number type to
another, you initialize a new number of the
desired type with the existing value.
• In the example below, the constant
twoThousand is of type UInt16, whereas the
constant one is of type UInt8.
• They cannot be added together directly,
because they are not of the same type.

Swift
• Instead, this example calls UInt16(one) to create
a new UInt16 initialized with the value of one,
and uses this value in place of the original:
• let twoThousand: UInt16 = 2_000
• let one: UInt8 = 1
• let twoThousandAndOne = twoThousand + UInt16(one)
• Because both sides of the addition are now of type
UInt16, the addition is allowed. The output constant
(twoThousandAndOne) is inferred to be of type UInt16,
because it is the sum of two UInt16 values.

Swift
• SomeType(ofInitialValue) is the default way to call
the initializer of a Swift type and pass in an initial
value.
• Behind the scenes, UInt16 has an initializer that
accepts a UInt8 value, and so this initializer is
used to make a new UInt16 from an existing
UInt8.
• You can’t pass in any type here, however—it has
to be a type for which UInt16 provides an
initializer.

Swift
• Integer and Floating-Point Conversion
• Conversions between integer and floating-point
numeric types must be made explicit:
• let three = 3
• let pointOneFourOneFiveNine = 0.14159
• let pi = Double(three) + pointOneFourOneFiveNine
• // pi equals 3.14159, and is inferred to be of type Double
• Here, the value of the constant three is used to create a new value
of type Double, so that both sides of the addition are of the same
type.
• Without this conversion in place, the addition would not be
allowed.

Swift
• Integer and Floating-Point Conversion
• Floating-point to integer conversion must also be
made explicit. An integer type can be initialized
with a Double or Float value:
• let integerPi = Int(pi)
• // integerPi equals 3, and is inferred to be of type Int
• Floating-point values are always truncated when
used to initialize a new integer value in this way.
This means that 4.75 becomes 4, and -3.9
becomes -3.

Swift
• Type Aliases
• Type aliases define an alternative name for an existing type. You
define type aliases with the typealias keyword.
• Type aliases are useful when you want to refer to an existing type
by a name that is contextually more appropriate, such as when
working with data of a specific size from an external source:
• typealias AudioSample = a href="" UInt16 /a
• Once you define a type alias, you can use the alias anywhere you
might use the original name:
• var maxAmplitudeFound = AudioSample.min
• // maxAmplitudeFound is now 0
• Here, AudioSample is defined as an alias for UInt16. Because it is an
alias, the call to AudioSample.min actually calls UInt16.min, which
provides an initial value of 0 for the maxAmplitudeFound variable.

Swift
• Type Aliases
• Type aliases define an alternative name for an existing type. You
define type aliases with the typealias keyword.
• Type aliases are useful when you want to refer to an existing type
by a name that is contextually more appropriate, such as when
working with data of a specific size from an external source:
• typealias AudioSample = UInt16
• Once you define a type alias, you can use the alias anywhere you
might use the original name:
• var maxAmplitudeFound = AudioSample.min
• // maxAmplitudeFound is now 0
• Here, AudioSample is defined as an alias for UInt16. Because it is an
alias, the call to AudioSample.min actually calls UInt16.min, which
provides an initial value of 0 for the maxAmplitudeFound variable.

Swift
• Booleans
• Swift has a basic Boolean type, called Bool.
Boolean values are referred to as logical, because
they can only ever be true or false. Swift provides
two Boolean constant values, true and false:
• let orangesAreOrange = true
• let turnipsAreDelicious = false
• The types of orangesAreOrange and
turnipsAreDelicious have been inferred as Bool
from the fact that they were initialized with
Boolean literal values.

Swift
• Booleans
• As with Int and Double above, you don’t need to declare constants or
variables as Bool if you set them to true or false as soon as you create
them.
• Type inference helps make Swift code more concise and readable when it
initializes constants or variables with other values whose type is already
known.
• Boolean values are particularly useful when you work with conditional
statements such as the if statement:
• if turnipsAreDelicious {
• print("Mmm, tasty turnips!")
• } else {
• print("Eww, turnips are horrible.")
• }
• // prints "Eww, turnips are horrible."

Swift
• Booleans
• Swift’s type safety prevents non-Boolean values from being substituted for Bool.
The following example reports a compile-time error:
• let i = 1
• if i {
• // this example will not compile, and will report an error
• }
• However, the alternative example below is valid:
• let i = 1
• if i == 1 {
• // this example will compile successfully
• }
• The result of the i == 1 comparison is of type Bool, and so this second example
passes the type-check.
• As with other examples of type safety in Swift, this approach avoids accidental
errors and ensures that the intention of a particular section of code is always clear

Swift
• Tuples
• Tuples group multiple values into a single
compound value. The values within a tuple
can be of any type and do not have to be of
the same type as each other.
• In this example, (404, "Not Found") is a tuple
that describes an HTTP status code. An HTTP
status code is a special value returned by a
web server whenever you request a web page.

