Read-only and non-computed variable properties in Swift - objective-c

I'm trying to figure out something with the new Apple Swift language. Let's say I used to do something like the following in Objective-C. I have readonly properties, and they cannot be individually changed. However, using a specific method, the properties are changed in a logical way.
I take the following example, a very simple clock. I would write this in Objective-C.
#interface Clock : NSObject
#property (readonly) NSUInteger hours;
#property (readonly) NSUInteger minutes;
#property (readonly) NSUInteger seconds;
- (void)incrementSeconds;
#end
#implementation Clock
- (void)incrementSeconds {
_seconds++;
if (_seconds == 60) {
_seconds = 0;
_minutes++;
if (_minutes == 60) {
_minutes = 0;
_hours++;
}
}
}
#end
For a specific purpose, we cannot touch the seconds, minutes and hours directly, and it's only allowed to increment second by second using a method. Only this method could change the values by using the trick of the instance variables.
Since there are no such things in Swift, I'm trying to find an equivalent. If I do this:
class Clock : NSObject {
var hours: UInt = 0
var minutes: UInt = 0
var seconds: UInt = 0
func incrementSeconds() {
self.seconds++
if self.seconds == 60 {
self.seconds = 0
self.minutes++
if self.minutes == 60 {
self.minutes = 0
self.hours++
}
}
}
}
That would work, but anybody could change directly the properties.
Maybe I already had a bad design in Objective-C and that's why the potential new Swift equivalent is not making sense. If not and if someone have an answer, I would be very grateful ;)
Maybe the future Access Control Mechanisms promised by Apple is the answer?
Thanks!

Simply prefix the property declaration with private(set), like so:
public private(set) var hours: UInt = 0
public private(set) var minutes: UInt = 0
public private(set) var seconds: UInt = 0
private keeps it local to a source file, while internal keeps it local to the module/project.
private(set) creates a read-only property, while private sets both, set and get to private.

There are two basic ways of doing what you want. First way is by having a private property and and a public computed property which returns that property:
public class Clock {
private var _hours = 0
public var hours: UInt {
return _hours
}
}
But this can be achieved in a different, shorter way, as stated in the section "Access Control" of the "The Swift Programming Language" book:
public class Clock {
public private(set) var hours = 0
}
As a side note, you MUST provide a public initialiser when declaring a public type. Even if you provide default values to all properties, init() must be defined explicitly public:
public class Clock {
public private(set) var hours = 0
public init() {
hours = 0
}
// or simply `public init() {}`
}
This is also explained in the same section of the book, when talking about default initialisers.

Since there are no access controls (meaning that you can't make an access contract that differs depending on who the caller is), here's what I would do for now:
class Clock {
struct Counter {
var hours = 0;
var minutes = 0;
var seconds = 0;
mutating func inc () {
if ++seconds >= 60 {
seconds = 0
if ++minutes >= 60 {
minutes = 0
++hours
}
}
}
}
var counter = Counter()
var hours : Int { return counter.hours }
var minutes : Int { return counter.minutes }
var seconds : Int { return counter.seconds }
func incrementTime () { self.counter.inc() }
}
This merely adds a level of indirection, as it were, to direct access to the counter; another class can make a Clock and then access its counter directly. But the idea — i.e., the contract we're trying to make — is that another class should use the Clock's top-level properties and methods only. We can't enforce that contract, but actually it was pretty much impossible to enforce in Objective-C too.

Actually access control (which does not exist yet in Swift) is not as enforced as you may think in Objective C. People can modify your readonly variables directly, if they really want to. They just do not do it with the public interface of the class.
You can do something similar in Swift (cut&paste of your code, plus some modifications, I did not test it):
class Clock : NSObject {
var _hours: UInt = 0
var _minutes: UInt = 0
var _seconds: UInt = 0
var hours: UInt {
get {
return _hours
}
}
var minutes: UInt {
get {
return _minutes
}
}
var seconds: UInt {
get {
return _seconds
}
}
func incrementSeconds() {
self._seconds++
if self._seconds == 60 {
self._seconds = 0
self._minutes++
if self._minutes == 60 {
self._minutes = 0
self._hours++
}
}
}
}
which is the same as what you have in Objective C except that the actual stored properties are visible in the public interface.
In swift you can also do something more interesting, which you can also do in Objective C, but it's probably prettier in swift (edited in the browser, I did not test it):
class Clock : NSObject {
var hours: UInt = 0
var minutes: UInt {
didSet {
hours += minutes / 60
minutes %= 60
}
}
var seconds: UInt {
didSet {
minutes += seconds / 60
seconds %= 60
}
}
// you do not really need this any more
func incrementSeconds() {
seconds++
}
}

