问题
I am switching an application from Objective-C to Swift, which I have a couple of categories with stored properties, for example:
@interface UIView (MyCategory)
- (void)alignToView:(UIView *)view
alignment:(UIViewRelativeAlignment)alignment;
- (UIView *)clone;
@property (strong) PFObject *xo;
@property (nonatomic) BOOL isAnimating;
@end
As Swift extensions don\'t accept stored properties like these, I don\'t know how to maintain the same structure as the Objc code. Stored properties are really important for my app and I believe Apple must have created some solution for doing it in Swift.
As said by jou, what I was looking for was actually using associated objects, so I did (in another context):
import Foundation
import QuartzCore
import ObjectiveC
extension CALayer {
var shapeLayer: CAShapeLayer? {
get {
return objc_getAssociatedObject(self, \"shapeLayer\") as? CAShapeLayer
}
set(newValue) {
objc_setAssociatedObject(self, \"shapeLayer\", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
}
}
var initialPath: CGPathRef! {
get {
return objc_getAssociatedObject(self, \"initialPath\") as CGPathRef
}
set {
objc_setAssociatedObject(self, \"initialPath\", newValue, UInt(OBJC_ASSOCIATION_RETAIN))
}
}
}
But I get an EXC_BAD_ACCESS when doing:
class UIBubble : UIView {
required init(coder aDecoder: NSCoder) {
...
self.layer.shapeLayer = CAShapeLayer()
...
}
}
Any ideas?
回答1:
Associated objects API is a bit cumbersome to use. You can remove most of the boilerplate with a helper class.
public final class ObjectAssociation<T: AnyObject> {
private let policy: objc_AssociationPolicy
/// - Parameter policy: An association policy that will be used when linking objects.
public init(policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN_NONATOMIC) {
self.policy = policy
}
/// Accesses associated object.
/// - Parameter index: An object whose associated object is to be accessed.
public subscript(index: AnyObject) -> T? {
get { return objc_getAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque()) as! T? }
set { objc_setAssociatedObject(index, Unmanaged.passUnretained(self).toOpaque(), newValue, policy) }
}
}
Provided that you can "add" a property to objective-c class in a more readable manner:
extension SomeType {
private static let association = ObjectAssociation<NSObject>()
var simulatedProperty: NSObject? {
get { return SomeType.association[self] }
set { SomeType.association[self] = newValue }
}
}
As for the solution:
extension CALayer {
private static let initialPathAssociation = ObjectAssociation<CGPath>()
private static let shapeLayerAssociation = ObjectAssociation<CAShapeLayer>()
var initialPath: CGPath! {
get { return CALayer.initialPathAssociation[self] }
set { CALayer.initialPathAssociation[self] = newValue }
}
var shapeLayer: CAShapeLayer? {
get { return CALayer.shapeLayerAssociation[self] }
set { CALayer.shapeLayerAssociation[self] = newValue }
}
}
回答2:
As in Objective-C, you can't add stored property to existing classes. If you're extending an Objective-C class (UIView is definitely one), you can still use Associated Objects to emulate stored properties:
for Swift 1
import ObjectiveC
private var xoAssociationKey: UInt8 = 0
extension UIView {
var xo: PFObject! {
get {
return objc_getAssociatedObject(self, &xoAssociationKey) as? PFObject
}
set(newValue) {
objc_setAssociatedObject(self, &xoAssociationKey, newValue, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN))
}
}
}
The association key is a pointer that should be the unique for each association. For that, we create a private global variable and use it's memory address as the key with the & operator. See the Using Swift with Cocoa and Objective-C
on more details how pointers are handled in Swift.
UPDATED for Swift 2 and 3
import ObjectiveC
private var xoAssociationKey: UInt8 = 0
extension UIView {
var xo: PFObject! {
get {
return objc_getAssociatedObject(self, &xoAssociationKey) as? PFObject
}
set(newValue) {
objc_setAssociatedObject(self, &xoAssociationKey, newValue, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN)
}
}
}
UPDATED for Swift 4
In Swift 4, it's much more simple. The Holder struct will contain the private value that our computed property will expose to the world, giving the illusion of a stored property behaviour instead.