Swift
• Tuples
• A status code of 404 Not Found is returned if you
request a webpage that doesn’t exist.
• let http404Error = (404, "Not Found")
• // http404Error is of type (Int, String), and equals
(404, "Not Found")
• The (404, "Not Found") tuple groups together an
Int and a String to give the HTTP status code two
separate values: a number and a human-readable
description. It can be described as “a tuple of
type (Int, String)”.

Swift
• Tuples
• You can create tuples from any permutation of types, and they can
contain as many different types as you like. There’s nothing
stopping you from having a tuple of type (Int, Int, Int), or (String,
Bool), or indeed any other permutation you require.
• You can decompose a tuple’s contents into separate constants or
variables, which you then access as usual:
• let (statusCode, statusMessage) = http404Error
• print("The status code is (statusCode)")
• // prints "The status code is 404"
• print("The status message is (statusMessage)")
• // prints "The status message is Not Found"

Swift
• Tuples
• If you only need some of the tuple’s values,
ignore parts of the tuple with an underscore
(_) when you decompose the tuple:
• let (justTheStatusCode, _) = http404Error
• print("The status code is (justTheStatusCode)")

Swift
• Tuples
• Alternatively, access the individual element values in a
tuple using index numbers starting at zero:
• print("The status code is (http404Error.0)")
• print("The status message is (http404Error.1)")
• // prints "The status message is Not Found"
• You can name the individual elements in a tuple when
the tuple is defined:
• let http200Status = (statusCode: 200, description: "OK")

Swift
• Tuples
• If you name the elements in a tuple, you can use the element
names to access the values of those elements:
• print("The status code is (http200Status.statusCode)")
• print("The status message is (http200Status.description)")
• // prints "The status message is OK"
• Tuples are particularly useful as the return values of functions.
• A function that tries to retrieve a web page might return the (Int,
String) tuple type to describe the success or failure of the page
retrieval.
• By returning a tuple with two distinct values, each of a different
type, the function provides more useful information about its
outcome than if it could only return a single value of a single type.

Swift
• Tuples
• NOTE
• Tuples are useful for temporary groups of
related values.
• They are not suited to the creation of complex
data structures.
• If your data structure is likely to persist
beyond a temporary scope, model it as a class
or structure, rather than as a tuple.

Swift
• Optionals
• You use optionals in situations where a value
may be absent.
• An optional says:
• There is a value, and it equals x
• or
• There isn’t a value at all

Swift
• Optionals
• Here’s an example of how optionals can be used
to cope with the absence of a value.
• Swift’s Int type has an initializer which tries to
convert a String value into an Int value.
• However, not every string can be converted into
an integer.
• The string "123" can be converted into the
numeric value 123, but the string "hello, world"
does not have an obvious numeric value to
convert to.

Swift
• Optionals
• The example below uses the initializer to try to
convert a String into an Int:
• let possibleNumber = "123"
• let convertedNumber = Int(possibleNumber)
• // convertedNumber is inferred to be of type "Int?", or "optional Int”
• Because the initializer might fail, it returns an optional Int, rather
than an Int. An optional Int is written as Int?, not Int.
• The question mark indicates that the value it contains is optional,
meaning that it might contain some Int value, or it might contain no
value at all. (It can’t contain anything else, such as a Bool value or a
String value. It’s either an Int, or it’s nothing at all.)

Swift
• Optionals
• nil
• You set an optional variable to a valueless state by
assigning it the special value nil:
• var serverResponseCode: a href="" Int /a ? = 404
• // serverResponseCode contains an actual Int value
of 404
• serverResponseCode = nil
• // serverResponseCode now contains no value

Swift
• Optionals
• nil
• NOTE
• nil cannot be used with nonoptional constants
and variables.
• If a constant or variable in your code needs to
work with the absence of a value under
certain conditions, always declare it as an
optional value of the appropriate type.

Swift
• Optionals
• If you define an optional variable without
providing a default value, the variable is
automatically set to nil for you:
• var surveyAnswer: a href="" String /a ?
• // surveyAnswer is automatically set to nil

Swift
• Optionals
• If Statements and Forced Unwrapping
• You can use an if statement to find out whether an optional
contains a value by comparing the optional against nil. You perform
this comparison with the “equal to” operator (==) or the “not equal
to” operator (!=).
• If an optional has a value, it is considered to be “not equal to” nil:
• if convertedNumber != nil {
• print("convertedNumber contains some integer value.")
• }
• // prints "convertedNumber contains some integer value."

