Swift 2.2 Design Patterns CocoaConf Austin 2016

Swift 2.2 Design Patterns
COCOACONF AUSTIN
APRIL 2016
Skyline Credit: User:Argash [GFDL (http://www.gnu.org/copyleft/fdl.html) or
CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
@CarlBrwn

[:]?
So, who is this idiot, anyway?
INTRODUCTION
@CarlBrwn

These mean a lot of different things to
different people.
Hopefully they’re of some use to you.
WIDELY RECOGNIZED,
BUT NOT UNDISPUTED

These mean a lot of different things to
different people.
Hopefully they’re of some use to you.
WIDELY RECOGNIZED,
BUT NOT UNDISPUTED
[:]?
Critical Concept in
Software Engineering?
Some think so, others disagree.
[:]?
Simple Enumeration of
Weaknesses in C++
See http://www.norvig.com/design-
patterns/
[:]?
Just Shared Vocabulary?

So, Let’s talk about Swift
(2.2 Version)
PATTERNS ARE
LANGUAGE SPECIFIC
[:]?
Critical Concept in
[:]?
Weaknesses in C++
patterns/
[:]?

So, Let’s talk about Swift
(2.2 Version)
& Cocoa
PATTERNS ARE LANGUAGE
(& FRAMEWORK) SPECIFIC
[:]?
Critical Concept in
[:]?
Weaknesses in C++
patterns/
[:]?

Design Patterns in Swift 2.2CocoaConf Austin 2016
PATTERN SOURCES FOR TODAY
8


8

Classic Cocoa Design Patterns as
implemented in Swift

8

Some Gang of Four Patterns that lend
themselves to Swift

8

themselves to Swift
Patterns Apple Documents for Swift

8

themselves to Swift
Patterns from Other Languages

8

themselves to Swift
Patterns from Other Languages
Interesting/Potential Swift-speciﬁc Patterns


DELEGATE PATTERN
9

Cocoa is said to favor “Composition over Inheritance.” Delegation
is one of the hallmarks of that strategy.
UITableViewDataSource and UITableViewDelegate are
examples.
CLASSIC COCOA PATTERN


DELEGATE PATTERN
9

Cocoa is said to favor “Composition over Inheritance.” Delegation
is one of the hallmarks of that strategy.
UITableViewDataSource and UITableViewDelegate are
examples.
CLASSIC COCOA PATTERN
[:]?
Eases Construction
This is a big reason why we don’t have
to use the “Factory Pattern” in Cocoa/
Swift

DELEGATE PATTERN IN SWIFT
10

var myDS : UITableViewDataSource?
let myTVC = UITableViewController()
let myTV = myTVC.tableView
let num = myDS?.tableView(myTV, numberOfRowsInSection: 0)
let firstHeader = myDS?.tableView?(myTV, titleForHeaderInSection: 0)

DELEGATE PATTERN IN SWIFT
10

var myDS : UITableViewDataSource?
let myTVC = UITableViewController()
let myTV = myTVC.tableView
let num = myDS?.tableView(myTV, numberOfRowsInSection: 0)
let firstHeader = myDS?.tableView?(myTV, titleForHeaderInSection: 0)
1
Test Delegate Optional
The delegate itself must
not be nil (check with ?
and don’t (EVER) use !)
2
Test Method Exists
Put ? after method call to
test for @optional
implementation


ERROR HANDLING PATTERN
11

In Cocoa, methods that produce errors take an NSError pointer
parameter last parameter.
In Swift 2, this is converted to a thrown ErrorType
NEW IN SWIFT 2 - HOOKS INTO COCOA’S (NSError**)


ERROR HANDLING PATTERN
11

In Cocoa, methods that produce errors take an NSError pointer
parameter last parameter.
In Swift 2, this is converted to a thrown ErrorType
NEW IN SWIFT 2 - HOOKS INTO COCOA’S (NSError**)
[]
Not JAva’s Throw
try on each line that might
throw
catch for blocks

ERROR HANDLING PATTERN IN SWIFT
12

- (BOOL)removeItemAtURL:(NSURL *)URL error:(NSError **)error {
NSFileManager *fileManager = [NSFileManager defaultManager];
NSURL *URL = [NSURL fileURLWithPath:@"/path/to/file"];
NSError *error = nil;
BOOL success = [fileManager removeItemAtURL:URL error:&error];
if (!success) {
NSLog(@"Error: %@", error.domain);
}
}

