Fragment arguments without hassle !

FragmentArgumentDelegate.kt

package be.brol

import android.os.Binder
import android.os.Bundle
import android.os.Parcelable
import android.support.v4.app.BundleCompat
import android.support.v4.app.Fragment
import kotlin.reflect.KProperty

/**
 * 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
 */
object FragmentArgumentDelegate {

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

    operator fun <T : Any> setValue(thisRef: Fragment, property: KProperty<*>, value: T) {
        var args = thisRef.arguments
        if (args == null) {
            args = Bundle()
            thisRef.arguments = args
        }
        val key = property.name

        when (value) {
            is String -> args.putString(key, value)
            is ByteArray -> args.putByteArray(key, value)
            is CharArray -> args.putCharArray(key, value)
            is CharSequence -> args.putCharSequence(key, value)
            is Bundle -> args.putBundle(key, value)
            is Binder -> BundleCompat.putBinder(args, key, value)
            is Parcelable -> args.putParcelable(key, value)
            is java.io.Serializable -> args.putSerializable(key, value)
            else -> throw IllegalStateException("Type ${value.javaClass.canonicalName} of property ${property.name} is not supported")
        }
    }
}

WeatherCityFragment .kt

package be.brol

import android.os.Bundle
import android.support.v4.app.Fragment
import android.widget.Toast

/**
 * Example usage of FragmentArgumentDelegate
 */
class WeatherCityFragment : Fragment() {

    private var cityId: String by FragmentArgumentDelegate

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        Toast.makeText(activity, cityId, Toast.LENGTH_SHORT).show()
    }

    companion object {
        fun newInstance(cityId: String) = WeatherCityFragment().apply {
            this.cityId = cityId
        }
    }
}

I added the following changes compared to the original version:

• The delegate is a singleton object, so each fragment argument will be backed by the same instance. The argument will not be cached but read directly from the HashMap/ArrayMap inside the Bundle which should be fast enough.
• To write primitive types like int or long, putSerializable() is used. Because calling the generic setValue() method will already box the primitive value to an object which implements Serializable. Then we can avoid unboxing that object and reboxing it again because the value ends up being stored as an object in a HashMap/ArrayMap inside the bundle. So single boxing instead of double boxing and shorter code.