Swift
• Optionals
• If Statements and Forced Unwrapping
• Once you’re sure that the optional does contain a value, you can
access its underlying value by adding an exclamation mark (!) to
the end of the optional’s name. The exclamation mark effectively
says, “I know that this optional definitely has a value; please use it.”
This is known as forced unwrapping of the optional’s value:
• if convertedNumber != nil {
• print("convertedNumber has an integer value of
(convertedNumber!).")
• }
• // prints "convertedNumber has an integer value of 123."

Swift
• Optionals
• Optional Binding
• You use optional binding to find out whether an optional contains a
value, and if so, to make that value available as a temporary
constant or variable. Optional binding can be used with if and while
statements to check for a value inside an optional, and to extract
that value into a constant or variable, as part of a single action.
• Write an optional binding for an if statement as follows:
• if let constantName = someOptional {
• statements
• }

Swift
• Optionals
• You can rewrite the possibleNumber example from the Optionals section
to use optional binding rather than forced unwrapping:
• if let actualNumber = Int(possibleNumber) {
• print("'(possibleNumber)' has an integer value of
(actualNumber)")
• } else {
• print("'(possibleNumber)' could not be converted to an
integer")
• }
• // prints "'123' has an integer value of 123"
• This code can be read as:
• “If the optional Int returned by Int(possibleNumber) contains a value, set a
new constant called actualNumber to the value contained in the optional.”

Swift
• Optionals
• If the conversion is successful, the actualNumber
constant becomes available for use within the
first branch of the if statement. It has already
been initialized with the value contained within
the optional, and so there is no need to use the !
suffix to access its value. In this example,
actualNumber is simply used to print the result of
the conversion.

Swift
• Optionals
• You can use both constants and variables with
optional binding. If you wanted to manipulate
the value of actualNumber within the first
branch of the if statement, you could write if
var actualNumber instead, and the value
contained within the optional would be made
available as a variable rather than a constant.

Swift
• Optionals
• You can include multiple optional bindings in a
single if statement and use a where clause to
check for a Boolean condition:
• if let firstNumber = Int("4"), secondNumber = Int("42") where
firstNumber < secondNumber {
• print("(firstNumber) < (secondNumber)")
• }
• // prints "4 < 42"

Swift
• Optionals
• Implicitly Unwrapped Optionals
• As described above, optionals indicate that a
constant or variable is allowed to have “no value”.
• Optionals can be checked with an if statement to
see if a value exists, and can be conditionally
unwrapped with optional binding to access the
optional’s value if it does exist.

Swift
• Optionals
• These kinds of optionals are defined as
implicitly unwrapped optionals. You write an
implicitly unwrapped optional by placing an
exclamation mark (String!) rather than a
question mark (String?) after the type that
you want to make optional.

Swift
• Optionals
• Implicitly unwrapped optionals are useful when
an optional’s value is confirmed to exist
immediately after the optional is first defined and
can definitely be assumed to exist at every point
thereafter. The primary use of implicitly
unwrapped optionals in Swift is during class
initialization,

Swift
• Optionals
• An implicitly unwrapped optional is a normal optional behind the scenes, but can
also be used like a nonoptional value, without the need to unwrap the optional
value each time it is accessed. The following example shows the difference in
behavior between an optional string and an implicitly unwrapped optional string
when accessing their wrapped value as an explicit String:
• let possibleString: a href="" String /a ? = "An optional string."
• let forcedString: a href="" String /a = possibleString! // requires an
exclamation mark
•
• let assumedString: a href="" String /a ! = "An implicitly unwrapped optional
string."
• let implicitString: a href="" String /a = assumedString // no need for an
exclamation mark

Swift
• Optionals
• An implicitly unwrapped optional is a normal optional behind the scenes, but can
also be used like a nonoptional value, without the need to unwrap the optional
value each time it is accessed. The following example shows the difference in
behavior between an optional string and an implicitly unwrapped optional string
when accessing their wrapped value as an explicit String:
• let possibleString: String? = "An optional string."
• let forcedString: String = possibleString! // requires an exclamation mark
•
• let assumedString: String ! = "An implicitly unwrapped optional string."
• let implicitString: String = assumedString // no need for an exclamation mark

Swift
• Optionals
• You can think of an implicitly unwrapped optional
as giving permission for the optional to be
unwrapped automatically whenever it is used.
• Rather than placing an exclamation mark after
the optional’s name each time you use it, you
place an exclamation mark after the optional’s
type when you declare it.

Swift
• Optionals
• NOTE
• If an implicitly unwrapped optional is nil and
you try to access its wrapped value, you’ll
trigger a runtime error.
• The result is exactly the same as if you place
an exclamation mark after a normal optional
that does not contain a value.