Source
extension UIViewController {
struct Holder {
static var _myComputedProperty:Bool = false
}
var myComputedProperty:Bool {
get {
return Holder._myComputedProperty
}
set(newValue) {
Holder._myComputedProperty = newValue
}
}
}
回答3:
So I think I found a method that works cleaner than the ones above because it doesn't require any global variables. I got it from here: http://nshipster.com/swift-objc-runtime/
The gist is that you use a struct like so:
extension UIViewController {
private struct AssociatedKeys {
static var DescriptiveName = "nsh_DescriptiveName"
}
var descriptiveName: String? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.DescriptiveName) as? String
}
set {
if let newValue = newValue {
objc_setAssociatedObject(
self,
&AssociatedKeys.DescriptiveName,
newValue as NSString?,
UInt(OBJC_ASSOCIATION_RETAIN_NONATOMIC)
)
}
}
}
}
UPDATE for Swift 2
private struct AssociatedKeys {
static var displayed = "displayed"
}
//this lets us check to see if the item is supposed to be displayed or not
var displayed : Bool {
get {
guard let number = objc_getAssociatedObject(self, &AssociatedKeys.displayed) as? NSNumber else {
return true
}
return number.boolValue
}
set(value) {
objc_setAssociatedObject(self,&AssociatedKeys.displayed,NSNumber(bool: value),objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
回答4:
The solution pointed out by jou doesn't support value types, this works fine with them as well
Wrappers
import ObjectiveC
final class Lifted<T> {
let value: T
init(_ x: T) {
value = x
}
}
private func lift<T>(x: T) -> Lifted<T> {
return Lifted(x)
}
func setAssociatedObject<T>(object: AnyObject, value: T, associativeKey: UnsafePointer<Void>, policy: objc_AssociationPolicy) {
if let v: AnyObject = value as? AnyObject {
objc_setAssociatedObject(object, associativeKey, v, policy)
}
else {
objc_setAssociatedObject(object, associativeKey, lift(value), policy)
}
}
func getAssociatedObject<T>(object: AnyObject, associativeKey: UnsafePointer<Void>) -> T? {
if let v = objc_getAssociatedObject(object, associativeKey) as? T {
return v
}
else if let v = objc_getAssociatedObject(object, associativeKey) as? Lifted<T> {
return v.value
}
else {
return nil
}
}
A possible Class extension (Example of usage):
extension UIView {
private struct AssociatedKey {
static var viewExtension = "viewExtension"
}
var referenceTransform: CGAffineTransform? {
get {
return getAssociatedObject(self, associativeKey: &AssociatedKey.viewExtension)
}
set {
if let value = newValue {
setAssociatedObject(self, value: value, associativeKey: &AssociatedKey.viewExtension, policy: objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
}
}
This is really such a great solution, I wanted to add another usage example that included structs and values that are not optionals. Also, the AssociatedKey values can be simplified.
struct Crate {
var name: String
}
class Box {
var name: String
init(name: String) {
self.name = name
}
}
extension UIViewController {
private struct AssociatedKey {
static var displayed: UInt8 = 0
static var box: UInt8 = 0
static var crate: UInt8 = 0
}
var displayed: Bool? {
get {
return getAssociatedObject(self, associativeKey: &AssociatedKey.displayed)
}
set {
if let value = newValue {
setAssociatedObject(self, value: value, associativeKey: &AssociatedKey.displayed, policy: objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
}
var box: Box {
get {
if let result:Box = getAssociatedObject(self, associativeKey: &AssociatedKey.box) {
return result
} else {
let result = Box(name: "")
self.box = result
return result
}
}
set {
setAssociatedObject(self, value: newValue, associativeKey: &AssociatedKey.box, policy: objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
var crate: Crate {
get {
if let result:Crate = getAssociatedObject(self, associativeKey: &AssociatedKey.crate) {
return result
} else {
let result = Crate(name: "")
self.crate = result
return result
}
}
set {
setAssociatedObject(self, value: newValue, associativeKey: &AssociatedKey.crate, policy: objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
}
回答5:
You can't define categories (Swift extensions) with new storage; any additional properties must be computed rather than stored. The syntax works for Objective C because @property in a category essentially means "I'll provide the getter and setter". In Swift, you'll need to define these yourself to get a computed property; something like:
extension String {
public var Foo : String {
get
{
return "Foo"
}
set
{
// What do you want to do here?
}
}
}
Should work fine. Remember, you can't store new values in the setter, only work with the existing available class state.
回答6:
My $0.02. This code is written in Swift 2.0
extension CALayer {
private struct AssociatedKeys {
static var shapeLayer:CAShapeLayer?
}
var shapeLayer: CAShapeLayer? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.shapeLayer) as? CAShapeLayer
}
set {
if let newValue = newValue {
objc_setAssociatedObject(self, &AssociatedKeys.shapeLayer, newValue as CAShapeLayer?, objc_AssociationPolicy.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
}
}
}
}
I have tried many solutions, and found this is the only way to actually extend a class with extra variable parameters.
回答7:
I prefer doing code in pure Swift and not rely on Objective-C heritage. Because of this I wrote pure Swift solution with two advantages and two disadvantages.
Advantages:
Pure Swift code
Works on classes and completions or more specifically on
Anyobject
Disadvantages:
Code should call method
willDeinit()to release objects linked to specific class instance to avoid memory leaksYou cannot make extension directly to UIView for this exact example because
var frameis extension to UIView, not part of class.
