Announcements
• Lab 2 is due next Monday (Sept 23rd) by 11:59 PM – Make sure your lab does not contain any warnings
when you submit it
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Todays Topics
• Additional Swift Concepts • Views
• Drawing
• Text & Images
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Lazy Initialization of Properties (CS193p)
• Lazy properties do not get initialized until someone accesses them
• You can allocate objects, execute a closure, or call a method lazy var theResult = LotsOfWorkObject()
lazy var someProperty: Type = {
// construct the value of someProperty here
return (the constructed value) }()
lazy var myProperty = self.initializeMyProperty()
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Initialization in Swift
• Classes and structures must set all of their stored properties when created
• Various way to set properties (without an init)
– Define default values
– Properties may be Optional (so they start out as nil) – Initialize a property by setting a closure
– Use lazy instantiation
• Use an init when values can not be set using the previous examples
– You can have as many init methods in your class or struct – Each init will have different arguments
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Initialization (CS193p)
• Some init methods are for free
– Free init() given to all base classes
• A base class has no superclass
– If a struct has no initializers, it will get a default one will all properties as arguments
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Initialization (CS193p)
• What can I do with an init?
– Set property values, even those that already had defaults
– Constant properties (those declared with let) can be set
– You can call other init methods in your own class or stuct
using self.init(args)
– In a class, you can also call super.init(args)
• There are some rules for calling inits from other inits in a class
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Class Initialization Requirements (CS193p)
• After init completes all properties must have values (Optionals can be nil)
• A class has two types of inits
– Convenience and designated
• Designated init
– Must (and can only) call a designated init in its immediate superclass
– You must initialize all properties introduced by your class before calling a
superclass’s init
– You must call a superclass’s init before you assign a value to an inherited
property
• Convenience init
– Must (and can only) call an init in its own class
– Must call that init before it can set any property values
– The call of other inits must be completed before you can access properties or
invoke methods
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Initialization (CS193p)
• Inheritinginit
– If you do not implement any designated inits, you will inherit all of you superclass’s designated inits
– If you override all of your superclass’s designated inits, you’ll inherit all its convenience inits
– If you implement no inits, you will inherit all of your superclass’s inits
– Any init inherited by these rules qualifies to satisfy any of the rules on the previous slide
• Requiredinit
– A class can mark one or more of its init methods as required – Any subclass must implement those init methods
• They can be inherited per rules above
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Failable init (CS193p)
• If an init is declared with a ? after the word init, it returns an Optional init? (arg1: Type1,..) {
} // might return nil here (means init failed) • Example
Let image = UIImage(named:”foo”) //image is Optional UIImage
• Typically use if-let for these cases If let image = UIImage(named: “foo ” ) {
// image was successfully created } else {
// failed to create image }
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Demo
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Other Swift Concepts
• Swift Syntax – Guard
– Defer
• Swift Error Handling
– https://docs.swift.org/swift-
book/LanguageGuide/ErrorHandling.html#
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Guard
• Used to transfer program control out of a scope, if one or more conditions are not met
– Early exit
• Improves readability of code – Avoid the if let, if let, if let
guard condition else {
statements }
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Defer
• Used to execute a set of statements just before code execution leaves the current block of code
– “defers” execution until the current scope is exited
defer {
statements }
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Demo
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Error Handling
• Swift supports throwing, catching, propagating and manipulating recoverable errors at runtime
• Helpful when an operation does not complete execution or fails to provide useful output
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Represent and Throw Errors
• Errors are represented by values of types conforming to the Error Protocol
enum VendingMachineError: Error { case invalidSelection
case insuffientFunds(coinsNeeded: Int)
case outOfStock }
throw VendingMachineError.insufficientFunds(coinsNeeded: 5)
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Four Ways to Handle Errors
• Propagate errors from a function to the code calling it
• Handle the error using a do-catch statement
• Handle the error as an optional value (try?)
• Assert the error will not occur (try!)
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Propagating Errors
• To specify that a function, method, or initializer can throw an error, you write the throws keyword
func canThrowError() throws -> String func cannnotThrowErrors() -> String
func buy(itemNumber: Int) throws -> String { if itemNumber > 20 {
throw VendingMachineError.invalidNumber }
return “Coke” }
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Error Handling – Do-Catch
• When handling errors in code use a do-catch statement of the following form:
do {
try expression
statements
} catch pattern 1 {
statements
} catch pattern 2 where condition {
statements
} catch {
} statements //all other error conditions
• Not necessary to catch all conditions here, as error will propagate to surrounding scope
– Must be caught by some surround scope
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Handling Error as an optional value
• Use the try? to handle an error by converting it to an optional – Ifanerroristhrownwhileevaluatingthetry?itwillreturnnil.
func someThrowingFunction() throws -> Int {
// … }
let x = try? someThrowingFunction()
• Equivalent to writing the following code let y: Int?
do {
y = try someThrowingFunction() } catch {
y = nil }
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Disable Error propagation
• If you know that a function will not throw an error, you can disable error propagation.