Related

Objective-C enum value into Swift enum

Before you vote on this question I would like to how dumb what I am trying to do is. Maybe I still don't understand properly enums.
So, I am working on a project that uses an Obj-C framework. This framework contains enums:
typedef enum : NSInteger
{
kImageSizeUnknown = 0,
kImageSize75,
kImageSize110,
kImageSize170,
kImageSize220,
kImageSize300,
kImageSize450,
kImageSize720,
kImageSize1080,
/* Size aliases */
kImageSizeThumbnail = kImageSize75,
kImageSizeSmall = kImageSize170,
kImageSizeMedium = kImageSize450,
kImageSizeLarge = kImageSize720,
kImageSizeXLarge = kImageSize1080
} GnImageSize;
I want somehow be able to declare a Swift enum that returns values of the Obj-C enum (That might be the silly part).
That is how I have at the moment.
enum GNImageSize:Int, CaseIterable{
case thumbnail
case sizeSmall
func toGnImageSize() -> GnImageSize {
switch self {
case .thumbnail:
return kImageSizeThumbnail
case .sizeSmall:
return kImageSizeSmall
}
}
static func toGnImageSize(sizeType:GNImageSize) -> GnImageSize {
switch sizeType {
case .thumbnail:
return kImageSizeThumbnail
case .sizeSmall:
return kImageSizeSmall
}
}
}
However when I do:
enum GNImageSize:Int, CaseIterable{
case thumbnail = GnImageSize.kImageSizeThumbnail
case sizeSmall = GnImageSize.kImageSizeSmall
}
I get the following error message:
Raw value for enum case must be a literal
Thank you.
EDIT
Function that comunicates with Obj-c functions
func getArtworkURL(forImageType imageType:GNImageSize, shouldFindAlternatives:Bool, highQualityFirst:Bool)->URL?{
if let asset = coverArt()?.asset(GnImageSize(rawValue: imageType.rawValue)), let assetURL = asset.urlHttp(){
return URL(string:assetURL)
}
else{
if shouldFindAlternatives{
if highQualityFirst{
for size in GNImageSize.allCases.reversed(){
if let asset = coverArt()?.asset(GnImageSize(rawValue: size.rawValue)), let assetURL = asset.urlHttp(){
return URL(string:assetURL)
}
}
}
else{
for size in GNImageSize.allCases{
if let asset = coverArt()?.asset(GnImageSize(rawValue: size.rawValue)), let assetURL = asset.urlHttp(){
return URL(string:assetURL)
}
}
}
}
}
return nil
}
Where
-(nullable GnAsset*) asset: (GnImageSize)imageSize;
The five size aliases have the (raw) values 1, 3, 6, 7, 8 so declare a Swift enum
enum GNImageSize : Int {
case thumbnail = 1
case small = 3
case medium = 6
case large = 7
case xLarge = 8
}
To use the Int value in Swift use for example
GNImageSize.thumbnail.rawValue
Alternatively create a custom enum with static properties to map the types
enum GNImageSize {
static let thumbnail = GnImageSize(0)
static let small = GnImageSize(3)
static let medium = GnImageSize(6)
static let large = GnImageSize(7)
static let xLarge = GnImageSize(8)
}
I don't understand that in 2018 ObjC frameworks still use the Stone-age syntax typedef enum : NSInteger { ... } Foo; rather than Swift compliant syntax typedef NS_ENUM (NSInteger, Foo) { ... }; The latter syntax exists for 6 years (iOS 6, macOS 10.8).
That raw-value style enum grammar error.
raw-value-assignment → = raw-value-literal
raw-value-literal → numeric-literal | static-string-literal | boolean-literal
So, only numeric(numbers like -7, 0x10, 0b010), static string(characters in quotes, like "foo") and boolean(true or false) literals are allowed there.
Anything else won't work.

