Hi there, if I have the following code: ``` ...

# announcements

[robinchew]

Hi there, if I have the following code:

val f: (Context) -> View = ::TextView
        val text = f(activity)

how do I check that text is an 'instance' of f?

[Andreas Sinz]

What do you mean with instance of f?

[robinchew]

Like how to find a relationship between f and text.

[Andreas Sinz]

You need to find out if text was returned by f?

[robinchew]

In Python, when you do:

f = TextView
text = f(activity)

you can check text is an instance of f by doing:

isinstance(text, f)

[vnsgbt]

val isString = when (foo) { is String -> true }

[Andreas Sinz]

in kotlin you can do text is TextView

[robinchew]

Is it possible to extract TextView out of f?

[robinchew]

Cos it's not going to be TextView all the time, it could be other views

[Andreas Sinz]

f is just an ordinary function that creates an instance of TextView, so you cant get the type out of it

[robinchew]

It sounds like if I want to keep a list of classes to instantiate dynamically, I have to keep both the class and constructor function.

[Andreas Sinz]

you could do something like

when(text) {
    is TextView -> ...
    is Button -> ...
    ...
}

[Andreas Sinz]

why do you need to instantiante different classes dynamically?

[vnsgbt]

you can also give Context -> View an alias, and use is to check it.

[robinchew]

Building nested data structure that represents UI elements

[robinchew]

Anyway say I have a list of views:

val manyViewClasses = listOf(TextView, SomeView)

And I loop them to instantiate:

val manyViews = manyViewClasses.map {cls ->
    cls(activity)
}

It won't work, because I'm missing contsructors

[robinchew]

It would work if manyViewClasses was:

val manyViewClasses = listOf(::TextView, ::SomeView)

[robinchew]

And just for example I want to compare manyViews and manyViewClasses like so:

manyViews.zip(manyViewClasses).forEach {(view, cls) ->
    if (view.class == cls) {
        println('match')
    }
}

[robinchew]

That can't be done, because cls is a function, not a class

[robinchew]

So this is my workaround:

val manyViewClasses = listOf(::TextView, ::SomeView)

val manyViews = manyViewClasses.map {con ->
    con(activity)
}

val manyViewClasses2 = myChangesTo(manyViewClasses)

manyViews.zip(manyViewClasses2).forEach {(view, con) ->
    val newView = con(activity) // <- workaround
    if (view::class == newView::class) {
        println('match')
    }
}

[Andreas Sinz]

and why do you need to know whether view has the same class as con?

[Dico]

This is dynamic programming

[Dico]

Kotlin is a statically typed language

[Dico]

You should consider making an interface View and then implement it with class TextView : View and class OtherView : View

[Dico]

I see no point trying to have a list of classes, doing map and zip operations on them and seeing if the type is in there in a loop.

[Dico]

Moreover, you did not explain what the point is despite multiple requests to do so

[Dico]

So I can't really give you an equivalent of what you're trying to achieve that might be considered more idiomatic

[robinchew]

Thanks for the suggestion @Dico. What I'm trying to do a simple naive diffing between 2 sets of views.

So if I have existing views rendered which looks simply like:

listOf(TextView, SomeView)

and I try to render a similar

listOf(TextView, SomeView)

on top of it, nothing will be applied because it's considered the same.

[robinchew]

But if instead I try to render something slightly different, say:

listOf(TextView, ImageView)

My program can see that the 2nd View has changed, and replaces SomeView with ImageView, while leaving TextView as is.

[robinchew]

Again this is a simple example, a bit more closer to my actual code to represent a set of views is more like:

m(::LinearLayout,
     children(
         m(::TextView, attrs(text("Hello"), color("red"))),
         m(::SomeView)))

m(::LinearLayout,
     children(
         m(::TextView, attrs(text("Hello"), color("red"))),
         m(::ImageView, attrs(url("<http://www.site.com/image.gif|www.site.com/image.gif>")))))

[Andreas Sinz]

You could create your own DSL https://kotlinlang.org/docs/reference/type-safe-builders.html

[Andreas Sinz]

I'd just create the second View-Tree and compare it with the existing one and see how it works performance wise. Only when creating instances of the different View is very expensive, i'd try to optimize it with Builders that know what kind of View they create

[Dico]

interface View {
    val type: ViewType
}

enum class ViewType(val ktype: KClass<out View>, val ctor: () -> View) {
    TEXT(TextView::class, ::TextView),
    // more
    ;
}

[Dico]

Something like that would probably be considered a bit more idiomatic for what you described

[Dico]

I would prefer for there to exist a constraint between the 2 properties of the enum for better type safety, but it's 5 am

[robinchew]

Thanks @Andreas Sinz @Dico. The DSL would be ideal, and the interface/enum could be a good compromise.

[robinchew]

I would still be keen to see how to make the extra constraint for better type safety.

[robinchew]

I'll see if I can apply your suggestions for my little project https://github.com/robinchew/mvil