I don't find Kotlin class delegates very useful. I...

# android

[spierce7]

I don't find Kotlin class delegates very useful. I've found many uses for property delegates, but I have never found the class delegation in Kotlin to be what I needed. I've been programming in Kotlin for almost 4 years, and haven't used it once. Has anyone found it to be useful, and can they explain how they use it?

[drofwarcs]

I also have a hard to finding a good use for this feature. In my case, its turns out that this feature is really a form of the Decorator pattern instead of just a plan delegate pattern, which is why the class thats using by must also conform to the interface. If there was a similar kotlin feature like this that did not implement/conform the interface methods in the class using by, I would be using this like crazy. The current way it works comes in handy for places where you would use the decorator pattern.

[pankajrai]

Yes it is extremely useful for example by lazy delegate which does object initialization when its used for the first time thereafter directly use its value. Observer/Vetoable are great way to observer value and restrict what value should be assigned to an object.

[pankajrai]

lazy initialization make your object immutable that means no overhead included to allocate space for variable memory as its value never change

[trevjones]

I typically only use it as a way to reduce code duplication around things like rx disposable management or viewholder code where a custom view isn’t any more appropriate. ie) Foo(...): DisposeBucket by DisposeBucket.Impl()

[kenkyee]

Why wouldn't you use it for class composition?

[rkeazor]

I found it useful

[rkeazor]

It favors composition over inheritance. I think the problem most people have , is that they are used to the inheritance model so its can be somewhat weird to wrap your head around

[rkeazor]

in essence you can delegate the actual implmentation of Object. so for instance, in a normal MVP(passive view flavor) architecture you would have a interfaces that seperate the layer of abstraction between the presenter and Activity. However you would have to have the actual implementation as a instance variable

[rkeazor]

in both the activity and the presenter. but class delegation removes this need

[rkeazor]

The only downside I see with it , was perhaps the same downside you get from Multiple inheritance, and default methods..Is that if you do it to much you don’t know where things are coming from lmaoo

[rkeazor]

but i think its a cool feature

[spierce7]

Every time I use it, I find it's limitations crippling. I've seen 2 uses for it

1. When I want to delegate all methods from an interface to another class, but then I'm asking myself why am I not just extending the class, since this 95% of the time, the class definitely won't be extending anything else. I agree learned to favor composition over inheritance, but the limitations of the class delegate in Kotlin have ensured that I can't use it for that in all the instances I've come across.

2. If I split an interface into multiple smaller interfaces, and then kind of patched implementations together of the implementations using the class delegates. I could see it useful for this, effectively using this as a decorator pattern of some sort. I'm not sure if this would actually work well though.

I'm sure it is useful, I just haven't seen a way that I've found it to be useful. I'm confident that if someone showed me a real example of how its been useful for them it would probably grow me as a programmer, as I think it's likely there is just a mental block there now.

[rkeazor]

yea you can extend it, but remember Java and Kotlin favor single Inheritance.So if you have a class that extends a base class, and you want to inherit from another class as well, you will have to create a instance variable of that class

[louiscad]

I found a very compelling use case for it on Android that also involves companion objects. I'll share it back here this week (or maybe today)

[louiscad]

Here's what you can do with interface implementation delegates, companion objects, and reified type parameters for Android

[spierce7]

?

[louiscad]

Oh… silly me, forgot the link… Here it is: https://github.com/LouisCAD/Splitties/blob/master/intents/README.md @spierce7
Feel free to jump straight to the example