OptaPlaner simple example cant find feasible solution

to get familiar with optaplanner i created a simple test project. I only have one Solution and one Entity class. The Entity has only one value between 0 and 9. There should only be odd numbers and the sum of all should be less then 10 (this are just some random constraints i came up with).
As Score i use a simple HardSoftScore. Here is the code:
public class TestScoreCalculator implements EasyScoreCalculator<TestSolution>{
#Override
public HardSoftScore calculateScore(TestSolution sol) {
int hardScore = 0;
int softScore = 0;
int valueSum = 0;
for (TestEntity entity : sol.getTestEntities()) {
valueSum += entity.getValue() == null? 0 : entity.getValue();
}
// hard Score
for (TestEntity entity : sol.getTestEntities()) {
if(entity.getValue() == null || entity.getValue() % 2 == 0)
hardScore -= 1; // constraint: only odd numbers
}
if(valueSum > 10)
hardScore -= 2; // constraint: sum should be less than 11
// soft Score
softScore = valueSum; // maximize
return HardSoftScore.valueOf(hardScore, softScore);
}
}
and this is my config file:
<?xml version="1.0" encoding="UTF-8"?>
<solver>
<!-- Domain model configuration -->
<scanAnnotatedClasses/>
<!-- Score configuration -->
<scoreDirectorFactory>
<easyScoreCalculatorClass>score.TestScoreCalculator</easyScoreCalculatorClass>
</scoreDirectorFactory>
<!-- Optimization algorithms configuration -->
<termination>
<secondsSpentLimit>30</secondsSpentLimit>
</termination>
</solver>
for some reason OptaPlanner cant find a feasible solution. It terminates with LS step (161217), time spent (29910), score (-2hard/10soft), best score (-2hard/10soft)... and the solution 9 1 0 0.
So the hardScore is -2 because the two 0 are not odd. A possible solution would be 7 1 1 1 for example. Why is this ? This should be a really easy example ...
(when i set the Start values to 7 1 1 1 it terminates with this solution and a score of (0hard/10soft) how it should be)
Edit:
The Entity class
#PlanningEntity
public class TestEntity {
private Integer value;
#PlanningVariable(valueRangeProviderRefs = {"TestEntityValueRange"})
public Integer getValue() {
return value;
}
public void setValue(Integer value) {
this.value = value;
}
#ValueRangeProvider(id = "TestEntityValueRange")
public CountableValueRange<Integer> getStartPeriodRange() {
return ValueRangeFactory.createIntValueRange(0, 10);
}
}
The Solution class
#PlanningSolution
public class TestSolution {
private List<TestEntity> TestEntities;
private HardSoftScore score;
#PlanningEntityCollectionProperty
public List<TestEntity> getTestEntities() {
return TestEntities;
}
public void setTestEntities(List<TestEntity> testEntities) {
TestEntities = testEntities;
}
#PlanningScore
public HardSoftScore getScore() {
return score;
}
public void setScore(HardSoftScore score) {
this.score = score;
}
#Override
public String toString() {
String str = "";
for (TestEntity testEntity : TestEntities)
str += testEntity.getValue()+" ";
return str;
}
}
The Main Program class
public class Main {
public static final String SOLVER_CONFIG = "score/TestConfig.xml";
public static int printCount = 0;
public static void main(String[] args) {
init();
}
private static void init() {
SolverFactory<TestSolution> solverFactory = SolverFactory.createFromXmlResource(SOLVER_CONFIG);
Solver<TestSolution> solver = solverFactory.buildSolver();
TestSolution model = new TestSolution();
List<TestEntity> list = new ArrayList<TestEntity>();
// list.add(new TestEntity(){{setValue(7);}});
// list.add(new TestEntity(){{setValue(1);}});
// list.add(new TestEntity(){{setValue(1);}});
// list.add(new TestEntity(){{setValue(1);}});
for (int i = 0; i < 4; i++) {
list.add(new TestEntity());
}
model.setTestEntities(list);
// Solve the problem
TestSolution solution = solver.solve(model);
// Display the result
System.out.println(solution);
}
}
It gets stuck in a local optima because there is no move that takes 1 from entity and gives it to another entity. With a custom move you can add that.
These kind of moves only apply to numeric value ranges (which are rare, usually value ranges are a list of employees etc), but they should probably exist out of the box (feel free to create a jira for them).
Anyway, another way to get the good solution is to add <exhaustiveSearch/>, that bypassing local search and therefore the local optima. But that doesn't scale well.

