Features suggestions: Live data transformation

[theerasan]

Since Android Team announces LiveData and Transformation. We use it a lot in our project but somehow the transformation is not enough so we create our own extension to solve our issue, so we would like to share

import android.arch.lifecycle.LiveData
import android.arch.lifecycle.MediatorLiveData
import android.arch.lifecycle.Transformations

fun <X, Y> LiveData<X>.map(func: (source: X) -> Y) = Transformations.map(this, func)

fun <X, Y> LiveData<X>.switchMap(func: (source: X?) -> LiveData<Y>) = Transformations.switchMap(this, func)

fun <X, Y, Z> LiveData<X>.zip(stream: LiveData<Y>, func: (source1: X?, source2: Y?) -> Z): LiveData<Z> {
    val result = MediatorLiveData<Z>()
    result.addSource(this, { x -> result.setValue(func.invoke(x, stream.value)) })
    result.addSource(stream, { y -> result.setValue(func.invoke(this.value, y)) })
    return result
}

fun <X> LiveData<X>.merge(stream: LiveData<X>): LiveData<X> {
    val result = MediatorLiveData<X>()
    result.addSource(this, { x -> result.setValue(x) })
    result.addSource(stream, { x -> result.setValue(x) })
    return result
}

fun <X, Y> LiveData<X>.mapNotNull(func: (source: X?) -> Y?): LiveData<Y> {
    val result = MediatorLiveData<Y>()
    result.addSource(this, { x ->
        val y = func.invoke(x)
        if (y != null) result.setValue(y)
    })
    return result
}

fun <X> LiveData<X>.filter(func: (source: X?) -> Boolean): LiveData<X> {
    val result = MediatorLiveData<X>()
    result.addSource(this, { x -> if (func.invoke(x)) result.setValue(x) })
    return result
}

fun <X> LiveData<X>.filterNotNull(func: (source: X) -> Boolean): LiveData<X> {
    val result = MediatorLiveData<X>()
    result.addSource(this, { x ->
        if (x != null && func.invoke(x)) result.setValue(x)
    })
    return result
}

fun <X> LiveData<X?>.filterOutNull(): LiveData<X> {
    val result = MediatorLiveData<X>()
    result.addSource(this, { x ->
        if (x != null) result.setValue(x)
    })
    return result
}

fun <M, S> MediatorLiveData<M>.observeOnce(source: LiveData<S>, func: (data: S?) -> Unit) {
    this.removeSource(source)
    this.addSource(source, {
        func.invoke(it)
        this@observeOnce.removeSource(source)
    })
}

[JakeWharton]

This project is only extensions for the Android framework APIs and not the AndroidX libraries. If you want to propose this functionality to LiveData (which has no reason to be a Kotlin extension) please file a feature request at b.android.com/new under the AndroidX component.