– If the error is thrown you will get a runtime error let photo = try! loadImage(atPath: “./Resources/John Appleseed.jpg”)
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Demo
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Views
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View Fundamentals
• Rectangular area on screen
• Draws content
• Handles events
• Subclass of UIResponder (event handling class)
• Views arranged hierarchically
– every view has one superview
– every view has zero or more subviews
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View Hierarchy – UIWindow
• Views live inside of a window
• UIWindow is actually just a view
– adds some additional functionality specific to top
level view
• One UIWindow for an iOS app
– Contains the entire view hierarchy
– Set up by default in Xcode template project
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View Hierarchy – Manipulation
• Add/remove views in Storyboard or using UIView methods
func addSubview(UIView) func removeFromSuperview()
• Manipulate the view hierarchy manually:
func insertSubview(UIView , at: Int)
func insertSubview(UIView, belowSubview: UIView) func insertSubview(UIView, aboveSubview: UIView) func exchangeSubview(at: Int, withSubviewAt: Int)
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View-related Structures
• CGPoint
– location in space:{ x , y }
– sometimes used as an origin
• CGSize
– dimensions: { width , height }
• CGRect
– location and dimension: { origin , size }
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Rects, Points and Sizes
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Creation Function
CGPoint(x: Double,y: Double)
CGSize(width: Double, height: Double)
CGRect(x: Double,y: Double ,width: Double, height: Double)
View-related Structure
Example
var point = CGPoint(x: 100.0, y: 200.0)
point.x = 300.0 point.y = 30.0
var size = CGSize (width: 42.0, height: 11.0);
size.width = 100.0 size.height = 72.0
var rect = CGRect (x:100.0, y: 200.0, width: 42.0, height: 11.0)
rect.origin.x = 0.0 rect.size.width = 50.0
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UIView Coordinate System
• Origin in upper left corner • y axis grows downwards
• Units are points, not pixels
– Points are units of coordinate system
– Pixels are min size unit of drawing
– Typically 2 pixels per point +y • var ContentScaleFactor
+x
0,0
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Location and Size
•
0,0
400
Views location and size expressed in two ways
– –
Frame is in superviews coordinate system Bounds is in local coordinate system
550
What about View B?
• View A frame: • Origin: 0,0
• Size: 550 x 400
• View A bounds :
• Origin: 0,0
• Size 550 x 400
• View B frame:
• Origin: 200, 100
• Size 200 x 250
• View B bounds:
• Origin: 0,0
• Size: 200 x 250
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Frame and Bounds
• Which to use?
– Usually depends on the context
• If you are using a view, typically you use bounds
• If you are implementing a view, typically you use frame
• Matter of perspective
– From outside its usually the frame
– From inside its usually the bounds
• Examples:
– Creating a view, positioning a view in superview – use frame
– Handling events, drawing a view – use bounds
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Creating Views
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Where do views come from?
• Commonly placed in Storyboard
• Drag out any of the existing view objects (buttons, labels, etc)
• Or drag generic UIView and set custom class
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Manual Creation
• Views are initialized using UIView.init(frame: ) let theFrame = CGRect(x:0, y:0, width:200, height:150)
let myView = UIView(frame: theFrame)
• Example:
let frame = CGRect(x:20, y:45, width: 140, height: 20) let myLabel = UILabel(frame:frame)
myLabel.text = “Hello Class” view.addSubview(myLabel)
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Defining Custom Views
• Subclass UIView
• For custom drawing, you override:
func draw(_ rect: CGRect)
• For event handling, you override:
func touchesBegan(_ touches: Set
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Drawing Views
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draw: Method
• – draw: does nothing by default
– If not overridden, then backgroundColor is used to fill
• Override – draw: to draw a custom view – rect argument is area to draw
• When is it OK to call draw:?
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Be Lazy
• draw: is invoked automatically – Dont call it directly!
• Being lazy is good for performance
• When a view needs to be redrawn, use: setNeedsDisplay
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Demo
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CoreGraphics and Quartz 2D
• UIKit offers very basic drawing functionality – UIRectFill(CGRectrect)
– UIRectFrame(CGRectrect)
• CoreGraphics: Drawing APIs
• CG is a C-based API, not Objective-C
• CG and Quartz 2D drawing engine define simple but powerful graphics primitives
– Graphicscontext – Transformations – Paths
– Colors
– Fonts
– Paintingoperations
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CG Wrappers
• Some CG functionality wrapped by UIKit • UIColor
– Convenience for common colors
– Easily set the fill and/or stroke colors when drawing
UIColor.red.set()
// drawing will be done in red
• UIFont
– Access system font
– Get font by name
– Get preferred font for a given text style
• Bestwayforfontincode
class func preferredFont(forTextStyle style: UIFontTetStyle) -> UIFont
– A few examples of Text Styles
• UIFontTextStyle.headline • UIFontTextStyle.body
• UIFontTextStyle.footnote
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Simple draw(_:) example
• Draw a solid color and shape override func draw(_ rect: CGRect) {
let bounds = self.bounds
UIColor.gray.set() UIRectFill (bounds)
What shape is this?
let myShape = CGRect(x: 10, y: 10, width: 50, height: 100) UIColor.red.set()
UIRectFill(myShape)
UIColor.black.set() UIRectFrame(myShape)
}
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Drawing More Complex Shapes
• Common steps for draw:
– Get current graphics context – Define a path
– Set a color
– Stroke or fill path
– Repeat, if necessary
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Paths
• CoreGraphics paths define shapes
• Made up of lines, arcs, curves and rectangles
• Creation and drawing of paths are two distinct operations
– Define path first, then draw it
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Drawing Shapes using Bezier Paths
• First create a Bezier Path let path = UIBezierPath()
• Move around, add lines or arcs to path path.move(to: CGPoint(x:60,y:40)) path.addLine(to: CGPoint(x:100,y:50))
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Simple Example
override func draw(_ rect: CGRect){
let path = UIBezierPath()
path.move(to: CGPoint(x: 75,y: 10)) path.addLine(to: CGPoint(x: 10,y: 150)) path.addLine(to: CGPoint(x: 160,y: 150)) path.close()
UIColor.red.setFill() UIColor.black.setStroke() path.lineWidth = 3.0
path.stroke()
path.fill()
}
What shape is this?
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More Drawing Information
• UIView Class Reference
• CGContext Reference
• Quartz 2D Programming Guide
• Lots of samples in the iPhone Dev Center
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