How to convert this Swift syntax into Objective C?

I'm converting a swift project to objective c, but i get some trouble because i don't know how to convert follow code. Please help me. Thanks!
public enum UPCarouselFlowLayoutSpacingMode {
case fixed(spacing: CGFloat)
case overlap(visibleOffset: CGFloat)
}
and
fileprivate var currentPage: Int = 0 {
didSet {
let character = self.items[self.currentPage]
self.infoLabel.text = character.name.uppercased()
self.detailLabel.text = character.movie.uppercased()
}
}
The first (an enum with associated values) has no direct equivalent in Objective-C. For your particular example, you could use something like this:
typedef NS_ENUM(NSInteger, UPCarouselFlowLayoutSpacingMode) {
UPCarouselFlowLayoutSpacingModeFixed,
UPCarouselFlowLayoutSpacingModeOverlap
};
typedef struct {
UPCarouselFlowLayoutSpacingMode mode;
CGFloat amount;
} UPCarouselFlowLayoutSpacing;
You would just pass around values of type UPCarouselFlowLayoutSpacing. You could create helper functions to make these easier to create, e.g.
UPCarouselFlowLayoutSpacing UPCarouselFlowLayoutSpacingMakeFixed(CGFloat spacing) {
UPCarouselFlowLayoutSpacing value;
value.mode = UPCarouselFlowLayoutSpacingModeFixed;
value.amount = spacing;
return value;
}
UPCarouselFlowLayoutSpacing UPCarouselFlowLayoutSpacingMakeOverlap(CGFloat visibleOffset) {
UPCarouselFlowLayoutSpacing value;
value.mode = UPCarouselFlowLayoutSpacingModeOverlap;
value.amount = visibleOffset;
return value;
}
For the second, you can override the setter method of your Objective-C class's currentPage property:
- (void)setCurrentPage:(NSInteger)page {
_page = page;
MovieCharacter *character = self.items[page];
self.infoLabel.text = character.name.localizedUppercaseString;
self.detailLabel.text = character.movie.localizedUppercaseString;
}

"Binary/Unary operator '++/<' cannot be applied to an operand of type" AND "Use of unresolved identifier '=-'"