ERROR HANDLING PATTERN IN SWIFT
12

func removeItemAtURL(URL: NSURL) throws {
let fileManager = NSFileManager.defaultManager()
let URL = NSURL.fileURLWithPath("/path/to/file")
do {
try fileManager.removeItemAtURL(URL)
} catch let error as NSError {
print("Error: (error.domain)")
}
}
- (BOOL)removeItemAtURL:(NSURL *)URL error:(NSError **)error {
NSFileManager *fileManager = [NSFileManager defaultManager];
NSURL *URL = [NSURL fileURLWithPath:@"/path/to/file"];
NSError *error = nil;
BOOL success = [fileManager removeItemAtURL:URL error:&error];
if (!success) {
NSLog(@"Error: %@", error.domain);
}
}


OBSERVER PATTERN
13

Key-Value Observing lets you observe any well-behaved Cocoa
property (getter/setter).
It works in Swift *As Long As* the target isKindOf: NSObject
KVO IN SWIFT

OBSERVER PATTERN IN SWIFT
14


14

objectToObserve.addObserver(self, forKeyPath: "myDate", options: .New,
context: &myContext)

14

override func observeValueForKeyPath(keyPath:String?, ofObject object: AnyObject?,
change: [ String : AnyObject ]?, context: UnsafeMutablePointer <Void>) {
if context == &myContext {
if let newValue = change?[NSKeyValueChangeNewKey] {
print("Date changed: (newValue)")
}
} else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change,
context: context)
}
}

14

override func observeValueForKeyPath(keyPath:String?, ofObject object: AnyObject?,
change: [ String : AnyObject ]?, context: UnsafeMutablePointer <Void>) {
if context == &myContext {
if let newValue = change?[NSKeyValueChangeNewKey] {
print("Date changed: (newValue)")
}
} else {
super.observeValueForKeyPath(keyPath, ofObject: object, change: change,
context: context)
}
}
objectToObserve.removeObserver(self, forKeyPath: "myDate", context:
&myContext)

SWIFT-NATIVE KVO *MAYBE* IN SWIFT 4
15



TARGET-ACTION PATTERN
16

In Objective-C, we use @selector to wrap the method name
when adding it as a target.
In Swift 1.x, we just used a String and still had no compiler
typo checking.
In Swift 2.2, we have #selector (SE-0022) and we FINALLY have
compiler type checking.
OSS FTW!!
ATTACH AN ACTION TO A UI ELEMENT

TARGET-ACTION PATTERN IN SWIFT
17


17

@objc func tappedButton(sender:UIButton!) {
print("tapped button")
}

17

}
#if swift(>=2.2)
myButton.addTarget(self, action: #selector(tappedButton),
forControlEvents: .TouchUpInside)
#else
myButton.addTarget(self, action: "tappedButton",
#endif

17

}
#if swift(>=2.2)
myButton.addTarget(self, action: #selector(tappedButton),
#else
myButton.addTarget(self, action: "tappedButton",
#endif
1
Legacy Objective-C
Only @objc functions can
be made #selectors


SINGLETON PATTERN
18

Fairly common in Cocoa, like [UIApplication
sharedApplication]
In Swift, they’re much simpler to declare (and like ObjC, never
dealloc’ed).
A static type property is guaranteed to be lazily initialized only
once, even when accessed across multiple threads
simultaneously.
SINGLETONS: GLOBAL SHARED INSTANCE

SINGLETON PATTERN IN SWIFT
19

class Singleton {
static let sharedInstance = Singleton()
}

SINGLETON PATTERN IN SWIFT
19

class Singleton {
static let sharedInstance = Singleton()
}
[]
Well, That Was Easy
Obj-C is so much more
complicated
[]
Not All Shared Instances Are Singletons
e.g. NSFileManager


“INTROSPECTION PATTERN”
20

Apple’s Docs call this a pattern. It’s pretty much the same
functionality as we discussed for Delegation.
In my mind, “is this a kind of X class” and “does this implement Y
method” are very limited forms of Introspection.
DYNAMICALLY EXAMINE TYPES AT RUNTIME


“INTROSPECTION PATTERN”
20

Apple’s Docs call this a pattern. It’s pretty much the same
functionality as we discussed for Delegation.
In my mind, “is this a kind of X class” and “does this implement Y
method” are very limited forms of Introspection.
DYNAMICALLY EXAMINE TYPES AT RUNTIME
[]
Not Very Useful
Way harder in pure Swift
than ObjC. Most attempts
bridge NSObjects.