EDIT:
import UIKit
var extensionPropertyStorage: [NSObject: [String: Any]] = [:]
var didSetFrame_ = "didSetFrame"
extension UILabel {
override public var frame: CGRect {
get {
return didSetFrame ?? CGRectNull
}
set {
didSetFrame = newValue
}
}
var didSetFrame: CGRect? {
get {
return extensionPropertyStorage[self]?[didSetFrame_] as? CGRect
}
set {
var selfDictionary = extensionPropertyStorage[self] ?? [String: Any]()
selfDictionary[didSetFrame_] = newValue
extensionPropertyStorage[self] = selfDictionary
}
}
func willDeinit() {
extensionPropertyStorage[self] = nil
}
}
回答8:
Why relying on objc runtime? I don't get the point. By using something like the following you will achieve almost the identical behaviour of a stored property, by using only a pure Swift approach:
extension UIViewController {
private static var _myComputedProperty = [String:Bool]()
var myComputedProperty:Bool {
get {
let tmpAddress = String(format: "%p", unsafeBitCast(self, to: Int.self))
return UIViewController._myComputedProperty[tmpAddress] ?? false
}
set(newValue) {
let tmpAddress = String(format: "%p", unsafeBitCast(self, to: Int.self))
UIViewController._myComputedProperty[tmpAddress] = newValue
}
}
}
回答9:
With Obj-c Categories you can only add methods, not instance variables.
In you example you have used @property as a shortcut to adding getter and setter method declarations. You still need to implement those methods.
Similarly in Swift you can add use extensions to add instance methods, computed properties etc. but not stored properties.
回答10:
I also get an EXC_BAD_ACCESS problem.The value in objc_getAssociatedObject() and objc_setAssociatedObject() should be an Object. And the objc_AssociationPolicy should match the Object.
回答11:
I tried using objc_setAssociatedObject as mentioned in a few of the answers here, but after failing with it a few times I stepped back and realized there is no reason I need that. Borrowing from a few of the ideas here, I came up with this code which simply stores an array of whatever my extra data is (MyClass in this example) indexed by the object I want to associate it with:
class MyClass {
var a = 1
init(a: Int)
{
self.a = a
}
}
extension UIView
{
static var extraData = [UIView: MyClass]()
var myClassData: MyClass? {
get {
return UIView.extraData[self]
}
set(value) {
UIView.extraData[self] = value
}
}
}
// Test Code: (Ran in a Swift Playground)
var view1 = UIView()
var view2 = UIView()
view1.myClassData = MyClass(a: 1)
view2.myClassData = MyClass(a: 2)
print(view1.myClassData?.a)
print(view2.myClassData?.a)
回答12:
Here is simplified and more expressive solution. It works for both value and reference types. The approach of lifting is taken from @HepaKKes answer.
Association code:
import ObjectiveC
final class Lifted<T> {
let value: T
init(_ x: T) {
value = x
}
}
private func lift<T>(_ x: T) -> Lifted<T> {
return Lifted(x)
}
func associated<T>(to base: AnyObject,
key: UnsafePointer<UInt8>,
policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN,
initialiser: () -> T) -> T {
if let v = objc_getAssociatedObject(base, key) as? T {
return v
}
if let v = objc_getAssociatedObject(base, key) as? Lifted<T> {
return v.value
}
let lifted = Lifted(initialiser())
objc_setAssociatedObject(base, key, lifted, policy)
return lifted.value
}
func associate<T>(to base: AnyObject, key: UnsafePointer<UInt8>, value: T, policy: objc_AssociationPolicy = .OBJC_ASSOCIATION_RETAIN) {
if let v: AnyObject = value as AnyObject? {
objc_setAssociatedObject(base, key, v, policy)
}
else {
objc_setAssociatedObject(base, key, lift(value), policy)
}
}
Example of usage:
1) Create extension and associate properties to it. Let's use both value and reference type properties.
extension UIButton {
struct Keys {
static fileprivate var color: UInt8 = 0
static fileprivate var index: UInt8 = 0
}
var color: UIColor {
get {
return associated(to: self, key: &Keys.color) { .green }
}
set {
associate(to: self, key: &Keys.color, value: newValue)
}
}
var index: Int {
get {
return associated(to: self, key: &Keys.index) { -1 }
}
set {
associate(to: self, key: &Keys.index, value: newValue)
}
}
}
2) Now you can use just as regular properties:
let button = UIButton()
print(button.color) // UIExtendedSRGBColorSpace 0 1 0 1 == green
button.color = .black
print(button.color) // UIExtendedGrayColorSpace 0 1 == black
print(button.index) // -1
button.index = 3
print(button.index) // 3
More details:
- Lifting is needed for wrapping value types.