I am translating an Obj-C app to Swift and having trouble dealing with some syntax. I believe I have declared the variable types correctly so I don't know why I'm be getting these errors. Maybe some blocks are located incorrectly inside classes/functions when they should be outside or something. I would love it if you could review my code. I'm new to programming so what may be a clear and explicit explanation to you probably will still be vague for me so please show with examples using existing names.
Thanks
"Unary operator '++' cannot be applied to an operand of type 'Int?'"
and
"Binary operator '<' cannot be applied to an operand of type 'Int? and Float'"
and
"Use of unresolved identifier '=-'"
import UIKit
import Foundation
import AVFoundation
let minFramesForFilterToSettle = 10
enum CurrentState {
case statePaused
case stateSampling
}
class ViewController: UIViewController, AVCaptureVideoDataOutputSampleBufferDelegate {
let session = AVCaptureSession()
var camera : AVCaptureDevice?
var validFrameCounter: Int = 0
var pulseDetector: PulseDetector!
var filter: Filter!
var currentState = CurrentState.stateSampling // Is this initialized correctly?
override func viewDidLoad() {
super.viewDidLoad()
self.pulseDetector = PulseDetector()
self.filter = Filter()
// TO DO startCameraCapture() // call to un-used function.
}
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
}
let NZEROS = 10
let NPOLES = 10
class Filter {
var xv = [Float](count: NZEROS + 1, repeatedValue: 0)
var yv = [Float](count: NPOLES + 1, repeatedValue: 0)
func processValue(value: Float) -> Float {
let gain: Float = 1.894427025e+01
xv[0] = xv[1]; xv[1] = xv[2]; xv[2] = xv[3]; xv[3] = xv[4]; xv[4] = xv[5]; xv[5] = xv[6]; xv[6] = xv[7]; xv[7] = xv[8]; xv[8] = xv[9]; xv[9] = xv[10]; xv[10] = value / gain;
yv[0] = yv[1]; yv[1] = yv[2]; yv[2] = yv[3]; yv[3] = yv[4]; yv[4] = yv[5]; yv[5] = yv[6]; yv[6] = yv[7]; yv[7] = yv[8]; yv[8] = yv[9]; yv[9] = yv[10];
yv[10] = (xv[10] - xv[0]) + 5 * (xv[2] - xv[8]) + 10 * (xv[6] - xv[4])
+ ( -0.0000000000 * yv[0]) + ( 0.0357796363 * yv[1])
+ ( -0.1476158522 * yv[2]) + ( 0.3992561394 * yv[3])
+ ( -1.1743136181 * yv[4]) + ( 2.4692165842 * yv[5])
+ ( -3.3820859632 * yv[6]) + ( 3.9628972812 * yv[7])
+ ( -4.3832594900 * yv[8]) + ( 3.2101976096 * yv[9]);
return yv[10];
}
}
let maxPeriod = 1.5 // float?
let minPeriod = 0.1 // float?
let invalidEntry:Double = -11
let maxPeriodsToStore:Int = 20
let averageSize:Float = 20
class PulseDetector {
var upVals: [Float] = [averageSize]
var downVals: [Float] = [averageSize]
var upValIndex: Int?
var downValIndex: Int?
var lastVal: Float?
var periodStart: Float?
var periods: [Double] = []
var periodTimes: [Double] = []
var periodIndex: Int?
var started: Bool?
var freq: Float?
var average: Float?
var wasDown: Bool?
func reset() {
for var i=0; i < maxPeriodsToStore; i++ {
periods[i] = invalidEntry
}
for var i=0; i < averageSize; i++ { // why error when PulseDetector.h said averageSize was an Int?
upVals[i] = invalidEntry
downVals[i] = invalidEntry
}
freq = 0.5
periodIndex = 0
downValIndex = 0
upValIndex = 0
}
func addNewValue(newVal:Float, atTime:Double) -> Float {
// we keep track of the number of values above and below zero
if newVal > 0 {
upVals[upValIndex!] = newVal
upValIndex++
if upValIndex >= averageSize {
upValIndex = 0
}
}
if newVal < 0 {
downVals[downValIndex] =- newVal
downValIndex++
if downValIndex >= averageSize {
downValIndex = 0
}
}
// work out the average value above zero
var count: Float
var total: Float
for var i=0; i < averageSize; i++ {
if upVals[i] != invalidEntry {
count++
total+=upVals[i]
}
}
var averageUp = total/count
// and the average value below zero
count=0;
total=0;
for var i=0; i < averageSize; i++ {
if downVals[i] != invalidEntry {
count++
total+=downVals[i]
}
}
var averageDown = total/count
// is the new value a down value?
if newVal < (-0.5*averageDown) {
wasDown = true
}
// original Objective-C code
PulseDetector.h
#import <Foundation/Foundation.h>
#define MAX_PERIODS_TO_STORE 20 // is this an Int?
#define AVERAGE_SIZE 20 // is this a Float?
#define INVALID_PULSE_PERIOD -1 // done
#interface PulseDetector : NSObject {
float upVals[AVERAGE_SIZE];
float downVals[AVERAGE_SIZE];
int upValIndex;
int downValIndex;
float lastVal;
float periodStart;
double periods[MAX_PERIODS_TO_STORE]; // this is an array!
double periodTimes[MAX_PERIODS_TO_STORE]; // this is an rray !!
int periodIndex;
bool started;
float freq;
float average;
bool wasDown;
}
#property (nonatomic, assign) float periodStart; // var periodStart = float?
-(float) addNewValue:(float) newVal atTime:(double) time; // declaring a method called addNewValue with 2 arguments called atTime and time that returns a float