API AVAILABILITY CHECKING “PATTERN”
21

Swift does this at Compile time rather than Runtime (like ObjC).
Calling this a “Pattern” seems like a bit of a stretch to me, but
Apple does.
It is indisputably useful, though.
DECIDE WHETHER AN API IS AVAILABLE


API AVAILABILITY CHECKING “PATTERN”
21

Swift does this at Compile time rather than Runtime (like ObjC).
Calling this a “Pattern” seems like a bit of a stretch to me, but
Apple does.
It is indisputably useful, though.
DECIDE WHETHER AN API IS AVAILABLE
[]
Also for Swift Version #s
Starting with Swift 2.2

API AVAILABILITY CHECKING “PATTERN” IN SWIFT
22

if ([CLLocationManager
instancesRespondToSelector:@selector(requestWhenInUseAuthorization)]) {
// Method is available for use.
} else {
// Method is not available.
}

22

let locationManager = CLLocationManager()
if #available(iOS 8.0, OSX 10.10, *) {
locationManager.requestWhenInUseAuthorization()
}
guard #available(iOS 8.0, OSX 10.10, *) else { return }
} else {
}

22

if #available(iOS 8.0, OSX 10.10, *) {
}
guard #available(iOS 8.0, OSX 10.10, *) else { return }
} else {
}
#if swift(>=2.2)
let tapped = #selector(tappedButton)
#endif


VALUE TYPES/STRUCTS
23

Swift structs can have methods and implement protocols much
like classes, but they’re always passed by value (copied) rather
than passed by reference like objects (which makes them
generally thread-safe, amongst other things).
SWIFT HAS RICH VALUE-TYPES


UBIQUITOUS IMMUTABILITY
24

Pretty much all Swift variables can be declared immutable (with
‘let’), not just certain types like ObjC.
Immutable variables are safer, thread-safe and side-effect free,
which is one claim to fame for functional languages.
NOT JUST FOR STRING/DICT/ARRAY ANYMORE


SEPARATION OF ERRORS
25

Swift has several mechanisms to keep error checking outside and
separate from your business logic.
Checks often get moved from runtime to compile time, and
syntax allows for checks to be isolated to avoid further
complicating already-intricate code.
KEEP ERROR CHECKING AWAY FROM YOUR ALGORITHMS


SEPARATION OF ERRORS 1: if let
26

Much safer than finding out the hard way at runtime.
Unless you use (!)
You don’t use (!), do you? (At least not in shipping apps…)
ENFORCES NIL-CHECK AT COMPILE TIME

SEPARATION OF ERRORS IN SWIFT 1: if let
27

if let obj = obj {
//business logic about object goes here
}


SEPARATION OF ERRORS 2: guard
28

Good for checking at the top of a method
Avoids the “if-let pyramid of doom”
LIKE if-let, BUT WITH CONTINUING SCOPE

SEPARATION OF ERRORS IN SWIFT 2: guard
29

guard let obj = obj as? NSString
where obj.length > 0 else { return }
//business logic about object goes here


SEPARATION OF ERRORS 3: defer
30

Make sure something happens as you exit a scope
SCHEDULE SOMETHING FOR LATER


SEPARATION OF ERRORS 3: defer
30

Make sure something happens as you exit a scope
SCHEDULE SOMETHING FOR LATER
[]
Dearly Departed
@synchronized(dict) {
//use dict safely
}

SEPARATION OF ERRORS IN SWIFT 3: defer
31

let lock = NSLock()
func useLock() {
lock.lock()
defer { lock.unlock() }
//use lock
}


SEPARATION OF ERRORS: try?
32

The try syntax gets scattered all over your code.
Not exactly as good, but at least better than NSError** (or god
forbid C’s return codes and errno).
TRY IS A COUNTER-EXAMPLE THOUGH

SEPARATION OF ERRORS IN SWIFT: try?
33

let url = try fileManager.URLForDirectory(.ApplicationDirectory,
inDomain: .UserDomainMask, appropriateForURL: nil, create: false)
try fileManager.removeItemAtURL(
url.URLByAppendingPathComponent("/path/to/file", isDirectory: true))
if let path = url.URLByAppendingPathComponent("/path/to/file").path {
let files = try fileManager.contentsOfDirectoryAtPath(path)
//do something with files
}


MONADS (A LA HASKELL MAYBE)
34

Very complicated, at least if you believe most people.
I remember being amazed - *Really amazed* at WWDC 2014
when I realized Apple stuck a Haskell Maybe Monad into Swift,
and used all of 1-character to do it.
FUNCTIONAL PROGRAMMING CONCEPT

”MAYBE” MONAD
35


”MAYBE” MONAD SWIFT OPTIONAL
36


36

Optional


36

Optional

! ?