- Default associated object behavior is retain. If you want to learn more about associated objects, I'd recommend checking this article.
回答13:
Another example with using Objective-C associated objects and computed properties for Swift 3 and Swift 4
import CoreLocation
extension CLLocation {
private struct AssociatedKeys {
static var originAddress = "originAddress"
static var destinationAddress = "destinationAddress"
}
var originAddress: String? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.originAddress) as? String
}
set {
if let newValue = newValue {
objc_setAssociatedObject(
self,
&AssociatedKeys.originAddress,
newValue as NSString?,
.OBJC_ASSOCIATION_RETAIN_NONATOMIC
)
}
}
}
var destinationAddress: String? {
get {
return objc_getAssociatedObject(self, &AssociatedKeys.destinationAddress) as? String
}
set {
if let newValue = newValue {
objc_setAssociatedObject(
self,
&AssociatedKeys.destinationAddress,
newValue as NSString?,
.OBJC_ASSOCIATION_RETAIN_NONATOMIC
)
}
}
}
}
回答14:
I tried to store properties by using objc_getAssociatedObject, objc_setAssociatedObject, without any luck. My goal was create extension for UITextField, to validate text input characters length. Following code works fine for me. Hope this will help someone.
private var _min: Int?
private var _max: Int?
extension UITextField {
@IBInspectable var minLength: Int {
get {
return _min ?? 0
}
set {
_min = newValue
}
}
@IBInspectable var maxLength: Int {
get {
return _max ?? 1000
}
set {
_max = newValue
}
}
func validation() -> (valid: Bool, error: String) {
var valid: Bool = true
var error: String = ""
guard let text = self.text else { return (true, "") }
if text.characters.count < minLength {
valid = false
error = "Textfield should contain at least \(minLength) characters"
}
if text.characters.count > maxLength {
valid = false
error = "Textfield should not contain more then \(maxLength) characters"
}
if (text.characters.count < minLength) && (text.characters.count > maxLength) {
valid = false
error = "Textfield should contain at least \(minLength) characters\n"
error = "Textfield should not contain more then \(maxLength) characters"
}
return (valid, error)
}
}
回答15:
Here is an alternative that works also
public final class Storage : AnyObject {
var object:Any?
public init(_ object:Any) {
self.object = object
}
}
extension Date {
private static let associationMap = NSMapTable<NSString, AnyObject>()
private struct Keys {
static var Locale:NSString = "locale"
}
public var locale:Locale? {
get {
if let storage = Date.associationMap.object(forKey: Keys.Locale) {
return (storage as! Storage).object as? Locale
}
return nil
}
set {
if newValue != nil {
Date.associationMap.setObject(Storage(newValue), forKey: Keys.Locale)
}
}
}
}
var date = Date()
date.locale = Locale(identifier: "pt_BR")
print( date.locale )
回答16:
if you are looking to set a custom string attribute to a UIView, this is how I did it on Swift 4
Create a UIView extension
extension UIView {
func setStringValue(value: String, key: String) {
layer.setValue(value, forKey: key)
}
func stringValueFor(key: String) -> String? {
return layer.value(forKey: key) as? String
}
}
To use this extension
let key = "COLOR"
let redView = UIView()
// To set
redView.setStringAttribute(value: "Red", key: key)
// To read
print(redView.stringValueFor(key: key)) // Optional("Red")
回答17:
How about storing static map to class that is extending like this :
extension UIView {
struct Holder {
static var _padding:[UIView:UIEdgeInsets] = [:]
}
var padding : UIEdgeInsets {
get{ return UIView.Holder._padding[self] ?? .zero}
set { UIView.Holder._padding[self] = newValue }
}
}
回答18:
I found this solution more practical
UPDATED for Swift 3
extension UIColor {
static let graySpace = UIColor.init(red: 50/255, green: 50/255, blue: 50/255, alpha: 1.0)
static let redBlood = UIColor.init(red: 102/255, green: 0/255, blue: 0/255, alpha: 1.0)
static let redOrange = UIColor.init(red: 204/255, green: 17/255, blue: 0/255, alpha: 1.0)
func alpha(value : CGFloat) -> UIColor {
var r = CGFloat(0), g = CGFloat(0), b = CGFloat(0), a = CGFloat(0)
self.getRed(&r, green: &g, blue: &b, alpha: &a)
return UIColor(red: r, green: g, blue: b, alpha: value)
}
}
...then in your code
class gameController: UIViewController {
@IBOutlet var game: gameClass!
override func viewDidLoad() {
self.view.backgroundColor = UIColor.graySpace
}
}
来源：https://stackoverflow.com/questions/25426780/how-to-have-stored-properties-in-swift-the-same-way-i-had-on-objective-c