-(float) getAverage; // declaring a method called getAverage that returns a float
-(void) reset; // declaring a method that returns nothing
#end
PulseDetector.m
#import <QuartzCore/QuartzCore.h>
#import "PulseDetector.h"
#import <vector>
#import <algorithm>
#define MAX_PERIOD 1.5
#define MIN_PERIOD 0.1
#define INVALID_ENTRY -100 // is this a double?
#implementation PulseDetector
#synthesize periodStart;
- (id) init
{
self = [super init];
if (self != nil) {
// set everything to invalid
[self reset];
}
return self;
}
-(void) reset {
for(int i=0; i<MAX_PERIODS_TO_STORE; i++) {
periods[i]=INVALID_ENTRY;
}
for(int i=0; i<AVERAGE_SIZE; i++) {
upVals[i]=INVALID_ENTRY;
downVals[i]=INVALID_ENTRY;
}
freq=0.5;
periodIndex=0;
downValIndex=0;
upValIndex=0;
}
-(float) addNewValue:(float) newVal atTime:(double) time {
// we keep track of the number of values above and below zero
if(newVal>0) {
upVals[upValIndex]=newVal;
upValIndex++;
if(upValIndex>=AVERAGE_SIZE) {
upValIndex=0;
}
}
if(newVal<0) {
downVals[downValIndex]=-newVal;
downValIndex++;
if(downValIndex>=AVERAGE_SIZE) {
downValIndex=0;
}
}
// work out the average value above zero
float count=0;
float total=0;
for(int i=0; i<AVERAGE_SIZE; i++) {
if(upVals[i]!=INVALID_ENTRY) {
count++;
total+=upVals[i];
}
}
float averageUp=total/count;
// and the average value below zero
count=0;
total=0;
for(int i=0; i<AVERAGE_SIZE; i++) {
if(downVals[i]!=INVALID_ENTRY) {
count++;
total+=downVals[i];
}
}
float averageDown=total/count;
// is the new value a down value?
if(newVal<-0.5*averageDown) {
wasDown=true;
}
// is the new value an up value and were we previously in the down state?
if(newVal>=0.5*averageUp && wasDown) {
wasDown=false;
// work out the difference between now and the last time this happenned
if(time-periodStart<MAX_PERIOD && time-periodStart>MIN_PERIOD) {
periods[periodIndex]=time-periodStart;
periodTimes[periodIndex]=time;
periodIndex++;
if(periodIndex>=MAX_PERIODS_TO_STORE) {
periodIndex=0;
}
}
// track when the transition happened
periodStart=time;
}
// return up or down
if(newVal<-0.5*averageDown) {
return -1;
} else if(newVal>0.5*averageUp) {
return 1;
}
return 0;
}
-(float) getAverage {
double time=CACurrentMediaTime();
double total=0;
double count=0;
for(int i=0; i<MAX_PERIODS_TO_STORE; i++) {
// only use upto 10 seconds worth of data
if(periods[i]!=INVALID_ENTRY && time-periodTimes[i]<10) {
count++;
total+=periods[i];
}
}
// do we have enough values?
if(count>2) {
return total/count;
}
return INVALID_PULSE_PERIOD;
}
#end
Your problem is that you didn't copied the defines:
#define MAX_PERIODS_TO_STORE 20 // is this an Int?
#define AVERAGE_SIZE 20 // is this a Float?
#define INVALID_PULSE_PERIOD -1 // done
You have to change your defines so they work in your Swift code.
Check this answer how to replace the Objective-C #define to make Swift-Workable.
Also you could just change the defines to variables and initialize your variables with them.
First, a bit on optionals. Variables that end with a '?' are Optional, meaning that they are allowed to be nil (basically not exist). The compiler will not know at compile time whether this variable exists or not, because you are allowed to set it to nil.
"Unary operator '++' cannot be applied to an operand of type 'Int?'"
You seem to have read that last word as Int, but it is Int? which is significant. Basically, since it is an optional (as indicated by the question mark), the compiler knows it can be nil. You cannot use ++ on nil, and since optionals can be nil, you cannot use ++ on optionals. You must forcibly unwrap it first:
downValIndex!++ //note the exclamation point for unwrapping
"Use of unresolved identifier '=-'"
=- isnt a thing. -= is a thing. So
downVals[downValIndex] -= newVal
downVals[downValIndex] = downVals[downValIndex]-newVal //equivalent to above
"Binary operator '>=' cannot be applied to an operand of type 'Int? and Float'"
The compiler thinks you have an optional int on the left of the < and a Float on the right. Assuming you want two Ints, you must unwrap the left and make sure the right is cast to be an int (something like this). If you want two floats instead, cast or define as floats instead of ints.
if downValIndex! >= averageSize as! Int { //casting to Int
You should just be defining averageSize as an int though
var averageSize:Int = 10 //or whatever number
Also, you have lots of optionals. If any of them can be defined to something at compile time, it will make your life easier as you won't need to unwrap them everywhere. Alternately you could implicitly unwrap them (only do this if you are absolutely sure they will never be nil).
var implicitlyUnwrappedOptional:Int!

