More fun with Kotlin delegates - delegates

Question

If you know Google's experimental Android Architecture Components, you probably know MutableLiveData. Trying to make it a bit more fun to use I came with:
class KotlinLiveData<T>(val default: T) {
val data = MutableLiveData<T>()
operator fun getValue(thisRef: Any?, property: KProperty<*>):T {
return data.value ?: default
}
operator fun setValue(thisRef: Any?, property: KProperty<*>, value:T) {
if (Looper.myLooper() == Looper.getMainLooper()) {
data.value = value
} else {
data.postValue(value)
}
}
}
And then I can:
var name : String by KotlinLiveData("not given")
name = "Chrzęszczybrzęczykiewicz"
But alas - that makes data which is needed i.e. to register Observer inaccessible:
name.data.observe(this, nameObserver) // won't work :(
Any idea if I can get it somehow?

Answer 1

You can access the delegate object of the property and get the MutableLiveData<T> from it:
inline fun <reified R> KProperty<*>.delegateAs<R>(): R? {
isAccessible = true
return getDelegate() as? R
}
Then the usage is:
::name.delegateAs<KotlinLiveData<String>>?.data?.observe(this, nameObserver)
To reference a member property, use this::name or someInstance::name.
This solution requires you to add the Kotlin reflection API, kotlin-reflect, as a dependency to your project. Also, due to the type erasure, the .delegateAs<KotlinLiveData<String>> call is not type-safe: it can only check that the delegate is KotlinLiveData<*> but not that its type argument is String.

Answer 2

Thanks to hotkey's solution, here's some better code:
class KotlinLiveData<T>(val default: T, val liveData : MutableLiveData<T>? = null) {
val data = liveData ?: MutableLiveData<T>()
operator fun getValue(thisRef: Any?, property: KProperty<*>):T {
return data.value ?: default
}
operator fun setValue(thisRef: Any?, property: KProperty<*>, value:T) {
if (Looper.myLooper() == Looper.getMainLooper()) {
data.value = value
} else {
data.postValue(value)
}
}
}
inline fun <reified R> KMutableProperty0<*>.getLiveData(): MutableLiveData<R> {
isAccessible = true
return (getDelegate() as KotlinLiveData<R>).data
}
inline fun <reified R> KMutableProperty0<*>.observe(owner: LifecycleOwner, obs : Observer<R>) {
isAccessible = true
(getDelegate() as KotlinLiveData<R>).data.observe(owner,obs)
}
Now I can:
someViewModel::name.observe(myActivity, Observer<String>{...})
with
someViewModel.name = "Kowalski, Leon"
working as expected
This class enables using LiveData with Android Data Binding out of the box.

Answer 3

the simplest way you can achieve is make the delegator to a field, for example:
#JvmField val dataOfName = KotlinLiveData("not given")
var name : String by dataOfName
then you can using live data in the class, for example:
dataOfName.data.observe(this, nameObserver)
name = "Chrzęszczybrzęczykiewicz"
OR you can write some syntax suglar, for example:
var name : String by live("not given").observe(this, nameObserver)
Note you can make nameObserver lazily too, for example:
val observers by lazy{mutableListOf<Observer>()}
var name : String by live("not given").observe(this){data->
observers.forEach{it.dataChanged(data)}
}
then you can do something like as below:
observers+= nameObserver;
name = "Chrzęszczybrzęczykiewicz"
observers-= nameObserver;