Frgment arg helper

FragmentArg.kt

/**
 * Eases the Fragment.newInstance ceremony by marking the fragment's args with this delegate
 * Just write the property in newInstance and read it like any other property after the fragment has been created
 *
 * Inspired by Jake Wharton, he mentioned it during his IO/17 talk about Kotlin
 */
class FragmentArg<T : Any?> : kotlin.properties.ReadWriteProperty<Fragment, T?> {

    var value: T? = null

    override operator fun getValue(thisRef: android.support.v4.app.Fragment, property: kotlin.reflect.KProperty<*>): T {
        if (value == null) {
            val args = thisRef.arguments ?: throw IllegalStateException("Cannot read property ${property.name} if no arguments have been set")
            @Suppress("UNCHECKED_CAST")
            value = args.get(property.name) as T
        }
        return value ?: throw IllegalStateException("Property ${property.name} could not be read")
    }

    override operator fun setValue(thisRef: android.support.v4.app.Fragment, property: kotlin.reflect.KProperty<*>, value: T?) {
        if (thisRef.arguments == null) thisRef.arguments = android.os.Bundle()

        val args = thisRef.arguments
        val key = property.name

        when (value) {
            is String -> args?.putString(key, value)
            is Int -> args?.putInt(key, value)
            is Short -> args?.putShort(key, value)
            is Long -> args?.putLong(key, value)
            is Byte -> args?.putByte(key, value)
            is ByteArray -> args?.putByteArray(key, value)
            is Char -> args?.putChar(key, value)
            is CharArray -> args?.putCharArray(key, value)
            is CharSequence -> args?.putCharSequence(key, value)
            is Float -> args?.putFloat(key, value)
            is Bundle -> args?.putBundle(key, value)
            is Binder -> args?.let { BundleCompat.putBinder(it, key, value) }
            is android.os.Parcelable -> args?.putParcelable(key, value)
            is java.io.Serializable -> args?.putSerializable(key, value)
            else -> throw IllegalStateException("Type of property ${property.name} is not supported")
        }
    }
}

    private var filterId by FragmentArg<Long>()

    companion object {
        fun newInstance(filterId: Long = ABSENT_ID) = ThisFragment().apply {
            this.filterId = filterId
        }
    }

Thanks for sharing. It's beautiful) You should definitely make a pull request to android ktx.