36

Optional

! ?


?

36

Optional

! ?


?
!

36

Optional

! ?


?
!
[]
“!” Considered HARMFUL
I mean that in a “GOTO” sense.


MAPREDUCE
37

Without this, the Internet as you know it would never have come
to exist, and least not when and how it did.
SEMINAL PAPER BEHIND EARLY GOOGLE
http://static.googleusercontent.com/media/research.google.com/es/us/archive/mapreduce-osdi04.pdf

MAPREDUCE — MAP
38



MAPREDUCE — MAP
38







 



MAPREDUCE — MAP
38







 

[]
Rule of thumb
Same count,
Different Type

MAPREDUCE — MAP IN SWIFT
39
let names = ["RedStripeOnWhite.jpg","OrangeStripeOnWhite.jpg",
"YellowStripeOnWhite.jpg","GreenStripeOnWhite.jpg",
"BlackHorizontal.jpg","BlueStripeOnWhite.jpg",
“IndigoStripeOnWhite.jpg”,"VioletStripeOnWhite.jpg"]
let images = names.map {
UIImage(named: $0)
}

MAPREDUCE — MAP IN SWIFT
39
let names = ["RedStripeOnWhite.jpg","OrangeStripeOnWhite.jpg",
"YellowStripeOnWhite.jpg","GreenStripeOnWhite.jpg",
"BlackHorizontal.jpg","BlueStripeOnWhite.jpg",
“IndigoStripeOnWhite.jpg”,"VioletStripeOnWhite.jpg"]
let images = names.map {
UIImage(named: $0)
}
1
Real WorlD: JSON
I use this a lot to map
JSON Dict to Object/Struct,
but this is more visual

MAPREDUCE — FILTER
40


40










40









[]
Rule of thumb
Same Type,
Lower Count

MAPREDUCE — FILTER IN SWIFT
41

let colors = images.filter { !mono($0) }

MAPREDUCE — FILTER IN SWIFT
41

let colors = images.filter { !mono($0) }
1
Real WorlD: JSON
I use this a lot to throw out
JSON Dicts I don’t need

MAPREDUCE — REDUCE
42



42








42






[]
Rule of thumb
Different Type,
Count == 1

43

let rainbowImage = ciImages.reduce(CIImage()) { (s, img) -> CIImage? in
let f = CIFilter(name: "CISourceOverCompositing")
f?.setValue(s, forKey: kCIInputBackgroundImageKey)
f?.setValue(img, forKey:kCIInputImageKey)
return f?.outputImage
}

44

}

45

}

46

}

47

}

48

}

49

}


MAPREDUCE - WAIT, WAIT, WAIT!!!
50

I have yet to see a case where map/reduce worked consistently
faster on iOS than the equivalent for..in loop, and often it can be
quite a bit slower.
BUT - learn it anyway.
ABOUT SWIFT’S MAPREDUCE PERFORMANCE…


MAPREDUCE - SO WHAT’S THE POINT?
51

…or threads. Or at least, that was the design intent.
I still use it (although I don’t have to), because I’m used to it from
other languages (and I use it with dispatch_groups, despite the
fact they don’t fit on slides).
But Soon (SWIFT 4?? - see earlier email), we’ll have a Swift release
that focuses on asynchronous functionality. And THEN, it will
really, really matter.
MAPREDUCE SPLITS WORK BETWEEN MACHINES


PROTOCOL DEFAULT IMPLEMENTATIONS
52

Protocol Implementations allow for much more useful protocols,
and for much more flexible implementations.
WHOLE NEW WORLD


PROTOCOL EXTENSIONS (W/IMPLEMENTATIONS)
53

WHOLE NEW WORLD


PROTOCOL EXTENSIONS (W/IMPLEMENTATIONS)
53

WHOLE NEW WORLD
1
Examples are Contrived
And they take a while to
set up, and I’m expecting
this is already going long
2
Big Topic
There’s a whole (VERY
GOOD) WWDC talk on just
this.

Speaking to you all is always a pleasure.
Thank You

Swift 2.2 Design Patterns CocoaConf Austin 2016

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Swift 2.2 Design Patterns CocoaConf Austin 2016