Detecting a change in a compound property

Objective-C would not allow you to run the following code:
myShape.origin.x = 50
This made it easy to detect changes in the origin, since someone using your class was forced to write myShape.origin = newOrigin, and thus you could easily tie in to the setter of this property.
Swift now allows you to perform the original, formerly-disallowed code. Assuming the following class structure, how would you detect the change to the origin in order to execute your own code (e.g. to update the screen)?
struct Point {
var x = 0
var y = 0
}
class Shape {
var origin: Point = Point()
}
Update: Perhaps I should have been more explicit, but assume I don't want to modify the Point struct. The reason is that Shape is but one class that uses Point, there may very well be hundreds of others, not to mention that the origin is not the only way a Point may be used.
Property observers (willSet and didSet) do fire when sub-properties of that property are changed. In this case, when the x or y values of the Point structure change, that property will be set.
Here is my example playground code:
struct Point : Printable
{
var x = 0
var y = 0
var description : String {
{
return "(\(x), \(y))";
}
}
class Shape
{
var origin : Point = Point()
{
willSet(newOrigin)
{
println("Changing origin to \(newOrigin.description)!")
}
}
}
let circle = Shape()
circle.origin.x = 42
circle.origin.y = 77
And here is the console output:
Changing origin to (42, 0)!
Changing origin to (42, 77)!
Doesn't this work?
class Shape {
var origin: Point {
willSet(aNewValueForOrigin) {
// pre flight code
}
didSet(theOldValueOfOrigin) {
// post flight code
}
}
}
Edit: revisited code and added name of arguments to reflect what to expect.
You can use Property Observers also works for structs
Link to the part on the ebook
class StepCounter {
var totalSteps: Int = 0 {
willSet(newTotalSteps) {
println("About to set totalSteps to \(newTotalSteps)")
}
didSet {
if totalSteps > oldValue {
println("Added \(totalSteps - oldValue) steps")
}
}
}
}
let stepCounter = StepCounter()
stepCounter.totalSteps = 200
// About to set totalSteps to 200
// Added 200 steps
stepCounter.totalSteps = 360
// About to set totalSteps to 360
// Added 160 steps
stepCounter.totalSteps = 896
// About to set totalSteps to 896
// Added 536 steps
Use didSet, e.g.,
struct Point {
var x = 0
var y: Int = 0 {
didSet {
println("blah blah")
}
}
}
class Shape {
var origin: Point = Point()
}
let s = Shape()
s.origin.y = 2