FR: Adding methods to classes without subclassing

[willlarche]

Idea comes from Objective-C's "Categories".

Would be great to be able to add methods, both instance and static, to a class without having to create a new type. Here's how it works in Objective-C (I bet you could make it less verbose!) The - method is an instance method and + is the static one:

Source File: UILabel+MyNewInits.h

@interface UILabel (MyNewInits)

- (instancetype)initThatSpecialWay;
+ (instancetype)specialLabel;

@end

@implementation UILabel (MyNewInits)

- (instancetype)initThatSpecialWay {
  self = [super init];
  if (self) {
    self.text = @"Wuthering, Wuthering Heights";
  }
  return self;
}

+ (instancetype)specialLabel {
  UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
  label.text = @"Heathcliff, it's just me. It's Cathy.";

  return label;
}

@end

Consuming File: SomeView.m

#import "UILabel+MyNewInits.h"

@interface KateBushView : UIView
@end

@implementation KateBushView

- (void)awakeFromNib {
  [super awakeFromNib];

  [self addSubview:[[UILabel alloc] initThatSpecialWay]];
  // Or, like:
  [self addSubview:[UILabel specialLabel]];
}

@end

[natebosch]

See language discussion: dart-lang/language#40 which has linked proposals that I think covers at least the instance method side.

cc/ @lrhn

[lrhn]

This is definitely related to dart-lang/language#41 (static extension methods), but as I read it, it is actually more powerful. These extension methods (at least the instance ones) are introduced on the class the same way as members introduced by the class itself.
In Dart, that would mean that the methods can be overridden virtually in sub-classes. It also means that they can conflict with existing members or other categories on the same class.

(Static extension methods can also be extended to add static methods to classes, if that is actually useful in Dart).

[willlarche]

Yeah, it's more than just static. And collisions have been a super rare occurance. They can be caught by the compiler tho.

Overriding them is a cool idea I never thought of. Seems like a less-useful case but interesting.