DEV Community: amay077

Kotlin's scope functions in C#

amay077 — Tue, 19 Dec 2017 17:49:33 +0000

Kotlin's scope functions(.let, .also, .apply, etc) are super useful.

I want to these methods in C#.
So I just define it!

static class ObjectExtensions 
{
    // Kotlin: fun <T, R> T.let(block: (T) -> R): R
    public static R Let<T, R>(this T self, Func<T, R> block) 
    {
        return block(self);
    }

    // Kotlin: fun <T> T.also(block: (T) -> Unit): T
    public static T Also<T>(this T self, Action<T> block)
    {
        block(self);
        return self;
    }   
}

Usage

var text = resultCode.Let(x => {
    switch (x) {
        case 0:
            return "success";
        default:
            return $"error({x})";
    }
});

var model = new MyModel().Also(m => {
    m.Initialize();
    m.Load(path);
});

I hope .NET Framework provide these methods.

Gets first index of match to condition in List

amay077 — Mon, 11 Dec 2017 15:52:27 +0000

Extension method

inline fun <T> Iterable<T>.firstIndexOrNull(predicate: (T) -> Boolean): Int? {
    return this.mapIndexed { index, item -> Pair(index, item) }
            .firstOrNull() { predicate(it.second) }
            ?.first
}

Just using .mapIndexed and .firstOrNull .

Usage

val arr = arrayListOf("a", "b", "c")

arr.firstIndexOrNull { it == "b"} // -> 1
arr.firstIndexOrNull { it == "z"} // -> null

RxJava combineLatest quick example by Kotlin

amay077 — Tue, 28 Nov 2017 11:57:13 +0000

RxJava's combineLatest is very convenient but I can use it difficultly because it has many overloads.
So I wrote the code snippet for combineLatest.

val name = PublishSubject.create<String>()
val age = PublishSubject.create<Int>()

// Can not omit Type parameters and BiFunction 
Observable.combineLatest<String, Int, String>(
        name, age, BiFunction { n, a -> "$n - age:${a}" })
        .subscribe({
            Log.d("combineLatest", "onNext - ${it}")
        })

// If you introduce RxKotlin then you can use type inference
Observables.combineLatest(name, age) { n, a -> "$n - age:${a}" }
        .subscribe({
            Log.d("combineLatest", "onNext - ${it}")
        })

// Also we can use Observable array for 1st parameter 
// but second parameter to be array, it's not cool.
Observable.combineLatest(arrayOf(name, age), {
    val n = it[0] as String
    val a = it[1] as Int
    "$n - age:${a}" })
        .subscribe({
            Log.d("combineLatest", "onNext - ${it}")
        })


name.onNext("saito")
age.onNext(24)
name.onNext("yoshida")