How to implement the "didset of swift" in objective-c?

Question

Swift (from the book 《iOS Animations by Tutorials:Chapter 12》 released by http://www.raywenderlich.com/):

let photoLayer = CALayer()

@IBInspectable
  var image: UIImage! {
    didSet {
      photoLayer.contents = image.CGImage
    }
}

How can I implement the above syntax in objective-c? I know only to set the property of photoLayer and image like below:

@property (strong, nonatomic) CALayer *photoLayer;
@property (strong, nonatomic) IBInspectable UIImage *image;

But i do not know how to implement didset{...} parts using objective-c syntax, please help!

check this...http://stackoverflow.com/questions/26674111/ib-designable-ibinspectable-interface-builder-does-not-update — Bhavin Bhadani, Sep 24 '15 at 08:12
in objc calling a nil object works, but in swift it could be `photoLayer.contents = image != nil ? image.CGImage : nil` — Daij-Djan, Sep 24 '15 at 08:15

score 44 · Accepted Answer · edited Aug 10 '21 at 09:52

44

override the setter and implement the setter yourself.

- (void)setImage:(UIImage *)image {
    if (_image != image) {
        _image = image;
        photoLayer.contents = image.CGImage;
    }
}

edited Aug 10 '21 at 09:52

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answered Sep 24 '15 at 08:13

Daij-Djan

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Amin Negm-Awad · Answer 2 · 2017-04-23T04:38:16.243

Swift needs special language features for this, because of its static nature, one cannot subclass at runtime.

In Objective-C this is not necessary, so you have no language feature for this. You can use key-value observation.

But if the observing instance is the instance whose property changed, you can do it with subclassing as mentioned by @Daij-Djan. (Basically this is what you do in Swift.) Additionally in Objective-C you have the ability to observe the properties of a different instance of a different class.

How to implement the "didset of swift" in objective-c?

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