LiveDataExt.kt

package your.awesome.domain

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

fun <T> LiveData<T>.filter(matcher: (T?)->Boolean): LiveData<T> {
    val result = MediatorLiveData<T>()

    result.addSource(this, {
        if (matcher.invoke(it)) {
            result.value = it
        }
    })

    if (matcher.invoke(this.value)) {
        result.value = this.value
    }

    return result;
}

fun <T1, T2, S> LiveData<T1>.combineLatest(source2: LiveData<T2>,
                                           func: (T1?, T2?) -> S?): LiveData<S> {
    val result = MediatorLiveData<S>()
    result.addSource(this, {
        result.value = func.invoke(this.value, source2.value)
    })
    result.addSource(source2, {
        result.value = func.invoke(this.value, source2.value)
    })

    result.value = func.invoke(this.value, source2.value)

    return result
}