Lazy read-only properties

Question

Created Nov ’15

Replies 2

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I frequently use lazily loaded read-only properties. A typical use case for this would be a class that has an icon image, which should only be loaded once it is requested. The Obj-C code would typically look like this:

@interface DocumentItem : NSObject
@property(readonly) NSImage *icon;
@end


@implementation DocumentItem
-(NSImage*)icon {
     if (!_icon) _icon = [NSImage imageNamed:@"DocumentIcon"];
     return _icon;
}
@end

I saw that Swift has built-in support for lazy loading and was thrilled!

class DocumentItem : NSObject {
     lazy var icon : NSImage? = NSImage(imageNamed:"DocumentIcon")
}

Much shorter! But, it turns out that this isn't read-only, and I can't seem to override it in a subclass. If I try, I get a cryptic error about conflicting objective C selectors related to setIcon: or something.

What I really want is to use "let" instead of "var", but for some reason this isn't possible

class DocumentItem : NSObject {
     lazy let icon : NSImage? = NSImage(imageNamed:"DocumentIcon")
}

Someone on Stack Overflow suggested using a private setter:

class DocumentItem : NSObject {
     private(set) lazy var icon : NSImage? = NSImage(imageNamed:"DocumentIcon")
}

But that still doesn't let me override the property in a subclass.

Is there some way to tell Swift to either make a "var" property readonly, or to make a "let" property lazy?

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Answer 1

CharlesS OP

Nov ’15

Seems to work for me:

import Cocoa

class Foo: NSObject {
  private(set) lazy var icon : NSImage? = NSImage()
}


class Bar: Foo {
  override var icon : NSImage? {
      get { return NSImage() }
      set { fatalError("Don't call this!") }
  }
}

0

Answer 2

jabakobob OP

Nov ’15

Hm. Obviously I wanted to override the lazy property with another property like this:

import Cocoa
class DocumentItem: NSObject {
  lazy var icon : NSImage? = NSImage(imageNamed:"DocumentIcon")
}
class SpecialDocumentItem: DocumentItem {
  override lazy var icon : NSImage? = NSImage(imageNamed:"SpecialDocumentItem")
}

However, that doesn't work. Looking at the error messages, maybe I'm doing something wrong; since it seems that overriding vars just isn't possible.

A solution I found is to use computed properties, since I can override them just fine, and do the lazy loading somewhere else:

let documentIcon : NSImage? = NSImage(imageNamed:"DocumentIcon")
let specialDocumentIcon : NSImage? = NSImage(imageNamed:"SpecialDocumentItem")

class DocumentItem: NSObject {
  var icon : NSImage? { return documentIcon }
}
class SpecialDocumentItem: DocumentItem {
  override var icon : NSImage? { return specialDocumentIcon }
}

Since global constants are always loaded lazily, it seems that this does what I want. Unfortunately this does spread the code out, so it makes it harder to follow, but it's almost as clean as the Obj-C version

0