Swift
• Optionals
• You can still treat an implicitly unwrapped
optional like a normal optional, to check if it
contains a value:
• if assumedString != nil {
• print(assumedString)
• }
• // prints "An implicitly unwrapped optional
string."

Swift
• Optionals
• You can also use an implicitly unwrapped optional
with optional binding, to check and unwrap its
value in a single statement:
• if let definiteString = assumedString {
• print(definiteString)
• }
• // prints "An implicitly unwrapped optional
string.”

Swift
• Optionals
• NOTE
• Do not use an implicitly unwrapped optional
when there is a possibility of a variable
becoming nil at a later point. Always use a
normal optional type if you need to check for
a nil value during the lifetime of a variable.

Swift
• Error Handling
• You use error handling to respond to error
conditions your program may encounter during
execution.
• In contrast to optionals, which can use the
presence or absence of a value to communicate
success or failure of a function, error handling
allows you to determine the underlying cause of
failure, and, if necessary, propagate the error to
another part of your program.

Swift
• Error Handling
• When a function encounters an error
condition, it throws an error. That function’s
caller can then catch the error and respond
appropriately.
• func canThrowAnError() throws {
• // this function may or may not throw an
error
• }

Swift
• Error Handling
• A function indicates that it can throw an error
by including the throws keyword in its
declaration.
• When you call a function that can throw an
error, you prepend the try keyword to the
expression.

Swift
• Error Handling
• Swift automatically propagates errors out of their current
scope until they are handled by a catch clause.
• do {
• try canThrowAnError()
• // no error was thrown
• } catch {
• // an error was thrown
• }
• A do statement creates a new containing scope, which
allows errors to be propagated to one or more catch
clauses.

Swift
• Error Handling
• Here’s an example of how error handling can be used to respond to
different error conditions:
• func makeASandwich() throws {
• // ...
• }
• do {
• try makeASandwich()
• eatASandwich()
• } catch Error.OutOfCleanDishes {
• washDishes()
• } catch Error.MissingIngredients(let ingredients) {
• buyGroceries(ingredients)
• }

Swift
• Error Handling
• In this example, the makeASandwich()
function will throw an error if no clean dishes
are available or if any ingredients are missing.
• Because makeASandwich() throws, the
function call is wrapped in a try expression.
• By wrapping the function call in a do
statement, any errors that are thrown will be
propagated to the provided catch clauses.

Swift
• Error Handling
• If no error is thrown, the eatASandwich() function
is called.
• If an error is thrown and it matches the
Error.OutOfCleanDishes case, then the
washDishes() function will be called.
• If an error is thrown and it matches the
Error.MissingIngredients case, then the
buyGroceries(_:) function is called with the
associated [String] value captured by the catch
pattern.

Swift
• Assertions
• In some cases, it is simply not possible for
your code to continue execution if a particular
condition is not satisfied.
• In these situations, you can trigger an
assertion in your code to end code execution
and to provide an opportunity to debug the
cause of the absent or invalid value.

Swift
• Assertions
• Debugging with Assertions
• An assertion is a runtime check that a Boolean
condition definitely evaluates to true.
• Literally put, an assertion “asserts” that a condition is
true. You use an assertion to make sure that an
essential condition is satisfied before executing any
further code.
• If the condition evaluates to true, code execution
continues as usual; if the condition evaluates to false,
code execution ends, and your app is terminated.

Swift
• Assertions
• If your code triggers an assertion while running in
a debug environment, such as when you build
and run an app in Xcode, you can see exactly
where the invalid state occurred and query the
state of your app at the time that the assertion
was triggered.
• An assertion also lets you provide a suitable
debug message as to the nature of the assert.

Swift
• Assertions
• You write an assertion by calling the Swift standard
library global assert(_:_file:line:) function. You pass this
function an expression that evaluates to true or false
and a message that should be displayed if the result of
the condition is false:
• let age = -3
• assert(age >= 0, "A person's age cannot be less
than zero")
• // this causes the assertion to trigger, because age
is not >= 0

Swift
• Assertions
• In this example, code execution will continue only if
age >= 0 evaluates to true, that is, if the value of age is
non-negative.
• If the value of age is negative, as in the code above,
then age >= 0 evaluates to false, and the assertion is
triggered, terminating the application.
• The assertion message can be omitted if desired, as in
the following example:
• assert(age >= 0)

Swift
• Assertions
• When to Use Assertions
• Use an assertion whenever a condition has the potential to
be false, but must definitely be true in order for your code
to continue execution. Suitable scenarios for an assertion
check include:
• An integer subscript index is passed to a custom
subscript implementation, but the subscript index value
could be too low or too high.
• A value is passed to a function, but an invalid value
means that the function cannot fulfill its task.
• An optional value is currently nil, but a non-nil value is
essential for subsequent code to execute successfully.

IOS Swift language 2nd tutorial

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